Forecasting of water demand is fundamental to the success of managing an irrigation project. A system-wide irrigation demand forecasting model, SIDFOM, using an electronic spreadsheet was developed to estimate cumulative irrigation demands. The model considers potential irrigation demand and effective rainfall. The potential irrigation demand is a function of evapotranspiration, infiltration, water supply for nursery and for transplating, irrigation efficiencies, and mid-season drainage. The effective rainfall is estimated based on the water budjet analysis. Four irrigation districts were selected to evaluate the model applicability, and the reservoir releases were compared satisfactorily with the forecasted irrigation demands. The SIDFOM is found to be adequate both for evaluating the reservoir operation strategies and for operating water management system.