In 1985 the Mayor's Intergovernmental Task Force on New York City Water Supply Needs (Task Force) was formed to assess long-range water supply needs and the adequacy of planning efforts to meet those needs. The Task Force concluded in their June 1992 final report that New York City (NYC) was operating the water supply system at a supply deficit of 200 to 300 million gallons per day. The Task Force recommended implementing conservation measures to reduce the demand and valuating potential additional sources of supply. To plan for the demand that exceeds the estimated safe yield and to provide for the necessary operating flexibility to take transmission facilities out-of-service for periodic maintenance, Department of Environmental Protection (DEP) contracted for studies on additional source of supply. The end result was the evolution of the Water Supply Simulation System computer model along with some preliminary evaluations demonstrating the planning and management capabilities of the model. This paper describes several enhancements made to the model and several evaluations performed to aid the NYC DEP in planning the future of the water supply system.