APPROACHES FOR ASSESSING GROUND-WATER AVAILABILITY UNDER COMPETING DEMANDS AND CLIMATE CHANGE

The Yakima River basin in eastern Washington, like many areas of the arid American West, struggles with issues of water allocation. The 1 billion dollar agricultural economy in the basin lowlands is one of the largest in the United States and primarily is based on the diversion of about 6,000 ft³/s of surface water. The mountainous uplands generate the snowmelt runoff for irrigation and fish habitat, making riverine transport of surface water of paramount importance. Surface water in the basin is fully appropriated in average years and over-appropriated in dry years, but there are increasing demands for water for municipal, fisheries, agricultural, industrial, and recreational uses. These demands must be met through the use of groundwater, increased storage, greater conservation, and or purchasing water rights. In some areas, groundwater pumping has caused water level declines of more than 300 ft, potentially reducing streamflow in reaches with senior surface-water rights or instream flow requirements for endangered species. A variety of analytic tools have been developed to address the issues of water management under existing conditions, future growth scenarios, and potential regional climate change, including a comprehensive assessment of groundwater. The hydrologic and hydrogeologic framework that has been developed is integrated into an overlapping series of groundwater models. The models have been designed to evaluate (1) surface water effects from existing pumpage and potential new pumpage, (2) effects of projected changes in climate on groundwater use, (3) potential improvements in irrigation efficiencies on water availability, (4) artificial recharge and aquifer storage systems, and (5) the relative utility of various aquifer management strategies. In combination, these approaches may help to accommodate municipal, agricultural, and ecological needs of the basin within the physical limitations of the hydrologic system.