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 water available for crop production is transpired and water use efficiency (WUE) is low. Changes in crop management practice to reduce evaporation from the soil surface (E-5) have been successful in some locations but unsuccessful elsewhere. This paper outlines a conceptual framework for assessing the potential for improved crop management to reduce E-5 and summarizes results from Syria, Kenya, and Niger. The results show that factors such as evaporative demand, amount and frequency of rainfall, soil texture, and the distribution of roots interact to influence the sensitivity of E-5 to management practices that modify canopy area and root growth. Using a simulation model we demonstrate the quantitative effect of these interactions and show that the scope for reducing E-5 is greatest in clay soils in locations with frequent rain and low evaporative demand and least on sandy soils in regions with sporadic rainfall and high evaporative demand. The distribution of roots has a marked influence on the rate of drying of the soil surface and thereby on soil hydraulic conductivity which becomes more important as evaporative demand increases relative to rainfall.