Tampa Bay Water, Florida's largest wholesale public water supplier, has historically relied on groundwater to meet approximately 70 percent of the region's public water supply needs of 240 million gallons per day (mgd). The state has mandated a reduction in groundwater use because of environmental effects observed near the wellfields. The 2000 through 2001 drought magnified the importance of a reliable water supply in the Tampa Bay, Florida, region to meet current and future municipal water supply needs. To meet current and future demands and to diversify their water supply, Tampa Bay Water has developed new surface water supplies as an alternative source. Tampa Bay Water has also developed other sources from desalinization of seawater and through system interconnections. New surface water sources were made operational beginning in 2002 to replace traditional groundwater. Unlike groundwater supplies, the natural variability in surface water flows has caused new challenges in developing long-term plans. Tampa Bay Water's Enhanced Surface Water System (ESWS) is composed of withdrawals from two rivers and a 15-billion-gallon offline storage reservoir. The surface withdrawals are operational and the reservoir is currently under construction. This paper describes a combination of statistical methods that were applied for evaluating future water supply availability. These statistical methods included long-term trend analysis, seasonal analysis using Fast Fourier Transforms (FFT), time series analysis using Box-Jenkins autoregressive, integrated, moving-average (ARIMA) modeling, and Monte Carlo simulations. Mathematical models of the 25-year forecast for the ESWS were developed, including error bounds. The availability or non-availability of the large reservoir was also analyzed to determine its impact on reducing variability. The reservoir dramatically altered the expected supply time series characteristics; however, these statistical approaches were still applicable in this situation. This work was used by Tampa Bay Water to help plan for near-term and long-range capital improvements. Collectively, these models provide a thorough approach to analyze and represent a highly variable and complex water supply system. Includes reference, tables, figures.