J.B. Converse (Converse) Lake is the primary source of drinking water for the city of Mobile, Alabama. Concerns regarding the ability of the reservoir to meet current and future water demands during drought conditions have prompted this study. The 1991 through 2006 water years included a drought that occurred during 2000, and drought conditions currently (2007) are affecting the area. To assist officials of the Mobile Area Water and Sewer System in planning for future demands for drinking water in the Mobile metropolitan area, the firm yield for Converse Lake was estimated by the U.S. Geological Survey.

The firm yield of Converse Lake was estimated using the Massachusetts Department of Environmental Protection’s firm-yield-estimator (FYE) model, which recently was refined by the U.S. Geological Survey. The model uses a mass-balance approach to determine the maximum average daily withdrawal rate that can be sustained during a period of record that includes a drought of record. If the reservoir is in contact with an aquifer, the FYE also includes routines that estimate the volume of ground-water and surface-water exchange between the aquifer and the reservoir.

The average daily firm yield for Converse Lake was estimated to be 79 million gallons per day using the FYE routine that does not include ground-water exchange between the reservoir and the adjacent aquifer. Observed lake levels and withdrawals during the drought of 2000 indicate that more than 74 million gallons per day of water were withdrawn without complete depletion of reservoir storage. Therefore, it is likely that ground-water exchange with the reservoir may supplement available reservoir storage. If water exchange occurs between the aquifer and the reservoir, an increase in the volume of water available to the reservoir may occur during a drought. To quantify the potential ground-water contribution to reservoir storage, an analytical solution was applied to the FYE simulation of Converse Lake to estimate ground-water exchange between the reservoir and the aquifer. Aquifer properties required by the FYE were estimated by model calibration to observed water levels that occurred during the drought of 2000. When ground-water exchange between the reservoir and adjacent aquifer is included, the average daily firm yield increased to 83 million gallons per day.

The estimate of 83 million gallons per day incorporates both total surface-water flow and ground-water exchange components. This analysis indicated that direct ground-water interaction contributes about 5 percent of the firm yield of Converse Lake. However, the average daily firm yield of 83 million gallons per day, based in part on calibrated values for aquifer transmissivity and storage, can be used only as a guideline until these aquifer properties can be defined better by field investigation in the Converse Lake watershed.