runoff, erosion, and the sustainability of ancient irrigation...

The Hydrology-Geomorphology Interface: Rainfall, Floods, Sedimentation, Land Use (Proceedings of the Jerusalem Conference, May 1999). IAHS Publ. no. 261, 2000. 75

Runoff, erosion, and the sustainability of ancient irrigation systems in the Central Negev desert

LESLIE SHAN AN Institute of Earth Sciences, Hebrew University ofJerusalem, Givat Ram Campus, Jerusalem 91904, Israel

Abstract Runoff and erosion processes in desert watersheds were investigated by studying ancient irrigation systems discovered in the 100 mm rainfall region of the central Negev. Catchments delivering flood waters to these areas ranged in size from small plots to large watersheds. Runoff from small watersheds (less than 50 ha) varied from 4—12 mm year"1 compared to 0.5-2.5 mm year"1 for large watersheds (greater than 1000 ha). Even in extreme drought years, small watersheds produced at least 1.4 mm of runoff, while large watersheds experienced "dry" years (i.e. without any runoff event) about once in every three years. Ancient irrigation systems using runoff from small watersheds were much more efficient "water-harvesting" projects than those diverting flash flood flows from large watersheds. Rates of erosion from small watersheds averaged 3.6 mm century"1 (54 t km"2 year"'), originating mainly as sheet erosion on the hillsides. Rates of erosion from large watersheds, where main wadis are stable broad depressions with deep loessial soils and a good winter vegetation cover, are about 4.6 mm century"1 (701 km"2 year"1). In large watersheds where wadi incision and headcutting processes are active, rates of erosion can be expected to range from 7.6-12.6 mm century"1 (115-180 t km"2 year"1). Key words climatic changes; erosion; Negev Desert; runoff; sustainable irrigation systems

INTRODUCTION

Numerous studies have reported that the technique of using runoff and flash floods for irrigation has been practised for more than 3000 years in China, India, Egypt, Iraq and Israel. Parts of these ancient irrigated areas still produce reasonable yields but extensive sections are now barren and desolate wasteland. There is no general agreement on the cause of the decline of these systems. Some investigators have favoured a theory of increasing aridity and climatic change. Our research lends support to the conclusion following the dendro-archaeological studies of Lipshitz & Waisel (1978), that the climate of the Negev has not changed significantly during the past 5000 years. The majority of studies on other ancient systems demonstrate that three principle factors, acting singly or in combination, led to the deterioration and abandonment of projects: - The strong central authority that planned and operated the systems was replaced by

one that was incapable of managing them. - The accumulation of sediment in canals and fields reduced the amount of water

available for irrigation, and the heavy burden of maintaining the projects led to their abandonment.

- Inadequate drainage, water logging, and the accumulation of salts in the soils reduced yields below financially viable levels.

76 Leslie Shanan

This paper focuses on the lessons learned from the ancient irrigation systems in the Central Negev desert and shows how: (a) the presence or absence of a strong central authority, and (b) the processes of runoff and erosion affected their sustainability.

THE CENTRAL NEGEV DESERT

The Central Negev, the southern desert of Israel, is dotted with extensive remains of ancient habitation and agricultural systems (Fig. 1). In this 100-150 mm annual rainfall region, irrigation based on the utilization of surface runoff from the meagre winter storms was developed to a high technical degree, reaching its peak during the Nabatean-Roman-Byzantine domination of the region from about the second century BC to the seventh century AD. Desert agriculture using hillside runoff was already

Fig . 1 The Negev , showing (I) lowland foothills and (II) central h ighlands. The dashed and dotted line indicates the boundary be tween the Negev and Jordan and Sinai. The triple dots indicate the ruins of ancient cities: Haluza (Halutsa) , Rucheiba (Rehovoth) , Nizzana (Auja-Hafir), Shivta (Subeita) , Kurnub (Mamshi t ) and Avda t (Abda) .

Runoff, erosion, and the sustainabilily of ancient irrigation systems in the Central Negev desert 11

practised during the Israelite Period (eighth and ninth centuries BC) at the time of the Judean Kings (Evenari et al., 1982). The densest settled areas have been discovered in the lowlands and foothills and the highlands (Fig. 1).

The lowlands and foothills cover about 150 000 ha. The morphological structure of this subregion is made up mostly of Eocene limestone hills separating wide rolling plains, with elevations ranging from 200-450 m a.m.s.l. A number of large wadis whose sources are in the highlands, cut through the plains (Evenari et al, 1982). The hillsides are generally covered with shallow, gravelly, saline soils with immature profiles (Table 1).

The highlands cover about 200 000 ha and are composed of a series of parallel anticlines of Cenomanian Turanian limestones and cherts. Elevations vary between 450-1000 m a.m.s.l. (Evenari et al, 1982). Adjacent to the main wadis (gullies) lie relatively narrow flood plains, and near the watershed divides, where the wadis have not cut down to stable base levels, there are a number of expansive plains (Table 1).

Table 1 S u m m a r y of ecological conditions in the Negev Highlands (after T a d m o r & Hillel , 1956).

Habitat % o f t h e highlands area

Soils Plant associations Water available for plant g rowth (mm)

Rocky slopes

Loessial plains

Wad i beds 3

8 0 - 9 0 Shallow, gravelly, saline

10 -15 Deep loessial soils; salts leached to 30 c m or more

Deep loessial soils or gravel and silt fill

Artemisietum herbae-albae and 1 0 - 6 0 Zygophylletum dumosi

Anabasidetum hausknechtii and 2 0 - 5 0 Haloxylonetum articulati

Retama roetam associat ion with many annuals

Gravel ly wadis : 6 0 - 1 0 0 , loessial wadis : 4 0 0 - 6 0 0

"RUNOFF FARM" SYSTEMS

In order to investigate the techniques that enabled ancient desert agriculture to exist under extremely marginal climatic conditions, a research team (the late Professor M. Evenari, the late Professor N. H. Tadmor and the author) was established in 1954. During 1954-1959 we surveyed and studied more than 100 ancient farming systems and irrigation projects (Evenari et al, 1982). We discovered, amongst other findings, that all the ancient agricultural projects in this desert were based on utilizing runoff from small and large watersheds—hence the term "runoff farm" systems. In order to evaluate the hydrological and agricultural potential of these methods, in 1958-1959 the research team reconstructed two ancient runoff farms, one at Shivta and the other at Avdat, where there are extensive remains of ancient systems (Fig. 1). Scientific research at the reconstructed farms continued for 25 years (Evenari et al, 1982).

The selection of the sites was partly influenced by a controversy concerning man-made heaps of stones that had been placed in mounds and long strips covering thousands of hectares in the Central Negev desert (Fig. 2). These structures had been observed in the 1870s, but were not investigated until the 1950s.

Various theories regarding the purpose of the mounds have been reviewed by Evenari et al. (1982). Palmer in 1871, had concluded that they were associated with vine cultivation on the hillsides because his Bedouin guide had called them tuleilat el enab,

78 Leslie Shanan

Fig . 2 A n oblique aerial photo of 2000 year old patterns of stone strips, near Nizzana. The remnants of ancient terraced fields, surrounded by stone fences, can be seen in the background in the broad loessial depression at the foot of the slopes. (Photo by N. Tadmor , 1954).

i.e. grapevine mounds. His theory failed to explain how the grapes grew on such extremely shallow saline soils, without any irrigation in this 100 mm rainfall region. Some investigators overcame this difficulty by proposing that the additional moisture was obtained due to the mounds acting as "air-wells", with dew condensing on the stones. Observations in gravel mounds rebuilt by us, after the ancient model, neither collected dew (in sealed containers to prevent evaporation) nor did they improve the soil moisture conditions below them as compared with the surrounding soil.

Mayerson, in 1959, suggested that the ancient farmers irrigated the tens of thousands of hectares of hillside vines by carrying water from wells or cisterns, completely disregarding the amount of water that would be required or the human effort involved. Previously, Kedar (1957) had proposed an entirely different theory and argued that the main function of the mounds was not viticulture, but to increase erosion from the hillsides and so accelerate soil accumulation in the cultivated valleys.

Concurrent with these theories, we proposed that the purpose of the mounds, and also the strips (Fig. 2, which the previous investigators never recorded or referred to) was to increase runoff, not erosion from the hillsides, with the aim of collecting the maximum possible runoff from the slopes. It was in this atmosphere of debate regarding the runoff and erosion processes in the Negev that the two ancient farms were reconstructed and experiments superimposed on them to study, inter alia, the hydrological and climatic conditions of the area.

This paper focuses on case studies related to the techniques used by the ancient farmers in their endeavours to establish sustainable irrigation systems in this harsh

Runoff, erosion, and the sustainability of ancient irrigation systems in the Central Negev desert 79

desert region, and refers to the results of our research work over three decades (1954-1985) that deal with runoff and erosion processes of the region.

The case studies include: (a) diversion systems from large watersheds, (b) runoff farm systems associated with small watersheds, (c) research data from two reconstructed ancient runoff farms, and (d) sedimentation measurements in two large watersheds not directly related to ancient agricultural systems.

The erosion data is presented as "depth per unit of time", namely, mm per century (mm century"1) together with the conventional unit, tons per square kilometre per year (t Ion"2 year"1). The unit 1.0 mm century"1, is approximately equal to 14 t Ian"2 year"1, assuming an average sediment bulk density value of 1.45.

Diversion systems

Case study 1: The Nahal Lavan system Nahal Lavan (Wadi Abiad) (Figs 3and 4) is the largest wadi in the vicinity of the ancient town of Shivta (Fig. 1) and drains from the high plateau of the Matred Plain through an area of barren rocky Cenomanian-Turonian hills. Torrential floods have cut a deep wadi into the alluvial plain that is narrow in the upper reaches but in the lower reaches, widens out into extensive flood plains. Today, the wadi flows in a gravel-bed watercourse typical of the area. Numerous remnants of ancient walls and terraces are found in the alluvial flood plain (Fig. 5). At a point where the drainage area of Wadi Abiad is about 53 km 2, an area covering 200 ha of terraces was studied in detail (Fig. 4).

Because of the superimposition of many terrace systems one on the other, it is often difficult to differentiate between projects of different periods. However, the size and capacity of the spillways, canals and drop structures, provided a key to their understanding. Three types of spillway, all serving to lower water from one terrace to the next, were found: (a) spillways with crest lengths of 30-60 m for handling flows of 10-30 m 3 s"1 (Fig. 6). (b) spillways with a crest length of 3-8 m for flows in the range of 1-5 m 3 s"1. (c) small spillways up to 1 m wide, for flows less than 1 m 3 s"1. Using these criteria, three different types of developments were distinguished. The earliest flood irrigation devices were discovered on the west bank of the wadi where massive stone spillways with 30-60 m crest lengths are the common structure. These spillways were connected to low earth embankments, stretching across the plain, of which only faint traces remain today in the form of low banks (Fig. 3). The spillways are of such large capacity that they were capable of handling the entire flood. The topographic location of this system indicated that it was used when Wadi Abiad was a shallow depression and the earth embankments were built to spread the runoff waters across the broad flood plain. The wide stone spillways served to control and direct the water as it passed from higher to lower elevations. This flood plain water-spreading system was in use before Wadi Abiad had become a deep gravel-bed watercourse.

Water-spreading systems with spillways of 3-8 m crest length and diversion canals able to handle 1-5 m 3 s"1, were found mainly on the northeast bank of the wadi, in the middle and upper reaches of the survey area (Fig. 3). Some of these canals are more than 1 Ion long, 5-10 m wide and aligned with a gradient of 0.4-0.5%. Each diversion

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ANCIENT AGRICULTURE IN WADI ABIAD

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Cotiarime feetra) Biycrmn dail Orep structure.splmy Or Control structure Dislriiufm Hitch Cistern

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m Ngvsf. âr WitcMewer F i g . 3 A map of Wadi Abiad (Nahal Lavan) surveyed b y L. Shanan and N. H. Tadmor in 1955. No te the series of diversion canals on bo th banks of the w a d i leading part o f the flood flows to the terraced fields. Stage I sys tems are located on the southwes t side of the presen t day wadi , and stage II and III systems on the northeast side. Note h o w a wadi has cut through Stage II sys tem towards the lower end of the surveyed system. This tr ibutary wadi was "captured" b y W a d i Abiad in relatively recent t imes (probably about the R o m a n Per iod, 50 B C - 1 5 0 A D ) .

Runoff, erosion,

and the sustainability

of ancient irrigation

systems

in the Central

Negev desert

81

82 Leslie Shanan

Fig . 5 A wadi (gully) bank stabilization wall in Wad i Ab iad (Fig. 4) . Note that the wall was buil t during at least three different per iods . The terraced fields are 4.5 m above the wadi bed at this point.

F ig . 6 A mass ive spil lway about 50 m long on the southwest flood plain of Wad i Abiad (Fig. 4) , belonging to Stage I development of a large wadi diversion sys tem (900 B C - 7 5 0 B C , Middle Bronze II Per iod) .

canal irrigates an area of about 2-4 ha. The original stone diversion dams have been washed away. Most of these terraces are still in excellent condition.

The walls of the diversion canals and the associated terrace walls, were built in stages (Fig. 3) because the silt and sediment that accumulated in the fields and canals


compelled the irrigators periodically to raise the terrace walls and diversion structures. Some of the walls reach 5 m in height.

An interesting example of river-capture was found cutting through the system (Fig. 3). The water-spreading system on the northeastern side of Wadi Abiad was once a continuous project—despite the fact that today, a 4 m deep tributary wadi joins Wadi Abiad from the north. The alignment and elevation of the walls on both sides of this tributary wadi, as well as the sizes and capacities of the structures, clearly indicate that they once belonged to the same system. In earlier times this tributary was not connected to Wadi Abiad at all, but continued to flow west to a separate water-spreading system. This tributary wadi was "captured" by Wadi Abiad in relatively recent times, probably after the late Byzantine period (about 650 AD).

The third use of the area was as "runoff farms" (see below) connected to the adjoining small watersheds and not to the main wadi. These farms adapted the existing structures and stone walls of the diversion system to their needs. They no longer exploited the floods in Wadi Abiad but used runoff from small catchments in the adjoining hills. It is in connection with these runoff farms that the small spillways are found.

Most of the flood plain, particularly the area lying west of the wadi is badly damaged because it has been used for military manoeuvres since the 1960s.

Case study 2: Wadi Kurnub Wadi Kumub cuts a narrow, steep gorge through a limestone ridge 2 km south of the ancient town of Kurnub (Fig. 7). At the point where the gorge opens onto the Tureibeh plain, the ancient settlers constructed a large channel to divert part of the Wadi Kumub flood waters (generated over a 27 km 2

drainage basin). The diversion chamiel is a solidly built stone structure 5-9 m wide, with a gradient of 1:2000 over its 400 m length. The channel led the water to an extensive (10-12 ha) system of terraced fields that are all still in good condition. Excess water from each terrace flowed to the next lower one, through drop structures.

The diversion structure in the wadi diverted large quantities of silt into the terraced areas and, in a manner similar to that described above for the Wadi Abiad systems, the level of the terraced fields rose continually, forcing the farmers from time to time to raise the level of the terrace walls and diversion structures. The three types of development—flood plain, diversion system, and runoff farm—as described for the Wadi Abiad system, were also discovered in the Wadi Kumub system.

Runoff farm systems

The term "runoff farm" (Evenari et al, 1982), denotes a group of adjoining terraced fields surrounded by a stone-wall fence, forming an integral unit of about 0.5-2.0 ha of cultivated land. A house, cistern and/or watch-tower are often found within the boundaries of this fence, and are indicative of a sedentary agricultural population (Fig. 8). The hillside surrounding the farm served as a catchment from which conduits channelled runoff water to the terraced fields. Catchment and cultivated areas are thus seen as a clearly defined unit—an integral part of an overall plan of watershed subdivision.

In the desert, rainfall only wets the hillside soil to a shallow depth, and is soon lost by evaporation. Ancient runoff fanners had to depend on winter runoff water from the surrounding slopes to supplement the meagre rainfall. Cultivation practices of the

Fig . 7 M a p of the Kurnub (Mamshit) system, surveyed by L. Shanan and N. H. Tadmor in 1954. The remains of ancient walls related to the evolut ion of the wadi and the terraces during three diversion stages dating from 900 B C to 600 A D were clearly visible in the field and are shown on the m a p . Note the divers ion ditch leading runoff from two small catchments to a farm unit belonging to the Byzantine Period (300-650 AD).

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F i g . 8 A n oblique aerial photo of a 2000 year old runoff farm near Nizzana . The homes tead and the stone fence surrounding the terraced fields are clearly seen. A wadi (gully) has b roken through the farm, destroying sections of the terraced walls and fields. (Photo by N . Tadmor , 1954.)


farmer in more humid regions aimed to prevent hillside runoff and enhance the infiltration of rain into the soil. The desert runoff farmer, operates the contrary principle. He aims to minimize infiltration on the slopes, maximize runoff, and lead the runoff from a relatively large area on the slope to small cultivated field in the bottomlands.

Agricultural development under the physiographic conditions of the Negev (shallow, gravelly soils and steep gradients), required not only a mastery of the techniques of using surface runoff for irrigation, but also an understanding of the skills of land reclamation. One of the important aspects of ancient desert settlement was terracing the small narrow wadis (Table 1), and it is in these wadis that runoff farm units or groups of farm units are found.

Case study 3: Runoff farms using runoff from small watersheds Figure 9 is a map of a number of farm units in the Avdat region where the fields were irrigated by a

Fig. 9 A detailed map (surveyed by L. Shanan and N . T a d m o r in 1955) of a number of runoff farm systems in the Avda t area. The contour interval is 10 m. The farm units, collecting conduits and the mound and strip systems are easily discernible in the fields. See text for further explanation.

86 Leslie Shanan

system of hillside collection channels. The small eastern valley A (Fig. 9), is terraced in its lower reaches and gets part of its water from the upstream wadi. The terraces, receive additional runoff from stone built channel systems on the adjoining hills. The western valley B (Fig. 9), is terraced from its lower end to the head gully. Water is collected from adjoining slopes that have been partly cleared of stones, to form patterns of stone mounds, strips, and conduits.

The total area of all these systems is about 100 ha, of which 3 ha were cultivated. The ratio between the water-collecting and the water-receiving area is about 33:1.

The catchments were subdivided into subcatchments, with conduits transporting runoff from specific parts of the slopes to specific fields. A single conduit generally collected water from a relatively small area, 0.1 ha to 1.5 ha in size. The runoff was thus divided into small controllable streams of water, suited to the dry stone structures built by the ancient farmers. These small flows could be easily controlled during a flood period.

Figure 10 is a detailed map of a complete runoff farm unit, which is also shown in the centre of the flood plain on the southwest bank of Wadi Abiad in Fig. 3. The stone mounds and strip systems (A, B and C, Fig. 10) increased the runoff from the catchment. Five runoff collecting channels (a, b, c, d and e, Fig. 10) directed the runoff to the terraced fields. The ratio of the catchment to the cultivated area is a low 3:1 compared to the normal average of 20:1, because the cultivated terraces were originally part of a larger project planned to receive runoff water from the early 900 BC-750 BC diversion system shown in Fig 3.

The earliest runoff farm units were found in the Mishor Haruach and Matred Plains, dating to the Israelite III period (850 BC-600 BC). The farms were generally constructed near forts and water cisterns located along the caravan routes (Evenari et al, 1982). Negev (1979) dated several runoff farm units in the Avdat area to the late Nabatean period (c. 100 AD). He suggests that the Nabateans had applied the technology of collecting hillside runoff water for filling their cisterns, to irrigating the terraced fields.

Runoff farms were used continuously until about the seventh century AD, when the rising tide of Islam swept the region. They were abandoned because the Arab civilization had neither religious, economic, or military motives for maintaining them (Negev, 1979).

RECONSTRUCTED ANCIENT FARMS

Case study 4: Reconstructed farms at Avdat and Shivta

Detailed descriptions of studies carried out at the two reconstructed ancient runoff farms have been published elsewhere (Shanan & Schick, 1980; Evenari et al, 1982). The hydrological studies include 20 runoff plots and 13 watersheds.

Avdat The watersheds include eight catchments ranging in size from 1 ha to 345 ha. The large watershed is a third order basin in which many of the ancient terraced walls have collapsed. The other seven watersheds are ancient subdivisions of a 30 ha watershed that resulted from reconstructing the ancient "water-harvesting" hillside collecting ditches. The subcatchments vary in size from 1 to 7 ha.


i l o u r l l n r

88 Leslie Shanan

subcatchments of the watershed. In Nahal Haroeh, a detention-storage dam was constructed in 1954-1955 by the author to use flood flows from a 43 km 2 catchment area (Fig. 11). Nativ (1976) provides details of the watershed, the dam and its construction, and daily and seasonal rainfall and runoff measurements.

Most of the rainfall in the Negev is localized, often coming in convective cells; the typical cell diameter is less than 10 km. The proportion of the area receiving rainfall on a given day may be as low as 20% (Sharon, 1972). Rainfall data were recorded at Sde Boqer at a point 9 Ion south of the catchment centre and 3 Ion outside the watershed boundary. Because of the spotty, erratic, and unequal distribution of the storms on the

Fig. 11 A vertical aerial photograph of the Naha l Haroeh D a m , constructed near Sde Boker in 1 9 5 4 - 1 9 5 5 . Downs t r eam of the d a m are the terraced fields that receive this supplemental irrigation. Note also the ancient terraced fields in some wadis , particularly the broad terraced sys tem southeast of the dam. A tributary wadi cut back from Naha l Haroeh and completely bypassed the terraced fields, p robably during the late Byzant ine Period, about 600 A D .


watershed, the recorded rainfall values are only approximate estimates of the probable average values for the catchment. Annual rainfall for the period 1951-1981 ranged from 30 mm to 170 mm and averaged 93 mm.

The maximum storage capacity of the dam is 250 000 m 3 and the water is used for irrigation. Seepage and evaporation losses are high (15 cm day"1) and an estimated 20% of the flood flows are wasted through the spillway that functions about once every 7-10 years.

Case Study 6: Nahal Boqer

The Nahal Boqer watershed (35 km 2) adjoins Nahal Haroeh and is physiographically similar to Nahal Haroeh. Observations relevant to this paper included measurements of a wadi "back-cutting" process that has been followed by the author since 1951. We have observed similar wadi incision processes in Ramat Matred, Mishor Haruach and Nahal Lavan areas.

RESEARCH RESULTS AND OBSERVATIONS

Runoff processes

Small watersheds and runoff-plots Based on two decades of comprehensive rainfall and runoff measurements in small watersheds and runoff plots in the Avdat and Shivta reconstructed farms, the runoff processes have been studied in detail (Shanan & Schick, 1980; Evenari et al, 1982). The principle factors affecting runoff in these two regions include: soil cover, seasonal infiltration rates, basin shape, basin size, hillside slope, differential slope contribution, overland flow lengths, and channel losses.

Our studies showed that both stoim and seasonal runoff are mainly functions of catchment size and slope gradients (Table 2). Average seasonal yields from the three catchment sizes studied ranged from 2.4 mm (for third order catchments) to 26 mm (for small plots, 80 m 2 in size). One of the most important factors contributing to these differences in yield is the initial loss (i.e. the threshold rainfall needed to initiate runoff, Table 2). The initial loss in the third order basin (7.0-8.0 mm) can be accounted for in the following manner:

crust wetting 2.5 mm overland flow losses 2.5-3.0 mm wadi channel loss 2.0-2.5 mm

Table 2 Average annual runoff and initial losses from catchments in the Central Negev .

Catchment Area (ha) Average annual runoff (mm) Initial loss (mm)

Plots: 1% slope 0.08 26 2.5 10% slope 0.08 22 2.5 2 0 % slope 0.08 11 2.5

Sub-catchments 1-7 4 - 1 2 5.5 Third-order catchments 345 2.4 7 .0 -8 .0

90 Leslie Shanan

These conclusions explain why ancient runoff farming systems based on small watersheds were relatively efficient water harvesting projects. In the ancient systems, subdividing the first order basins by artificial channels reduced initial losses from 5.5 mm to about 3.0 mm and so increased the frequency of runoff events and the seasonal water yield.

The results of the runoff plot studies showed that slope gradient is an extremely important factor affecting runoff. The 10% gradient, for example, produced about 60% more total annual runoff than the 20% gradient. Gradient is not only an indicator of the topographic condition of an area, but is an index summarizing the physiographic site conditions including soil depth, stone cover, rock outcrops and vegetation cover. Moderate slopes with low infiltration rates were found to be the principal contributors of runoff in the Avdat area. Unequal areal distribution of rainfall further reduced the runoff contribution from peaks of hills, because valley side slopes were found to receive almost twice the rainfall received by the peaks and knolls.

Stone clearing was found to have a significant effect on runoff. Stone clearing increased average annual yields by 24% and 49% from 10% and 20% slope plots, respectively. The studies indicate that this increase can be explained by regarding infiltration as a two-phase process that takes into account the movement of air escaping upwards through the soil profile and water infiltrating downwards. This explanation supports the theory that the purpose of the gravel mounds, tuleilat el enab, found in the region, were a result of stone clearing by ancient farmers to increase the runoff yield (Evenari et al, 1982).

Avdat large watershed (345 ha) Runoff from this watershed was erratic with about 50%o of the years producing less than 0.5 mm. However, in about 10% of the years, runoff exceeded 10 mm. The average annual yield was 2.4 mm (Table 2).

This explains why the catchment-to-cultivated area ratio in the large diversion projects is at least 35:1 (compared to 20:1 for the small watershed runoff farms), indicating that the large systems are relatively inefficient "water-harvesters".

These studies contributed towards understanding of the water balance of the Upper Nahal Zin Basin. By evaluating the differential contribution from first, second, and third order catchments, the research showed that the amount of water available for plant growth both on hillsides and on wadi bottoms was minimal. On the hillsides, only about 30-40% of the annual rainfall penetrates below the soil crust and becomes available for plant growth because a 10-15 mm rainfall wets only 5-10 cm depth, depending on local soil cracks and fissures. The threshold conditions required for third order basins to contribute to the regional water table indicate that: (a) a 2-3 h flow in the wadi wets an average depth of about 40-60 cm; (b) that after a 3-4 h flow, the maximum saturation depth in the centre of the wadi does not exceed 1.2 m; and (c) flood water penetrates to the deeper gravel layers in the wadi beds of third order streams only during exceptional floods lasting more than 8 h.

Nahal Haroeh dam (43 km 2 watershed) Based on 35 years of records at Kibbutz Sde Boqer (1955-1991), average rainfall was calculated as 95 mm. In 13 out of the 35 years (37%) there was no runoff, i.e. about one out of every three years is likely to be a drought year. However, the spillway functioned five times during the same 35 year period, averaging once in seven years (15%). The average annual runoff was 2.4 mm.


The records also indicated that: (a) a threshold daily rainfall of 8 mm was required to initiate a runoff event in Nahal

Haroeh; (b) a daily rainfall of about 32 mm produced a large enough flood for the spillway to

operate. Daily runoff, for rainfall amounts of up to 50 mm day"1, can be represented by the equation:

RD = K{PD-%) where, RD = daily runoff [mm], PD = daily rainfall (

92 Leslie Shanan

Erosion from the Shivta cistern watershed Sediment accumulated in the cistern over the 20 year period (1960-1980) at an average rate of 0.042 mm year"1 from this 1.2 ha watershed. Average annual rainfall was 93 mm year"1 and runoff 12 mm year"1.

1 - 2 1

Total sediment yield was therefore equivalent to 4.2 mm century" (63 t Ion" year" ). Taking into account that only 10% of the total sediment yield was coarse particle

bed load (see above), these low sediment values explain why the silt-traps built by the ancient settlers at the entrance to these runoff-collecting cisterns (Evenari et al., 1982) had capacities of generally less than 100 1. The Shivta cistern, for example, produced an average of about 80 kg (50 1) bed load sediment per year, a quantity that could be caught and stored in the silt-trap. Cleaning it once per year, or after each storm, was a simple maintenance task.

Erosion processes in large watersheds with diversion systems All the diversion projects studied showed a remarkable similarity in their evolution that is characterized by three development stages: - Stage I, Flood plain development Many of the major present-day gravel-bed wadis

were wide shallow depressions in the alluvial plains, prior to 1500 BC. Intensification of the agricultural use of these fertile areas necessitated the construction of the stone walls to prevent the flood flows concentrating in the lowest depressions and creating gullies. These walls were, over time, extended to spread the water to extensive sections of the flood plain. The main spillways of these systems were characterized by 30-60 m wide openings capable of passing the entire flood (Fig. 3).

- Stage II, Diversion system At some period these flood plain spreading projects were abandoned and the system deteriorated through lack of maintenance. After abandonment, the floods cut a 1-3 m deep gully through the flood plain. The next settlers in the area developed the technique of diverting the flood water by constructing low stone barrages in the wadi and building canals to lead the water to the flood plain. Each diversion canal served a relatively small area (2-6 ha) and in most cases the new settlers built on the remnants of earlier walls and structures they found in the floodplains. The wadi continued to erode, and silt from eroding banks and from upstream head-cutting gullies was deposited in the terraced fields. This sediment raised the level of the fields until a stage was reached when the height of the terraced walls had to be increased and a new diversion structure and canal constructed further upstream to irrigate the fields at their higher elevations. The wadi bed was eventually several metres below the terraced fields in the flood plain, and channelled between high stone retaining walls (Fig. 3). Any break in these walls would cause serious damage to the whole system. The construction of these structures required an understanding of hydrology and hydraulics. Thus, the period of these diversion systems must have been one in which the science of engineering was well developed and the projects were under the control of a central authority that was able to manage the entire watershed and to enforce rules for distributing the water during the short flood periods that occurred two or three times every year.

- Stage III, Runoff farms The area was again abandoned at some point in time. The systems may have become unmanageable because of the silting problem or exceptional floods may have destroyed the main structures. The next settlers in the


area no longer relied on using flood water from the main wadis, but used runoff from small watersheds adjoining their terraced fields to obtain supplementary irrigation water. These runoff farms usually adapted older walls and structures to their new requirements and used only small sections of the original diversion system area that were situated close to hillsides.

Sediment in the Nahal Haroeh dam (43 km 2 basin) Sediment yields in the dam (Fig. 11) were calculated on the basis of a revised survey made in 1976 by the Department of Agriculture (E. Ador) and on flood flows and discharges recorded by Kibbutz Sde Boker (Nativ, 1976, and personal communication, R. Yahel of Sde Boker) for the period 1955-1991.

Average annual runoff was 3.0 mm year"1 and the total volume of sediment that accumulated in the dam during a 21 year period (1955-1976) was 23 000 m 3. Assuming a trap efficiency for the dam of 55% (25-35% delivered to irrigation and 15-20% wasted over the spillway), average annual sediment yield was 0.046 mm year"1

(4.6 mm century"1, 70 t km"2 year"1). This relatively small sediment yield is due to the physiographic and ecological

conditions prevailing in Wadi Haroeh. It is a stable, wide depression with a lush winter vegetation cover, and in sections of the wadi the ancient terraces are still in good condition. The relatively low gradients of the main wadi—about 1%—have also contributed to the stability of the wadi and few gullies have cut back into the flood plains.

Gully headcutting in Nahal Boqer (40 km 2 basin) At the lower end of Wadi Boqer, the flood waters flow in a wide depression that in some sections was stabilized by ancient stone walls built to keep the flood spread across a 30-50 m wide zone that was covered with perennial and annual vegetation (Table 1). Base levels of the area have not changed appreciably since ancient times, perhaps for 5000-10 000 years. However, in 1950, at the lower end of the valley, probably because a stone stabilizing wall broke during an exceptional flood, an ever deepening and widening gully began cutting back into the flood plain destroying the vegetation and ancient walls as it moved upstream.

In 1951 the gully was about 1-2 m wide and 1 m deep. Thirty years later it had become a wadi 30 m wide with vertical banks 2.5 m high, and it had cut back some 500 m into the original stable floodplain. These observations indicate that an average of about 1250 m 3 soil had eroded annually during the 30 year upstream advance of the headcut. This is equivalent to 0.031 mm year"1 on the entire watershed or 3.1 mm century"1 (46 t km"2 year"1). This is of the same magnitude as the rate of erosion from small watersheds.

This degradation in the main wadi results in initiation of a correlated incision process in tributary wadis, and a cycle of upstream headwater erosion can be expected to further increase the total rate of erosion from the watershed.

Sedimentation in ancient terraced fields The silt-laden flood water used for irrigation brought about cumulative changes in the levels of the terraced fields. The silt deposited in diversion canals and in fields raised their levels about 2 m during a 600 year period, averaging about 3.3 mm year"1 (33 cm century"1).

The rate of rise in the levels of the fields irrigated from diversion canals can also be estimated from the annual water use by agricultural crops in the ancient systems

94 Leslie Shanan

which was about 300-400 mm year"1 (Evenari et ai, 1982). Given that the average silt content of the flood waters reaching the fields is about 1% by weight, the amount of silt deposited in the fields was about 3-4 mm year"1, i.e. 30-40 cm century"1.

This magnitude of rise would require a concomitant raising of the terraced walls at the same rate (Fig. 5). These estimated sedimentation rates are considerably higher than those reported for ancient irrigation systems in Mesopotamia (Iraq) which averaged about 20 cm century"1 over a 5000 year period (Jacobson & Adams, 1966). The situation was less acute in the runoff farm systems, because the rate of sedimentation from the small watersheds was only 6 cm century"1 (see above).

DISCUSSION

Rates of erosion

The rates of erosion reported above, are summarized in Table 3. These observations indicate that erosion from small watersheds averaged an extremely low 3.0-4.2 mm century"1

(45-63 t km"2 year"1). The large Nahal Haroeh watershed, where the main wadi is a shallow wide loessial depression with a good winter annual and perennial vegetation cover, produced about 4.6 mm century"1 (70 t km"2 year"1) of sediment. Incremental sediment load resulting from gullies headcutting back into a stable deep loessial wadi (Nahal Boqer), was equivalent to additional 3.1 mm century"1 (46 11cm"2 year"1).

Large watersheds, where gully erosion and headcutting processes are active, can be expected to produce at least 7.6-9.5 mm century"1 (115-1521 km"2 year"1) of sediment as estimated in Table 4. In catchments where the headstream erosion process is also taking place in the tributary wadis, rates of erosion may increase by an additional, say, 3.1 mm century"1 and reach a total of 12.6 mm century"1 (180 t km"2 year"1).

Rates of erosion reported for other regions in the Negev are given in Table 5. Several factors account for the relatively lower rates of erosion in the Central Negev, principally:

T a b l e 3 Eros ion rates in the Central Negev: summary .

Watershed Area Rates of erosion: ( m m century" 1) (t km" 2 year" 1)

Avda t small watersheds 1-7 ha 3.0 45 Shivta cistern 1.2 ha 4.2 63 Nahal Haroeh d a m 43 k m 2 4.6 70 Nahal Boqer headcut 35 k m 2 3.1 46

T a b l e 4 Est imated rates of erosion in subcatchments of a large watershed with active headcutt ing.

Source Est imated rate of erosion: ( m m century"') (t km" 2 year"')

Small watersheds 3 .6-4 .0 5 4 - 6 0 Stable wadis 1.0-2.0 15 -30 Act ive headcuts 3 .1-3 .5 4 6 - 6 2 Total 7 .6-9 .5 115 -152


Table 5 Rates of erosion in three selected regions in the Negev .

Region and reference Annua l rainfall (mm)

Watershed area ( k m 2 )

Descript ion Erosion ra tes : ( m m century"')

(t km" 2 year" 1)

Eilat, Nahal Yael 31 0.5 Natural watershed 11.5 170 (Shick & Lekach, 1993) Machtesh HaKatan 80 7.3 Natural watershed 7.5 120 (Greenbaum & Lekach, 1997) Beersheba (Lemanim) 220 2.9 4 7 % contour ploughed; 16.0 259 (Laronne, 1989) 51 % loess-mantled

slopes

(a) The number of days per year with a rainfall greater than 5 mm averages about 6, of which only about 3 exceed 10 mm. Rainfall of 25 mm day"1 has a probability of less than once every few years. The maximum intensities of these rainfalls are much lower than in the rest of Israel (Evenari et al, 1982).

(b) Overland flow distances are short, seldom exceeding 40 m. (c) The loess soil of the area forms a crust after 2-3 mm of rainfall have wetted the

surface, and it remains relatively stable under the low-velocity laminar flow conditions prevailing before runoff concentrates in rills and gullies.

(d) The stone cover, particularly in the hamada areas, protects the soil surface and acts like a mulch.

(e) In the deep loess wadis, annual winter and perennial vegetation stabilizes the depressions. Erosion rates increase significantly however, after a head-cutting incision process has been initiated.

The ancient farmer, by subdividing the watersheds into relatively small subcatchments achieved two important advantages: (a) He increased the amount of runoff that could be harvested from the hillsides by

reducing the overland flow distances and seepage losses. (b) He decreased the rates of erosion from the watersheds by minimizing or

eliminating the development of rill, gully, and wadi erosion. Herzog (1998) discovered an ingenious Israelite III period (850 BC-600 BC)

water supply system at Tel Beersheva that diverted flash floods from a large 25 km 2

watershed into a tunnel leading to four underground cisterns with a total storage capacity of 500 m 3. The system was abandoned after 200 years because of serious sedimentation problems in the tunnel and the cisterns.

During the Hellenistic period (350 BC-167 BC), one of the cisterns (with a capacity of about 100 m 3) was again put into use to store runoff water, collected this time not from a large watershed, but from a small watershed.

The Tel Beersheva water cistern complex is the only system discovered in the Negev that was supplied with runoff water from a large watershed. All other cisterns, dating from the Israelite period through to the Byzantine period (850 BC-650 AD), used runoff from small watersheds (Evenari et al, 1982). Apparently after about 600 BC, the engineers realized that the high silt and sediment loads carried by flash floods in large watersheds, caused rapid rates of sedimentation in the diversion canals, tunnels, and cisterns. Unable to meet the heavy burden of maintaining these diversion systems, they decided to use runoff flows only from small watersheds to fill their cisterns.

96 Leslie Shanan

Role of steep rocky slopes in producing runoff

Yair (1983), based on studies in a small watershed near Sde Boqer, concluded inter alia, that from the point of view of runoff production, a greater emphasis should be placed on the role played by steep rocky hillsides and less on the gentle slopes and the stoneless bare soils. Evenari et al. (1982) had recognized the relative contribution of the rocky hillslopes, but their runoff plot results showed that significant amounts of runoff from these areas could only reach the fields if two conditions were met, separately or in combination: (a) the overland flow distances of the natural catchments were reduced to 40 m or less

by dividing the hillsides into subcatchments with cross-slope collecting conduits; (b) the annual runoff yield could be increased 20-60% when the stones were cleared

from the surface and placed in mounds and/or strips. First, it is important to point out that Yair's Sde Boqer experimental site is not typical of watersheds where ancient agricultural systems are found and the results may not be applicable to the ancient runoff-collecting systems for the following reasons: (a) At Yair's experimental site there are neither runoff farm systems or gravel mound

and/or strip systems. Several terraced wadis and wadi stabilization walls are found in a few subcatchments of the Sde Boker watershed. However, ancient agricultural systems had been constructed on no more than 1.3% of the Boqer-Ashalim catchment area, compared to 2.7%, 5.4%, and 3.1% for the Shivta, Avdat and Nizzana watersheds, respectively (Table 6). The ratio of the catchment-to-cultivated area (Table 6) reflects different densities of development and a diversity in landforms. In the Avdat watershed, the systems are predominately runoff farms with mounds, strips, and collecting conduits enhancing runoff production from the hillsides, with an average catchment-to-cultivated area ratio of 18:1; in the Shivta and Nizzana watersheds, flood plain diversion systems adjacent to the main wadis are the principal form of development with catchment-to-cultivated area ratios of 38:1 and 32:1, respectively. However, the catchment-to-cultivated area ratio for Sde Boqer is 84:1 and reflects the paucity and infrequent occurrence of stabilization walls in the watershed.

(b) This low level of runoff farm development in the Sde Boqer watershed is one of the reasons why Yair (1983) did not find any "agricultural installations" in the 13 km 2 loessial plain of Mishor Zin. A second reason for the absence of development is that, although Mishor Zin is only 8 km due north of Avdat city, it is separated from the Avdat area by the deep canyon of Nahal Zin, about 150 m deep and 2 km wide in parts. Access to Mishor Zin from Avdat necessitates a 15 km trek circumventing this canyon (Fig. 12).

T a b l e 6 Ancien t agricultural cultivated areas in selected watersheds in the Negev (after Kedar , 1967).

Watershed Catchment area ( k m 2 )

Cult ivated area (ha)

% watershed cult ivated

Rat io of ca tchment- to-cultivated area

Sde Boquer , Ashal im 255 303 1.2 84:1 Shivta 188 495 2.7 38:1 Avda t 125 678 5.4 18:1 Nizzana-Ruth-Lotz 560 1750 3.2 32:1


Fig . 12 A vertical aerial photo of the deep wide canyon of Naha l Zin, cutt ing through Mishor Zin. Nor th of the canyon, the buildings of the Ben Gurion Universi ty, Sde Boker Campus , can be seen on the brink of Mishor Zin. No te the ancient runoff farm systems located on that section of Mishor Zin situated south of the canyon are only 4 k m distant from the ancient town of Avdat.

The predominant intensive runoff farm developments in the Negev (particularly the gravel mound and strip systems and the collecting conduits), are found mainly within 5-6 km distance of the main ancient towns (Avdat, Shivta and Nizzana). Farmers were not prepared to journey more than 6 km to reach and tend their irrigated fields or walk tens of km to maintain their water collecting systems. Furthermore, they were not prepared to live too far from the main centres for security reasons. (In many developing countries in which the author has worked, it was observed that villages with 3000-5000 inhabitants cultivate irrigated lands extending over 300-600 ha but located no more than 3 km from the village, for the same two reasons). On the southern side of the Nahal Zin canyon, and only a 4 km trek from Avdat, is a continuation of the landforms of Mishmar Zin (Fig. 12). On this loessial plain,

98 Leslie Shanan

ancient runoff farms with collecting conduits on the almost stoneless gentle sloping area are clearly seen in the photo and in the field. This loess plain was found by the ancient farmers to be satisfactory from the point of view of its water harvesting potential and its distance from the town of Avdat.

(c) The physiographic and ecological conditions of the Sde Boqer watershed differ significantly from those at Shivta and Avdat. First, the Sde Boqer geological formations are Cretaceous limestones, dolomites and chalks; those at Avdat and Shivta are younger Eocene limestones, "hamadas" and conglomerate "hamadas". Second, the plant associations present are also significantly different. In the Sde Boqer experimental site they are primarily Vartenia phionedes-Originum dayi, indicative of relatively high soil moisture conditions (Yair, 1983); in Shivta the Zygophyllum dumosum association dominates while Artemesia herba alba represents the common association in the Avdat area (Evenari et al, 1982).

(d) The studies by Evenari et al. (1982) were carried out at Avdat and Shivta on an experimental layout superimposed on reconstructed ancient agricultural runoff collecting systems, while the experimental site at Sde Boqer was in no way comiected to any ancient runoff inducement and collecting systems.

(e) The Avdat runoff plots, from which Evenari et al. (1982) drew their conclusions regarding the effect of slope, cover, and rainfall, were carefully designed and constructed in four separate blocks with the slope of each plot uniform as well as equal within blocks. The site was chosen so that the geological, pedological, and ecological conditions were uniform. The experiment was planned in a random block design, with four treatment replicates and control plots. The runoff plots of the Sde Boqer site in contrast, differ widely in their shape, size, dominant hillslopes, stone cover, and geology. The site comprised three limestone formations: Dorim, Shivta, and Netser, each with its particular ecological environment. The majority of plots include two different geological formations (Yair, 1983). In addition, overland flow lengths vary from 55-76 m and hillside gradients from 12-29%, Furthermore there are wide variations in the slope gradients within the plots themselves. Consequently, in the Sde Boqer experimental site, it is impossible to separate out from the data, the effects of interrelated variables in an analytical, statistical, or simulation analysis of the complex nonlinear relationships. This problem is discussed in further detail below.

(f) The runoff plots at Avdat were uniform in size and shape (20 m long and 4 m wide), and the slope was uniform in each plot and equal within blocks. The geological formation is also unifonn on the site. The overland flow length on all plots was standardized (20 m) and the rainfall micro-distribution was recorded with a representative number of recorders.

The lack of uniformity and wide variation in attributes between and within the Sde Boqer plots results in basic differences in the statistical populations under study. For example, Yair et al. (1980, Table 1) found that 82% of the total erosion and 46% of the runoff originated on three plots representing only 35% of the area, due to the differential bioturbation activity of porcupines and isopods that was concentrated on these plots.

In contrast, the experimental design of the runoff plots of Avdat limited the number of variables under study, and evaluated them in a manner that overcame the


problem of equifmality (i.e. similar changes resulting from disparate combinations of input, throughput, and output acting over different periods of time).

Ancient terraces—sediment traps or stabilization walls?

An important aspect of understanding the runoff and erosion processes in the Negev, is the question of whether the ancient stone terraces on the hillsides and in the valley bottoms were constructed to trap the silt washed down from the hillsides, or whether they were built to stabilize existing soils in the wadis and depressions by preventing gullies "cutting-back" into these potentially productive areas. This issue has been referred to widely in the literature during the past four decades. There are several major objections to the hypotheses that the walls were sediment collecting structures: (a) Our studies have shown that erosion from small and large watersheds in the Negev

does not exceed 5 mm century"1 and 10 mm century"1, respectively. Assuming a catchment-to-cultivated area ratio of 30:1, soil accumulates behind the terraces at a rate of 15-30 mm century"1 This means that the ancient farmers would have to wait at least 200 years until they had trapped 30-60 cm of soil behind their terraces before they could expect to produce any viable agricultural crop.

(b) Our archaeological discoveries in the field support the hypothesis that the soils in the wadis and depressions pre-date the construction of the runoff collecting systems: (i) The three-stage evolution of the large wadi floodplains described previously, shows that the first walls were built specifically to stabilize the wide depressions and spread the flood flows across the floodplains (Figs 2, 8 and 11) and so prevent the development of gullies in the depressions. (ii) Many of the flood plain development projects were operated during the Israelite Period (1200 BC-1000 BC), at least a millennium before the Roman-Byzantine period of development. (iii) We discovered stone mound systems superimposed on the Roman road just north of Avdat, showing that the Roman road predated these runoff collecting systems (Evenari et al, 1982). (iv) Numerous examples were recorded of wadi incision and "wadi capturing" (Figs 2, 8 and 11) clearly indicating that many of these head-cutting processes occurred after the areas had been abandoned (probably after 650 AD) when the systems became dilapidated through lack of maintenance and gullies broke through the terraced walls.

(c) Terraces to enable the intensive cultivation of hillsides and bottomlands have been built continuously throughout the ages, for example, in Middle America and Peru during the first millennium AD, in the USA during the Navaho period about 800 AD, as well as more ancient examples in China, Nepal and North Africa. Modern terrace development continues in many regions, particularly those bordering the Mediterranean. All these projects are constructed only on sites with soil profiles of at least 50 cm depth, and soils that can be cultivated intensively and adapted to profitable upland crops, vines, olives and orchards. They are always built as stabilizing structures and are never constructed on barren areas.

100 Leslie Shanan

It must be pointed out that the hillside terraces in the ancient runoff farm systems in the Negev desert are always constructed in hillside depressions or wadis, and the runoff from the hillsides collected and led in conduits to the terraced fields. However, in humid regions (1000-2500 mm annual rainfall)—for example in Nepal, Korea, the Philippines and other southeast Asian countries—hillside terraces are constructed as level-bench terraces to collect and hold all the rainfall falling directly on the hillside slopes. Hence, these are not runoff farm systems. The level-bench terraces are usually constructed of earth embankments and the fields used for rice, upland crops or orchards. Similarly, in the 500-100 mm annual rainfall region, for example in the Jerusalem Hills in Israel and in other countries in the Mediterranean region, the walls of the bench terraces on the rocky slopes are constructed of stone, the fields are not always level and are used for growing vines, olives, orchards and grain production.

Stabilization terrace walls in both arid and humid regions enhance the infiltration of water into the soil profile and so contribute to improving yields and production. Based on the records of the Nizzana Papyri (Evenari et al, 1982), the yields of barley in the ancient farm systems in Nizzana (Fig. 1) in the seventh century AD were recorded as being 8.0-8.7 times the amount originally sown, compared to yields obtained at Avdat in the 1960s of about 10-11 fold increases. The higher modem yields were due probably to the use of fertilizers. Based on these production estimates, terracing in the Negev desert enabled hillside and valley fields to be cultivated intensively, either to increase the range carrying capacity 10-20 times on steep slopes, or for grain production, of about 1-3 t ha"1 barley (grain) on the deeper soils of the gentle slopes (Evenari et al, 1982).

Gravel mounds and strips-a unique phenomenon

The tuleilat et enab man-made gravel-stone mounds and strips (Figs 2, 9 and 10), are confined to specific areas in the Negev. They have not been observed in any other part of the region, nor reported elsewhere in the world. They are therefore a unique and remarkable aspect of the Negev desert. Their occurrence in the central Negev Highlands is always associated with hillside runoff-collecting conduits and together they form an integral part of the runoff farm systems. Our research has shown that they result from the ancient settlers clearing stones from the hillside slopes and placing them in mounds and/or strips. The bared soils on the slopes increased the seasonal amount of runoff harvested from the hillsides, particularly from the rocky slopes and the rock outcrops.

Why is their occurrence a unique phenomenon? We concluded that a combination of many physiographic, environmental and sociological conditions must exist in a particular area to justify their establishment, operation and maintenance. These preconditions include: (a) Availability of land with agricultural potential which only requires the addition of

200-300 mm year"1 of supplemental water to make it productive and economically viable.

(b) Location of these potential agricultural areas close to existing towns (within a 6 km radius) so that the farmer does not spend more than about three hours a


work-day, walking to and from his fields to cultivate, sow, irrigate, harvest, and keep guard over his crops and his water-collecting systems.

(c) The soil in the fields is at least 1.5 m deep so that the water holding capacity of the profile is not less than 250 mm, the average depth of water that would be supplied by a flood irrigation.

(d) The only reliable source of water for this supplementary irrigation is runoff from the adjoining hillsides.

(e) Runoff from hillsides can be increased significantly if the stones are cleared from the hillside surfaces; the cleared soils comprise impermeable rock outcrops and/or have the characteristic of forming an impermeable crust after a few mm of rainfall have wetted the surface, so that most of the subsequent rain becomes runoff.

(f) The natural overland flow distances are less than 40 m, or alternatively, the catchments can be subdivided into subcatchments with collecting conduits that also serve to reduce the overland flow length to less than 40 m.

(g) Rainfall occurs mostly in the winter months when the soil and rainfall temperatures are 0-5°C. Infiltration rates at these low temperatures are significantly less than the rainfall intensities of the average storm, thus ensuring runoff occurring even with light rainfalls of 5 mm depth.

(h) The rates of erosion from the catchments are low, less than 4 mm century"1

(60 t km"2 year"1) so that sedimentation in the ditches, fields and cisterns is not a critical problem.

(i) The area is under the control of a strong central authority that has the political will and competence to plan and operate the systems and enforce regulations for water rights and water distribution procedures. The agricultural sector includes farmers who are willing to introduce and use new techniques,

(j) An economic and social structure that does not rely only on agriculture for its livelihood but insures a satisfactory and stable income from several productive economic sectors. This includes: desert caravan convoys continually moving through the region along international trade routes; military camps and defence installations, for local and regional security objectives (like the large Nabatean army camp for about 1000 permanent soldiers at Avdat); churches and monasteries serving as local and national religious centres of learning and study found in all the ancient towns (Fig. 1); and a class of entrepreneurs who are prepared to initiate new economic ventures in the region (like the horse-breeding enterprise at Kurnub during the Nabatean period (about 100 BC), or the large Nabatean factory at Avdat manufacturing exquisite hand-painted delicate pottery of high quality and supplying to the entire region, or the luxurious public bath-house projects at Avdat and Kurnub serving the caravan convoys along the regional trade routes (Negev, 1979).

The concurrent presence of all these circumstances in the central Negev Highlands enabled the ancient settlers to introduce runoff inducing water-harvesting systems into limited areas near the ancient towns of Avdat, Shivta and Nizzana.

Analytical solutions to the runoff process

The scientist often finds himself having to chose between simple or elaborate analytical mechanisms for understanding complex relationships and has to select from

102 Leslie Shanan

a number of widely differing analytical methodologies—mathematical, statistical and simulation modelling. Systems that can be solved mathematically are generally very simple sub-processes and essentially only of academic interest, with limited practical value. Analytical analysis is nevertheless important because it gives an insight into fundamental aspects of a problem.

Methods of studying the behaviour of involved, even chaotic interacting systems, have been developed using advanced statistical methods and/or with the simulation of continuous and parallel systems. In simulation modelling, the state of a system at any particular point in time, is expressed quantitatively; changes in the system are described in mathematical statements or as input data. Ecological, mathematical and programming aspects are interwoven into a simulation model, and the use of continuous-system-modelling (CSMP) languages has been developed specifically for this purpose (de Wit & Goudrian, 1978; Shanan & Schick, 1980).

Two independent methodologies have been used to analyse the complex process of runoff from the Negev desert watersheds: a multivariate analysis for predicting annual runoff yield, and a digital simulation model for predicting individual storm and total seasonal runoff (Shanan & Schick, 1980; Evenari et al, 1982) and are briefly reviewed below.

Multivariate analysis Annual runoff for watersheds varying in size from micro-catchments (less than 0.1 ha) to third order watersheds (up to 300 ha) were correlated with watershed size, annual rainfall, hillside slope, and stone cover. The results are presented as a nomogram in Fig. 13 (Evenari et al, 1982, Fig. 91). Yair (1983) used Fig. 13 to extrapolate an extreme value for predicting the threshold amount of annual rainfall needed to initiate runoff production from a 1.5 ha catchment with a hillside slope exceeding 20%. He concluded that the nomogram predicts that a minimum annual rainfall of 75 mm is required for the catchment to provide "a minimum amount

0 100 200 300 Fig . 13 N o m o g r a m of rainfal l-runoff relat ionships at Avdat , showing effect of size, slope, and surface cover of the catchments on annual runoff. Scale 0 - 3 0 0 , runoff ( m ha" 1 ); scale 0 - 1 5 0 , rainfall (mm) (after Evenar i et al, 1982).


of water". He then commented that annual rainfall amounts of less than 70 mm are common in the Negev, and if the nomogram result is correct, then the ancient runoff farm systems were inefficient users of runoff. Unfortunately, Yair used the nomogram erroneously. Instead of applying the curve titled "Watersheds 5-70%", he applied the curve titled "Microcatchments >20%." If he had used the nomogram correctly, he would have concluded that the threshold annual rainfall for this specific watershed to produce runoff is 25 mm (not 75 mm). During the 38 years of records at Avdat, the lowest recorded rainfall (1962-1963) was 28 mm, a one-time occurrence. Even in 1962-1963, all the watersheds produced significant amounts of runoff, averaging 1.4 mm (giving an average coefficient of runoff of 5.0% for the extreme drought year).

Simulation modelling The simulation model (Shanan & Schick, 1980) developed in CSMP language to simulate the runoff process, used storm rainfall as input, and runoff measurements from plots and catchments as output. The structure of the model was verified in three stages, with an optimization algorithm minimizing the sum of the squares of the residuals between observed and simulated events. In the first stage, the model was confined to runoff plot data so that time and areal variations in rainfall and the effects of overland flow would be minimized. Functions to account for the effects of slope, stone cover, infiltration, and evaporation were evaluated. In the second stage, the model was expanded to simulate the processes on seven sub-basins that were considered as a combination of plots, modified by areal rainfall distribution patterns and overland flow conditions. Finally, in the third stage the model was adapted to a third order basin, which was considered an assemblage of sub-basins, modified for main channel losses.

The basic premise of the model states that runoff is initiated after a crust is formed. Infiltration rates of the soils are considered to be greater than rainfall intensities until the crust is puddled and saturated. This threshold requirement, called the "maximum saturation deficit", is regarded as including depression and interception storage. When the crust is partly saturated, the amount of water required to bring it to saturation is called the "saturation deficit". Runoff is initiated when two conditions are fulfilled: (a) the crust is saturated (saturation deficit is zero), and (b) the rate of rainfall exceeds the infiltration rate of the crust.

The sub-basin model was modified to include the effect of areal rainfall distribution, differential contribution of areas, seasonal infiltration rates as functions of soil and rainfall temperatures, raindrop impact, stone cover, hillside slope, overland flow and channel losses. Maximum saturation deficit values are about 2.5 mm and infiltration rates of the saturated crust about 1.0 mm h"1. A normal distribution in the areal variation of infiltration rates was found to give the best fit for the simulated results of the plots. The simulation of storm and annual runoff was performed in two stages: (a) parameter development stage: trial and error runs of 63 storms during the 1964-1967 period; and (b) validation stage: simulation runs of 42 storms during the 1967-1969 period using the "best fit" values developed from the first period.

Although the problem of equifmality was successfully resolved in the plot stages of the model, it was not solved for the sub-basins and the third order basin because of the differential effects of hillside slope, slope distribution, overland flow distances, and channel losses. Nevertheless, the model gives satisfactory results and the inter-

104 Leslie Shanan

100 9 0 80 70 60 50 iO 30 20 !0 0

/,Area with infiltration capacity

equal or less than indicated value

Fig . 14 N o m o g r a m (after Evenari et al., 1982), showing the distribution of infiltration rates on a runoff plot as influenced by season, stone cover, s lope, and moisture content of the crust. M a x i m u m saturation deficit ( M D E F ) is the amount of water required to saturate the crust to initiate runoff. The n o m o g r a m shows that M D E F is 3.4 m m , and 4.3 m m for the spring, winter, and autumn seasons respectively. T w o examples are given in the figure: for a 6.0 m m rainfall in the spring season ( ) on a plot with a 10% slope and with stone cover, the m e a n infiltration rate of the plot would be about 4.0 m m h"1 varying from a m i n i m u m of 2 m m h " ' to a m a x i m u m of 6 m m If1 for different points of the plot; and for a 6.0 m m rainfall in the winter season ( ) for the same 10% slope and stone cover, the m e a n infiltration rate would be 3.2 m m h " 1

varying from a m i n i m u m of 1 m m h"1 to a m a x i m u m of 5.0 m m h"'.

relationship between the factors affecting infiltration rates on plots (cumulative rainfall, maximum saturation deficit, season, stone cover, hillside slope angle, and a normal distribution of infiltration rates) is presented in nomogram form (Fig. 14).

The model includes: (a) functions for evaluating the sensitivity of the results so as to enable the research scientist to decide which parameters mainly control the complex process and so guide him to further experiments and studies, and (b) functions for formulating the model stochastically. Consequently, the model has applicability both as a research tool and an engineering planning technique.

THE SUSTAIN ABILITY OF THE ANCIENT IRRIGATION SYSTEMS

Our studies highlighted the factors that enabled several civilizations (Israeli, Nabatean, Roman and Byzantine) to establish irrigation projects in the Central Negev desert during a 1500 year period from about 850 BC to 650 AD. Their operation depended on the ability of the settlers to understand the complex environmental conditions of the


desert (particularly the climate, pedology and hydrology), to master innovative technologies that enabled them to exploit hillside runoff and flash flood flows in the wadis, and to establish irrigation layout and design criteria for constructing dry stone structures to serve as stabilizing terrace walls, diversion canals, distribution systems, and erosion-control measures.

The research also highlighted the environmental and socio-economic constraints following in the wake of this development and limiting the sustainability of many of the projects. These included: (a) Environmental constraints: although erosion, transportation and deposition of

sediment are processes that have occurred throughout geological time, man's interference with the balance of nature results in changes in the relative levels of the flood plains and the wadi bottoms at accelerated rates of 10-30 cm century"1. This necessitated the raising of the height of the terraced walls and diversion structures. The increasing burden of maintaining the projects (cleaning the canals and raising the heights of the terraced walls) eventually exceeded the capabilities of the farmers, and the larger systems were abandoned.

(b) Socio-economic constraints: the planning, operation and maintenance of the projects required a central authority to manage entire watersheds and possess the power to enforce the laws for distributing the water during the short flash flood periods. The projects were abandoned when the central authority was no longer interested or capable of carrying out these duties.

Furthermore, the economic and financial viability of irrigated agriculture based on small family-sized plots of less than 1 ha in size, was dependent on the overall economic viability of the community in the region. Agriculture was economically sustainable provided that it was integrated into a regional economy that comprised several economic sectors including: large-scale military installations and/or army camps protecting the security of the region; regional and international trade routes passing through the area that served as a ready cash market for local goods and services (such as fresh foods and bath-house facilities); the existence of local entrepreneurs of industries for exporting goods to other regions (horse-breeding, exquisite pottery, etc.); and the presence of large-scale regional religious institutions of learning.

Understanding the reasons for the failure of irrigation projects in the past is a prerequisite to proposing feasible ways of improving the sustainability of irrigation projects today. During the last four decades, numerous attempts have been made by governments and international agencies to improve the present short-comings in irrigation projects, particularly in developing countries (Shanan, 1998). Unfortunately, the results of improvement schemes have fallen far below the planners' expectations. The lessons learned from the ancient irrigation projects in the Central Negev desert can be of great value to planners who are searching for ways to make present-day irrigation projects sustainable over long periods of time.

REFERENCES

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Evenari, M., Shanan, L. & Tadmor, N. H. (1982) The Negev: The Challenge of a Desert (second edn). Harvard University Press, Cambridge, Massachusetts, USA.

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Greenbaum, N. & Lekach, Y. (1997) The potential water resource and efficiency of detention storage reservoirs in the Machtesh Hakatan: final report (in Hebrew). Department of Geography, Hebrew University of Jerusalem, Israel.

Herzog, Z. (1998) Water supply in Iron Age Tel Beersheva. Paper presented at the First International Congress in Rome on the Archaeology of the Ancient Near East. Department of Archaeology, Tel Aviv University, Israel.

Ionides, M. G. (1939) Report on the Water Resources of Jordan and their Development. Publication of the Government of Transjordan, London.

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Agricultural Land (ed. by J. Boardman, I. Foster & J. Dearing), 481-492. John Wiley & Sons, London, UK. Lipschitz, N. & Waisel, Y. (1987) The climatic and vegetation history of the last 5000 years in the Sinai Penninsula: a

dendroarchaeological study. In: Sinai (ed. by Y. Grados). Israel Ministry of Defense, Geographic Research and Publications, Tel Aviv, Israel.

Nativ, R. (1976) Seepage losses in the Nahal Haroeh Dam Near Kibbutz Sde Boqer (in Hebrew). MSc Thesis, Institute of Desert Research, Ben Gurion University, Sde Boker Campus, Israel.

Negev, A., (1979) The Nabateans in the Negev. In: The Land of the Negev (ed. by A. Shmueli, Y. Gradus & F. Mor), 226-269. Tel Aviv University and Ben Gurion University, Israel.

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Shanan, L. (1998) Irrigation development: Proactive planning and interactive management. In: The Arid Frontier (ed. by H. J. Bruins & H. Lithwick), 251-276. Kluwer Academic Press, Dordrecht, The Netherlands.

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runoff, erosion, and the sustainability of ancient irrigation...

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