Increasing population is stressing already limited water supplies in the arid southwest US and elsewhere. Large-scale water supply management system is difficult given the complexities of water management policies. The objective of this paper is to develop a set of general water supply planning tool that can assist decision makers with long range planning decisions. The tool consists of simulation and optimization packages. The simulation model is geared to a general audience and developed in a dynamic simulation environment using Powersim Studio. A genetic Algorithm (GA) is linked with the simulation package to determine optimal system expansion and operation policies. Future conditions are assumed to be known with certainty for a sequence of operation periods. Copyright ASCE 2005.