Water Budgets for Coeur d’Alene Lake, Water Years 2000–2005

The Coeur d’Alene Lake water budgets accounts for surface-water inflows, precipitation to the lake surface, surface-water outflows, evaporation from the lake surface, losses from the Coeur d’Alene Lake and Spokane River to the Spokane Valley-Rathdrum Prairie (SVRP) aquifer, lake storage change, consumptive use from domestic and municipal withdrawals, and a residual value. This water budget is expressed in equation 1 and illustrated in figure 3.

(1)

Annual budgets for water years 2000–2005 are reported in figure 4 and table 1. The largest gain to the water budgets is from the surface-water inflows from the six gaged tributaries, making up most of the total mean annual inflow. The largest loss to the water budgets is the outflow to the Spokane River, which serves as the natural outlet of the lake.

Mean annual inflow to Coeur d’Alene Lake for the study period, including precipitation, was about 167,110 million ft³ (3,836 thousand acre-ft). Mean annual outflow, using Spokane River flows adjusted for Coeur d’Alene wastewater treatment facility effluent, and including evaporation, was about 167,850 million ft³ (3,853 thousand acre-ft). Losses from Coeur d’Alene Lake and the Spokane River to the SVRP aquifer were 7,250 million ft³ (167,000 acre-ft). Mean annual precipitation into Coeur d’Alene Lake was 3,267 million ft³ (75,000 acre-ft), which exceeded mean annual evaporation of 2,483 million ft³ (57,000 acre-ft). Consumptive use for surface-water and groundwater withdrawals was 265 million ft³ (6,100 acre-ft). Mean annual change in storage resulted in a net loss of about 49 million ft³ (1,100 acre-ft). The residual value was about -8,310 million ft³ (191,000 acre-ft).

Inflows to Coeur d’Alene Lake from the two largest tributaries, Coeur d’Alene and St. Joe Rivers, account for almost 1.5 times the total volume of the lake, about 100,300 million ft³ (2.3 million acre-ft) (Woods and Beckwith, 1997). During 2000–2005, the two rivers contributed a combined mean annual inflow of about 151,184 million ft³ (3.5 million acre-ft). Inflow from gaged tributaries was 92 percent of the mean annual total inflows, and ungaged inflows 6 percent of the total mean annual inflows. Mean annual precipitation was about 2 percent of the total inflows. The Spokane River carried 94 percent of the mean annual total outflows, and evaporation was about 1.5 percent of the mean annual total outflows. Estimates for the losses to the SVRP aquifer from Coeur d’Alene Lake and the Spokane River were 4 percent of the total mean annual outflows. Consumptive use was 0.2 percent of the total mean annual outflows. Net change in storage was 0.03 percent of the total mean annual outflows. The mean annual residual value was 5 percent of the total inflows and nearly 5 percent of the total outflows. Each water year has a negative residual value, 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.