Irrigation Planning 2: Choosing Optimal Acreages within a Season

Three problems involved in planning and operating an irrigation system are (1) the short‐run problem of allocating a quantity of water in time over an irrigation season, (2) the intermediate‐run problem of deciding what area of crop to plant at the beginning of an irrigation season, and (3) the long‐run problem of deciding what area of land to develop for irrigation. This paper concerns the intermediate‐run problem. Crop growth is simulated with a simple crop growth model over a number of years on a large acreage supplied with water from a reservoir subject to stochastic inflows; the demand for water by the crop also being stochastic. Irrigation decision rules at each of a number of decision points in time are taken from the output of a simulation‐dynamic programing model used to solve the short‐run problem. The approach used allows for the abandonment for the rest of the season of portions of the area initially irrigated but does not allow for temporary abandonment, and reinstatement during the season, which is a limitation. The results show the best irrigated acreage to plant to be an approximately linear function of the reservoir contents at the beginning of the season. The function is found to be highly sensitive to changes in the profitability of the best dryland alternative enterprise. The cost of planting suboptimal acreages is found to be high.