, John J. Vaccaro and Theresa D. Olsen, U.S. Geological Survey
Process-based models that compute distributed water budgets on a watershed scale have demonstrated an ability to accurately calculate recharge rates at varying spatial-temporal scales using readily available databases. Two models in the U.S. Geological Survey’s Modular Modeling System (MMS) were employed to estimate recharge: Precipitation-Runoff Modeling System (PRMS) and Deep Percolation Model (DPM).The two models were used to simulate the two different hydrologic regimes of the Yakima River Basin, Washington. PRMS is a process-based, deterministic, distributed-parameter model designed to analyze the effects of climate and land use on streamflow and basin hydrology. PRMS is designed for mountainous, snow-dominated environments. DPM originally was developed as a tool for estimating daily ground-water recharge over a broad array of landscapes and spatial-temporal scales. DPM is best suited for areas dominated by agricultural irrigation.
Ground-water recharge to the Yakima River Basin aquifer system was estimated for predevelopment and current land-use and land-cover conditions. Daily values of recharge were estimated using climate data for water years 1950-2003. The mean annual recharge for predevelopment conditions was estimated to be about 12 in. (about 4 million acre-ft) for the entire 6,200 mi2 basin. The mean annual recharge for current conditions was estimated to be about 16.4 in. (about 5.4 million acre-ft) for the entire basin. The increase in recharge primarily is due to the application of irrigation water to croplands. Mean annual actual evapotranspiration was estimated to have increased by more than 1.2 million acre-ft due to irrigation.Resulting recharge rates from the models were subsequently used as input into six ground-water-flow models. This coupling of surface-water and ground-water models created a more physically deterministic representation of the flow system and allowed an independent assessment of the recharge estimate.
