In furrow irrigation method that is wetted less area from farm is more capability with different slopes. Thus, this method need to land grading and construction and maintenance less than other methods, and it is more acceptable among farmers. Furrow irrigation management to obtain high efficiency and uniformity is difficult, therefore, to investigate the variation of the input discharge, the cut-off time and furrow length that is effect on the efficiency and uniformity are very important. One of the most effective steps to improve the efficiency of surface irrigation is make design correctly. Considering to water scarcity in Iran, application of deficit irrigation or water stress on some crops is inevitable. We need to provide appropriate design for deficit irrigation. All of the design methods are presented for the full irrigation. The objective of this study is determiniation of optimal design variables and depth of applied water for deficit irrigation condition. The objective function in this method was net income, that is need to production function. Corn and potato were cultivated in Shahrekord and determined the their production function using a randomized complete block experimental design with 7 treatments of different irrigation levels and 3 replications. Different irrigation levels were the 40, 55, 70, 85, 100, 115 and 130 percent soil moisture deficit. The costs and benefits functions based on design variable and depth of applied water, were dertrmined. To optimize the design variables (length of the furrow, the input discharge and cut-off time) and depth of applied water in deficit irrigation condition was used Lingo software. The results showed that, for the potato the use of net irrigation depth of 576 mm (equivalent to 89% of full irrigation) and 102 m,0.43liter per second and 247 minutes respectively for the length of furrow, input discharge and cut-off time,obtained the highest profits. Also this results for corn forage were 535 mm (equivalent to 79% of full irrigation) and 85 meters, 0.39 liter per second and 188 minutes, respectively, for the length of furrow, input discharge and cut-off time. The results of this design were compared to full irrigation for deferent soil textures.The results showed an increase in the permeability of the soil, caused length of furrow and the cut-off time decrease, while the flow rate increases and depth of applied water or percent of deficit irrigation was constant. It was stated thatin an optimum design of furrow irrigation to calculate the design parameters , cost of irrigation canal , irrigation water costs and sales prices are the priority . Keywords: Optimization of design of furrow irrigation, Deficit irrigation, furrow irrigation, Production function, Optimum Applied Water.
