Methodology and strategies for multi-season farm-level irrigation decisions under limited water conditions

The problem of ground water mining has resulted in many different responses, including limitation of ground water withdrawals. Current irrigation management techniques may be inadequate for making farm-level decisions under water limiting conditions. Thus, objectives were formulated to: (1) describe a general methodology for making optimum water use decisions, and (2) develop a multi-season water allocation model that could be used for producer planning purposes. A general methodology was developed to assist in farm-level irrigation management decisions under a wide variety of irrigation situations. The management decisions which were conceptually addressed included irrigation system modifications, cropping pattern selection, multi-season water allocations and single season irrigation scheduling. However, only the cropping pattern selection and multi-season water allocation decision were addressed empirically in this study. The cropping pattern model utilized a non-linear optimization methodology to determine the optimal crop acreages and planned annual water allocation. A non-linear optimization technique was selected to enable the direct consideration of non-linear water-yield response functions. The multi-season, field-level water allocation planning model was designed to address the temporal planning decisions associated with a limited multi-year water allocation. The model used a two-state dynamic program model to optimally allocate water between years and to make irrigation system disinvestment decisions. The model also determined the optimal spatial water distribution in a multi-field environment by the use of a second, single-state dynamic programming model. The models were tested using a representative farm case in South Central Nebraska. It was found that the optimum cropping pattern was sensitive to relative crop prices and grain yields. The optimum temporal water distribution was found to be quite sensitive to the water allocation level and the water allocation period, as well as to the cost of irrigation system disinvestment. In total the methodology developed was found to be an effective and efficient approach to analyzing producer irrigation management alternatives under water limiting conditions