Use of Different Irrigation Water Deficit Schemes for Economic Operation of a Reservoir

Some common method, such as 75% of full demand (75%), shortage index (SI), modified shortage index (MSI) and planned deficit at specific dry years (PD) were compared with proposed method of constant water deficit (CWD) for a 30×10⁶m³ reservoir of the Ardak dam located northwest of Mashhad (Khorasan province of Iran). In each case the optimum downstream area for a cropping pattern of wheat, barley, sugar beet and corn was determined. The amount and net value (NV) of actual yield, net present value (NPV) and benefit to cost (B/C) ratio were computed for each scenario, using dated water production functions of different crops, accompanied with suitable sensitivity indices. Optimum intra-seasonal water allocation under PD was made by applying the nonlinear programming scheme. The results of this study showed that the methods of 75%, SI, and MSI are neither appropriate since they do not account for actual crop yield obtained under water deficit, and nor are they practical in real time. On the other hand, the inappropriateness of the PD method is attributed to its impossible utilization for an actual reservoir and also to its theoretical shortcomings. In contrast, the proposed method of CWD for the whole period of operation can be used in reservoir operation management, while it is coupled with optimum intra-seasonal water allocation based on a nonlinear programming scheme. this study also shows that the CWD method is more appropriate for the economical operation of the reservoir.