Water-Harvesting Efficiency in Arid and Semiarid Areas

Abstract

Water-harvesting techniques were, and are, often applied in arid areas. Cultivated fields are irrigated by collecting overland flow from adjoining hillslopes. The purpose of the present chapter is to analyze water-harvesting efficiency in the northern Negev Desert of Israel, where annual rainfall varies from 280 mm in the north to 70 mm in the south. The hypothesis advanced is that runoff water-harvesting efficiency is primarily controlled by surface properties rather than by the absolute amounts of storm and annual rain amounts, the controlling factor for runoff generation being the extent of rocky versus soil-covered surfaces. Rocky surfaces respond quickly to rainfall, due to their low water absorption capacity. At the same time, soil covered surfaces, with a high porosity and high water absorption capacity, represent efficient sinks. As extensive rocky surfaces are more widespread in arid than in semiarid areas, where soil cover is more extensive, it appears that rocky arid areas are more suitable for runoff water-harvesting than climatically wetter semiarid areas. Hydrological data collected at two instrumented watersheds, located one in an arid rocky area, and the second in a semiarid soil-covered area, support the hypothesis. The implications of data obtained for runoff water-harvesting under changing climatic conditions are analyzed. A drier climatic regime can be expected to improve runoff water-harvesting perspectives in a semiarid area, while reducing such perspectives in rocky arid areas. An opposite trend for both areas is assumed during a transition to wetter climatic conditions.