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

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 (54 t km -2 year -1 ), 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 (70 t 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 ).