Annual water budgets were developed for Sebago Lake in southwestern Maine. The inflow components of the water budget are direct precipitation to the surface of the lake and surface-water inflow. Mean annual inflow to Sebago Lake during water years 1996-99 was 35,100 million cubic feet. The outflow components of the water budget are evaporation from the surface of the lake, municipal water-supply withdrawals by the Portland Water District, and surface-water outflow. Mean annual outflow during water years 1996-99 was 28,200 million cubic feet. The two largest components of the water budgets are the surface-water components - surface-water inflow was 84.0 percent of the mean annual inflow budget and surface-water outflow was 87.3 percent of the mean annual outflow budget. Changes in lake storage also were included in the water budgets.
The sum of inflow minus outflow volumes, adjusted for changes in lake storage, do not balance for each water year. This remainder volume is the residual in the water budget calculation. The mean annual residual for the 4 years is 5,860 million cubic feet and is relatively consistent in magnitude and sign (positive) each water year, indicating either a systematic overestimation of inflows to or underestimation of outflows from Sebago Lake. The residual also could be partially composed of ground-water flow, a budget component not accounted for in this study.
Errors associated with budgeting precipitation, evaporation, and net changes in storage are relatively small. The largest potential errors in calculating the water budget for Sebago Lake are those associated with surface-water inflow and outflow, because they are the two largest elements of the budget, and ground-water flow (net in or out) because it was not computed.