ESTIMATES OF GROUND-WATER RECHARGE TO THE YAKIMA RIVER BASIN AQUIFER SYSTEM, WASHINGTON, FOR PREDEVELOPMENT AND CURRENT LAND-USE AND LAND-COVER CONDITIONS

Ground-water recharge to the Yakima River Basin aquifer system, Washington, was estimated for predevelopment (estimate of natural) and current (multi-year composite) land-use and land-cover conditions using two different models. Daily values of recharge were estimated for Water Years 1950–1998 using four previously developed watershed models for the generally forested upland areas and for Water Years 1950–2003 using 17 deep percolation models applied to the semiarid to arid parts of the basin.

The mean annual predevelopment recharge was estimated to be about 12 in. or 5,500 ft3/s (about 4 million acre-ft) for the entire 6,200 mi² basin. About 97 percent of the recharge occurs in the 3,663 mi² included in the watershed models, but much of this quantity is not available to recharge the bedrock hydrogeologic units. Only about 0.4 in. or 187 ft3/s (about 0.14 million acre-ft) was estimated to occur in the 2,554 mi² included in the deep percolation models. In the 21 modeled areas, recharge ranged from 0.08 in. (1.2 ft3/s) to 34 in. (2,825 ft3/s).

The mean annual current recharge was estimated to be about 16.4 in. or 7,505 ft3/s (about 5.4 million acre-ft) for the entire basin. The increase in recharge is principally due to the application of irrigation water to croplands. The annual quantity of irrigation was more than 4 times the annual precipitation for some of the modeled areas. Mean annual actual evapotranspiration was estimated to have increased by more than 1,700 ft3/s (about 1.2 million acre-ft) due to irrigation.