Summary and Conclusions

The U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources, calculated annual water budgets for Coeur d’Alene Lake and a mean annual budget for study period water years 2000–2005. The calculation and associated uncertainty of the following major water-budget components were included: (1) inflows, including precipitation to the lake surface and all gaged and ungaged tributary surface-water inflows; (2) outflows, including evaporation from the lake surface, consumptive use of withdrawals from the lake and wells near the lake, and the flow of the Spokane River, which is the main outlet of the lake; and (3) changes in lake storage, seepage from Coeur d’Alene Lake and the Spokane River to the Spokane Valley-Rathdrum Prairie (SVRP) aquifer system, and a residual value.

Mean annual inflow to Coeur d’Alene Lake for the study period, including precipitation, was about 167,110 million cubic feet (3,836 thousand acre-feet). Mean annual outflow for the study period, including evaporation, was about 167,850 million cubic feet (3,853 thousand acre-feet). Losses from Coeur d’Alene Lake and the Spokane River to the SVRP aquifer was 7,250 million cubic feet (167,000 acre-feet). Mean annual precipitation into Coeur d’Alene Lake was 3,267 million cubic feet (75,000 acre-feet), which exceeded mean annual evaporation of 2,483 million cubic feet (57,000 acre-feet). Consumptive use for surface-water and groundwater withdrawals was 265 million cubic feet (6,100 acre-feet). Change in storage resulted in a net loss of about 49 million cubic feet (1,100 acre-feet). The residual value for water years 2000–2005 was about -8,310 million cubic feet (191,000 acre-feet).

Inflow from gaged tributaries was 92 percent of the mean annual total inflows, ungaged inflow was 6 percent, and mean annual precipitation was about 2 percent. The Spokane River outflow was 94 percent of the mean annual total outflows, evaporation from the lake surface about 1.5 percent, losses to the SVRP aquifer from Coeur d’Alene Lake and the Spokane River 4 percent, consumptive use 0.2 percent, and net change in storage 0.03 percent. The mean annual residual value was 5 percent of the total inflows and nearly 5 percent of the total outflows. A negative residual value was calculated each year, but the residual values are within an acceptable margin of error for the budget components that are the largest percentage of total inflows and outflows.

Total inflow and outflow volumes, adjusted for changes in lake storage, losses to the SVRP aquifer, and consumptive use resulted in a negative (net loss) residual value for each water year. The mean annual residual value for the six water years was -8,310 million cubic feet (190,000 acre-feet).The residual value is assumed to consist of unqualified water-budget components, such as groundwater fluxes around the lake, and uncertainties associated with all other measured and estimated water-budget components.