msc. thesis: the irrigation system as an open system c.lieveld...

MSc. Thesis: The Irrigation System as an Open System C.Lieveld

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Table of Content PREFACE

INTRODUCTION................................................................................................................................. 3

CHAPTER 2 GENERAL INFORMATION................................................................................... 4

2.1 ARGENTINA ............................................................................................................................. 4 2.2 SANTIAGO DEL ESTERO ........................................................................................................... 6

CHAPTER 3 RESEARCH PROBLEM.......................................................................................... 9

CHAPTER 4 DESCRIPTION OF THE BASIN AND IRRIGATION SYSTEM......................... 11

4.1 THE RIO DULCE RIVER BASIN............................................................................................... 11 4.1.1 The upper basin ............................................................................................................. 12 4.1.2 The middle basin ........................................................................................................... 12 4.1.3 The lower basin ............................................................................................................. 13 4.1.4 La Mar Chiquita............................................................................................................ 14

4.2 DESCRIPTION OF THE PROYECTO RIO DULCE ....................................................................... 16 4.2.1 Geological description of the PRD................................................................................ 16 4.2.2 Development of the area................................................................................................ 17 4.2.3 Design of the PRD system ............................................................................................. 19 4.2.4 The current PRD............................................................................................................ 20 4.2.5 The water management.................................................................................................. 25 4.2.6 Further remarks on the PRD area................................................................................. 28 4.2.7 Problems in the area ..................................................................................................... 29

CHAPTER 5 METHOD OF RESEARCH................................................................................... 30

5.1 THEORY ................................................................................................................................. 30 5.1.1 The hydrological cycle .................................................................................................. 30 5.1.2 The irrigation water balance......................................................................................... 32 5.1.3 Fractions ....................................................................................................................... 35 5.1.4 Combining the models ................................................................................................... 36 5.1.5 Interaction between the two systems ............................................................................. 38

5.2 METHOD OF APPLICATION ..................................................................................................... 40 5.2.1 The total incoming volume (I) ....................................................................................... 40 5.2.2 Evaporated fraction (EF) .............................................................................................. 42 5.2.3 Surface Fraction (SF).................................................................................................... 48 5.2.4 Sub-surface Fraction (GF) ............................................................................................ 51 5.2.5 Summary of the Assumptions......................................................................................... 53

CHAPTER 6 RESULTS OF THE APPLICATION.................................................................... 55

6.1 THE PRD................................................................................................................................ 55 6.3 ZONE 2 ................................................................................................................................... 64 6.4 ZONE 3 ................................................................................................................................... 69 6.5 ZONE 4 ................................................................................................................................... 73 6.6 ZONE 5 ................................................................................................................................... 77 6.7 DISCUSSION ........................................................................................................................... 81

CONCLUSIONS & RECOMMENDATIONS.................................................................................. 87

BIBLIOGRAPHY ............................................................................................................................... 91


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ANNEX 1: MAPS OF ARGENTINA

ANNEX 2: CLIMATIC DATA

ANNEX 3: MAPS OF THE PRD

ANNEX 4: DATA USED

ANNEX 5: RESULTS


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Introduction Today twice the volume of water is extracted in comparison to 50 years ago (Gleick, cited from WCD report). A growing population and a rising level of economic activity both increase human demand for water and water-related services. The fresh water supply, however, remains the same. Currently water managers and planners apply non-structural measures, such as increasing the efficiency of needs and allocation, in order to solve the growing gap in water supplies. These measures aim at improving the productivity of the available water. This mode of thinking extends itself to the irrigation system, which is regarded as the largest water user in the basin. Increasing the efficiency of an irrigation system though may not have the intended effect, namely making more water available for other users in the river basin. Allen et al (1997), in their article on water conservation, explain that taking measures to increase the efficiency comes from thinking of the irrigation system as a closed system, thus isolated from its surroundings, when in fact it is not. The subject of this rapport is the irrigation system as an open system, thus understanding the irrigation system as a part of its surrounding. Great emphasis is put on the interactions between the irrigation system and the river basin and an attempt is made to quantify the flows between them. To this end a waterbalance is used. The perception of the open irrigation system was applied to the Proyecto Rio Dulce an irrigation system in the Rio Dulce river basin. The irrigation project itself lies in the province of Santiago del Estero in the country Argentina. The river basin lies in the provinces of Salta, Catamarca, Tucumán, Santiago del Estero and Cordoba. This thesis is built up of several components, its structure follows. Chapter 2 gives some background information on the country Argentina and the province of Santiago del Estero. Chapter 3 takes a closer look at the closed system and the open system and gives a formulation of the subject researched in this thesis. Chapter 4 gives a description of the Rio Dulce river basin and the irrigation system Proyecto Rio Dulce, upon which the open system perspective is applied. Chapter 5 takes a closer look at the theory in order to formulate a model that can be applied to quantify the interactions between basin and system. In the second part of this chapter the method of application is presented with which the interaction can be quantified. The method of application is applied to the Proyecto Rio Dulce and in chapter 6 the results are presented and discussed. Finally, in the last chapter conclusions are drawn and recommendations for further refining the water balance are presented.


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CCHHAAPPTTEERR 22 GGEENNEERRAALL IINNFFOORRMMAATTIIOONN In this chapter some background information is given on Argentina and the province of Santiago del Estero. It serves as a preliminary orientation of the social and physical context of which the Proyecto Rio Dulce is a part.

2.1 Argentina

Figure 2.1: Worldmap with the location of Argentina

Argentina is a vast country. República Argentina as it is officially named is a republic that lies in the southern part of the continent South America. The total area of the country is 2.766.890 km², which makes it the second largest country in South America and the eighth largest country in the world, immediately behind India. The country has a length of about 3.500 km and a maximum width of 1.400 km. It is about 65 times as big as the Netherlands. In the east Argentina borders on Brazil and Uruguay in the northeast it borders on Paraguay, in the north on Bolivia and in the west it borders on Chili. In the east and southeast lies the Atlantic Ocean. In the far south of Argentina lies the Beagle canal. The Andes in the west forms a natural line between Argentina and Chili. Argentina does not own any territory outside of the continental territory, but has claims on the Falkland Islands (Islas Malvinas), South Georgia, the Southern Orkeny islands (Orkadas del Sud) as well a sector of the Antarctic continent. The name Argentina is derived from the Latin word argentum, which means silver. The capital of this country is Buenos Aires. The official name of the capital is: “Ciudad de Nuestra Señora de Buenos Aires”. Argentina consists of the Federal Capital and 23 provinces: Buenos Aires, Córdoba, Corrientes, Entre Rios, Mendoza, Tucumán, Santa Fé, Santiago del Estero, Formosa, Misiones, Jujuy, Rio Negro, Catamarca, Chubut, Chaco, Salta, La Rioja, San Juan, San Luis, La Pampa, Neuquén, Santa Cruz and the Tierra del Fuego with a couple of south Atlantic islands and a part of the Antarctica that together form a province. Argentina has a population of 36.260.130 people (2001). The biggest city is Buenos Aires, which is inhabited by approximately 3 million people and roughly 11 million people live in the Greater Buenos Aires (the metropolitan area next to the city of Buenos Aires), which makes it


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one of the largest conglomerates in the world. The second and third largest cities are Córdoba and Rosario each consisting of 1,3 million inhabitants.

Figure 2.2: Political Division of Argentina

Geography Thanks to its longitudinal position, between the Tropic of Cancer and Antarctica, the country has a great diversity of climates and landscapes (mountainous regions, lakes, steppe plateaus and areas with tropical vegetation) but is chiefly characterized by lowlands (the pampas). The country can roughly be divided into 4 great natural territories:

• The “Pampa Humeda”, a vast fertile plain, which lies in a radius of 500 to 750 km around Buenos Aires. About two thirds of the total Argentinean population lives in this area. It encompasses the provinces of Buenos Aires and parts of the provinces Córdoba, Santa Fé and La Pampa. It is the primary agriculture and cattle-breeding region in Argentina and harbours three fourths of the countries industry.

• The Andes area that encloses the eastern slopes of the Cordillera, the Pre-Cordillera and the highlands in the northwest.

• The Chaco, which is an area in the north with forests and dessert like lowlands.

• Patagonia, a thinly populated and bare plateau south of the Rio Colorado, including the Tierra del Fuego that is separated from the American continent by the Strait Magelhaens.

The most important rivers are the Parana and the Uruguay, which both end in a broad estuary, Rio de la Plata. More information on Argentina can be found in annex 1.


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2.2 Santiago del Estero

The province of Santiago del Estero is located between 61° and 64° Western longitude and 30° and 26° Southern latitude. It has a total area of 136.351 km² (roughly 4 times the size of

the Netherlands) and is surrounded by the provinces Chaco, Catamarca, Córdoba, Salta, Santa Fe and Tucumán.

Figure 2.3: Santiago del Estero

This province was inhabited by 819.062 people1 in 2002 and consists of 27 departments (see annex 2). The capital has the same name as the province, Santiago del Estero, and lies in the department Capital. The important cities in the province, the departments in which they lie and their number of inhabitants are presented in the following table.

City Department Number of inhabitants

1. Santiago del Estero Capital 148.357

2. La Banda Banda 46.994

3. Frías Choya 20.901

4. Termas de Rio Hondo Rio Hondo 20.652

5. Añatuya Taboada 15.025

6. Quimilí Moreno 8.972

7. Villa San Martin Loreto 6.237

8. Fernandez Robles 6.062

9. Clodomira Banda 5.945 Table 2.1: The nine biggest cities in Santiago del Estero (Source: INTA)

1 From Ministerio del Interior; website: www.mininterior.gov.ar


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The most important departments, when regarding the concentration of population and economic development, are Capital and Banda. Five of the nine biggest cities (those highlighted in blue) lie within the irrigation system Proyecto Rio Dulce, the subject of this thesis. See annex 1C for a map of Santiago del Estero and its departments. Climate Santiago del Estero is a semi arid region. There are mild winters and hot summers. This is due to the narrowness of the southern part of the South American continent, which prevents the development of cold continental air masses such as are experienced in the northern hemisphere. In the winter (June to August) the wind comes from the south and at night temperatures may drop below freezing. Day temperatures in winter may rise above 20°C. In the summer, the wind blows from the north and is often very hot and dry. Temperatures may rise above 40°C during the day and stay above 25°C at night. Santiago del Estero is situated in the so-called South American Heat Pole, with a maximum absolute temperature of 47°C. The average annual temperature varies from 19° C to 22°C from S to N (see annex 2). The annual precipitation, mainly summer rains (November- April) ranges from 500 mm to 850mm; main annual precipitation in the Proyecto Rio Dulce area equals 593,3 mm. Winters are almost totally dry; Santiago del Estero has an average of six dry months (April/May-September/October)(Torres Bruchman 1981). The province has a hydric deficit all around the year, because evapotranspiration exceeds the precipitation even during the wet season. There is spatial variability in the precipitation pattern. The highest precipitation is in the east. Maximum values are in the southeast (800 mm/year), decreasing towards the center (550 mm/year) and increasing again to the northwest (600 mm/year). The hydric balance is in general negative, also in the spring and summer. The potential evapotranspiration oscillates between the 900 and 1.100 mm annually. For further information concerning the climate see annex 2. Geography The province is for the greatest part a plain without orografic elevations of importance. Its

slope goes in the direction from NW (highest NW point is 300 m above M.S.L) to SE (lowest SE point is 180 m above M.S.L.). The original material, the climatic characteristics and the dominantly flat landscape have the result that there are soils of limited development and an instable structure and are greatly susceptible to hydric and eolic erosion. The province is crossed by 5 rivers: Dulce, Salado, Horcones, Urueña and Albigasta (see annex 1C) The most important of these rivers are the Rio Dulce and the Rio Salado due to their great discharge and permanent transportation of water. The Rio Dulce is the more important of the two because of the economic and human importance of its route. The greatest part of the water streaming through these two rivers is used for agriculture and cattle farming, thus providing the economic foundation of the province.

Figure 2.4: Rivers in Sgo. Del Estero


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Economic activity The most important economic sectors in the province are agriculture, livestock farming and forestry. The most important activities within the agriculture is the cultivation of cotton, soy, maize and vegetables (especially onions) and cattle farming. The industrial sector has had limited development and the principal activity has been the cultivation of cotton. Agriculture

• Cotton Santiago del Estero delivers the second largest production of cotton in the country. The greater part of the cotton is produced in the east of the province in the departments Moreno, Juan F. Ibarra, Alberdi and Taboada, under rain fed conditions. The rest is produced in the departments Banda, Capital, Robles, Figueroa and Silipica (PRD) under irrigation.

• Soy This crop is principally cultivated in unirrigated areas in the departments Belgrano, Moreno, Taboada, Rivadavia, Pellegrini and Jiménez. Between 1991 and 2002 the area sown has grown eightfold. The past year the area sown with this crop was 614.000 has. The mean yield was 2.000 kg/ha. Due to a lack of processing factories, the crop is exported to other provinces for further processing.

• Maize This crop is cultivated within the PRD with a yield of 4.000 kg/ha and under rain fed conditions in the departments Moreno, Rivadavia, Juan F. Ibarra, Belgrano and Taboada with a yield of 3.000 kg/ha.

• Vegetables The cultivation of vegetables takes place in the PRD. The principal crops are onions, sweet potato, Cucurbitaceae (squash, watermelon and melon), tomato and sweet maize. The most important of these is onions, of which Santiago del Estero produces around 17% of the national production. The cultivated vegetables are intended for local markets and the Central Market.

Livestock farming Within the livestock-farming sector, the most important forms are cattle farming and goat farming. The former is practiced in the departments Moreno, Belgrano, Rivadivia and Taboada because the climatic conditions in these areas are more favourable for this type of farming. Forestry Santiago del Estero is the most important producer of charcoal. The production consists of about the half of the total national production. Charcoal is chiefly produced in the eastern departments Copo, Alberdi, Moreno, Ibarra, Taboada and Belgrano. For this purpose the wood from native forests are used. The majority of the production is used for consumption and a small percentage is exported. Export The income from exported products was around the 14 million euros in 2002. Of the total export around 94% consisted of primary products, the unprocessed products, especially cereals and oleaginosas, but also vegetables. Around 5% of the exported products were of manufactured products from agricultural crops (especially the cotton fibers) and 1% were industrially manufactured products. The countries to which products were exported in 2002 were:

• Brazil 34%

• European Union 30%

• Asia 22%

• Chile 8%

• The rest of the world 6%


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CCHHAAPPTTEERR 33 RREESSEEAARRCCHH PPRROOBBLLEEMM Today around 3.800 km³ of fresh water is withdrawn annually from the world’s lakes, rivers and aquifers. This is twice the volume extracted 50 years ago (Gleick, cited from WCD report). A growing population and a rising level of economic activity both increase human demand for water and water-related services. While the demand continues to rise, the basic amount of fresh water supply provided by the hydrological cycle does not. The water management problem thus consists of bridging the gap and managing the water resources in such a way that the needs of the population are met. Prior to the 1980’s the solution for water problems was sought in increasing the water supply: adapting the natural hydrological cycle through physical infrastructure (reservoirs, interbasin transfer through pipelines and aqueducts etc) and tapping groundwater on an increasingly larger scale. These solutions are increasingly opposed due to their environmental, economic and social consequences. Currently solutions are sought in non-structural measures such as increasing the efficiency of needs and allocation in order to improve the productivity of the available water. In other words planners focus on doing more with what is available rather than building new infrastructure to capture and transport water. Irrigation and non structural solutions An estimated 75 % of all the water that is withdrawn from rivers or underground resources is used for irrigation.2 Analysts have estimated that the overall project efficiency of agricultural water use worldwide is about 40%3, i.e. that only 40% of the water withdrawn from the rivers or underground resources is used beneficially and the rest is lost or wasted. In regions where water is scarce, the irrigation sector is a great candidate for the non-structural measures described above. Measures are focused on increasing the efficiency so less water is used in the irrigation system and more water is available for other river basin users.

Frederiksen (1996) however argues that it is a great misconception that “improved agricultural efficiency will yield substantial quantities of new water [for other river basin water users]”. Frederiksen et al (1997) believe this mistaken belief is caused by the use of the term irrigation efficiency. Thus it is useful to take a closer look at this term. The efficiency of an irrigation system is calculated in order to monitor how well the irrigation system performs the task it was built for, namely the collection, transportation and distribution of water. This term is defined as the ratio of the total amount of water needed (output) and the total amount of water available (input). The outcome tells the percentage of the water that is used beneficially, thus used for the intended purpose (providing the crops with the needed water), and how much non-beneficially – due to canal infiltration and operational spills. The goal beforehand is to improve the efficiency of the system and as a consequence decrease the quantity non-beneficially used water so this water becomes available for other users within the basin. To this end measures such as lining canals to prevent infiltration, designing ever more advanced distribution and offtake structures to deliver the precisely calculated water requirements in order to decrease operational spills and excess water delivery, new methods of irrigation such as drip irrigation etc are proposed. According to Frederiksen et al (1997): “The term ‘irrigation efficiency’ in irrigation evaluations focuses on beneficial needs, performance, and water movement in and out of the boundaries of an area (garden, farm or irrigation project). An efficiency term, while useful for identifying

2 Worldbank (at www.worldbank.org)

3 Postel, 1997 cited from Gleick 2000


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needed improvements in irrigation systems and practices, ignores the destiny of the unused water“. Frederiksen (1996) explains that although most irrigation projects have an efficiency of only 40%, about 50 – 60 % of the ‘losses’ are passed on to downstream irrigation and urban users or to underlying aquifers for subsequent use. An example is given in Frederiksen et al (1997) of irrigation projects in eastern Idaho, which have an irrigation efficiency of 12 to 40 %. A large fraction of the unused water enters groundwater systems via deep percolation (in the springtime). The local groundwater systems join the massive Snake Plain Aquifer system, which discharges water back into the Snake River in the late summer and fall via springs along the way. These return flows have had great benefits for the fisheries sector and the hydropower production. The example makes it clear that a reduction in irrigation diversions will result in a reduction in the return flows back to the resource and could lead to a reduction of outflow for downstream users. Perry (1999) also cites examples in which the improvement of the irrigation efficiency has not resulted in the expected water savings. In one example in a European country a shift was made to sprinkler and drip irrigation. This measure resulted in a severe depletion of aquifers on which its urban supplies were dependent. In another example in a South American country the water rights to the water saved from increasing the efficiency by upstream users were sold to downstream users who depended on the return flow. What the examples demonstrate is when there is an insufficient understanding of the destiny of the unused water, what may seem as saving water could in fact be a transition in water users, thus from groundwater users to surface water users. The destiny of the unused water is ignored because: “…it is common to draw ‘lines’ around project boundaries and to neglect the real interconnections between in-project ‘losses’ and existing river systems gains”4. Thus the irrigation system is perceived as isolated from its surroundings, which is why it is also referred to as a closed irrigation system. Efficiency is a product of this perception as are the measures taken. Regarding an irrigation system as such and formulating water allocation policies based on this perception can lead to unforeseen problems in basin management as can be seen from the above cited examples. This is because there is no real understanding of how the total river system functions, how surface and groundwater interact on a basin level and how the irrigation system, among many other systems in the basin, interacts with the river basin. Therefore an effort is made to regard and understand the irrigation system as an open system, thus neglecting the boundaries. The open system is defined as a system in interaction with the basin within which it lies. The question then rises: “How is an open system approached?”. The purpose of this report is to answer this question by providing a basis upon which the irrigation system can be regarded as such. Secondly, to try and quantify the interaction between an irrigation system (Proyecto Rio Dulce) in the province of Santiago del Estero of the country Argentina and the river basin within which it lies. Research Problem Defining the open irrigation system as a system in interaction with the basin wherein it lies and taking into account the characteristics of the Proyecto Rio Dulce irrigation system, how does the Proyecto Rio Dulce behave as an open system within the river basin context? Research objective The purpose of this research is thus to approach the Proyecto Rio Dulce as an open irrigation system by:

• Providing a basis upon which the irrigation system can be approached as such and

• Determining the interaction between river basin and irrigation system.

4 Allen, RG; Willardson, LS; Frederiksen, HD (1997)


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CCHHAAPPTTEERR 44 DDEESSCCRRIIPPTTIIOONN OOFF TTHHEE BBAASSIINN AANNDD IIRRRRIIGGAATTIIOONN SSYYSSTTEEMM In this chapter a closer look is taken at the Proyecto Rio Dulce and the river basin within which it lies. The first paragraph describes the Rio Dulce river basin, important regulation structures relevant to the Proyecto Rio Dulce and the major users in the basin. The second paragraph describes the Proyecto Rio Dulce irrigation system, which from hence forward will be referred to with the acronym PRD.

4.1 The Rio Dulce River Basin The Rio Dulce basin has an area of approximately 67.500 km² and runs through the provinces of Salta, Catamarca, Tucumán, Santiago del Estero and Córdoba in North West Argentina.

Figure 4.1: Rio Dulce river basin


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The basin can be divided into four different sections, which are all discussed in this paragraph:

• The upper basin

• The middle basin

• The lower basin

• La Mar Chiquita

4.1.1 The upper basin

This is area I in figure 4.1 marked by a blue box. This part of the river basin located before the Embalse Rio Hondo. The water in the river is collected in the districts of Salta, Catamarca and Tucumán. This area is mountainous and the climate in this region can be considered as humid with an average rainfall of 800 mm up to 1.000 mm per year. The distribution of precipitation in this region is irregular both in space and time. Of the total rainfall, 90% is concentrated in the mountainous area in the west of the province during the humid periods. The greatest portion of water that flows through the Rio Dulce is accordingly derived from this area. In 1968 the provinces Santiago del Estero, Tucumán and Córdoba agreed to divide the use of the average module of the river (calculated to be 3.600 hm³/year) in the following manner: Tucumán 32%, Santiago del Estero 49% and Córdoba 14%. In volumetric units, Santiago del Estero receives 1.773 * 106 m³, using 1.548 * 106 m³/year for the design of the PRD (Romanella, 1971).5 Embalse Rio Hondo The water from the upper basin is stored in the reservoir of the Rio Hondo. The Agua y Energía Electrica (AyEE) presented plans for this dam in 1957 and it was completed in 1968. The reservoir covers approximately 33.000 ha with 1.740 hm³ of storage capacity. Its functions are flood control, storage of water for irrigation purposes and electricity generation (Angella). It forms an important part of the irrigation infrastructure of the PRD. The reservoirs capacity can only provide for annual regulation. Therefore in a year with less than average rainfall or management problems, the reservoir cannot fully meet the diversion requirements for the total irrigable area (Ertsen). In the table below some technical information on the dam is given.

Collected water from upper basin Characteristics Rio Hondo Mean annual runoff 3.253 hm³/year Max. capacity 1.740 hm³

Max. annual runoff 7.519 hm³/year Max. height 274 m a.s.l.

Min. annual runoff 402 hm³/year Total height 43,25 m

Max. monthly runoff 2.492 hm³/month Length concrete dam 206 m

Min. monthly runoff 0,52 hm³/month Length earth dam 4.119 m

Mean annual flow rate 103,1 m³/s Max. mean daily discharge 2.916 m³/s

Min. mean daily discharge 0,0 m³/s

Max power capacity 15.200 kW Table 4.1: Technical information on the Rio Hondo dam

4.1.2 The middle basin

This is area II marked by a yellow box in figure 4.1. The middle section reaches from the Rio Hondo dam to the smaller Los Quiroga dam. Here the river enters the province of Santiago del Estero and has a change of name from Rio Salí in the section before the Rio Hondo dam to Rio Dulce below the dam. This part of the basin has gentle slopes (0,7 – 0,3 ‰) and less

5 Prieto


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rain (500-800 mm). As most soils in semi-arid areas, soluble salts can be found in the subsurface horizons, because percolated rain is not enough to leach the salts. In this section there are two users: the PRD irrigation system and the city of Santiago del Estero. Proyecto Rio Dulce The dam of Los Quiroga diverts the portion of river water needed for the PRD into the system. The remaining discharge proceeds to the lower basin. Since this irrigation area is the focus of study, a more detailed description will be given of this water user in the following paragraph.

Santiago del Estero and La Banda About half of the total population in the province of Santiago del Estero lives here. The houses use groundwater, which is pumped up from the ground with a hydro fore. In some neighborhoods water, however, is taken directly from the San Martin.

4.1.3 The lower basin

This is area III marked by the purple box in figure 4.1. This area is a vast plain with a slope of 0,01%. The Rio Dulce runs through this area on its way to its final destination, La Mar Chiquita, due to the low slope of the landscape it is not clearly defined and has a strong tendency to diverge evermore eastward. Here the water tables are generally shallow with high salt contents, including salts leached from upstream areas. It is in this area that bañados

(Spanish word for wetlands) are regularly produced. Important water users in this area are the smaller irrigation system and the cattle herders.

Downstream Irrigation system Further downstream of the PRD, especially in the department of Malbran and Atamisqui there is irrigation on a smaller scale. The farms range from medium size to large (areas of up to 100.000 ha). Here an extensive form of cattle farming is practised. During the years some of the farmers started planting soy on their farms because there is a great demand for it on the world market. Water is thus needed as drink water for the cattle, for the grass on which the cattle feeds to grow and for the soy. This water is derived from the Río Dulce and the Río Salado. This area has great water problem. The Río Dulce has an abundance of water in the summer (December-March) of such a quantity that it sometimes floods the lands of the farmers and drowns the cattle. In this period the farmers store water in reservoirs for drier periods. In the winter (June- August) it hardly ever rains. The farmers hardly have enough water for their cattle and use the water previously stored in the reservoirs, which is often not sufficient.

The bañados del Rio Dulce This is an important wetland in the southern part of the province of Santiago del Estero. It ultimately ends in the Mar Chiquita. According to the PERD on the bañados (1982) about 1.400.000 ha of area was at one point or another covered with the bañados. In the flood periods (winter), the river flows more or less channelled until the south of Loreto, just before Villa Atamisqui. Once the discharge supersedes its riverbeds, the floods begin and the water flows into the streams of the Saladillo and the Utis. When the riverbeds of the Utis are superseded, the bañados are produced. The result of a very high volume of water is an inundation. In the north of this area starting from Villa Atamisqui the bañados are of short duration. From the center of this area to the south the drainage conditions are gradually limited, which permits a greater permanence of the bañados. The bañados are more evident to the south of provincial route nº 15, due to the rain and accumulations from hydrological cycle.


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Figure 4.2: The bañados ( source: M. Pagot)

4.1.4 La Mar Chiquita

This is area IV marked by the green box in figure 4.1.This lake is the largest in Argentina and one of the largest saline lakes in the world, with waters varying between saline and hyper saline. The lake has an area of 6.000 km² and is shallow (water depth is around 10 m).

Figure 4.3: Mar Chiquita and the part of the bañados protected by RAMSAR

Banados

La Mar Chiquita


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The most important contributor to the lake is the Rio Dulce. Other contributors are the Primero and Segundo Rivers (coming from the Sierras de Cordoba Mountains, in Cordoba Province), which contribute about 17% of the discharge of the Rio Dulce. The lake is considered one of the most important wetlands in Argentina and in the Chaco ecoregion in terms of the richness of its biodiversity in a range from freshwater to very saline. Therefore La Mar Chiquita and part of the bañados (the part that lies in the province of Córdoba) are protected by the convention of RAMSAR (see figure 4.3).


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4.2 Description of the Proyecto Rio Dulce The physical conditions of the area, the state of the infrastructure and the irrigation culture among system managers and farmers are important factors that determine how the incoming water is divided over atmosphere, surface and ground within the irrigation system. The focus of this paragraph is to describe these factors for the PRD.

4.2.1 Geological description of the PRD

In chapter 2 the climatological conditions in Santiago del Estero and consequently for the PRD have been described already. In the previous paragraph the context, the river basin, within which the PRD lies, has been described. Here the focus will be on the geological conditions of the PRD, specifically the grounds, their quality and the subterranean flow. The PRD is situated between 27°51’ and 28°30’ Southern Latitude and 64°20’ and 63°45’ Western Longitude. It takes in the departments Capital, Loreto and Silípica on the right margin and Robles, Banda and San Martín on the left margin. Observing the hydro-geological map of the whole province of Santiago del Estero shown in annex 1 the general direction of the of the subterranean flow is from the NW to the SE, consequently from the highest point of the province to the lowest point. The little white arrows represent the subterranean flow. For the specific area of the PRD the map shown in annex 2 shows the different hydro-geological units. As can be seen from the map the PRD for the largest part lies on an alluvial fan (2). To the north of the PRD and partially covering zone I lies the Saladillo de Huyamampa (1). South of the alluvial fan is the zone of the old riverbanks of the Rio Dulce (4). Alongside the fan, to the west and east, and partially covering zone IV lie the löss planes. As mentioned above, the greater part of the PRD lies on an alluvial fan. The fan has a general gentle slope of about 1o/oo in the direction W to NE, E and SE. Within this area there are local slopes that range from 7 o/oo to 0,3 o/oo (see annex 2 for the isoheight map of this area). It occupies a great part of the departments: Capital, Banda, Robles, San Martín and Silípica and stretches from the provincial capital in the West to the city Fernandez in the east and from the city Clodomira in the north to the population of Arraga in the south. The layer has a depth of around 200 m and consists only of clastic material. The soil ranges from porous textures like sands and rough gravel in the apex of the fan to finer textures of medium to fine sands, clay and loams at the edges. Accordingly the permeability will also decrease from the apex to the edges of the fan. The fan consists of multiple aquifers. The phreatic aquifer ranges in depth from 3 to 10 m. The direction of the subterranean flow is from west to east, where the piezometric level decreases from – 4,80m to –7,80 m. The chemical quality in the phreatic aquifer is good, although large parts of the fan are salty to brackish. From west to east the percentage of the ion sulphate increases from 550 mg/l to 960 mg/l. In the upper, including the top aquifer, the concentration of arsenic is high. It supersedes the level possible for human consumption. Soil studies conducted in 1968 by Nijhenson show that from the total area of the PRD, 45% was found without limitations due to salinity and sodicity; 16,3 % had a moderate salinity (EC: 4-16 deci-Siemens per meter-dS/m) and required a mean application for leaching of 500 mm per year; 8,8% had moderate to severe salinity problems and moderate sodicity and


17

required rather higher water depth for leaching (500-1.500 mm). Finally, 7,4% of the soils were considered unsuitable for production due to a high sodicity.6

To the north of the fan lies the Saladillo de Huyamampa, a depression. The phreatic level varies from a depth of 5 m in marginal areas to 2 m in the center and south of the Depression of the Saladillos. This conditions the subterranean flow, which has the direction north � south and accordingly the groundwater flow into the PRD. This depression has a high concentration of salts. The dry residue also increases from the marginal areas to the center of the depression. The concentration of these salts oscillates in value from 0,6 to 60,8 g/l, the low concentrations occurring when the area is recharged by precipitation. The zone to the south of the fan is an alluvial plain that digresses from the Rio Dulce from the south of the capital city of the province. Here the slope diminishes creating a meandering river of which some spirals are abandoned. The abandoned spirals form geomorphologic units that are hydraulically still connected to the Rio Dulce and form good reservoirs of fresh water of pluvial origin. These sedimentary units are located at brief depths and in some cases at a depth of not more than 25 or 30 m. The units are integrated by sands that are medium fine to rough. Generally the units contain water of low salinity, with characteristic volumes of 5 m³/h to 10 m³/h. The specific discharges are low. The Löss plain to the East and West of the alluvial fan are part of the alluvial fan of the Rio Salado that in digressing formed a series of riverbanks that are at present dry and filled with fine sediments. Its main characteristic presents the effect of superposition of Aeolian-Fluvial packages that demonstrate the first aspect of the direction SW-NE of the arboreal cords and the second by the direction of old channels that disappear in the direction of the east.

4.2.2 Development of the area

Around1552 the first Spanish settlers came to the area and established themselves on the primitively irrigated areas of the Pre-Colombian era. They planted new crops on the land and expanded the planted area. Around 1577 these settlers dug their first irrigation ditch (acequia) in Santiago del Estero, which would be repeatedly destroyed by the Rio Dulce until a permanent canal was constructed around 1650. In 1680 an irrigator’s register was established. Individual landowners dug acequias from the river directly to their land. In 1870 there were 73 of these acequias. Most of these canals were longer than 10 km; some extending up to 50 km with a width of 6 m. With these acequias around 8000 ha of land was irrigated, officially. In practice this figure would have been higher. In 1878 a canal, La Cuarteada was dug with the intention to divert floodwaters from the Rio Dulce to the Rio Salado. The canal, however, only succeeded in inundating the surrounding area. Soon afterwards, individual agriculturalists began to build acequias from La Cuarteada to their property, thus changing the character of the canal (flood diversion � irrigation canal). In 1886 an intake structure was constructed for La Cuarteada. Soon afterwards it was washed away by the Rio Dulce. In this year a development plan for the acequias in the area of La Cuarteada was formulated.

6 Angella (1999)


18

In 1898 a new intake structure was built. This structure was the largest in Argentina in those days. Around 1905 the first public irrigation system in Santiago del Estero was implemented at the left bank of the Rio Dulce (see figure 4.4). Water was diverted through an intake structure into a main canal at the end of which it was distributed into three secondary canals: Canal Norte, Canal Sud and Canal La Cuarteada. This system would become the foundation for the existing infrastructure. It irrigated about 38.500 ha. A further 14.500 ha were irrigated from private acequias. In 1913 a communal canal, Canal San Martin, with a length of 64 km was dug on the right bank of the river for the farmers there.

Figure 4 .4: The first public irrigation system (left bank) and the right bank system

The irrigation systems on both riverbanks derived water when the flow and water level of the river were sufficiently high. There was plenty of water available in the wet season (December- April), but the water availability was limited in the dry season (July – October). Another problem was that the quantity of water available differed from year to year. Due to this temporal variability of the Rio Dulce, farmers were not certain they would receive enough water. Also around 1923 more (European) farmers settled in the area. This meant there was more ground to be irrigated and less water to irrigate them with. With the situation as it was then, the water was not enough to sustain the needs of the farmers.


19

These events lead to the building of a permanent diversion weir Dique Los Quiroga in 1947.

Figure 4.5: Los Quiroga

The weir at first only diverted water into the canal system on the left bank. In periods of low flow, however, the canal system on the right bank would not receive enough water. So in 1954 the canal systems on both banks were connected via a siphon. The overall water availability was further improved by building a reservoir in northwest Santiago, the Embalse del Rio Hondo (1957-1968). The reservoir deals with the annual regulation of the river, hence supplying water to the irrigation systems all year round. It, nonetheless, has insufficient capacity to meat the total diversion requirements in a dry year. In 1966 the Proyecto Rio Dulce, an ambitious project to modernize the irrigation system, was formulated by Agua y Energía Electrica (AyEE) and a provincial institution Corporación de Rio Dulce (CRD). The plan implied among other things:

• Modernizing the distribution canals. The canals were to be extended and a tertiary distribution network would be built. Also 48.000 ha were to be provided with drainage facilities.

• Rehabilitation of the distribution canals

• Changes in the way water is handled

• Institutional reorganization of Operation and Management

• Financial advise and support for the modernization of the infrastructure

• Irrigation management on the properties

• Colonization programs

• Help for the commercialisation of the products Implementation of PRD was not finished. This left its zones with an uneven development of its infrastructure for water control.

4.2.3 Design of the PRD system

Around 1965, before the project, an area of 60.000 ha was irrigated with Rio Dulce water. In 1968 the National Government decided to implement the Proyecto Rio Dulce with the intention of modernizing the water management and agricultural practices in the area. As mentioned above the AyEE and the CRD formulated the plan. The Project consisted of:

• Improvement of the water control by modernizing the infrastructure. This meant the construction of the Rio Hondo dam, the redesign of the irrigation and drainage canal network and the modernization of the water control structures.

• The introduction of a water control organisation AyEE, which would bare the responsibility for the operation and maintenance of the system.


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The design of the PRD was based on the inter-provincial accord between the provinces Santiago del Estero, Córdoba and Tucumán mentioned in the previous paragraph. This accord, which was signed in 1968, stated that Santiago would receive 49% of the water in the Rio Dulce, which comes down to an amount of 1.773 hm³. Of the water assigned to the

province 1.548 hm³ (≈ 87%) was at the disposal of the PRD. Based on this quantity of water the maximum irrigable area was determined. According to the Harza Feasibility report for the PRD, the amount of water available would be sufficient to irrigate 180.000 has if the water drained could be reused downstream. Without the reuse of drainage water the area would be 118.000 ha (116.200 ha on the left bank and 11.800 ha on the right bank). The maximum irrigable area, however, was ultimately determined to be 122.000 ha. The calculations were based on the water requirements of a pattern of selected crops and an efficiency of the project of 49% (70% efficiency of the plots and 70% of the conveyance and distribution system). The water delivery strategy was to cover all of the crop requirements. During the implementation of the PRD the pre-existing irrigation rights that were in use or had been used 3 years previous to 1968 were respected. Some concessions were redistributed. The redistributed concessions had a limit on the area one owner could irrigate: a maximum accumulated area of 50 ha. This was done with the intention of letting as many farmers as possible profit from the benefits of irrigation. This decision, however, lead to a dispersion of the irrigable land for which a vast canal network, large enough to provide an area of 350.000 ha of land with water, was built. The irrigation distribution schedule applied by the AyEE in the beginning was a fixed application time and stream size with a delivery frequency interval of 30 days; a delivery time of 50 minutes per hectare (min/ha) and a delivery stream size is 300 litres per second (l/sec). Theoretically this means that per month an irrigation volume of 900 m³/ha was permitted leading to a gross application depth of 90 mm. In 1971 another irrigation distribution plan was proposed for the PRD. Irrigation water would be distributed using a rotation schedule fixed in time, with a variable flow and an interval of 30 days. The proposed maximum irrigation flow was 300 l/s during 75 min/ha (135 mm/ha) in December (the month of maximum irrigation needs). With this time the required irrigation flows in the other months were calculated. The intention was to make a transition from the previous schedule to this one. This transition, however, never occurred.

4.2.4 The current PRD

The project formulated by the AyEE and the CRD for the irrigation system of the PRD was never fully implemented leading to an uneven development of the system. Consequently the system as is does not function as planned. The infrastructure was never completely modernized, the maximum irrigable area was never fully utilized and distribution is not executed as planned. What follows is a description of the system and how it functions. The organization of water allocation, the state of the physical infrastructure and the totality of the way the water is handled in reality by the farmers have a great impact on how the water is divided over surface, subsurface and atmosphere within the system and accordingly on the river basin.


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Water rights The irrigation rights in the PRD originally followed the principle of first appropriation and from the beginning were bound to the land owned. The following table demonstrates how the water rights were distributed.

Parcel size [has]

Number with WR

% Area with WR [ha]

Mean area with WR [ha]

Total area [ha]

%

0 ≤ x<5 2.745 30,4 4.518 1,6 7.314 2,6

5 ≤ x< 10 2.172 24,0 9.514 4,2 14.466 5,1

10 ≤ x< 25 2.548 28,2 23.613 9,3 39.583 13,9

25 ≤ x< 50 804 8,9 15.387 19,1 26.513 9,3

50 ≤ x<100 354 3,9 11.038 31,2 23.844 8,3

100 ≤ x< 500 334 3,7 18.491 55,4 63.732 22,3

500 ≤ x<1000 43 0,5 2.853 66,3 28.547 10,0

≥ 1000 35 0,4 4.496 128,5 81.656 28,6

Total 9.035 100,0 89.550 285.655 100,0 Table 4.2: The distribution of the permanent water rights in 30-10-’97 (Source: UER)

The full potential of the system has never been exploited. As can be seen from figure 4.6 the irrigated area has always been under the 122.000 ha. Since 1970 there has even been a decline in the cropped area. Thus there was no limitation to the amount of water available and not all the water reserved for the PRD was used. This was the technical reason for the AyEE agency to create a temporary and revocable annual water right Permiso Precario Temporario Annual (PRETA). This water right is given for one year and can be revoked if the concession conditions change. PRETAs allow the Agency to allocate annually the portion of water that is not used by permanent water rights holders. As can be seen in figure 4.6 the use of the temporary concessions has increased throughout the years.

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

68/6

9

69/7

0

70/7

1

71/7

2

72/7

3

73/7

4

74/7

5

75/7

6

76/7

7

77/7

8

78/7

9

79/8

0

80/8

1

81/8

2

82/8

3

83/8

4

84/8

5

85/8

6

86/8

7

87/8

8

88/8

9

89/9

0

90/9

1

91/9

2

92/9

3

93/9

4

94/9

5

95/9

6

96/9

7

97/9

8

98/9

9

99/0

0

00/0

1

01/0

2

02/0

3

Area Cultivated (ha)

TOTALES PRETAs

Source: Intendencia de Riego - AyEE

UER - Pcia. De Sgo. Del Estero

Figure 4.6: Area cultivated (Source: AyEE & UER)


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Physical infrastructure The infrastructure for the PRD begins with the Embalse Rio Hondo, which was completed in 1968. It has a storage capacity of 1.740 hm³. About 15 km downstream it is followed by the diversion dam Los Quirogas. The small dam, Los Quirogas (has a 100 m³/sec flow capacity outlet to the main canal), diverts water from the Rio Dulce into the main canal of the PRD, La Matriz. From here water is allocated over the different secondary canals that transport it to one of the 5 different zones. Water leaves the system at three points without being used:

• The Descargador (just before the siphon which connects the right and left bank)

• Jume Esquina, which releases its water to the Rio Salado. In total there are five different zones in the PRD. The division was based on the fysiographic conditions and the natural drainage basins within the system. This division facilitates research, execution and administration in the system. The secondary and tertiary canals of the different zones are (also see annex 3B): Zone I (the blue canals) o Alto C o Alto Prolongacion o La Cuarteada

� CT1 � CT2 � CT3 � MT4 � AT2

o Canal Norte � NT4 (Norte Tercero 4) � NT6 (Norte Tercero 6)

Zone II (the green canals) o Sud Primera Seccion

� El Puestito � El Alambrado � Bajo Grande � Bayo Muerto

o Canal Suri Pozo o Beltran I o Ppal A Fernandez

o Del Este o Mistol o Ejido o Jume Esquino o Sud Segunda Seccion Zone III (the canals in yellow) o Municipal o Canal A Villa Robles o Pinto o Beltran II Zone IV (the red canals) o Canal San Martin o Contreras Lopez o Maco Manogasta Zone V (the purple canals) o Secondario Simbolar

� T2 � T3a/b � T4 � T6

� T7


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Figure 4.7: PRD Canal Network

Zone I (± 19,000 ha) This zone lies farthest to the north of the system on the left bank. It was one of the first parts of the system to be irrigated. In total in zone 1 there are approximately 129 km of secondary and tertiary canals (see figure 4.7). In Zone I the irrigation network was completely modernized. Canals were lined until the quaternary levels and dense network of primary and secondary drains was built. The measuring and off-take devices in this zone are:

• Measuring device secondary canal: Long throated flume

• Measuring device tertiary canal: Long throated flume

• Measuring device distribution canal: Long throated flume

• Off-take to secondary canal: Sliding gates

• Off-take to tertiary canal: Sliding gates

• Off-take to distribution canal: Sliding gates

Zone II (± 46,000 ha) This zone lies directly under zone I on the left bank and stretches out to the southeast above Route 34. In this zone there is in total about 192 km of secondary canals (see figure 4.7). Zone II was not modernized. The canals remain unlined, but are, however, maintained by the larger farmers. Few drains were built in this zone. The measuring and off-take devices in this zone are listed below. As can be seen there is a main control and off-take structure, but the measuring devices are limited to the secondary canals.


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• Measuring device tertiary canal: No measuring device

• Measuring device distribution canal: No measuring device



• Off-take to distribution canal: Sliding gates and on/off gates

Zone III (± 15,000 ha) This zone lies on the left bank between Route 34 and the Rio Dulce. Here there are about 107 km of secondary canals (see figure 4.7). Modernization also did not reach zone III. The canals are unlined but as in zone II are maintained by the larger farmers. There were few drains were built and the measuring and off-take devices are equal to those in zone II. The measuring and off-take devices in this zone are:


• Measuring device tertiary canal: No measuring device




• Off-take to distribution canal: Sliding gates and on/off gates

Zone IV (± 19,000 ha) This is the only zone that lies on the right bank of the Rio Dulce. Water is diverted to this zone through a siphon. It is one of the more modern zones, which can be seen by the measuring and off-take devices used in the zone. In zone 4 there are about 152 km of canals (see figure 4.7), which are lined until the tertiary canals. The unlined canals are: Contrero Lopez and the Maco Mangosta. Also the tail of the San Martin canal is unlined. This is in total about 61 km. A dense network of primary and secondary drains was also constructed in this zone. The measuring and off-take devices in this zone are:

• Measuring device secondary canal: Neyrpic modules

• Measuring device tertiary canal: Neyrpic modules


• Off-take to secondary canal: Avio gate

• Off-take to tertiary canal: Avio gate

• Off-take to distribution canal: On/off gates

Zone V (± 7,500 ha) This zone lies farthest to the east, just under zone 1 on the left bank of the system. This is a relatively new irrigating zone that exists since 1975 and also relatively modern. This is the only zone that was designed for delivery on request. In total there is about 55 km of canal running through this zone (see figure 4.7). None of the canals in this zone are lined. The measuring and off-take devices in this zone are:

• Measuring device secondary canal: Neyrpic modules

• Measuring device tertiary canal: Neyrpic modules

• Measuring device distribution canal: Constant head orifice

• Off-take to secondary canal: Avio gate

• Off-take to tertiary canal: Avio gate

• Off-take to distribution canal: Sliding gates


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As can be seen from the inventory above of the infrastructure in zones I, IV and V are more developed than zones II and III. Regarding the canals, the expectations are that the infiltration in the zones II, III and V will be a factor to be reckoned with unlike in zones I and IV. Due to the vastness of the water distribution network in zones II, III and IV the chance of operational spills is increases. In figure 4.8 an impression is given of the canals and intakes.

Figure 4.8: (clockwise)La Matriz, secondary canal & intake, tertiary canal and field canal (Source M. Ertsen, Citg gallery)

4.2.5 The water management

There are two levels of water management in the irrigation system:

• System level management (meaning the collection, transportation and distribution of water)

• Tertiary level management System water management There is joint responsibility for the operation of the Rio Hondo dam, which is shared between the UER and the concessionaire of energy generation in the dam. Within the PRD system there is a joint administration between the Unidad Ejecutora de Riego (UER), a provincial government institution, and the Water Users Association. The UER (in existence since 1992) is responsible for the overall administration of the system and in particular the O&M of the system from the dam Los Quirogas until the beginning of the tertiary units, where the responsibility is transferred to the Users Association. The normal procedure for delivering water involves as little work as possible for the UER (Prieto). The gates are opened to the same height every turn, always the maximum height. Only the time varies according to the number of farms that irrigate in the turn. Before 1992 the time assigned to the comuneros (tertiary sector) was always fixed according to their water rights. From 1992 on it was necessary to pay water fees to use the water. As a result the delivery time to the comuneros was based upon their water rights and payment of the water fee. Farmers were able to refuse their turns, which frequently occurred with small


26

farmers. So a new procedure was adopted. Based on the real number of farmers in a comunero that will irrigate per turn and the area each of these farmers have with water rights, calculations are made for the delivery time to the comunero. The quantity of water to be delivered is calculated by the administrator of the zone and communicated to the Agency in advance. Zone V is an exception to the other zones. This is the only zone that was designed for delivery on request. Here farmers ask water delivery through their administrator at the zone’s district at least five days in advance. The UER then prepares an irrigation delivery schedule for this zone. Thus irrigation schedules are more frequently prepared for this zone than for the others. In general the system was built for a distribution of the type imposed allocation. Outside of critical periods, however, the system moves to a distribution of the type “controlled demand”, i.e. fitting the moment and sometimes the duration of the delivery to the demand of the agriculturists and the operational capabilities of the system thus diverging from the official distribution schedule. This is possible due to the great amount of water available to the system as explained in the section on water rights. Tertiary level management Little information was gathered on the actual practice of how farmers in the tertiary unit allocate water amongst themselves and within a growing season within each zone. This is regrettable because insight into how the farmers handle the water they receive is of great importance to the water balance of the individual zones and of the system in general. Per zone the handling of water will differ. What is known is that the responsibility for the distribution of the water from the tertiary unit on is the responsibility of the Water Users Association. They distribute the received water conform a pre-established order of irrigation that theoretically begins with the last user and advances towards the head of the canal. The associations are also responsible for maintaining the course of their water and collaborate in the maintenance of the superior channels during the annual maintenance break of the system. The associations, in common agreement between their members, can change the distribution pattern. The official order of distribution can be changed to take care of the emergencies of some of its members. As mentioned before the official irrigation distribution schedule allows for an irrigation gift of 90 mm/ha/month for a total of 11 turns in a year. In the wet periods (November - March = 5 turns) irrigation is used as a supplement. In the dry periods (April- October = 6 turns) irrigation is the principal source of water. During the period of one month (usually May sometimes June) irrigation activities are halted for system maintenance purposes. In actual practice farmers diverge from this official scheme by first of all, not irrigating each month. Secondly, they usually take more water than permitted per turn. A distinction can be made between the behaviour of small farmers and large farmers. Small farmers Three studies into the behaviour of small farmers delivered the following results:

• A study into the application efficiency (Ea) delivered the following results: o The mean application time was 2,2 hr/ha, instead of the 50 minutes permitted. o The mean irrigation depth was around 210 mm.

• A field survey of about 200 farmers done by Valeiro and Prieto (1991) had the following results:

o 50% of the farmers only applied a pre-seeding irrigation o 25% include a second irrigation during the flowering period o the other 25% added a third irrigation at bloom formation


27

• Studies done in the command area of the tertiary canal CT3 (all small farmers) by Ertsen in the growing season of 93/94 show:

o Farmers in this area applied irrigation twice (pre-seeding and flowering) o The official water delivery method allows 450 mm in 5 turns, the farmers however

applied this amount in two turns (2*220 mm/event) If these results are indicative of what happens the totality of the PRD, it could be said that for smaller farmers two or three turns a year are sufficient, but with every turn about two and a half times more water than permitted per turn is taken. Large farmers The larger farmers make use of 8 or 9 turns per year and sometimes irrigate a larger area than officially allowed. From the study on application efficiencies resulted that large farmers with a precise land levelling still apply more than 200 mm. To be able to apply 200-250 mm with a flow rate that is close to the official 300 l/s requires more time than the official 50 min/has. How the farmers have managed to increase the time remains unknown. Some hypotheses exist7: a) The farmers cultivated less area than officially stated on their water rights but irrigated according to the delivery time calculated for the maximum irrigable area. Thus they have extra time for the actual cropped area. b) The same situation as described in point a) but applied for the comunero and farmers distribute between them the extra time c) Higher duration for the whole command unit is arranged with gatemen and/or directly at the Agency district. All of these possibilities could be present in different zones or also in different times in the same command area. Field application There is not much difference in the water application method between small and large farmers, save for medium farmers that produce vegetables in the Zone V and some large farmers that apply furrow irrigation. The most common application method in the area is a local type of basin irrigation which is used for pre-seeding and later irrigation by small farmers while large farmers sometimes move to “furrow” irrigation after seeding their crops. Irrigation depth is roughly adjusted by the height of the levees and/or the depth water is allowed to reach before open the downstream border and the length of the irrigation unit.

Figure 4.9: Irrigation field, farm inlet and irrigated field (Source: M.Ertsen; Citg gallery)

7 Prieto


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Crops In total there are about 37 different crops cultivated in the PRD. These are:

1. Chard 20. Natural Pastures

2. Garlic 21. Parsley

3. Anquito 22. Sweet Pepper

4. Sweet Potato 23. Grapefruit

5. Safflower 24. Green Beans

6. Onion 25. Beet Root

7. Citrus 26. Cabbage

8. Cauliflower 27. Watermelon

9. Forages 28. Soybeans

10. Sunflower 29.Sorghum

11. Lettuce 30. Tomato

12. Lemon 31. Wheat

13. Maize 32. Verdeo de Inv.

14. Maize de Guinea 33. Grapes

15. Mandarin 34. Carrots

16. Peanuts 35. Zapallo

17. Sweet Melon 36. Alfalfa

18. Orange 37. Cotton

19. Potato

The most important crops however are cotton, maize, wheat and alfalfa. In zone 1 the most important crops are cotton alfalfa and wheat. In zone 2, 3 and 4 these are cotton, alfalfa and maize. In zone 5 the most important crops are cotton, carrots and onions.

4.2.6 Further remarks on the PRD area

Due to the concession of temporary water rights (PRETA’s), there is a high variation of the sum of annual cultivated area. This means that the cultivated area that is irrigated each year is variable. It also has consequences for the flow, both surface and groundwater, depending on the area of the parcels. It is also difficult to define the borders of the command area, because in general the larger farmers have their farms (both irrigated and non irrigated) in the downstream canal areas and these farms extend outside the borders of the command area. This can be seen in figure 4.10. The larger farms are on the outer rim of the PRD, while the small farms are positioned more in the core of the PRD. That is why Ertsen et al suggested it was more appropriate to speak in terms of a ‘water transition zone’ both in terms of irrigated areas as in water flows. Figure 4.10: Parcel distribution throughout PRD


29

Ertsen et al have identified three core irrigation areas within the PRD system. They are zone I, IV and V. Within these three core areas, not all the fields have irrigation rights. Thus both on scheme as on field level pockets of irrigation can be found.

4.2.7 Problems in the area

As in many irrigation systems, the PRD also suffers from secondary salinity and waterlogging. Of itself the irrigation system already had salinity problems in several areas as already mentioned in the geological description of the area. But secondary salinization is also a problem. This is not due to the quality of the water of the Rio Dulce, which has a salinity of 0,36 dS/m (measured by me) but rather to the rising water tables. The rising water tables are a result of infiltration on a system level but even more so due to the over-irrigation. As for the over-irrigation in the fields, in order to retain more water for dry periods, the irrigation is continued in the wet periods. The idea behind these actions is to create a hydraulic reserve in the soil. The quantity thus surpasses the natural drainage capacity of the soils (the hydraulic conductivity is moderate to slow). As mentioned above, there is no drainage or it is defect. All these factors cause the water table to rise. The increased groundwater levels causes major damage to the land and crops, because the root zones of the plants are saturated leading to the suffocation of the crops and secondly the groundwater contains a great amount of minerals causing the secondary salinization.


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CCHHAAPPTTEERR 55 MMEETTHHOODD OOFF RREESSEEAARRCCHH In this chapter a method to approach an open irrigation system is presented. Since an open system is described as a system that interacts with its context, the interaction between the irrigation system and the river basin within which it lies is explored in this chapter. In the first paragraph a closer look is taken at the river basin and the hydrological processes that take place here, the transportation process within the irrigation system and the interaction between them by qualitatively describing the processes involved. In the second paragraph the method of research is formulated including the formulas used and the assumptions made.

5.1 Theory The purpose of the first paragraph is to better understand the river basin, the irrigation system and the interaction between them by qualitatively describing the processes involved. This way it will be easier to derive a method of approach.

5.1.1 The hydrological cycle

Figure 5.1 shows the scheme of the natural hydrological cycle and the different processes that take place. Also see figure .2 for a diagram of the different processes. In the text different symbols for the different processes are given that can be found in the diagram.

Figure 5.1: Hydrological Cycle (Source: Savenije)

The radiation energy of the sun is the driving force behind this cycle. The greatest portion of water on earth is in the oceans and seas. Thus the starting point will be the effect of the sunrays on the sea surface, which is the evaporation of water in to the atmosphere (A). Through different processes this water condenses leading to the formation of clouds. Depending on the state of the atmosphere, the water precipitates through rainfall, snow or hail. The water then either falls directly in the ocean or it falls on land.


31

Surface Water reaches the land surface through rainfall (P). Part of this water is intercepted by the vegetation and returns into the atmosphere through evaporation from the leaves (I). The remaining rainwater infiltrates (F) into the soil until it reaches its capacity for infiltration. The precipitation that falls on the earth’s surface after this point is overland flow (Qs) and adds to the surface water bodies (lakes, rivers etc). The water bodies not only receive water from precipitation and overland flow, but through seepage as well. A portion of the surface water bodies is discharged (Q) into the ocean or sea. A portion leaves the basin by open water evaporation (O). When planning for water resources often only surface water resources are considered. Sub-surface After the rainstorm a part of the infiltrated water in the upper layer of the ground flows into the subsurface water bodies (subsurface flow). The infiltrated water can also be used by vegetation, through which the water reenters the atmosphere through the process of transpiration (T). If the soil moisture content is above the field capacity, part of the soil moisture percolates towards the groundwater. Balance between surface water and groundwater Underneath the earth’s surface there are saturated ground water bodies. These groundwater bodies have been developing for thousands of years and are in such a point of equilibrium that groundwater can flow freely by gravity, if it is unconfined, to a lake or a stream system (or to the ocean nearby) where it discharges. This discharge is referred to as seepage (Qg). The seepage thus adds to the volume of the water bodies. Most groundwater systems are in equilibrium with surface water systems. During dry seasons, most streams exist because of seepage. So when water is added to a groundwater system (the groundwater recharge R), over the long term, it is balanced by a similar amount of outflow to a surface system. This flow process is controlled by gravity. Such was also the case in the example cited in chapter 3 of the Snake Plain Aquifer in eastern Idaho. The process where water rises from the renewable groundwater to the unsaturated zone is called capillary rise. In figure 5.2 a diagram is shown of the different water resources and the different processes that contribute or subtract to the quantity of these resources. Hydrologists have divided the total water resources into different colors. The different resources have been given a color based on the space they take in the basin in order to distinguish the different types of water and their uses better. There are four different colors that are of importance:

• White water: This color refers to that part of the rainfall that returns directly to the atmosphere through evaporation from interception and from bare soil. (= I+Es ; Es is evaporation from bare soil).

• Green water: This is the water that is stored in the unsaturated zone, resulting directly from precipitation. It is consumed through the process of transpiration by vegetation. Green water contributes to the largest part of the world’s food and biomass production

• Blue water/ Deep blue water: This is the water that that occurs in rivers, lakes and aquifers. The blue water is the water that occurs as surface water in the water bodies. The deep blue water refers to the water that occurs as renewable groundwater in the aquifer. It is in the saturated zone. Renewable groundwater is groundwater that takes an active part in the hydrological cycle. It is impossible to regard these two resources as separate bodies of water, because they are interchangeable. As mentioned above the saturated groundwater bodies are in equilibrium with the surface water bodies and over the long term balance each other out.


32

Atmosphere

Surface Water bodies

Renewable

groundwater

Soil

P

I

T

F

R

Qs

O

Qg

Oceans

and Seas

A

Q

Figure 5.2: Diagram of the water resources and the different processes

5.1.2 The irrigation water balance

The purpose of irrigation is to assist the growth of crops or to increase the harvest in areas where the rainfall is not sufficient. Water is required for the crop growth, preparing the field for planting and for leaching. The irrigation system is the totality of infrastructure and distribution structures meant to collect the water from its source, distribute and transport the water to the fields. Often only a fraction of the water diverted into the system is used for the intended purpose. How great that fraction is, depends on the physical state of the irrigation system and the organization of the distribution and transport of the water. In this section this process is explored. For a diagram of the different flows within the irrigation system see figure 5.3. The figure reproduces what happens to the irrigation water as it follows its path through the irrigation system. Water enters the command area by rainfall (red), irrigation (blue) or possibly by subsurface flow (green). In the case of the irrigation water (the blue lines), water is diverted from the river into the conveyance system. Of the total amount of water (Vc) that enters the conveyance system, a portion leaves the conveyance system through evaporation, percolation, evapotranspiration for non-irrigation crops and operational spills. The water that leaves the system through evaporation and evapotranspiration ends up in the atmosphere. The part that leaves due to operational spills ends up in the drainage and depending on the layout of the irrigation system can be reused. The percolated water, adds to the groundwater resources. What remains of the water after these streams have left the system, is volume Vd. Vd is the volume of the water that enters the distribution system. The same story as for the conveyance system applies for the distribution system. What is left is Vf, the volume of water that irrigates the fields. Of Vf a certain portion enters the soil and is used for evapotranspiration of the plants, part percolates through the soil and ends up in the groundwater and a part becomes field runoff and ends up in the drainage. The ratio between these different portions depends on the irrigation method used. Another source of water supply is groundwater (the green lines). This applies directly to the


33

fields. Here water, through capillary rise reaches the root zone of the plant, where it can be utilized for evapotranspiration. The final source of water supply is precipitation. This source adds to the volume of water in the irrigation system and provides the fields directly with water. Of the total precipitation that falls upon the surface, depending on the type of rain storm, the type and execution of the water management and the rate of interception, only a portion (Pe) can effectively be used for the crops. Part of the precipitation that falls on the ground infiltrates into the unsaturated zone. The precipitation that falls after the maximum infiltration capacity is reached flows over the land and ends up in the drainage system (in which case it is transported out of the irrigation system) or maybe even in the conveyance or distribution system (in which case it can be used again). Part of the infiltrated water percolates into the groundwater system and adds to the groundwater storage, or is possibly transported out of the area by subsurface flow or the slower groundwater flow. The water collected in the drainage is transported out of the system. Depending on the layout of the system, this water is recycled within the system. Depending on the destination and quality of the drainage water it can be recycled within the river basin. As can be seen from figure 5.3 the water that enters the system (be it through discharge, precipitation or groundwater) redistributes itself, whereby not all of the water is used for the originally intended purpose. In the following section a model for the inventory of the water is presented.


34

Evapotrans-

piration Vm

Vc + V1

Vd

Vf

flow diverted or pumped from the

river or reservoir Vc

inflow from other sources (surface

and groundwater) V1

evaporation

operational spills

percolation

evapotranspiration of

non irr vegetation

evaporation

operational spills

percolation

evapotranspiration of

non irr vegetation

moisture

retained in

soil

percolated

through soil

Groundwater

Drainage

surface sub-surface natural

field runoff

CONVEYANCE SYSTEM

TERTIARY UNITS

DISTRIBUTION

FIELDS

AREA SERVED BY MAIN

AND(SUB-) LATERAL CANALS

TOTAL AREA

P

ground-

water

outflow

change in

groundwater

storage drainage

from the

area

Figure 5.3 The distribution of irrigation water


35

5.1.3 Fractions

In their paper “Water use definition and their use for assessing the impacts of water conservation”, Allen et al (1997) propose shifting from the use of “efficiency” to “fractions”. They divide the incoming discharge on the basis of two distinctions.

Figure 5.4: Division of the irrigation discharge into fractions

The first distinction is between water that is beneficially used and non-beneficially used water. Water is used beneficially if it serves the purposes to which the irrigation system is designed, i.e. for the crops, preparing the ground and leaching. Because interaction between the basin and the irrigation system is ignored, in a closed system the water that is not beneficially used is the water that is regarded as “lost”. The second distinction made is:

• Consumed fraction (CF) � Evaporated fraction (EF) (see the first column) � Nonreusable fraction (NRF) (see the second column) � Water exported out of the basin

• Reusable fraction (RF) (see the third column) The second distinction splits the water into consumed and reusable water. The consumed water is accordingly referred to as the consumed fraction. This water is lost due to its transformation into another physical state or the transformation of its quality. The water that is reusable is referred to as the reusable fraction.

• Crop ET

• Evaporation for climate control

• Non-reusable deep percolation for salt control (leaching)

• Water exported in produce from basin

• Reusable deep percolation for salt control (leaching)

• Phreatophyte ET

• Sprinkler evaporation

• Reservoir evaporation

• Excess wet soil evaporation

• Non-reusable excess deep percolation due to quality

• Excess deep percolation, runoff or spills to salt sinks

• Reusable excess deep percolation

• Reusable runoff

• Reusable canal spills

Consumed Fraction Reusable Fraction

EF NRF RF

Beneficia

l use

Non B

eneficia

l

use


36

Consumed fraction Evaporated Fraction This refers to that portion of the water that leaves the system due to evaporation and evapotranspiration. This water re-enters the atmosphere and is unavailable for further use in the irrigation system. Non-Reusable Fraction The second source of consumption is the non-reusable water. This water percolates into the ground and is lost because it enters a saline water body within the aquifer or degrades in quality to the point that it is economically non-reusable or physically beyond economic recovery. Whether percolated water is non-reusable or not is relative of the location of the irrigation system within the basin and the hydrological situation there. Is the irrigation system situated near to the sea, it is more likely that the greater part of the percolated water will be non-reusable. If, however, the irrigation system is located upstream in the river basin, it is more likely that the greater part of the water will be reusable. Another contributor to this category can also be reckoned water, of good quality, that is transported out of the basin for use in another basin. Reusable fraction The reusable water, finally, is the fraction that is available for reuse within the irrigation system or in the basin. This fraction consists of the water infiltrated/percolated from the canals and fields into the aquifers (the renewable groundwater) that is utilized by other downstream users, water collected in the drainage that is reused within or beyond the system. Whether the water is reusable or non-reusable is greatly dependant on its destination within the river basin.

5.1.4 Combining the models

In order to theoretically make a connection between system and basin, something will be borrowed from all three models described above. Borrowing from the idea of Allen et al (1997) the different processes within the water balance of the irrigation system are inventoried within fractions. The schematization of the hydrological process with use of the color scheme for water is borrowed from Savenije and the irrigation scheme from Wolters presented in figure 5.3. First, like Allen et al (1997), a distinction is made between water that is reusable and water that is non-reusable after distribution. Reusable water is defined as water that is of good quality and can be or is reused within the basin. Thus water of good quality that is transported to another basin is non-reusable. The second distinction made in the inventory is the based on the destination of the water. Three destinations are defined:

• The atmosphere; the portion of water that ends up here is referred to as the Evaporated Fraction (EF)

• The surface; water that remains or ends up on the surface is referred to as the Surface Fraction (SF)

• The subsurface; water that ends up here is referred to as the Groundwater Fraction (GF)


37

Combining these two aspects leads to the following model:

Atmosphere (EF) Surface water (SF) Sub-surface water (GF)

• Excess canal intake water

• Drainage water

• Runoff

• Deep percolation for salt control

• Canal spills

• Excess water applied to field

RF

• Crop evapotranspiration

• Canal evaporation

• Evapotranspiration from uncultivated areas

• Intercepted water

• Runoff

• Water exported in produce from basin

• Drainage water

• Canal spills

• Deep percolation for salt control

NRF

Table 5.1: New division of water

The Evaporated Fraction (EF) Evaporation is the process through which water, in a liquid state, is transformed into water in a vapor state (vaporization), and removed from the evaporating surface (vapor removal). Water evaporates from many different types of surfaces, such as lakes, rivers, pavements, soils and wet vegetation. The evaporated water returns to the atmosphere and is not reusable within the basin. This is the white and green water seen in figure 5.2. The green water here will be expanded to include irrigation water stored in the unsaturated zone. The processes through which this occurs are:

• Evapotranspiration

• Evaporation from the canals

• Interception Surface water (SF) This is the portion of water that remains on the surface, thus it adds to the volume of blue water. This water is either reusable or non-reusable depending on its quality and if it remains within the basin. To this category is reckoned:

• The excess discharge that enters the system (RF).

• Runoff (RF/NRF).

• Drainage water (NRF).

• Water exported in produce from the basin. Subsurface water (GF) This refers to the infiltrated water that does not remain in the unsaturated zone for the benefit of the plant. This water is reusable or non-reusable depending on the quality of the soil in which it infiltrates and the quality of the water it is added to. To this category can be reckoned:

• Canal spills.

• Water applied to the fields

• Leaching water Notice that unlike the model of Allen et al (1997) no distinction is made between beneficially used water and non-beneficially used water, because the purpose of this research is not to grade the system.


38

5.1.5 Interaction between the two systems

Next the division into fractions is processed into the hydrological scheme presented in figure 5.2. The irrigation system is a part of the river basin; therefore it lies within the basin. All the hydrological processes that occur within the basin also occur within the irrigation system. Water enters the system through precipitation (P), diverted discharge (Vc) and groundwater flow (Vg). It leaves the system through firstly the Evaporated Fraction, which is the collection of processes through which water returns to the atmosphere. These processes are the interception (I), evaporation (E) and transpiration (T). Secondly, it leaves the system through the Surface Fraction. The Surface Fraction is water that adds to the surface water resources of the river basin (blue water) or to the resources of another river basin. This water comes from excess discharge (Qe) and drainage (D). Last the Groundwater Fraction is the water from the irrigation system that adds to the basins groundwater resources (deep blue) through the process of natural drainage.

Surface

Soil

Water bodies

Renewable

Groundwater

Water bodies

Renewable

GroundwaterSalt water

Aquifer

Atmosphere

Ocean and seaO

ther Basin S

FGF

F

I E

Qe

D

Qg

R D

T

Irrigation System

River Basin

Q

Qg

P

Evaporated Fraction

Vc

Vg

Figure 5.5: Interaction between river basin and irrigation system

To further specify the irrigation system and have a better idea of how the water redistributes itself within the PRD the diagram in figure 5.6 was made. This is an adaptation of the diagram of the irrigation system from Wolters (figure 5.3). It has been adapted in order to be able to distinguish which processes in the irrigation system contribute to which fraction as the water is transported to the fields. In the diagram the factors that contribute to the Evaporated Fraction are in the yellow boxes as they end up in the atmosphere. The factors that contribute to the Surface Fraction are in

GF = Groundwater fraction SF = Surface water fraction Vc = Surface water for irrigation T = Transpiration I = Interception Qe = Excess Discharge Vg = Ground water flow

F = Infiltration P = Precipitation R = Groundwater recharge Q = Discharge Qg = Upward seepage E = Open water evaporation Qs = Overland flow


39

the blue boxes, as they end up in the water bodies. The factors that contribute to the Groundwater Fraction are in the dark blue boxes.

Precipitation

Vc

Ecanal

QRioSalado

Qexcess

Conveyance

Sys tem

Vzone-x

Ecanal

ETo-canal

Rcanal

Qexcess

V fie ld

SoilETc

R field

Drainage

Interception P-I

Zone x

Field

Figure 5.6: Adaptation of the irrigation scheme of Wolters


40

5.2 Method of application Using the model for the interaction between the river basin and the irrigation system defined above the link will be further concretized. This is done by quantifying the fractions as they form the important connection between basin and system. What are presented here are the different fractions, the processes that contribute to these fractions and the formulas to quantify the processes. Some of the data used in the calculation of the processes can be found in annex 4. Water balance The fractions are determined in the form of a water balance for the irrigation system. What the water balance essentially says is that the water entering the irrigation system redistributes itself over the different fractions, which all have a different destination. By using the fractions in the balance the distinction is made between atmosphere, surface and sub-surface.

NRGFNRSFRGFRSFEFI ++++= I = Incoming volume [dam³] EF = Evaporated fraction [dam³] RSF = Reusable Surface fraction [dam³] RGF = Reusable Subsurface (groundwater) fraction [dam³] NRSF = Non Reusable Surface fraction [dam³] NRGF = Non Reusable Subsurface (groundwater) fraction [dam³] Using this water balance, the fractions are determined for each month during three years (1995/96; 1996/97 and 1999/2000) spread out over a period of five years for the total PRD irrigation system as for each of the individual zones. For each of the zones the balance is calculated separately because each zone is distinct in its characteristics and as a consequence has a different mechanism from each other zone. In the following sections the fractions are quantified. In the last section a summary of the assumptions made in the method of approach is given.

5.2.1 The total incoming volume (I)

Part of the interaction between the river basin and the irrigation system is the water entering the system from the river basin. Here a closer look is taken at the total volume of water that enters the irrigation system. As mentioned in the previous paragraph, there are three possible sources of water for the irrigation system: precipitation, the irrigation discharge and groundwater flow. The precipitation enters from the atmosphere; the irrigation discharge enters from the surface water resources of the basin and the groundwater flow enters from the groundwater resources from the basin. Unfortunately, for the PRD, data is only available on the first two sources. Based on the geomorphological description in the previous chapter it is known that groundwater would flow into the system from the north (the Saladillo de Huyamampa) and that the size of this flow would vary with the seasons. In the wet season, the flow would be higher than in the dry season. In this section only the precipitation and the irrigation discharge are taken into consideration. These two sources combine to form the volume of water that enters the system that will be referred to as the incoming volume.


41

Precipitation There are five different stations (Santiago del Estero, La Banda, La Maria, Loreto and Fernandez) where the rainfall was measured within the PRD. One station, Loreto, only has data for the year 1999/2000 (see annex 4). The stations are located on the West and ranging from the center to the South of the system. There is no data of the Northern and Eastern part of the system. Outside the area of the PRD no data was collected from the stations of measurements. The stations are marked with a turquoise circle with a square of the same color in the middle.

Figure 5.7: Location of the stations.

Examining the values of the rainfall from station to station throughout the months, it is noticeable that in some cases the monthly rainfall differs no more than 2 mm in other instances the difference can be more than 20 mm. Especially for the stations Fernandez and Loreto, which lie most to the south of the system, the monthly rainfall is low in comparison to the other three stations which lie more to the north. Therefore the method used for determining the precipitation volume is the Thiessen polygon. With this method the total area of the PRD is divided in 4/5 sections. In each section the precipitation values of one station is valid.

Zone Stations

I La Banda

II La Banda/Fernandez

III La Banda/Santiago/La Maria/Fernandez

IV La Banda/Santiago/La Maria/(Loreto)

V Fernandez

The boundaries of the zones however do not coincide with the boundaries of the sections. Therefore the mean precipitation for each zone is calculated by multiplying the precipitation


42

value of a certain section with the area of its section that intersects the zone intersects. This is repeated for each part of the zone that lies within a particular section and dividing the sum of this by the area of the zone. For example:

FernadezzoneBandazone

FernandezzoneFernadezBandazoneBanda

zoneAA

APAPP

∩∩

∩∩

+

⋅+⋅=

22

22

2

Pzone2 = precipitation in zone 2 [mm/month] PBanda = precipitation measured at the station La Banda [mm/month] PFernadez = precipitation measured at the station Fernandez [mm/month]

Azone2∩Banda = the area of zone 2 that intersects section La Banda [ha]

Azone2∩Fernandez = the area of zone 2 that intersects section La Banda [ha] For precipitation of the total PRD, the precipitation of the different stations are multiplied by the area of their respective sections and added. The total area of the PRD divides this sum.

∑

∑ ⋅=

5

1

5

1

xzone

xzonexzone

PRD

A

AP

P

with PPRD = precipitation in PRD [mm/month] Pzone x = precipitation in zone x [mm/month] Azone x = area zone x [ha] Discharge Another source is the irrigation water received from the Rio Dulce. This flow is diverted into the canal La Matriz by Los Quirogas dam. La Matriz then transports this water to the different secondary canals. The UER measured the daily discharge of La Matriz and the secondary canals. Of the daily discharges, the mean monthly discharge for the total PRD system and the five zones is calculated.

5.2.2 Evaporated fraction (EF)

In this section, the different contributions to the evaporated fraction and how they were calculated are discussed. The formula for calculating this fraction is:

canaloicanalc ETEEETEF +++=

EF = Evaporated fraction [dam³] ETc = Crop evapotranspiration [dam³] Ecanal = Canal evaporation [dam³] Ei = Evaporation from interception [dam³] ETcanal = Evapotranspiration due to canal infiltration [dam³] Where the different processes occur can be seen in figure 5.6. Crop evapotranspiration (ETc) Crop water requirement is defined in the FAO Irrigation and Drainage paper-56 as the amount of water required to compensate the evapotranspiration from the cropped field.


43

Therefore the crop water requirement is equal to the crop evapotranspiration (ETc). It is the summation of the transpiration and the evaporation of water from the soil surface during the different stages of crop growth. Here the ETc under standard conditions will be calculated. This is the evapotranspiration from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions. This parameter is calculated with the formula:

coc KETET ⋅=

ETc = standard crop evapotranspiration [mm/day] ETo = reference crop evapotranspiration [mm/day] Kc = the crop factor [-] There are in total 37 different crops cultivated in the PRD. The Kc value of these crops was estimated with the help of report FAO Irrigation and Drainage paper-56: “Crop evapotranspiration – Guidelines for computing crop water requirements”. With the estimated Kc values ETc was calculated. In this case the procedure for calculating ETc was as advised in the report:

Figure 5.8: Method for calculating ETc

Reference crop evapotranspiration (ETo) As mentioned in the previous chapter, when the designers of the irrigation system determined the size of the irrigation system to be 122.000 ha, it was based on the demand that the crop water requirements would be met. The irrigation system has not fully developed its potential, thus sufficient water is available for the farmers and from the studies on small and large farmers mentioned in the previous chapter, farmers tend to over irrigate. Therefore it is assumed that the reference crop evapotranspiration can be used. The reference crop evapotranspiration is the evapotranspiration rate from a reference surface with abundantly available water. The reference surface is a hypothetical grass reference crop with specific characteristics.

Calculate reference ETo

Select stage lengths Verify and supplement locally

Select values for Kcini, Kcmid and Kcend

Construct Kc curve

ETc = Kc* ETo

Determining Kc


44

ETo is only affected by climatic parameters. Thus, ETo is a climatic parameter and can be computed from weather data. ETo expresses the evaporating power of the atmosphere at a specific location and time of the year and does not consider the crop characteristics and soil factors. To determine ETo, the program CROPWAT for Windows will be used which uses the FAO Penman-Monteith method described in FAO-56:

( ) ( )

( )2

2

034,01

273

900408,0

u

eeuT

GR

ETasn

o ⋅+⋅+∆

−⋅⋅+

⋅+−⋅∆⋅=

γ

γ

ETo = reference crop evapotranspiration [mm/day] Rn = net radiation at the crop surface [MJ m-2 day-1] G = soil heat flux density [MJ m-2 day-1] T = air temperature at 2 m height [°C] u2 = wind speed at 2 m height [m s-1] es = saturation vapor pressure [kPa] ea = actual vapor pressure [kPa] es - ea = saturation vapor pressure deficit [kPa]

∆ = slope vapor pressure curve [kPa °C-1]

γ = psychometric constant [kPa °C-1] In annex 4 the CLIMWAT data used and the resulting ETo values are shown. The necessary data for the program was downloaded from CLIMWAT from the FAO site. The crop factor (Kc) In FAO-56 two different types of crop factors were denoted: the single crop coefficient and the dual crop coefficient. The difference between them being that in the single crop factor both crop transpiration and the soil evaporation are incorporated into one single coefficient and with the dual crop coefficient the effects of crop transpiration and soil evaporation are calculated separately. The single crop coefficients here is replaced by two coefficients: Kcb(basal crop coefficient) and Ke (soil evaporation). The dual crop coefficient is only needed when values for Kc are needed on a daily basis for specific fields of crops and for specific years. Since this is not the case, the single crop coefficient is sufficient. As the crop develops, the ground cover, crop height and the leaf area change. During the various growth stages there will be differences in evapotranspiration, consequently the Kc for a given crop will vary over the growing period. Therefore growing period is divided into four different growth stages: initial, crop development, mid-season and late season. In figure 5.9 the growth curve of the crop is given. The curve represents the changes in the crop coefficient over the length of the growing season. In order to determine Kc for each month the curve must be constructed. To construct the curve the length of the growth stages as well as the values for Kc ini, Kc mid and Kc end must be determined for each crop. In the report FAO-56 guidelines for the stage lengths (initial, crop development, mid season and late season) are given. These were taken as given in the tables and can be found in annex 4b. The Kc values of each stage given in the FAO-56 report are average values in sub-humid climates (RHmin≈45% and u2 ≈ 2 m/s) and need to be adjusted to the conditions of the PRD. Not all of the crops were mentioned in the FAO – 56 tables of crop coefficients and stage


45

lengths. Because there is usually a close similarity in the coefficients among the members of the same crop group - as the plant height, leaf area, ground coverage and water management are normally similar – the first step for these crops was to determine to which crop group they belonged. The data of member crops were substituted for these crops.

Figure 5.9: The different stages in crop development

Kc ini depends upon the time interval between wetting events, the evaporation power of the atmosphere (ETo) and the magnitude of the wetting event. The value of Kc ini will also be taken as given in the table. The reason for this is that the time interval between wetting events due to rain and irrigation conjoined and their magnitudes are difficult to estimate. The value of Kc mid is dependent on the climatic conditions of the area. This value can be adjusted to the condition in the PRD using the following formula:

( ) ( )0,3

( ) 2 min0,04 2 0,004 453

cmid cmid FAO

hK K u RH

= + ⋅ − − ⋅ − ⋅ Kc mid = the crop factor during mid season [-] Kc mid(FAO) = reference crop factor during midseason for a sub-humid climate [-] u2 = velocity of the wind [m/s] RHmin = the minimum relative humidity [%] h = height of the crop [m] And RHmin is calculated using the formula:

( )( )

min

min

max

100%e T

RHe T

°= ⋅

°

where: RHmin = the minimum relative humidity [%] e°(T) = the saturation vapor pressure at the air temperature T [kPA] T = air temperature [°C]


46

Instead of Tmin the value of Tdew should have been used, but because this value was not available the minimum relative humidity was approximated by using Tmin.

( ) 17.270.6108 exp

237.3

Te T

T

⋅ ° = ⋅ + where: e°(T) = the saturation vapor pressure at the air temperature T [kPA] T = air temperature [°C] The value of Kc end is adjusted in the same way as Kc mid.

( ) ( )0,3

( ) 2 min0,04 2 0,004 453

c end c end FAO

hK K u RH

= + ⋅ − − ⋅ − ⋅ Kc end = the crop factor during late season [-] Kc end(FAO) = reference crop factor during late season for a sub-humid climate [-] u2 = velocity of the wind [m/s] RHmin = the minimum relative humidity [%] h = height of the crop [m] Evaporation from the canals Only the exact dimensions of the canal La Matriz are known. The length of the main canal is 23 km and has a width of 20 m. Further the lengths of the secondary and tertiary canals are known. In total there is a length of about 635 km of secondary and tertiary canals. Possible canal dimensions were calculated. The open water surface of a canal a full capacity was taken as the area from which canal evaporation takes place. In the experiment station La Maria the open water evaporation was measured in the years required in a class A pan (see annex 2a). These measurements are used to calculate the volume of open evaporation of the canals. Interception The definition of interception, according to Klaasen et al, is precipitation which is intercepted by vegetation and has evaporated before reaching the soil, or

TPI −= where I is interception, P is precipitation and T is the throughfall. Throughfall is the precipitation that does not touch the vegetation, canopy drip and stemflow (the latter two also known as the drainage). So it can be seen as the net precipitation that reaches the forest floor. The amount of interception is dependent on the following factors:

• The potential canopy storage, hence the amount of water a leaf can store.

• The area covered with leaves, expressed in the Leaf Area Index (LAI)8. This will differ in every stage of plant growth.

• The type of rain shower, a storm will lead to a more throughfall.

8 The LAI is the area of leaves present on an m² of ground. The unit for this parameter is m²/m²


47

Here an estimation will be made of the interception. In order to accurately determine the interception a model must be made using specific information concerning the growth behavior and morphology of the crops and the daily precipitation data. Here, however, assumptions are made concerning these different factors. These are:

• Precipitation � In annex 2 the average number of days within a month with precipitation are

mentioned. These can also be seen in the table below. It is assumed that this is standard for every year. Thus for each year in April there are 8 days with precipitation. The total precipitation for that month is divided over these days, to determine the rainfall on each occasion.

Month April May June July Aug Sep Oct Nov Dec Jan Feb March

Days with P

8 5 3 3 2 3 5 8 8 11 9 10

Table 5.2: Days with precipitation

• Plant growth As mentioned in the section concerning the crop factor, there are four stages in the growth of the crop: � The initial stage, where the seed begins to sprout. In this stage no leaves are

present, thus the LAI is 0. � The crop development stage where the plant further develops itself. In this stage

the leaves become apparent. Thus the interception increases with the growth of the plant. Therefore the LAI in this period increases from 0 to 2.

� The mid-season stage. Here the growth of the plant is completed. The maximum LAI is reached. Thus for this period the LAI remains at 2.

� The end stage. In this period the plant deteriorates, yet not completely. The LAI thus decreases from 2 to 1.

• Canopy storage � The storage canopy is 1 mm for each plant. � After each rain event it is assumed that the 1 mm of water evaporates from the

leaf surface covering the area at that moment.

• Other � No distinction is made between the different crops with regards to the amount of

leaves each plant has, or the surface of the leaves. � Of a plot about 90% of it is covered with the crop being cultivated.

Based on these assumptions the interception for each plant was calculated using the following formula:

cropP ALAIaDI ⋅⋅⋅=

I = interception [dam³] DP = days with precipitation [days]

a = storage canopy (≈ 1mm = 0,0001 dam) [dam]

Acrop = cropped area (≈ 0,90* A) [dam²] Explanation: The number of rainfall events is equal to the number of days with precipitation. During each of the rainfall events the plants intercept 1 mm of water. Of course in months with no precipitation the formula is not applied. The number of days is also altered for a low precipitation (=1-8 mm) total.


48

Evapotranspiration due to canal infiltration (ETcanal) In unlined canals infiltration occurs. Part of the infiltrated water is used for weeds or crops alongside the canal. To be able to calculate this factor certain assumptions are made. These are:

• Plants for a width of 2 m on both sides of the canal use the water infiltrated for the process of evapotranspiration.

• As these crops could be weeds or crops from a field, the reference crop evapotranspiration (ETo) is used to calculate the evapotranspiration of these crops.

5.2.3 Surface Fraction (SF)

The surface water is mainly that part of the incoming volume that remains or ends up on the surface adding to this resource. The surface fraction is divided into two categories: the reusable fraction and the non-reusable fraction. The distinction between these fractions is based upon the quality of the water and whether the water is used within the basin or exported to another basin. The reusable surface fraction is calculated as follows:

tQRSF excess ∆⋅=

where: RSF = Reusable surface fraction [dam³] Qexcess = the excess discharge [dam³/month] ∆t = time interval [months] The runoff was not included because the slopes are flat, hence this water will infiltrate quickly into the ground or evaporate. The non-reusable surface fraction is calculated with:

tQNRSF RS ∆⋅=

where NRSF = Non-reusable surface fraction [dam³] QRS = the discharge that goes to the Rio Salado [dam³/month] ∆t = time interval [months] To this category the drainage could also be added, but as there was no data available on this factor, it was not included in the equation. The water quality of one of the drainage canals was measured and the salinity was 6,47 dS/m, which is rather high and accordingly not suited for re-use in the irrigation system. Excess discharge (Qexcess) The excess discharge is the surplus water delivered to the PRD irrigation system. This water leaves the system through the canals. Although this parameter is referred to as excess discharge, the excess volume (Vexcess) is the factor that will be worked with in the balance. Contributing to the parameter Qexcess (excess discharge) for the totality of the PRD are two different sources:

• the flow leaving the system through the canal Descargador which returns the water to the Rio Dulce

• the excess discharge delivered to each zone on a monthly basis.


49

The formula with which the volume of excess discharge for the totality of the PRD is calculated is:

tQQV xzoneexcessadorDescexcess ∆

+= ∑ *

5

1

/arg

Vexcess = excess discharge volume delivered to the PRD [dam³] QDescargador = Descargador discharge [dam³/month] Qexcess/zone x = excess discharge in zone x [dam³/month]

∆t = time variation [month] The discharges of the Descargador are known. In order to calculate the excess discharge within a zone each of the canals is regarded individually. During its transportation to the fields, a portion of the water never arrives as a result of infiltration (where the canals are unlined) and operational spills. Accordingly the excess volume of water, which leaves each canal over the period of a month, is equal to the volume of water that enters the canal during that month from which is subtracted the irrigation gift, the infiltrated water when unlined canals occur and the evaporation from the canal. Although in the formula the infiltration was subtracted because it joins the groundwater resources, the operational spills were not because it forms part of the excess discharge delivered to the zone. Next the excess discharge is calculated for each canal individually is then summed up for the zone as can be seen in the below written formula.

[ ]( )∑=

∆++−=n

i

canalfieldicanalxzoneexcess tEFVQQ1

/ /

Qexcess/zone x = excess discharge in zone x [dam³/month]

∆t = time variation [month] Qcanal i = discharge in canal I [dam³/month] Vfield = irrigation gift [dam³] F = infiltration [dam³] Ecanal = canal evaporation [dam³] The irrigation gift was calculated for the total cropped area per zone per month. For the calculation of the irrigation gift two scenarios were made. The first scenario is based on the official distribution schedule, thus a fixed flow of 300 l/s during 50 minutes per hectare per month, which means that per month a farmer receives about 90 mm water per hectare. Scenario 1:

Month April May* June* July Aug Sep Oct Nov Dec Jan Feb March

Vfield [mm]

90 45 45 90 90 90 90 90 90 90 90 90

* one turn is divided over two months, thus is assumed that half of the area is irrigated in May and the other half in June. Thus

within the gift presented, the portion of the area is already calculated. The second scenario tries to imitate reality as much as possible. As mentioned in the previous chapter, in actual practice farmers do not irrigate according to the official distribution schedule. In general they do not irrigate each month and usually take more water than permitted per turn.


50

In order to imitate reality as much as possible, the actions of the farmers are regarded. As also mentioned in the previous chapter, based on the precipitation two seasons can be defined in a year. The first is the dry season (April/May – September/October) in which irrigation is the principle source of water. The second is the wet season (November - April) irrigation is a supplement. In the graph below the average discharge entering the PRD each month is shown. As can be seen, in accordance with what was mentioned above, in the months July to November (the dry months) the largest discharges enter the PRD irrigation system. The months August, September and October usually have the maximum discharge of the season. The reason why the discharge is high in the month November – usually a wet month – can be found in the explanation that farmers usually irrigate their parcels before the rainy season in case it turns out to be a dry year. In the months May, June, February and March usually the lowest discharges are found. For the first two months mentioned, their low discharges are due to maintenance. Often the maintenance begins in the middle of May and ends in the middle of June. Thus one turn is divided over the months May and June. The other two months are wet months, in which the precipitation covers the largest part of the water need on the field.

Total Q PRD per month

0,00

10,00

20,00

30,00

40,00

50,00

60,00

apr-95

aug-9

5

dec-9

5

apr-96

aug-9

6

dec-9

6

apr-97

aug-9

7

dec-9

7

apr-98

aug-9

8

dec-9

8

apr-99

aug-9

9

dec-9

9

apr-00

aug-0

0

dec-0

0

apr-01

aug-0

1

dec-0

1

apr-02

aug-0

2

dec-0

2

apr-03

aug-0

3

dec-0

3

Months

Q [m³/s]

PRD

Figure 5.10: Discharge entering the PRD from 1995-2003

Based on the points made above, the following scenario was created where:

• The largest irrigation gifts are in the dry season spread out from August to October.

• The pre season gifts of the wet season are spread out through November and December.

• The remaining five turns are according to normal procedures. Therefore the following scenario is made for the irrigation of the farmers:

Month April May* June* July Aug Sep Oct Nov Dec Jan Feb March

Vfield [mm]

90 45 45 90 225 225 225 150 150 90 90 90

* one turn is divided over two months, thus is assumed that half of the area is irrigated in May and the other half in June. Thus

within the gift presented, the portion of the area is already calculated.


51

The scenarios are not strictly followed. In those cases where the total irrigation gift to be delivered to the fields was higher than the discharge entering the zone, a moderation was made. As the total irrigation gift to the area was taken a value of 0,7*Q (taking operational spills into account). The reason for a lower incoming volume than the total irrigation gift could be that less area was irrigated or less water was diverted to the fields due to the rainy season. Discharge to the Rio Salado The Jume Esquina delivers a certain amount of water to the Rio Salado. It is uncertain whether the large farmers on the outskirts of the system use this water or if it ends up in the Rio Salado. In the case of the former, this water is reused within the basin and would have to be dealt into the Reusable Surface Fraction. In the case of the latter, the water remains reusable but not for the users within the Rio Dulce river basin and would have to be dealt into the Non-reusable Surface Fraction. However within this work it is assumed that this water is further not utilized and delivered as such to the Rio Salado.

5.2.4 Sub-surface Fraction (GF)

Through the process of infiltration, water from precipitation or through the irrigation gift enters the ground. This occurs in two different places: infiltration from unlined canals and infiltration from the parcels. In both cases a part of this water is initially used for plants, may it be crops or weeds. This part is thus a part of the Evaporated Fraction. The rest percolates into the deeper groundwater resources and adds to the Groundwater Fraction (-GF) The percolated water is either reusable or non reusable, depending on the quality of the groundwater it percolates into. As mentioned in chapter 4, within the PRD there are areas with salinity and sodicity problems. There are also problems with the high content of arsenal. Thus in some areas of the PRD the water will be of poor quality. Information was not collected on the distribution of lands of good, moderate and poor quality. Thus it is impossible to place the percolation into the RGF or the NRGF. A formula through which this fraction can be calculated is:

fieldcanal RRGFNRR +=)/(

in which (R/NR) GF = (Reusable/Non-Reusable) Groundwater Fraction [dam³] Rcanal = Groundwater recharge through infiltrated canal water [dam³] Rfield = Groundwater recharge through infiltrated field water [dam³] As reported in the previous chapter, the subsurface flow is in the direction west to east. In the report of Angella (1999), the author made an assumption of the natural drainage rate in the PRD. During a 105 day period in which no irrigation took place he measured the watertable on a ground with a soil type which is “average” for the area. Assuming a mean soil effective porosity of 14% he calculated that the drainage rate would be around 2 mm/day. This means that after the recharge around 60 mm of water flows in the direction west to east every month. Canal water recharge (Rcanal) In the unlined canals it occurs that part of the canal water enters the ground through infiltration. Part of the infiltrated water can return to the atmosphere through evapotranspiration from weeds growing alongside the canals or crops on farms that lay


52

alongside the canal. This process was discussed in the section on the Evaporated Fraction and named canal evapotranspiration (ETcanal). What is not used for ETcanal percolates into the groundwater resources. This water is referred to as the canal water recharge (Rcanal). This factor can be calculated with the following formula:

canalcanal ETFR −=

in which Rcanal = Groundwater recharge through infiltrated canal water [dam³] F = Infiltration [dam³] ETcanal = Canal evapotranspiration [dam³] How ETcanal is calculated has been mentioned already. Rests the calculation for the infiltration. For this calculation an assumption was made concerning the quantity of water infiltrating into the ground per day:10 cm/day over the total area of the canal. This would mean that 300 cm per month. This value was taken based on the assumption that 200 mm of water (an irrigation gift delivered to the field) infiltrates into the ground in one day. Because the fields are prepared by the farmer to take in water the infiltration rate here will be higher and because the canal area through which infiltration takes place is over dimensioned, a lower value was taken for the canal infiltration. The infiltration could then be calculated as:

canalcanal LPF ⋅⋅= 3,0

in which Pcanal = length of wet bottom of the canal [dam] Lcanal = length of the canal [dam] As the dimensions of the canals are unknown, a canal profile was calculated for each canal. These can be found in annex 4. Field Recharge (Rfield) After the irrigation gift is applied or the rainstorm has occurred, water infiltrates into the ground. The ground is at field capacity and the crops use this water for their growth. All the water that is not utilized by the plant percolates into the groundwater resources. This process is also referred to as groundwater recharge. With the following formula the groundwater recharge (R) was calculated:

cfieldifield ETVEPR −+−= )(

with Rfield = groundwater recharge [dam³] P = precipitation [dam³] Ei = interception [dam³] Vfield = Irrigation gift [dam³] ETc = crop water requirement [dam³] The recharged water flows in the direction of the Rio Salado basin that lies to the East of the PRD for the zones I, II, III and V. The recharged water flows into the Rio Dulce for zone IV that lies on the left bank of the Rio Dulce.


53

5.2.5 Summary of the Assumptions

Evaporated Fraction

• ETc � Crops are raised under standard conditions. � There is usually a close similarity in the coefficients among the members of the same

crop group, as the plant height, leaf area, ground coverage and water management are normally similar. Therefore data for certain crops were substituted with those of family members.

These crops are:

Crop Crop substitute Family

Chard Spinach Small vegetables

Garlic (length of growing

stage)

Green onion Small vegetables

Anquito Squash Cucurbitaceae

Zapallo Squash Cucurbitaceae

Maize de Guinea Maize Cereals

Parsley Carrots Small vegetable

Verdeo de invierno Oats Cereal

Forages Natural pastures Forages

Lemon Citrus Fruit tree

Mandarin Citrus Fruit tree

Orange Citrus Fruit tree

Grapefruit Citrus Fruit tree

• Ei � Precipitation

The number of days within a month with precipitation is standard for every year. That number is as stated below.

Month April May June July Aug Sep Oct Nov Dec Jan Feb March

Days with P

8 5 3 3 2 3 5 8 8 11 9 10

� Plant growth

� In the initial stage of crop growth thus the LAI is 0. � In the crop development stage the LAI in this period increases from 0 to 2. � In the mid-season stage the LAI remains at 2. � In the end stage the LAI decreases from 2 to 1.

� Canopy storage � The storage canopy is 1 mm for each plant. � After each rain event the total canopy storage evaporates.

� Other � All plants are equal with regards to the amount of leaves or the surface of the

leaves. � About 90% of the field is covered with the cultivated crop.

• ETcanal � Plants for a width of 2 m on both sides of the canal use infiltrated canal water for

evapotranspiration. � The reference crop evapotranspiration (ETo) is used to calculate the

evapotranspiration.


54

Surface Fraction

• Qexcess � All farmers in the PRD exert the same behaviour concerning the quantity of the

irrigation gift. Subsurface fraction

• Rcanal � Around 10 cm/day infiltrates into the ground.

Of the assumptions made there are uncertainties in: � The crop evaporation; the actual evapotranspiration could differ from the potential

evaporation calculated. This has a great impact on the field recharge consequently on the GF.

� The irrigation gift; this probably differs from zone to zone and year to year. Also there probably is a difference in the actual irrigation gift of the large farmer and small farmer. The irrigation gift has a great influence on the GF and the RSF.

� The infiltration rate; the infiltration rate estimated could be too low or too high and this impacts the field recharge.

� The canal dimensions; the canals are probably over dimensioned. This impacts the canal recharge.

This model is provides a basis to which further information can be added in order to further refine the model. After sufficient information is gathered on the true irrigation gift received each month, the actual crop evaporation, the infiltration rate and canal dimensions and the interception, the model can be calibrated by measuring the excess discharge and monitoring the groundwater levels.


55

CCHHAAPPTTEERR 66 RREESSUULLTTSS OOFF TTHHEE AAPPPPLLIICCAATTIIOONN The irrigation system is a dynamic system. From year to year the decisions taken by the farmer will change. Depending on the climate and other social and economic factors he will make decisions about the area he will cultivate, the crops he will plant, how often and when he will need to irrigate. All these factors influence the water balance and how the irrigation water will redistribute itself within the irrigation system. What is shown in this chapter is the redistribution of the total incoming volume, thus precipitation and irrigation water, for three years based on the acquired information and data on the system and the assumptions made on the behavior of farmers and system managers. The results of the redistribution of water in the total irrigation system as for each individual zone are regarded. For the total irrigation system and the zones the incoming volume, the fractions and the interaction with the rest of the zones and the river basins are discussed. For the tables and graphs with the results see annex 5. In this chapter only the relevant aspects will be regarded.

6.1 The PRD Incoming Volume Precipitation The average precipitation in Santiago del Estero is 593,3 mm. In the year 1995/96 the precipitation was about 370 mm, well below the average. The remaining two years the precipitation was above the average, with in the last year the highest precipitation rate (750 mm). The discharge In the table below the diverted discharge in the three years examined are presented and how this water was distributed over the different zones. What is also shown is the volume of water each zone receives per ha of cultivated area under irrigation (in mm/ha) for each zone and the total PRD.

1995/1996 1996/1997 1999/2000

Vtotal Vtotal Vtotal

hm³ mm/ha hm³ mm/ha hm³ mm/ha

Zone I 157,4 1.968 132,1 3.061 119,2 3.280

Zone II 411,2 2.047 400,0 1.764 381,2 1.354

Zone III 190,5 3.205 163,8 2.063 158,2 1.373

Zone IV 140,8 2.342 166,1 2.925 123,1 2.203

Zone V 90,0 809 88,7 822 86,0 1.403

Matriz 1.084,1 2.119 1.040,1 2.023 1.019,9 1.853

Table 6.1: Water distribution amongst the zones

14%

44%18%

14%

10%16%

42%

19%

14%

9% 14%

43%17%

17%

9%


56

What can be noticed from the table is the following: � The majority of the water in the PRD is diverted to zone II and the minority to zone V.

This is not surprising because zone 2 is the biggest zone and zone 5 the smallest. � The division of water over the zones is quite steady during the three years. � The maximum quantity of water available to the PRD is 1.548 hm³, which upon

calculations should be enough to irrigate 122.000 ha of cultivated land. This means that roughly there would be 1.269 mm water per ha cultivated land (factors as infiltration, operational spills etc have not been subtracted). Yet in the years 1995/96, 1996/97 and 1999/2000 this ratio has been superseded by respectively 167%, 159% and 146%. Which already indicates that more water is used than intended, because more water is available for the fields due to the fact that less area is cultivated than planned.

Area cultivated 1995/96

16%

38%

12%

12%

22%

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5


8%

45%

15%

11%

21%

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5


7%

51%21%

10%

11%

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Figure 6.1: Area Cultivated in the different zones

� The distribution of cultivated area is not in accordance with the water distributed. In zone

2 the greatest area is cultivated, in all three years. The least area is cultivated in zone 1 in 2 of the three years, yet it does not receive the smallest portion of water. Zone 5, though the smallest of the 5 zones, cultivates relatively more area than zones 1, 3 and 4, yet it receives the smallest portion of water, as already mentioned above.

Fractions In figure 6.2 the division of the total incoming volume that enters the PRD is shown. The division remains more or less steady throughout the years. During each year the EF has increased slightly. The area cropped has also increased a bit each year so this is not strange. The greatest fraction however is the RSF, which takes in more than half of the total volume of water. Taking a look at annex 5 it can be seen that a great portion of RSF comes from the discharge of the Descargador, thus it really never enters one of the zones but returns to the Rio Dulce.


57

From scenario1 to 2 the GF increases and the RSF decreases. The greater portion of the GF comes from field recharge in the last two years and increases with scenario 2. This is not a surprising development as more water is diverted to the fields. In the first year the greater part comes from canal recharge but this diminishes in scenario 2.

EF (NR/R)GF RSF NRSF

Scenario 2

1999/2000

Scenario 1

1995/1996 1996/1997

19%

63%

9%

9%

20%

57%

9%

14%

21%

56%

12%

11%

22%

50%

12%

16%

23%

55%

11%

11%

23%

48%

11%

18%

Figure 6.2: Division of the fractions for the PRD

Scenario 1 EF % RSF % GF % NRSF %

1995/96 306.888 19 991.705 63 140.994 3 150.595 9

1996/97 351.548 21 958.638 56 179.305 11 208.289 12

1999/2000 450.754 23 1.081.255 55 219.158 11 206.556 11

Scenario 2

1995/96 317.356 20 901.064 57 221.367 14 150.595 9

1996/97 372.398 22 846.151 50 271.198 16 208.289 12

1999/2000 464.892 23 941.134 48 347.818 18 206.556 11

Table 6.2: Fraction quantities in dam³ and %

In table 6.2 the values and percentages of the flows are presented. At the end of the chapter the total interaction of the PRD with its surroundings will be discussed.


58

6.2 Zone 1 In zone 1 the channels are all lined, thus in this zone there is no infiltration from the canals. This zone borders the basin in the north and northeast in the south and southwest it borders zone 2. In the west it borders the basin. Incoming Volume During the three years investigated, the total incoming volume diminishes as can be seen in figure 6.3 and table 6.3. The discharge entering the zone is far greater than the precipitation. The discharge is greatest during the dry season (July through October), which is also the season with the greatest incoming volume of the year.

Incoming Volume zone 1

0,00

5.000,00

10.000,00

15.000,00

20.000,00

25.000,00

30.000,00

Volume [dam³]

Q

P

Figure 6.3: The total incoming Volume in 95/96, 96/97 and 99/00

The area cultivated also decreases through the years. The cultivated area that was irrigated in the year 1999/2000 is less than half of the area in1995/96 to that of. Yet the discharge diverted into the zone per hectare of land cultivated with irrigation rights increases throughout these years.

Year Q [*10³ dam³] Acrop [10² a] Ratio [mm/ha] Ratio per month P [mm/year]

95/96 157,43 8.001 1.968 179 371

96/97 132,11 4.314,5 3.062 278 766

99/00 119,24 3.280,9 3.281 298 889 Table 6.3: Discharge, Cultivated area and ratio in zone 1

Fractions Figure 6.4 shows how the incoming volume divides itself throughout the years for both scenario 1 and 2. In table 6.4 the values are presented. For both scenarios the greater fraction is the RSF, to which around two thirds of the water is dealt. The RSF in zone 1 contributes 20 –23% for scenario 1 and 21-30% in scenario 2 to the RSF fraction of all five zones in the PRD (see annex 5, Results PRD). Of the total amount of water that enters zone 1 68-71% contributes to the RSF in the first scenario and 60-66% to the second scenario. In the balance, the irrigation gift per month depends on the flow


59

diverted to the fields and the total area cropped. A possible explanation for the large RSF fraction could be that both irrigation gifts assumed in scenarios 1 and 2 are too little or the area cropped is greater than used in the calculations. Vfield is probably greater than calculated.

EF RSF (NR/R)GF

1999/2000

Scenario 1

1995/1996 1996/1997

Scenario 2

19%

71%

10%

20%

66%

14%

22%

70%

8%

24%

61%

15%

27%

68%

5%

29%

60%

11%

Figure 6.4: Division of the fractions in zone 1

Scenario 1 EF % RSF % GF %

1995/96 49.933 27 124.035 68 9.566 5

1996/97 28.497 19 109.895 71 15.260 10

1999/2000 32.122 22 99.044 70 11.788 8

Scenario 2

1995/96 52.457 29 110.651 60 20.425 11

1996/97 30.924 20 100.774 66 21.954 14

1999/2000 34.107 24 86.901 61 21.946 15


The quantity of water that adds to the RSF decreases from scenario 1 to scenario 2. The RSF is what remains in the channels after the irrigation gift (Vfield) and the canal evaporation is subtracted. Thus the difference between scenario 1 and 2 can be explained by the irrigation gift, which is higher in scenario 2 than in scenario 1. Taking a closer look at the RSF throughout the months, as can be seen from figure 6.5, RSF is the greatest in the months July through October, thus in the dry season. In these months the greatest discharge is also diverted into the area, as can be noted from figure 6.3. However, only a small fraction of the incoming volume is diverted to the fields. This fact supports the assumption that the Vfield is greater than assumed in the scenarios.


60

RSF

0,00

5.000,00

10.000,00

15.000,00

20.000,00

25.000,00

scen. 1

scen. 2

Lar ge RSF

Lar ge RSF

Figure 6.5: RSF throughout the months of 95/96, 96/97 and 99/00

Of the total amount of water that enters the zone 19-27% in scenario 1 and 20-29% in scenario 2 returns to the atmosphere through the process of evaporation. The crop evapotranspiration is the greatest contributor to this fraction. Around 14% is due to evaporation from other sources. In comparing scenario 1 and 2 with each other, in scenario 2 the EF is greater. The reason for this is that in scenario 2 more water is diverted to the fields, therefore more water is available for crop evapotranspiration. Using the potential evapotranspiration, the water diverted to the fields in scenario1 is not always sufficient to fully cover the total crop evapotranspiration. In the months when this is not the case, the EF is highlighted with the color gray in annex 5 (results zone 1). In comparison to the other fields, the GF is quite low. This is because the canals in this zone are all lined, therefore the total (NR/R)GF comes from recharge from the fields. The excess water diverted to the fields percolate into the groundwater resources. In figure 6.6 the division of the GF throughout the months is shown. The GF is greatest in the wet season and small in the dry season. If indeed as suspected the irrigation gifts are larger than assumed in both scenarios 1 and 2, the EF and the GF would be greater than shown in the graphs. The EF would be greater in the case that more area is cropped than noted and the GF would be greater in the previous case as in the case that the irrigation gift is higher. That the irrigation gift is higher is not an unreasonable assumption if the researched irrigation culture is taken into consideration. Thus that farmers like to store water in the ground in the case the precipitation is lacking.


61

Groundwater Fraction

-1.000

0

1.000

2.000

3.000

4.000

5.000

6.000

7.000

8.000

9.000

GF scen 1

GF scen 2

Figure 6.6: Groundwater Fraction in 95/96, 96/97 and 99/00

Figure 6.7: The spatial division of the total incoming volume; scenario 1, year 1996/97 (d= dam³)

Field

Canal Rf = 15.260 d= 9,9%

Qex= 109.895 d= 71,5%

ETc+Ei = 27.215 d= 17,7%

Ecanal = 1.281 d = 0,9%

Vf = 21.450 d

Groundwater


62

In figure 6.7 is shown how the total incoming volume divides itself in scenario 1 over the fractions for one of the years used, namely 1996/1997. The size of the arrow gives an indication of the size of the flow. The color of the arrow indicates to which fraction the flow belongs, blue is for the surface fraction, yellow for the atmosphere and dark blue for the groundwater resources. Mentioned by each of the factors are the values and the percentage of the total incoming volume. The excess discharge (Qe) is the greatest factor here, consisting of 71,5% of the total incoming volume. This factor contributes to the surface fraction. Figure 6.8: The spatial division of the total incoming volume; scenario 1, year 1996/97 (d= dam³) In figure 6.8 is shown how the total incoming volume divides itself in scenario 2 over the fractions for one of the years used, namely 1996/1997. Mentioned by each of the factors are the values and the percentage of the total incoming volume. The excess discharge (Qe) is the greatest factor here, consisting of 65,6% of the total incoming volume. This is less than in scenario 1. Another difference with scenario 1 is that the evapotranspiration from the field and the groundwater recharge from the field are greater. Interaction with the rest of the system & river basin As for the interaction with its surroundings, the only contributor to the RSF in zone 1 is the excess discharge (Qe). It is also the largest factor, making the RSF the greatest fraction in the zone. It is not clear what the final destination of the irrigation water is after it leaves the zone, however, it leaves the zone via the canals Norte and La Cuarteada, which both flow in

Field

Canal Rf = 21.954 d= 14,3%

Qex= 100.774 d= 65,6%

ETc+Ei = 29.642 d= 19,3%

Ecanal = 1.281 d = 0,8%

Vf = 30.571 d

Groundwater


63

the direction northeast. This can be seen in figure 6.9. The size of the arrows is not indicative of the size of the flow.

Figure 6.9: Canals zone 1

The field recharge (Rf) adds to the groundwater resources. As mentioned in paragraph 4.2, the sub-surface flow occurs in the direction west to east. The ground has a drainage capacity of 2 mm/day, which means that 60 mm of groundwater can be drained out of an area per month. Depending on the position of the plot irrigated, the water naturally drained out of the area can be reused by other plots or weeds to the east of the plot within zone 1. If the plot lies on the borders of the zone, it could be reused within the second zone or the Rio Dulce basin. Thus on a subsurface level there is a possible interaction between zone 1 and zone 2 or the Rio Dulce river basin, depending on the location of the plot irrigated.


64

6.3 Zone 2 In zone 2 none of the canals are lined, thus there is canal infiltration. Zone 2 borders zone 1 to the northeast, the river basin and zone 5 to the east, to the south and southeast it borders the river basin and to the west it borders zone 3. Incoming Volume As can be seen in figure 6.10 the incoming volume increases throughout the years. In the last year, contrary to the two previous years, the precipitation contributes a lot to the incoming volume.

Incoming Volume Zone 2

0,00

10.000,00

20.000,00

30.000,00

40.000,00

50.000,00

60.000,00

70.000,00

80.000,00

Volume [dam³]

Q

P


Throughout the years the discharge that enters the zone decreases slightly while the cultivated area that is irrigated increases. The water per ha cultivated area that enters the zone decreases with the years.


95/96 354,74 19.996 1.774 161 352

96/97 337,16 22.598 1.492 136 606


Fractions Figure 6.11 shows how the incoming volume divides itself throughout the years for both scenario 1 and 2. In table 6.6 the values are presented. The greatest percentage of RSF from the all of the zones comes from zone 2. In scenario 1 this is 35-37% of the total RSF and 31-36% in scenario 2. While the RSF is the greatest fraction in 1995-96 this is no longer the case in 1999/2000 as the fraction decreases through the years. This is due to two factors the first being that the discharge delivered to the zone decreases through the years and the second that the area cultivated increases. This results


65

in a decrease of the ratio between the discharge and the cultivated area that is irrigated (see table (6.5). From scenario 1 to scenario 2 the RSF decreases due to the higher irrigation gifts applied in the second scenario.

EF RSF (NR/R)GF

Scenario 2

1999/2000

Scenario 1

1995/1996 1996/1997

30%

57%

13%

30%

49%

21%

38%

44%

18%

41%

32%

27%

47%

32%

21%

48%

19%

33%



1995/96 122.264 30 233.621 57 55.195 13

1996/97 161.183 38 184.344 44 73.669 18

1999/2000 225.761 47 155.482 32 99.045 21

Scenario 2

1995/96 123.875 30 198.876 49 88.329 21

1996/97 171.096 41 135.590 32 112.509 27

1999/2000 231.189 48 92.693 19 156.406 33


The EF increases from scenario 1 to scenario 2, the reason for this occurrence is that in the first scenario not all the crop needs calculated are met. In the second scenario this is also not the case. The months in which there is insufficient water to cover the needs of the crops are highlighted with gray in the tables in annex 5 (results zone 2). The crop evapotranspiration is the greatest contributor to this fraction. The other processes contribute roughly 4% to the EF. In comparison to the GF in the other zones, the GF in zone 2 is the largest. Of the total incoming volume of zone 2, in the first scenario 13-21% (≈1/10 to 1/5) adds to the groundwater resources and in the second scenario this is 21-27% (≈1/5 to ¼). There are two factors contributing to the GF and these are the field recharge and the canal recharge. The field recharge is in almost all of the cases the greatest contributor to the GF; the ratio between these two factors can be seen in annex 5 (results zone 2, the graph on Division GF over zones). From scenario 1 to 2 the GF increases due to the increase of the field recharge (Rfield).


66

Field

Canal Rf = 38.860 d= 9,3%

Qex= 184.344 d= 44,0%

ETc+Ei = 157.631 d= 37,5%

Rcanal = 34.809 d= 8,3%

Ecanal +ETcanal= 3.552 d = 0,9%

Vf = 115.334 d

Groundwater

Field

Canal Rf = 75.190 d= 17,9%

Qex= 135.590 d= 32,3%

ETc+Ei = 164.505 d= 39,2%

Rcanal = 37.319 d= 8,9%


Vf = 158.539 d

Groundwater


67

On the previous page the figures 6.12 and 6.13 are presented. The first figure shows how the total incoming volume divides itself in scenario 1 respectively scenario 2 over the fractions for one of the years used, namely 1996/1997. The size of the arrow gives an indication of the size of the flow. The color of the arrow indicates to which fraction the flow belongs, blue is for the surface fraction, yellow for the atmosphere and dark blue for the groundwater resources. Mentioned by each of the factors are the values and the percentage of the total incoming volume. Here both the excess discharge and the evapotranspiration from the fields that respectively contribute to the RSF and the EF are big factors. Interaction with the rest of the system & river basin

Figure 6.14: In and outflow zone 2

In figure 6.14 the green canals are the canals that run through zone 2. The thick black line is the main canal La Matriz which turns into Jume Esquina later on. The discharge enters the zone in three different places of which the first is canal Sud I, the second Suri Pozo and the third Sud II. The yellow arrows indicate these three places. The size of the arrows is not indicative of the discharge that enters the canals. The RSF leaves the zone in the areas indicated with blue arrows of which the first is at the end of the first canal, Sud I, and flows out in the northeast. The second outflow is at the end of the Suri Pozo and flows out in the southeast. In both cases the water reenters the river basin.


68

The field recharge (Rf) and canal recharge (Rcanal) add to the groundwater resources. As already mentioned, the sub-surface flow occurs in the direction west to east. The ground has a drainage capacity of 2 mm/day, which means that 60 mm of groundwater can be drained out of an area per month. Depending on the position of the plot irrigated, the water naturally drained out of the area can be reused by other plots or weeds to the east of the plot within zone 2. In figure 6.14 the dark blue arrows indicate the possible direction of the subsurface flow and how it would interact with its surroundings. If the plot lies on the eastern borders of the zone, it could be reused within the zone 5 or the Rio Dulce basin. Thus on a subsurface level there is a possible interaction between zone 1 and zone 5 or the Rio Dulce river basin, depending on the location of the plot irrigated.


69

6.4 Zone 3 The zone borders zone 2 in the east, the Rio Dulce in the west and to the south the Rio Dulce river basin. In zone 3 the canals are not lined, thus canal infiltration occurs in this zone. Incoming Volume The general trend of the incoming volume is that it decreases in this zone. This can be seen in figure 6.10 and table 6.7.


0,00

5.000,00

10.000,00

15.000,00

20.000,00

25.000,00

30.000,00

35.000,00

40.000,00

45.000,00

Volume [dam³]

Q

P


The greater part of the water is formed by the discharge, which by far exceeds the precipitation. In zone 3 the discharge decreases throughout the years, yet the cultivated area increases and in the last year it is twice the size it was in the first year. The ratio therefore decreases considerably throughout the years.


95/96 190,5 5.944 3.205 291 361

96/97 162,9 7.942 2.051 186 592


Fractions Figure 6.16 shows how the incoming volume divides itself throughout the years for both scenario 1 and 2. In table 6.8 the values are presented. Of the total RSF fraction of all the zones in the PRD, 18-23% in scenario 1 and 17-25% in scenario 2 contributes to this fraction. The greatness of this fraction varies from year to year in this zone as can be seen in figure 6.16. In the first year the RSF is the greatest fraction in scenario 1 as well as scenario 2. In the second year, the RSF is also the greatest fraction, yet not as great as in the previous year. In the third year it is no longer the greatest fraction.


70

This is due to the decrease in water available per ha cultivated area. From scenario 1 to scenario 2 the RSF decreases due to the higher irrigation gifts applied in the second scenario.

EF RSF (NR/R)GF

1999/2000

Scenario 2

Scenario 1

1995/1996 1996/1997

19%

70%

11%

21%

63%

16%

31%

56%

13%

34%

43%

23%

44%

38%

18%

46%

24%

30%



1995/96 39.549 19 144.831 70 23.177 11

1996/97 60.350 31 107.099 56 24.409 13

1999/2000 90.347 44 77.328 38 36.807 18

Scenario 2

1995/96 42.693 21 132.362 63 32.503 16

1996/97 65.107 34 82.795 43 43.956 23

1999/2000 95.283 46 48.526 24 60.673 30


The EF doubles from the first to the last year. This is consistent with the increase of the cultivated area. The greatest contributor to the EF is the crop evapotranspiration. The EF as the crop evapotranspiration increases from the first to the second scenario, because in the second scenario there is more water available for irrigation. The other factors contribute around 15% to the EF. As in zone 2 the GF increases from scenario 1 to scenario 2 due the higher irrigation gift delivered to the cultivated areas, which cause the field recharge to increase. The GF increases through the years with 9% in scenario 1 and 14% in scenario 1, this is mainly due to the increase in precipitation throughout the years.


71

Field

Canal Rf = 9.829 d= 5,1%

Qex= 107.099 d= 55,8%

ETc+Ei = 57.095 d= 29,8%

Rcanal = 14.580 d= 7,6%


Vf = 38.818 d

Groundwater

Field

Canal Rf = 29.412 d= 15,3%

Qex= 82.795 d= 43,2%

ETc+Ei = 61.907 d= 32,3%

Rcanal = 14.543 d= 7,6%


Vf = 63.213 d

Groundwater


72

On the previous page the figures 6.17 and 6.18 are presented. The first figure shows how the total incoming volume divides itself in scenario 1 respectively scenario 2 over the fractions for one of the years used, namely 1996/1997. The size of the arrow gives an indication of the size of the flow. The color of the arrow indicates to which fraction the flow belongs, blue is for the surface fraction, yellow for the atmosphere and dark blue for the groundwater resources. Mentioned by each of the factors are the values and the percentage of the total incoming volume. Here both the excess discharge that contributes to the RSF is the biggest factor. Its influence diminishes a bit in the second scenario, as the evapotranspiration from the fields increases and the field recharge triples. Interaction with the rest of the system & the river basin

In figure 6.19 zone 3 the canals of zone 3 are depicted in yellow. The line around the canal shows where the approximate boundary is of zone 3. As can be seen, the canals stay within the boundaries of zone 3, which would mean that there is no interaction on a surface level between zone 3 and the river basin. The RSF is reused within zone 3. As already mentioned, the sub-surface flow occurs in the direction west to east. The ground has a drainage capacity of 2 mm/day, which means that 60 mm of groundwater can be drained out of an area per month. Depending on the position of the plot irrigated, the water naturally drained out of the area can be reused by other plots or weeds to the east of the plot within zone 3. The dark blue arrows indicate the subsurface flow. Water from fields on the eastern border of zone 3 could flow into zone 2 on a subsurface level. Possibly fields irrigated by the canals Municipal, Los Romanos and Beltran 2 could have a field recharge that contributes to this interaction. Thus interaction on a subsurface level is possible between zone 3 and zone 2, depending on the location of the plot irrigated.

Figure 6.19: Canals in zone 3


73

6.5 Zone 4 Zone 4 is located at the right bank of the irrigation system. To the north it border the city of Santiago del Estero, to the east the Rio Dulce to the west and to the south it borders the Rio Dulce river basin. Roughly one third of the total length of the canals are unlined. Incoming Volume


0,00

5.000,00

10.000,00

15.000,00

20.000,00

25.000,00

30.000,00

Volume [dam³]

Q

P

Figure 6.20: The total incoming Volume in 95/96, 96/97 and 99/00 The discharge forms the greater part of the incoming volume. The greatest quantity of water entering the zone occurs in October and November. In 96/97 in December and January. Throughout the years the discharge diminishes, which is a common trend in almost all of the zones of the PRD. The cultivated area that is irrigated also decreases and so does the discharge per ha of cultivated area.


95/96 140,82 6.014 2.341 213 396

96/97 116,08 5.678 2.044 186 765


Fractions Figure 6.21 shows how the incoming volume divides itself throughout the years for both scenario 1 and 2. In table 6.10 the values are presented. Of the total RSF fraction of all the zones in the PRD, zone 4 remains at a steady 14% for scenario 1 as well as scenario 2. It is only in the last year that this percentage rises to 17%. In zone 4 half of the total incoming volume is RSF in scenario 1, while approximately 40% is RSF in scenario 2. The same trends can be noticed as in the zones above: in scenario 2 the RSF decreases and the EF and the GF increase for the same reasons as mentioned above.


74

What is noticeable is that the RSF follows the ratio between the diverted discharge and the cultivated area. In the year 1996/97 the ratio decreases only to rise again in the following year. The RSF follows the same pattern.

EF RSF (NR/R)GF

1999/2000

Scenario 2

Scenario 1

1995/1996 1996/1997

27%

57%

16%

29%

48%

23%

31%

47%

22%

31%

35%

34%

31%

49%

20%

32%

37%

31%



1995/96 43.199 27 90.935 57 24.846 16

1996/97 44.186 31 68.599 47 31.292 22

1999/2000 44.533 31 71.547 49 28.880 20

Scenario 2

1995/96 45.959 29 75.805 48 37.216 23

1996/97 44.186 31 51.587 35 48.304 34

1999/2000 46.901 32 52.643 37 45.417 31

Table 6.10: Fraction quantities in dam³ and % The evaporated fraction remains steady through the years and takes in about one third of the total incoming volume. The crop evapotranspiration is the greatest contributor to the EF, the other factors contribute around 14% to the EF. The GF does not follow the ratio between discharge and cultivated area being irrigated. In the second year when the ratio is at its lowest, the GF is the highest. The reason for this is the high precipitation in the second year. As in the other scenarios the percentage GF increases from the first to the second scenario, due to an increase in field recharge. On the following page the figures 6.22 and 6.23 are presented. The first figure shows how the total incoming volume divides itself in scenario 1 respectively scenario 2 over the fractions for one of the years used, namely 1996/1997. The size of the arrow gives an indication of the size of the flow. The color of the arrow indicates to which fraction the flow belongs, blue is for the surface fraction, yellow for the atmosphere and dark blue for the groundwater resources. Mentioned by each of the factors are the values and the percentage of the total incoming volume.


75

Field

Canal Rf = 17.625 d= 12,2%

Qex= 68.599 d= 47,6%

ETc+Ei = 42.955 d= 29,8%

Rcanal = 13.667 d= 9,5%


Vf = 32.954 d

Groundwater

Field

Canal Rf = 34.970 d= 24,3%

Qex= 51.587 d= 35,7%

ETc+Ei = 42.955 d= 29,8%

Rcanal = 13.334 d= 9,3%


Vf = 50.298 d

Groundwater


76

Interaction with the rest of the system & the river basin In figure 6.25 the canals of zone 4 are presented. They are colored red. Water at the end of the Maco Mangosta and the Contrera Lopez remain in zone 4. The water of the San Martin however leaves the system in the south (see the blue arrow). What happens to the water afterwards is unclear. As for the GF (see the dark blue arrows), the zone borders the Rio Dulce in the east. Water that enters the groundwater sources, depending on its location could possibly enter the Rio Dulce, especially the plots situated close to the Rio Dulce. From the other fields, which lie more to the east, the water will remain in zone 4.

Figure 6.25: Canals of zone 4


77

6.6 Zone 5 Zone 5 is a relatively new zone. It borders the Rio Dulce to the west. To the north, east and south of the system lies zone 2. all of the canals in this system are unlined. Incoming Volume Zone 5 is a relatively small and new zone. Throughout the years the discharge diverted to this zone decreases. The precipitation plays a greater role in the total incoming volume in the wet season. It seems the greater part of the water is necessary in the dry season. The discharge entering the zone in those months is greater than the discharge and the precipitation combined in the wet season.

Incoming Volume zone 5

0,00

5.000,00

10.000,00

15.000,00

20.000,00

25.000,00

Volume [dam³]

Q

P


Throughout the years the discharge diverted into the zone decreases slightly, while the cropped area decreases almost by half. The volume of water diverted per ha of ground steeply increases with the years.


95/96 89,97 11.120 809 73 339

96/97 88,69 10.790 822 75 484

99/00 86,30 6.128 1.408 128 669

Table 6.11: Discharge, Cultivated area and ratio in zone 4

The water used in the first two years is less than the 990 mm/ha of the official distribution schedule and all three years are far less than the 1515 mm/ha of scenario 2. Fractions Figure 6.27 shows how the incoming volume divides itself throughout the years for both scenario 1 and 2. In table 6.12 the values are presented.


78

EF RSF (NR/R)GF

1999/2000Scenario 1

1995/1996 1996/1997Scenario 2

44%

31%

25%

44%

18%

38%

48%

21%

31%

52%

9%

39%36%

10%

54%

36%

28%

36%



1995/96 49.309 44 34.599 31 28.099 25

1996/97 54.849 48 23.515 21 34.514 31

1999/2000 35.910 36 28.065 26 37.058 36

Scenario 2

1995/96 49.708 44 19.686 18 42.614 38

1996/97 58.561 52 10.219 9 44.099 39

1999/2000 35.910 36 10.581 10 54.542 54


In zone 5, contrary to the other zones, the RSF is the smallest fraction. In this zone the SF is a small fraction, in scenario 1 it is at most one third of the total incoming volume. This is due to the fact that the ratio is much smaller for this zone. It is even under the irrigation gift for the official distribution schedule, thus after the water is delivered to the fields, less water remains in the canals. The greater fraction in almost all of the cases is the EF. In the first and last year, however the EF does not increase whereas the GF does. The difference between scenarios 1 and 2 is that SF decreases and the GF in all cases and EF in some cases become greater. In the first scenario the irrigation gift was often not enough to cover the needs of the crops, according to the calculations. Therefore in the second scenario, with a higher irrigation gift, the needs were better met. So much so that the GF increased as well. Taking a look at the how the GF divides itself between the recharge from the fields and the recharge from the canals, it can be seen that in scenario 1 Rcanal is always greater, while in scenario 2 Rfield becomes greater. Because the irrigation gift is greater in scenario 2, it is logical that Rfield would increase as well.


79

Field

Canal Rf = 14.600 d= 12,9%

Qex= 23.515 d= 20,8%

ETc+Ei = 53.215 d= 47,1%

Rcanal = 14.600 d= 12,9%


Vf = 43.970 d

Groundwater

Field

Canal Rf = 27.554 d= 24,4%

Qex= 10.219 d= 9,1%

ETc+Ei = 56.927 d= 50,4%

Rcanal = 16.554 d= 14,7%


Vf = 60.635 d

Groundwater


80

On the previous page the figures 6.28 and 6.29 are presented. The first figure shows how the total incoming volume divides itself in scenario 1 respectively scenario 2 over the fractions for one of the years used, namely 1996/1997. The size of the arrow gives an indication of the size of the flow. The color of the arrow indicates to which fraction the flow belongs, blue is for the surface fraction, yellow for the atmosphere and dark blue for the groundwater resources. Mentioned by each of the factors are the values and the percentage of the total incoming volume. Here in both scenarios the evapotranspiration from the field is the largest fraction. An increase of the irrigation gift, for the largest part ends up in the groundwater resources. Interaction with the rest of the system & the river basin

Figure 6.30: Canals in zone 5

None of the canals in the zone leave the zone, therefore the RSF probably remains within the zone. What happens to it is unclear. As for the GF, the groundwater, when it drains out of the area, possibly ends up in the Rio Dulce river basin. This of course depends on the location of the field. Fields close to the western border of the system will more likely contribute more to the interaction between the zone and the Rio Dulce river basin.


81

6.7 Discussion To conclude this chapter some observations are made: The discharge is the greatest contributor of water to the basin and also to the individual zones.


82

In figure 6.31 is a drawing of the PRD and the flow of water in the main canal. The white arrows show the flow of the irrigation water as it is transported through the PRD. The blue arrows show where the water leaves the La Matriz to enter zone 1. The green arrows show where water is diverted to zone 2, the yellow arrow to zone 3, the red to zone 4 and the purple arrow to zone 5. The PRD system can be divided into 5 different systems each of the zones is different in their characteristics. As a result the redistribution of water differs from zone to zone as could be seen in the previous paragraphs. This is visible in comparing the division of the fractions from each zone with each other. Instead of focusing on the total system, it might be wiser to regard one system at a time and determine its interaction with the other systems and the river basin. The general trend in the scenarios is that the RSF decreases from scenario 1 to 2 and the GF and, in some cases where the crop needs are not met, the EF increases. The explanation for this is that more water is diverted to the fields thus leaving less water in the canals and more water for evapotranspiration and groundwater recharge. Here it is obvious that one of the important factors influencing the division in fractions is the irrigation gift applied to the fields. In almost all of the zones the RSF was the greater fraction. It is probable that the irrigation gift in reality is much higher than assumed in scenario 2. The irrigation system is dynamic and the rigid scenarios applied will vary from year to year and zone to zone. If the RSF in reality is lower due to higher irrigation gifts, the GF is higher. In this report the potential evapotranspiration was used to calculate the evapotranspiration of the crops. Schipper (2005) calculated the average actual evapotranspiration for the PRD. The values that resulted from his calculations are below those calculated for the reference crop evapotranspiration. Using the values for the actual evapotranspiration, in all three years the evapotranspiration for the total PRD (irrigated and non irrigated area ≈ 285.00 ha) was below the total incoming volume.

95/96-ETa

0,00

50.000,00

100.000,00

150.000,00

200.000,00

250.000,00

300.000,00

350.000,00

400.000,00

apr-95 mei-95 jun-95 jul-95 aug-95 sep-95 okt -95 nov-95 dec-95 jan-96 feb-96 mrt -96

Incoming Volume ETa

Figure 6.32: Actual Evapotranspiration for the total PRD in 1995/96


83

96/97- ETa

0,00

50.000,00

100.000,00

150.000,00

200.000,00

250.000,00

300.000,00

350.000,00

400.000,00

450.000,00

500.000,00


Volume [dam³]

Incoming Volume ETa


99/00- ETa

0,00

100.000,00

200.000,00

300.000,00

400.000,00

500.000,00

600.000,00

700.000,00

800.000,00


Volume [dam³]

Incoming Volume ETa


The total incoming volume is thus more than sufficient to supply the whole PRD irrigation system with water. Yet only one fifth of the 285.000 ha is irrigated. This leads to a surplus of water that can be reckoned either to the GF or the RSF. As can be seen in the following figures, the precipitation is enough supply the total PRD system (irrigated and rain fed) areas with water in the wet season (November- March). The discharge is, in the dry season (when irrigation provides the system with water, more than enough to supply an area of 285.000 ha with water. This period overlaps the period in zone 1 with the greatest RSF.


84

1995/1996

0,00

50.000,00

100.000,00

150.000,00

200.000,00

250.000,00

300.000,00

350.000,00

apr-95 mei-95 jun-95 jul-95 aug-95 sep-95 okt-95 nov-95 dec-95 jan-96 feb-96 mrt-96

Volume [dam³]

Pvolume ETa Qnetto

Figure 6.35: Precipitation, net discharge and actual evapotranspiration in 1995/1996

1996/1997

0,00

50.000,00

100.000,00

150.000,00

200.000,00

250.000,00

300.000,00

350.000,00

400.000,00

apr-96 mei-96 jun-96 jul-96 aug-96 sep-96 okt-96 nov-96 dec-96 jan-97 feb-97 mrt-97

Volume [dam³]

Pvolume ETa Qnetto



85

1999/2000

0,00

100.000,00

200.000,00

300.000,00

400.000,00

500.000,00

600.000,00

700.000,00

apr-99 mei-99 jun-99 jul-99 aug-99 sep-99 okt-99 nov-99 dec-99 jan-00 feb-00 mrt -00

Qnet to Pvolume ETa


Once again whether this water is added to the RSF or the GF, depends on the irrigation gift. The irrigation culture in the tertiary sector, as well as the interaction between farmer and the administrador will greatly influence this dynamic factor. Also of influence on this factor are the crops tilled and the area cultivated. From year to year the greatness of the fraction varies, what is obvious is that especially the RSF varies in accordance with the ratio between the diverted discharge and the area cultivated. These two are thus important factors in the division of the fractions. One of the characteristics mentioned in chapter 4 of the system is that in an area of about 350.000 ha only 122.000 ha are eligible for irrigation. Therefore there are pockets of land that are irrigated amongst land where rain fed agriculture is practiced. The interaction of the different zones with their surrounding has already been discussed in the previous paragraphs. Depending on the location of the farms, water entering the ground, will probably first spread in the surrounding area and will be consumed before ever reaching the borders of the irrigation system. The farms on the border could interact with the basin. However, Ertsen et al also spoke of unclear boundaries between the irrigation system and the basin. It is probable that the water is consumed in the transition zone between basin and irrigation system. In figure 6.37 is a summary of the interactions described in the previous paragraphs for the total PRD. The blue areas are where the surface water enters the river basin. The dark blue arrows show the interactions on a subsurface level. To better understand the interactions with the river basin, the zones 1, 5 and 2 should be studied with more detail, as they are the border zones. Studying zone 4 is also of interest as the RSF and GF could contribute to the discharge of the Rio Dulce.


86


87

CCOONNCCLLUUSSIIOONNSS && RREECCOOMMMMEENNDDAATTIIOONNSS The first part of this report entailed coming to a better understanding of the interaction between the river basin and the irrigation system. This interaction was defined in the form of a model, which combines the fraction model of Allen et al and the water color scheme of Savenije. Taking the irrigation system as focal point it was established that the inflow of water into the irrigation system came from three different sources: precipitation (atmosphere), discharge (surface) and groundwater flow (subsurface). For the outflow of water into the river basin a fraction model was defined redistributing the water between atmosphere, surface and sub-surface. A distinction was also made between reusable and non-reusable water. Using these distinctions a water balance was defined dividing the total incoming volume into 5 different fractions. These fractions formed the link between the irrigation system and the river basin. This model was applied to the Proyecto Rio Dulce. From the results could be gathered that the division of the water is dependent on: � The incoming volume � The area cultivated � The irrigation gift These factors change each year as a result of the decisions the farmer makes about the area he will cultivate, the crops he will plant, how often and when he will need to irrigate. This is especially noticeable in the differences between scenario 1 and scenario 2. The difference between these two zones lies in the irrigation gift given to the fields. Increasing the irrigation gift always leads to a decrease in the RSF and an increase in the GF and sometimes in the EF if scenario1 was not sufficient to cover the needs of the crops. Upon taking a closer look at the total PRD system it is noticeable that the total system can be divided into 5 different systems. Each of the zones is different in their characteristics. They all differ in size, physical infrastructure, dominant type of farm (small or large), crops tilled and probably in the manner in which water is handled. As a result the redistribution of water shall also differ from zone to zone. This is visible in comparing the division of the fractions from each zone with each other (see figure 7.1).

PRD-s2-95/96: Division RSF over zones

21%

36%

25%

14%4%

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

PRD-s2-95/96: Division GF over zones

9%

40%

15%

17%

19%

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Figure 7.1: Division of the fractions over the zones for scenario 2 in 95/96


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What has been presented in the results is only an estimation of how the water has been redistributed based on the information gathered and the assumptions made. In order to determine the real quantity of water that belongs to the different fractions the water balance must be refined further by gathering more information on the system. Information of value is:

• From the results the following can be concluded over the total PRD system: � Water enters the system through precipitation and discharge. Through subsurface flow

water will enter the system, but no data is available on the subsurface flow that enters the system.

� In the total PRD system the incoming volume is redistributed over 4 of the 5 possible fractions. These are the Evaporated Fraction (EF), the Reusable Surface Fraction (RSF), the Non-Reusable Surface Fraction (NRSF), the Ground Fraction that can be either reusable or non-reusable based on the location (R/NR GF).

� The average EF is 21% in scenario 1 and 22% in scenario 2. Its principal contributor is the crop evapotranspiration

� The average RSF is 58% in scenario 1 and 51% in scenario 2. Part of the RSF returns to the Rio Dulce. It is unclear to where the RSF in the form of excess discharge to the zones ends up within the Rio Dulce river basin. What is clear is that it can be used again.

� The NRSF is 10% in both scenario 1 and 2. This is the fraction of water that is transported to the Rio Salado by the Jume Esquina.

� The (R/NR) GF is 10% in scenario 1 and 16% in scenario 2. Whether this water can be reused or not depends the quality of the water it percolates into. What is known from the studies of Nijenhof is that 45% of the area was of good quality and the rest had moderate to severe problems with salinity and sodicity. The distribution of this area is however unknown. Furthermore the ground has a natural drainage of around 2 mm/ day and flows in the direction of West to East, thus in the direction of the Rio Salado. Due to its characteristics, among which the vastness of the system and the fact that only 14% makes use of the irrigation water and that this 14 % is spread in pockets within the system, it is probable that interaction between irrigation system and basin is minimal. Interaction is probably reserved to the plots that lie at the eastern border of the system.

Scenario 1 EF % RSF % GF % NRSF %

1995/96 306.888 19 991.705 63 140.994 3 150.595 9

1996/97 351.548 21 958.638 56 179.305 11 208.289 12

1999/2000 450.754 23 1.081.255 55 219.158 11 206.556 11

Scenario 2

1995/96 317.356 20 901.064 57 221.367 14 150.595 9

1996/97 372.398 22 846.151 50 271.198 16 208.289 12

1999/2000 464.892 23 941.134 48 347.818 18 206.556 11


Points of interaction are shown in figure 7.2 where the blue arrows show where the irrigation system is delivers water to the basin on a surface level and the dark blue arrows represent possible interaction on a subsurface level.


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Figure 7.2 : Interaction of the PRD with the basin and between the zones Based on the above stated we can say that the interaction between basin and irrigation system primarily takes place on the surface level.


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Further recommendations for future study are:

• Research should be limited to 1 zone at a time. If subject of research is the interaction between the river basin and the irrigation system, it would be wise to study zone 1, 2 and 5.

• Taking in account that the irrigation system is a dynamic system, it would be wiser to study a specific system during a shorter period of time, for example during the dry period of the wet period and in much greater detail.

• Of each zone should be collected: � The true irrigation gift received each month � The distribution of irrigated fields within a zone In order to determine the RSF better: � The true dimensions of the canals and the infiltration rate � The destination of the RSF � The excess discharge leaving the zone In order to determine the EF better: � The actual evapotranspiration In order to determine the GF � Groundwater inflows � The fluctuation of groundwater tables on the fields and near the canals � The quality of the groundwater � The crops planted and specific information on the anatomy of the plant � Groundwater levels on the borders of the zones � The direction of the groundwaterflow


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BBIIBBLLIIOOGGRRAAPPHHYY

Allen R.G., Willardson L.S. and Fredriksen H.D. (1997) Water Use Definitions and Their Use for Assessing the Impact of Water Conservation. Workshop Water Scarcity in Drought Areas. International Commission on Irrigation and Drainage, Oxford, England Allen, R.G, Pereira, L.S., Raes, D., Smith, M.;(1998) FAO Irrigation and Drainage paper-56:Crop evapotranspiration – Guidelines for Computing Crop Water Requirements; FAO Angella G.A.; Representative Farms of the Rio Dulce Project (PRD), Santiago del Estero, Argentina; Master of Science thesis, WAU (1999) Ertsen, M., Prieto, D.,Pradhan, T.M.S., Angello, G.; Historical Analysis of Waterflows in the Rio Dulce Catchment Ertsen M.W., Pradhan T.M.S.; Understanding the Influence of Irrigation Systems on Water Sharing Issues in a Catchment Context Frederiksen, H.D.; (1996) Water Crisis in Developing Worlds: Misconceptions about Solutions; Journal of Water Resources Planning and Management Gleick, P.H.; (2000) The Changing Water Paradigm: A Look at Twenty-first Century Water Resources Development; Water International Gleick, P.H.; (2002) Soft water paths; Nature Instituto Nacional de Ciencia y Técnica Hídricas, Seminario Nacional (1994); Situacíon Actual y Perspectives de las Areas Regadias en Argentina Klaasen, W., Lankreijer, J.M., Veen, A.W.L.; (1995) Rainfall Interception near Forrest Edge; Journal of Hydrology Liberal, El; Retrato de un siglo: Una visíon integral de Santiago del Estero desde 1898 Martín, R.A., Cortes, J., Storniolo, A.R., Thir, J.M.; Zonificacion Hidrogeologica de Santiago del Estero Pagot, M; (2003) Análisis y Simulación Hidrológica del sistema bañados del Rio Dulce; Universdad Nacional de Córdoba Perry, C.J.; (1999) The IWMI water resources paradigm- definitions and implications; Agricultural watermanagement Prieto, D.; (1998) Research Proposal: Modernisation and the Evolution of Irrigation Practices in the Rio Dulce Irrigation Project (PRD) in Santiago del Estero, Argentina: Un tarea de todos; Angueira, C., Prieto, D.; Sistema de Información Geográfica de Santiago del Estero; INTA Schipper, P; (2005); Water resources of the Rio Dulce in Santiago del Estero Simmers I.; Understanding Water in a Dry Environment, Hydrological Processes in Arid


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and Semi Arid Regions (2003); International Association of Hydrogeologists Vrugt, J.A., Dekker, S.C., Bouten, W.; (2003) Identification of Rainfall Interception Model Parameters from Measurements of Throughfall and Forest Canopy Storage; (Water Resources Research) Wolters W.; (1992) Influences on the efficiency of irrigation water use; TUDelft Savenije H.H.G.; Hydrology of Catchments, Rivers and Deltas (2003); TUDelft Websites INDEC; www.indec.gov.ar Government of Santiago del Estero www.sde.gov.ar Ministry of Interior in Argentina www.mininterior.gov.ar Worldbank; www.worldbank.org Servicio Meteorologico Nacional; www.indec.mecon.ar

msc. thesis: the irrigation system as an open system c.lieveld...

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