Scientific Investigations Report 2007–5007

Scientific Investigations Report 2007–5007

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Summary and Conclusions

Recharge was estimated for 6,207 mi2 of the Yakima River Basin aquifer system for predevelopment and current land-use and land-cover conditions to help in assessing water availability and water management and planning. Recharge was estimated using the Precipitation-Runoff Modeling System (PRMS) and the Deep Percolation Model (DPM) that are contained in the U.S. Geological Survey’s Modular Modeling System. The models estimated daily values of recharge for water years 1950-98 using four previously developed PRMS models for the mostly forested upland parts of the study area. Daily values were calculated for water years 1950-2003, using 14 DPMs applied to the semiarid to arid lowlands for predevelopment conditions, and using 16 DPMs for current conditions. The daily values of recharge were then aggregated to monthly, annual, mean monthly, and mean annual values.

The mean annual predevelopment recharge was estimated to be about 11.9 in., or 5,450 ft3/s (about 4 million acre-ft). About 97 percent of the recharge was calculated to occur in the upland areas included in the watershed models, but most of this recharge is not available to the bedrock hydrogeologic units. Only about 1.0 in., or 187 ft3/s (about 0.14 million acre-ft) of the total mean annual recharge was estimated to occur in the 2,554 mi2 area included in the deep percolation models. Mean annual recharge for all the modeled areas ranged from 0.08 in. (1.2 ft3/s) to 34 in. (2,822 ft3/s).

The mean annual recharge under current land-use and land-cover conditions was estimated to be about 15.6 in., or 7,132 ft3/s (about 5.2 million acre-ft). Septic-system drainfields provide another 17 ft3/s of recharge to the aquifer system. The increase in recharge from predevelopment conditions was principally due to the application of irrigation water to croplands. For some of the modeled areas, the irrigation application quantity was more than five times larger than the precipitation quantity. As a result, the ratio of recharge to total water input increased from less than 0.1 to more than 0.5. Actual evapotranspiration was calculated to have increased by more than 1,700 ft3/s (about 1.2 million acre-ft). Similar to recharge in the humid areas, additional recharge under current conditions may not be available to deeper aquifers because of shallow sub-surface discharge to drains and wasteways.

The main factors that control predevelopment ground-water recharge are the quantity of precipitation and the soil properties. For current conditions, the land use and land cover and the irrigation of croplands are additional factors. The mean annual predevelopment recharge has a potential error of about 10 percent, and for current conditions, the mean annual recharge error is estimated to be about 15 percent.

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