A procedure is presented for determining the optimum reservoir size for supplemental irrigation using Linear Programming technique. The volume of water stored depends on the available runoff during the time interval considered along the rainy season, and the water demand which also varies depending on stage of plant growth. The runoff amount is estimated using the well-known Soil Conservation Services (SCS) curve number method. The required input data for the model are the daily rainfall depth for each time interval, soil type, construction cost per unit volume of reservoir, loss of economical return per unit area of land left as a catchment, and the maximum allowable catchment area which depends on total available area of land considered. The computerized model consists of two parts; in the first part, the program formulates the input data in the form of Linear Programming problem, while in the second part the program solves the problem by the Simplex method. The model is applied using data for Al-Hader area in Nineva Governorate. The long-term mean annual rainfall of the selected site is about 150 mm. The results showed that the most economic design is the one in which the required harvesting area is about three quarters of the total area under consideration. In this case the yield is sub-maximal, but most economical, and the irrigation depth is only about 87.5% of the case that gives the maximum yield. The required reservoir volume for the most economic design is about 111 m3/ha of the land area considered.
Keywords: linear programming, modeling and simulation, reservoir sizing, supplemental irrigation, water harvesting.