A computer model is presented for quantifying the impacts of multi-purpose water management policy. The model is designed for the analysis of water availabilities throughout a river basin over extended time periods. It is capable of simulating the monthly storage, flow, and diversion of water in a complex river basin system. The prototype system is represented in the model by a network of interconnected nodes which characterize the reservoirs, tributary inflow points, and diversion points in the basin. Linkages between nodes represent the conveyance morphology of the basin. The model, MODSIM, is synthesized from previously existing models. A base or core model is selected from these models, and further modified so as to better conform to the attributes and capabilities considered desirable for the model.