WATER RESOURCE IN ARID ZONES

THE HYDROLOGICAL REGION 36 IN NORTHERN

M E X I C O

DESCROIX Luc* LOYER Jean-Yves** ESl!IWDA AVALOS Juan***

l ABSTRACT

The key of development of arid zones depends on the sufficient availability of water to ensure man's domestic, agricultural and industrial needs. Around the world, there is a considerable amount of territories, located near huge and sterile deserts, that face this crucial problem each year, each season and sometimes, daily: ín Africa, both Northern and Southern Sahara, the Near and Middle East... as well as Northern Mexico.

The water resources of the Hydrological Region no. 36, which insures the development of an important economical area ín the Northern Mexico ("LAGUNA"), must, in order to satisfy the requirements, complete the superficial supply of the Nazas river, using the underground water. Nowadays these water is out of balance due to an insufficient reloading. In addition to the quantitative aspect, the water's quality must be considered because they might present, within a certain time, specific problems, particularly in the hydro-agricultural field. Several solutions deserve to be studied and implemented in these regions in order to optimize the use of this essential resource for life.

* Researcher in Hydrology (ORSTOM), French Scientific Research

** Researcher in Soil Science (ORSTOM); *** Researcher in Hydrology (CENID-RASPA), Centro Nacional de

Investigacibn Disciplinaria en Relación Agua, Suelo, Planta y Atmósfera (Mexico)

Institute for Development trough Cooperation, París;

In most deserts in the world, "dryness", a phenomenon due to climatic variations on a geological scale, has made natural vegetation disappear and has transformed landscapes. Thus, some thousands years ago, a planetary cycle led to the transformation of -the African Sahara and the Near and Middle East deserts.

On the scale of much shorter climatic cycles over the 1960-1970 decade, a strong decrease in rainfalls caused a severe damaging into the environment of semi-arid and peridesertical areas, such as the African Sahel and the Brazilian Northeast.

This natural dryness has been increased by human pressure on these fragile environments : the exploitation of the scare wood resources, aiming at producing charcoal or growing crops, as well as overgrazing, have led to land bareness. Moreover, the albedo has raised and there is a strong risk of erosion. Some other minor factors could speed up the process of desertification.

Tough it seems illusory to bring big deserts back to life, peridesertical areas deserve special attention. These arid and semi-arid zones of transitions cover some 50 millions km2. Even if they have been rationally and carefully exploited, they now suffer from the fast growing of the population (2.5% /year, which means that it doubles every 30 years). On the other hand, rainfalls remain very low. Owing to both these factors, 200 O00 km2 are damaged every year.

Improvements in these areas depend on water supply. Populations are daily preoccupied by its availibility for agricultural and domestic needs. As to superfical and underground waters, we first need to make an quantitative and qualitative inventory in order to optimize their use for human, rural, urban and agricultural activities.

Furthermore, used waters should be recycled into natural environment to save this non renewable resource. So, their quality must therefore be controled and corrected if necessary.

THE HYDROLOGICAL SYSTES OF THE HYDROLOGICAL REGION No 36 IN N0R.LMER.N MEXICO

The hydrological areas in the arid Northern Mexico have an exit to the ocean or an endoreic runoff. One of them is the Hydro- logical Region No 36 (RH 361, (fig. 1).

Located on the south limit of the large Chihuahuan desert (350 O00 km21, it is very representative of the problems faced in peridesertical areas.

,

107' 106' 105' 104' 103' 102. IO

0, c

I c\ x PLUVIOMETRIA MEDIA CONSEGUIDA

CON LOS DATOS YA PROCESADOS ( CENID-ORSTOM I -

FIGURE I I . ANNUAL R A I N FALL AVERAGE

I

7'

6'

5'

4'

3'

* L I With 92 O00 b 2 , it covers two major hydrological basins : the Nazas and Aguanaval. They are runoff basins coming from the

!I Western Sierra Madre at 3000 meters high, and they supply in an external way a large low area formed by endoreic lagoons (CENID RASPA, ORSTOM, 1993). Yhis Hydrological Region is divided between two very contrasted parts : - an orderly runoff system, in the high part of the Nazas and Aguanaval catchments : 55 O00 km2, 1100 to 3000 meters high, an average slope over 10% and covered by annual isohyetes ranging from 250 to 900 mm. It is the supply zone (fig. 2 and 4 ) ; - a diffuse runoff system, like the Bolson of Mapimi, and an accumulation system in the Mayran and Viesca lagoons : 36 O00 km2, 1000 to 1100 meters high, an average slope below 5% and under the isohyetes ranging from 180 to 350 mm.

The high basin receives quite a lot of rain and supplies the arid low part which benefits still from major water resources.

Elsewere, low rainfalls prevent rain crops from being expanded as they are very delicate and require a minimum of 350 "/year.

I The central part of RH36, around the urban zones of Torreon, Gomez Palacio and Lerdo, is a proper "huerta" (garden) at the doorstep of the Chihuahuan desert. This rich area, called "La Laguna" has approximately 1.5 million inhabitants.

The 158 121 irrigated hectares of "La Laguna" (average 1982-1991) are divided between : - 92 457 hectares irrigated by the gravity waters of the basin (mainly from the Nazas river), - 65 664 hectares irrigated by underground waters.

The hydroagricultural activities have been developped as following : - The areas irrigated with gravity water have increased by 6% from 1932 to 1991 while the ones irrigated with underground water have decreased by 31% (especially by the end of the decade). - Basic crops and vegetable gardens have been expanded, . as well as forage crops (alfalfa in particular). Wineyards and walnut culture remain stable. - Cotton and industrial crops are on the decline.

"HE SUPERFICIAL BATER RESOURCES

The main renewable resource which we can rely on in the long run is the superficial water coming from the organised runoff system. The last can be divided between ten sub-basins (fig. 3 ) .

The Nazas river provides with an average of 1038 Mm3 / year, which means 89% of the total runoff water which enters "the Laguna", with an average rate of 33 Mm3/s : the specific rate is 1.81/s/km2 in Palmito, and 0.94 l/s/km2 in Los Angeles.

106' 105' 104' 103' 102' I O I ' 107' I I ! I I

I COEFICIENTES DE ESCURRIMIENTO POR SUB-CUENCA I I I

I

C O E F I C I E N T E D E

ZONA S I N ESCURRIMIENTO

ORGANIZADO

\

I

\ \. ,

- 2 2 v. FIGURE IV , DRAINING COEFFICIENT

I '

279

25'

24'

The Aguanaval basin provides with a total of 1 3 1 Mm3, with a rate of G.l m3/s, which means 0.21/s/km2, 11% of the total.

The total at the entry in endoreic "laguna" is 1169 Mm3/year.

The higher part of the basin supplies all the water flowing from the Nazas. The annual rainfall water upriver from Palmito dam ( 1 8 321 km2, 20% of the total superficie), is 497 m. The stream flow is 6 9 mm. The coefficient is 14% but it exceeds 15% in the dam stations upriver from Sardinas and Salome Acosta (fig. 4 ) .

The Nazas average flow decreases between Palmito reservoir and Francisco Zarco regulating dam (capacity : 400 Mm3). That's why, in spite of its large surface (150 O00 h 2 ) , and major rainfalls ( 3 0 0 to 500 m m ) , the intermediary basin has a runoff flow deficit. This is, in part, caused by the water consumption of two intermediary irrigated perimeters : Rodeo and Nazas. These 9000 ha perimeters consume an average of 5 3 . 4 Mm3 (average 1 9 8 8 - 1 9 9 2 ) . That's why we have negative runflow coefficients ( K I . The normal K of the intermediary sub-basin, Agustin Melgar, is thus close to o.

Yluviometric and hydrometric data traitement in "DIXLOI", (LEBEL, BOYEH, 19891 , helped to determine the following measures :

In Palmito rainfall is The maximum The maximum The maximum The minimal 'The minimal

high basin, which provides most of the water, the 497.5 mm. decennial rainfall is 693 mm (Gumbell centennial rainfall is 9 6 1 mm millenial rainfall is 1 2 2 4 mm decennial rainfall is 3 4 1 mm centennial rainfall is 262 mm

As to flowing stream, the average is 6 7 . 4 mm, which means a K of 1 3 . 6 :

The maximal decennial depth of runoff is 119 mm The maximal centannial depth of runoff is 1 9 2 mm The maximal millenial depth of runoff is 263 mm The minimal decennial depth of runoff is 2 3 . 3 mm The minimal centennial depth of runoff is 2 . 0 mm

Palmito dam and its large surface of free water (about 1 0 O00 ha) generates a water loss of 1 2 0 Mm3 by evaporation. Moreover, 40 Mm3 are lost owing to an infiltration of the reservoir. In Palmito, for a period of 47 years, there is a relation between rain and rate of . flow, but with a very low correlation coefficient. This is due to a great irregularity in rainfalls in time and space, which makes impossible any comparaison between two years with a similar rate of rain.

The equation of regression that associates both parameters is :

where QE = annual total volume of runoff and QP = total annual volume of rainfall (R2 = 0 .6741 , (fig. 5).

QE = 0.264 QP - 1 1 8 0

It can be deduced that runoff water exits from isoyhetes 350 mm. Though, this does not consider altitude (that makes evaporation and temperature decrease). Disregarding the altitude, we can relate the rate of forest vegetation cover with pluviometry ( R 2 = 0 . 8 6 2 ) and with runoff water flow coefficient K (R2= 0 . 8 9 3 ) (tab. 1)

This relationship has been established with datas frbm the whole runoff zone (Nazas and Aguanaval) and not with the mere Nazas basin. Now, considering as well the 1 0 sub-basins, there is a relationship between K and rainfall. K = 1 9 . 8 LP + 3 4 1 where LP = rainfall ín mm (K2 = 0 . 7 7 4 )

Table 1. Rainfall ( P ) , runoff coefficient ( K I , and percentage of forest cover for each sub-watershed in RH36.

Basin P Nazas :

Salome Acosta 636 Sard i nas 644 Palmito 5 0 1 Agustin Melgar 408 C. de Fernandez 347 Los Angeles 278

El Sauz 639 Cazadero 505 San Francisco 405 La Flor de Jimulco 362

Aguanaval :

K

1 5 . 1 1 7 . 2

9 O 4 . 4 O

8 . 3 6 .6

( 3 . 5 ) ( 2 )

Forest %

78 7.7 35

4 O O

52 1 7

O O

THE UNDEXGROUNI.) WATER EXPLOITATION

'l'he annual volume released from the dam is 1038 Mm3 (average 1941-1990) . A l l this volume is used for irrigation, in addition to 1 3 1 Y.ï3 from the Aguanaval. But this is just enough to irrigate 95 O00 ha. Since 1 9 2 0 , before dams were built, water has been pumped from the water table located in "La Laguna". The number of pumping wells went from 1 2 in 1 9 2 0 , up to 3060 in 1 9 9 1 (SARH, 1 9 9 2 1 , but hundreds of other wells are out of use. 2563 of them are strictly used for agriculture. The volume extracted from the water table raised from 75 Mm3 in 1 9 2 0 , up to 620 Mm3 in 1 9 3 8 , 1 7 4 0 Mm3 in 1 9 5 9 , and then went down to 1 2 1 4 Mm3 in 1980 . It eventuallly dropped to 1020 between 1982 and 1991 .

I

The average yield of pumping has gone down from 0.67 Mm3/year/we11 in 1959, to 0 . B Mm3/year/we11 in 1990, due to the overuse of the underground water resources.

The water table (Sanchez, 1985; Rigal, 1988; SARH, 19921, was located at 1108 meters high in 1950 and 1042 m ín 1986 : which means a decline of more than 60 m in 36 years. For the moment, water is being pumped from 80 to 130 meters deep. However, ín some areas, the decline is much quicker, especially away from major supply areas. In 12 years (1972 - 19841, for example, the water table has decreased from 40 to 60 meters around Francisco I Madero and Matamoros.

Owing to the modernization of the irrigation network between 1962 and 1972 (waterproofing), infiltration loses have been taken out. The annual recharge of the water table is estimated at 300 "3, and the overused volume is about 700 Mm3/year. Furthermore, water salinity and arsenic content are raising (GONZALEZ B., 1992). To avoid a decrease ín the water table level, and to maintain the present level, only 300 Mm3 of the underground water (corresponding to the recharge), should be consummed. Therefore, 20 O00 ha should be irrigated with underground water (against 65 O00 now).

WATER USE

Lately, the forage irrigated cultivations have experienced a noticeable evolution with a decrease ín cotton, which was the main activity until the mid-eighties. Today, these forage cultivations have made possible a very dynamic intensive milk cattle breeding (with free stabling). The Laguna is nowadays one of the biggest dairy producers ín the country (over 10% of the Federal production). This production has favourised the development of a major agro-industrial sector : milk conditionning, fabrication of dairy products : butter, cream, yogurts, cheese, etc ... 'Today, "La Laguna" is facing a very simple problem : the volume of water required to irrigate the 160 O00 ha is 2300 M m3/year : today, underground and superficial waters produce each half of this volume. But, all the water coming from the basin is being used for agricultural, industrial and domestic needs. As for the agro-industrial use :

- 1020 Mm3/year are being pumped (average 1980 - 1990) - 11'15 Mm3/year come from the catchment.

The agricultural consumption for an irrigated surface of 160 O00 ha, is about 2200 Mm3/year, which means that the used volume is 14 O00 m3/ha/year. It represents a water slice of 1400 "/year, in addition to 250 mm of annual rain, from which only the summer rain is useful. Along with this, there is a growing consumption

from families and industries (100 to 200 Mm3/year).

Surface waters are therefore wholy exploited : only the exceptional rainy years (1968 and 19911, have required to carry out the floods of Palmito dam. The later, with a total volume of 4400 Mm3, and a useful volume of 2800 Mm3, ensure the regular supply of 1000 Mm3 to the "Laguna", and is expected to be a reserve of water for inore than one year consumption.

Alluvions seal the reservoir surely but slowly : the reservoir only lost a volume of 323 Mm3 between 1947 and 1975 (11 Mm3/year). A specific damage of 608 m3/km2/year (about 1000 T/km2) , is low for a mountain region, especially in a semi arid area. Between 1975 and 1991, there was a 98 Mm3 sealing, 4.66 Mm3/year, 254 m3/h2/year (about 4OOT/km2/year).

In view of the bathymetric drilling realized in 1975, the dam had been heightened, but only to increase the total reserved volume, and not the useful volume. With the heightering of the dam, its capacity raised from 3500 to 4400 Mm3. Meanwhile, in 1968, was constructed Francisco Zarco regulating dam, in the "Canon de Fernandez" .

GLOBAL HYUHOLOGICAL BALANCE

All the runoff water is being ral needs : ab 1170 Mm3/year.come from the Nazas and Aguanaval basins.

sed for agricult t

The underground water is used as following, (SARH, 1992):

- 1085 MmJ are pumped for agropastoral use (against an average of 1020 Mm3 in 1982 - 19911, out of 2563 wells.

- 114 Mm3 are used for urban, domestic and industrial needs (313 wells 1.

The water consumption is twice higher than the renewable quantity. The later can be estimated at 1200 Mm3/year, divided as following :

- 300 Mm3 of natural recharge of the underground water reserve, coming mainly from the Nazas and Aguanaval valleys alluvions,

- 900 Mm3, that represent the water quantity that might be taken from Palmito reservoir at least 9 years out of 10 '(the minimum volume brought to the lake 9 years out of 10, is 425 Mm3).

There is therefore a remarkable overconsumption. But there is no reason that the volumes used by industries and families should decrease. They will probably increase quickly, due to the fast industrial development, the improvements in .living conditions and the demographic , growth in "la Laguna".

SURVE'Y TO PROSPECT

1 - In the supply zone 'lhe high basin of Nazas shows a continous damaging : the forest cover is now reducing, even if still occupies 60% of the total surface (more than 75% for the Sardinas and Salome Acosta sub- basins). The forest exploitation is quite important, but it seems rational and it does not damage much the natural environment.

To the contrary, most parts of the agropastoral surface and many sub-forest areas are suffering.from overgrazing : overexploited grass cover, trampled soil surface in many places. Erosion marks are more and more frequent : - around villages - in soft grounds (as Santiago's pit conglomerate rocks) - over plateau and glacis areas (herd ways)

This last point is quite worrying. It is logical indeed that the most populated areas should suffer from such a situation ; but if plane zones are affected by erosion, it means that even over areas with a little slope, a concentrated runoff flow can appear quickly, creating first erosion marks and then gullies. Apparently, there is no bad land yet, but in all the populated areas of Nazas high basin, there is a multiplication of erosion ravines. A similar phenomenon exists in dryer middle Nazas and Aguanaval basins, where overgrazing is remarkable.

Nazas high basin, as El Sauz sub-basin in the Aguanaval basin, provides with most of the water in "la Laguna". The "huerta" now depends on it. It seems impossible now to increase the disponible volume of water:

* Bringing water from other regions was contempled a few decades ago, but today these regions make use of most of their water and cannot export it.

* It is still possible to enlarge Palmito reservoir, but this would at best raise the annual volume from 900 Mm3 to 1000 or 1100 Mm3; a supplement equal to the forecasted increase in consumption over the mere construction of this reservoir.

*- A long run solution aims at maintaining, saving and extending forest areas, making sure that the development of the forest cover (such as been proved in Northern and Southern Mediterranean basins and in Arizona) , will reduce instantaneous runoff flow coefficients but also increase the annual volume of flowing runoffs. Overgrazing and trees cutting must therefore be controlled.

Objectively, it is only possible to contempla.te a 10% to 30% raise in the total volume of available water ( 4 0 to 50 years I ) . Anyway, in the long run, such a saving will be largely compensa.te by an increase in water consumption.

2 - In the consumption zone Carrying on exploiting underground water resource would not be rational, first because of its impact on nature, and second because it is never satisfactory to throw natural environment off balance. Other alternatives should be studied.

The present system of intensive cattle breeding demands a great water consumption. Moreover, it is obvious that water is overconsummed and even wasted. The common flood irrigation system is not the most economical one. The practical requirements of plants should be revaluated ín relation to the soil hydric characteristics, so as to calculate exactly the quantity and frequency of irrigation. That is why an efficient use of water resources deserve special efforts. It must be done in relation with crops yield and dynamic of salts.

The use of marnínal waters :

Some drainage waters evacuated towards the Mayran lagoon can, doubtless, be a complement to more extensive forage cultivations, that tolerate a certain salinity, (LOYER, 1991). For example, numerous types of Atriplex have been tested over the mexican territory and ín many other countries.

Some 3 Mm3/month domestic residual waters are released around urban concentrations. These waters are partly used for irrigation. They could be purified and totally recycled for agriculture.

The econdcal beriefí&. :

Selling good quality water at a fair price : this is hard to conceive on the social ground. It can only be done progressively, inciting producers to turn towards particular agricultural speculations (horticulture); the production could be exported at a fair price. Water is being paid for about 20% of its price. This may be one of the reason for such a waste. But it is also unthinkable to rise this price quickly ; it cannot be acheived without an agronomic and socio-economic approach, and on the bases of a transformation of the present system towards productions with a great added value that support the use of a good quality, but expensive and more.and more scarce water.

RELACION PLUVIOMETRIA GASTO (en millones de metros cubicos por ano)

d >

3500

3000

2500

2000

1500

1 O00

500 + +

+ + + i+ -#-

+ +

+ ++ +

-+ + + +

* +

+ + + + + *

+ +

f + u*+ +

O 5000 6000 7000 8000 9000 1000011000120001300Q14000

VOLUMEN PRECIPITADO ANUAL

FIGURE V: RELATION BETWEEN ANNUAL RAINFALL AND,ANNUAL RUNOFF

BIBLIOGRAPHY

ChXID RASPA - ORSTOM: "Análisis de los factores que influencia los escurrimientos y el uso del agua en la Región Hidrológica 3 6 " , 1993 (en publicación INIFAP). GONZALEZ BARRIOS J . L . : "Eaux d'irrigation et salinité des so l s en zone aride mexicaine: exemple dans la "Comarca Lagunera"; thèse USTL Montpellier II, France, 1992, 315p. LEBEL T., BOYER J. F.: "DIXLOI", Un ensemble de Programmes FORTRAN 77 pour l'ajustement de lois statistiques et leur représentation graphique. Laboratoire d'Hydrologie. ORSTOM France, 1989. LOYER J. L.: Salinité des eaux d'irrigation. Problèmes et Solutions. Conf. Intern. FAO, IAM Bari, Italie 25, 26 Jlt. 1991. RIGAI, G.: "Etude du problème de I'irrigation dans une r6gion agricole du Centre-Nord du Mexique"; Memoire de DAA, I.N.A., Paris, l o o p . , 1988. SANCHEZ V . : "Características generales del acuifero en la Comarca Lagunera" ; in Ciclo Internacional de Conferencias "aprovechamiento de aguas subterráneas en la agricultura"; Conacyt, Torreón, Coahuila; Mexico. 1985, p.13-32. SEC'ETAEUA DE AGRICULTURA Y RECURSOS HIDRAULICOS (SARH): "Programa de reconversión operativa: Región Laguna" (mimeograf iado 1 1992.

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