Deficit Irrigation Planning under Variable Seasonal Rainfall

The water use efficiency of irrigation water is increased by deficit irrigation. The optimum water use in deficit irrigation is obtained by an economic analysis using production and cost functions. However, there is uncertainty or risk associated with this estimation due to the fact that water production function is affected by a number of unpredictable factors among which the seasonal rainfall is an important one. Seasonal rainfall was considered in the economic analysis for optimum seasonal irrigation water application ww for cotton (a summer crop) and wheat (a winter crop). A modified equation for determination of ww for different years with different amounts of seasonal rainfall was presented based on the coefficients of production function for one growing season data. Based on this modified equation, the values of ww for different years decreased as the seasonal rainfall increased. Quadratic equations were presented to determine the value of ww based on the seasonal rainfall. For cotton in the study region, no irrigation water was required for 4 yr out of 28 yr since the seasonal rainfall was enough. For this crop, based on the pre-season rainfall, the seasonal rainfall can be estimated by the proposed equation from which the value of ww can be estimated and then, the cultivation area can be determined based on the total available irrigation water. However, for wheat, based on a given probability of occurrence for seasonal rainfall, i.e. 50 or 80%, the value of ww is estimated and the cultivation area can be determined from the total available irrigation water.