Decision support for complicated irrigation system is presented in this article. The optimization mathematical model was developed mainly based on water balance concept for both command areas and coupled river and canal delivery system to suitably determine planted area for each plant species under constraints of available water supplies and limited areas to meet the maximization of economic value for the lower Pak Phanang river basin, Southern Thailand. Due to very complex problem-solving type with many decision variables of 410 and constraints of 861, an open source Excel add-in, widely known as Open Solver, was applied to solve nonlinear mixed-integer problems. The developed model was calibrated by comparison of the required inflow quantities to satisfy the calculated agricultural water demand in the study and the observed runoff quantities at X.105 runoff station, upstream of study area, during dry season (Jan to Apr). Land use data in 2012 obtained from the Land Development Department of Thailand (LDD) was interpreted to determine agricultural water demand. It was found that there was satisfying agreement between both considered values with agricultural water requirement quantities of 21.54 MCM and observed runoff quantities of 20.98 MCM. We hence applied the developed model with 8 scenarios for guidance for irrigation management in the study area.
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