Assessment of Surface and Injection Recharge Strategies in the Yakima River Basin

The current water supply and storage capacity within the Yakima River Basin does not meet water supply demand in all years.  Water resources are vital to the agricultural economy and aquatic resources, especially anadromous fish.  We assisted the U.S. Bureau of Reclamation (Reclamation) and the Washington State Department of Ecology (Ecology) in evaluating the viability of water storage alternatives as part of an Environmental Impact Statement for the proposed 1.3 Million Acre-foot Black Rock Reservoir.  The alternatives were evaluated to estimate the extent to which they could improve in-stream flows while meeting irrigation and municipal needs.    A basin scale feasibility-level assessment of surface recharge and injection recharge (including ASR) was conducted. For surface recharge, the assessment included a numerical analysis of infiltration basin capacity, return flow dynamics based on analytical groundwater functions, a GIS-based analysis of potential site locations, and a cumulative return flow volume assessment at key management locations in the basin.      For injection recharge, the analysis included development of a three dimensional groundwater flow model (FEFLOW) that simulated the effects of displacement heads in deeper aquifers on the dynamic equilibrium and streamflow interactions in the shallow aquifers.  This allowed a calculation of streamflow benefits predicted to occur from injection under various scenarios.  The analysis indicated that surface recharge to alluvial aquifers could produce 10,100 to 14,400 AF of return flow during most years. During extreme dry years, benefits were smaller but carry-over effects from previous years were predicted.  The analysis suggested that, at one potential ASR site, replacing current municipal summer surface water diversions with ASR would result in a direct increase to streamflow of as much as 6,000 acre-feet (AF) from April to September. In addition, the indirect streamflow benefits from displacement heads could be 1,000 AF or more depending on how the system was operated.