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close this bookFreshwater Resources in Arid Lands (UNU, 1997, 94 p.)
close this folder2: Negev: land, water, and civilization in a desert environment
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View the documentIntroduction
View the documentCivilization in the Negev
View the documentFreshwater utilization
View the documentConclusions
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Freshwater utilization

Permanent rivers are totally absent in the Negev, and even springs or proper locations for digging shallow wells are few and far between. Hence, the major source of water for humans and animals could only be the collection of surface run-off obtained from sloping ground during winter rains, a task that has been called "water harvesting." The ability to collect and store potable water from runoff was the first imperative of desert settlement. This was done by means of cisterns, which are artificially constructed reservoirs filled by directed surface flows during each infrequent rainstorm. Building efficient cisterns became possible after the advent of watertight plaster, the recognition of suitable rock formations, and the proper construction of channels to collect and divert overland flow.

Where cisterns could be located along the rim of a natural watercourse, they were filled by flash floods. However, most cisterns in the Negev were hewn into hillsides and depended on the direct collection of run-off. Many hundreds of such cisterns were built in the Negev, and they are clearly discernible landmarks even today. A typical hillside cistern resembles a giant necklace, with the glistening white pile of excavated rock seeming to hang as a pendant from the two collection channels that ring the hill and curve down its sides from opposite directions. To the thirsty ancient traveller, to whom these cisterns beckoned from afar, no sight could be more gladdening.

Run-off water was also used for irrigating crops. The run-off from winter rains falling on adjacent slopes was gathered and directed to bottomland fields for periodic soakings, to accumulate and store sufficient moisture in the soil to produce crops. Although the Negev's average winter rainfall is only about 100 mm, the run-off farmers were able to gather and concentrate sufficient run-off from the barren slopes to develop intensive agriculture in the depressions and bottomlands, which constituted only some 5 per cent of the total area in the northern Negev Highlands subregion.

This ingenious type of agriculture is called "run-off farming." Whereas farmers in more humid regions aim to have the soil absorb all the rain where it falls, thus preventing run-off, the desert farmers worked on the opposite principle: their aim was to prevent the rain from penetrating the soil on the slopes, so as to produce the maximum possible amount of run-off. They then collected this run-off from a large area of slopes and directed it to a relatively small cultivated area in the bottomlands.

The cultivated area was usually divided into small field plots, which were levelled and terraced to ensure the efficient spreading of water as well as the conservation of both water and soil. The oldest version of run-off farming probably consisted of terracing the small creek beds that collected the run-off naturally. Terracing transformed the entire length of each creek into a continuous stairway, with stairs perhaps 10-40 metres wide and 20-50 centimetres high. The terrace walls were designed to spread the flood and to prevent erosion. The slowed-down cascade from one terrace to the next could thus irrigate the field plots sufficiently for a crop to be grown. Distinct groups or series of terraced plots, having definable catchment areas and surrounded by stone walls, formed integral farming units of perhaps several hectares of cultivated land. The remains of hundreds of such farm units are spread throughout the Negev Highlands, most commonly around the principal ancient towns.

Detailed observation of ancient run-off farm units reveals that each unit was served by a particular well-delineated portion of the watershed. An elaborate system of conduits was constructed to collect run-off from specific sections of the adjacent slopes, not merely for each farm but, indeed, for each terraced field within the farm. The complete farm unit comprised both the slope catchment (the run-off-contributing area) and the bottomland fields (the run-off-receiving area). Fields could be made productive only if associated with a catchment from slopes, since the meagre rainfall alone was far from sufficient for any crop. The larger the catchment, the greater the water supply one could expect and the corresponding plot of land that could be irrigated. Clearly defined catchment areas, allocated to serve particular farm units, constituted "water rights," as specified in the ancient documents found in the region.

Typical farm units consisting of 0.5-5 hectares were associated with 10-150 hectares of sloping watershed. The ratio of run-off-contributing catchment to runoff-receiving crop land varied from 20:1 to 30:1. If each hectare of sloping land contributed only 10 per cent of its annual rainfall of 100 mm, then the receiving crop land would have gotten approximately 25 x 10 mm = 250 mm. Added to its own reception of the annual 100 mm of rainfall, the plot would thus have received a total of 35 mm, just enough to produce a crop. If, however, the run-off yield constituted 20 per cent of annual rainfall, the amount of water received by the field could equal 500 plus 100, for a total of 600 mm, an amount equivalent to the rainfall of the relatively humid habitats along the Mediterranean coast of Israel.

The fraction of rainfall yielded by any given watershed varied, of course, from rainstorm to rainstorm and hence from year to year. Gentle showers contributed practically no run-off, whereas intense squalls might yield 30 per cent or more of their rain. So, even with all the alertness, ingenuity, skill, and diligence they could muster, the run-off-farmers of the Negev operated a risky business and had to face new uncertainties each season. It is all the more remarkable, therefore, that they were able to cope with all the difficulties and to sustain a viable agricultural economy for so long on such a scale. That scale is worth emphasizing: during its period of maximal development in the Byzantine era, the system of run-off farming encompassed practically all of the usable land in the northern Negev Highlands.

The Negev run-off farmers apparently did more than merely gather natural runoff: we have clear evidence that they actually tried to induce more of it. The hillsides in the Negev, as in many other deserts, are naturally strewn with a pavement of stones and gravel, and this covering inhibits and detains the flow of run-off over the surface. The ancient Negevites deliberately cleared the stones off the slopes and thus smoothed the surface and exposed the finer soil, to facilitate the formation of a self-sealing crust. Consequently, we find countless heaps, mounds, and strips of gravel on many hillsides, particularly in the vicinity of the old towns of Shivta, Ovdat, and Nitzana. Our own field trials in that region have shown that the practice of removing the surface gravel can increase the run-off yield by 8-20 per cent.

The ancient Negev dwellers also carried out larger-scale works to divert flood water from regional streams onto adjacent flat lands. However, such works were inherently more difficult to construct and maintain. Moreover, because of the totally unpredictable and occasionally violent nature of the flash floods, the harnessing of such floods was fraught with much greater risk than the handling of small and controllable flows off hillsides.

The ancient desert civilization of the Negev, however remarkable, is not entirely unique. Other deserts in the Middle East and elsewhere witnessed similar (though not identical) developments. One example is that of the Anasazi (Pueblo) Indians of the American South-West. Though removed in space and time from the Nabateans, the Anasazi had to contend with similar environmental conditions. The Anasazi (meaning "Ancient Ones" in Navajo) developed from about 100 C.E. in the area where the boundaries of Arizona, New Mexico, Colorado, and Utah intersect. The early Anasazis supplemented hunting and wild-seed gathering with the cultivation of maize, pumpkins, and beans. Later, hunting and gathering were abandoned and agriculture became the major occupation, and this included the production of cotton. The Anasazi developed a system of run-off utilization that resembled that of the Negevites to a remarkable degree.

Notwithstanding its early success, the Anasazi civilization began to decline in the twelfth century. Not much later, these remarkable people abandoned their great cliff houses, storage pits, and ceremonial chambers, and no one to this day is quite sure why. The demise may have resulted from the incursion of nomadic tribes from the north. Another possibility is that denudation of the catchments may have led to depletion of game and to accelerated erosion, gullying, and subsequent lowering of the water-table, all of which might have reduced the productivity of the land. More plausibly, the Anasazi may have succumbed to the worst enemy of all people living in arid regions - a severe and prolonged drought. The Anasazi system of run-off farming and grain storage was capable of coping with short-term periods of dryness, but probably could not survive a decades-long span of water deficiency. Such a prolonged drought would have been especially debilitating if it were preceded by an extended period of abundance and prosperity, during which the population might have grown beyond the number that could be sustained indefinitely in such a basically arid region.