Soil type, climatic regime, and the response of water use efficiency to crop management

In many rainfed regions of the world, only a small fraction of the total wa ter available for crop production is transpired and water use efficiency (W UE) is low. Changes in crop management practice to reduce evaporation from the soil surface (E-5) have been successful in some locations but unsuccess ful elsewhere. This paper outlines a conceptual framework for assessing the potential for improved crop management to reduce E-5 and summarizes result s from Syria, Kenya, and Niger. The results show that factors such as evapo rative demand, amount and frequency of rainfall, soil texture, and the dist ribution of roots interact to influence the sensitivity of E-5 to managemen t practices that modify canopy area and root growth. Using a simulation mod el we demonstrate the quantitative effect of these interactions and show th at the scope for reducing E-5 is greatest in clay soils in locations with f requent rain and low evaporative demand and least on sandy soils in regions with sporadic rainfall and high evaporative demand. The distribution of ro ots has a marked influence on the rate of drying of the soil surface and th ereby on soil hydraulic conductivity which becomes more important as evapor ative demand increases relative to rainfall.