Estimates of net infiltration at Yucca Mountain are used to define the upper boundary of flow models that will be used to evaluate the performance of the repository. Data and interpretations from several field studies at Yucca Mountain have led to a detailed understanding of the physical processes and properties that control net infiltration.

Because complex, interrelated, physical processes are involved, net infiltration is both temporally and spatially variable, requiring the use of modeling techniques to quantify the locations and rates of infiltration.

USGS has developed conceptual and numerical models of net infiltration for Yucca Mountain and the surrounding Death Valley region. The conceptual model describes the effects of precipitation, surface runoff and runon, evapotranspiration, and redistribution of water in the shallow unsaturated zone on estimated rates of net infiltration; precipitation and soil depth are the two most significant variables. The conceptual model was tested using a preliminary numerical model based on energy- and water-balance calculations.

The numerical model simulated net infiltration ranging from zero, for a soil thickness greater than 6 meters, to over 350 millimeters per year for thin soils at high elevations in the Spring Mountains. Estimated average net infiltration over the entire model domain is 7.8 millimeters per year. Comparison of model results with previous estimates of basinwide recharge indicates that the net-infiltration estimates obtained using this model may overestimate recharge. Although the model is preliminary and is not calibrated, it provides a first approximation of the spatial distribution and amount of net infiltration for the Death Valley region under current climatic conditions.

Detailed information on the infiltration model is provided in USGS Water-Resources Investigations Report 02-4010

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