By Joseph A. Hevesi, Alan L. Flint, and Lorraine E. Flint
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 02-4010
Sacramento, California 2001
Prepared in cooperation with the
NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF
ENERGY,
under Interagency Agreement DE-AI08-96NV11967
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A three-dimensional ground-water flow model has been developed to evaluate the Death Valley regional flow system, which includes ground water beneath the Nevada Test Site. Estimates of spatially distributed net infiltration and recharge are needed to define upper boundary conditions. This study presents a preliminary application of a conceptual and numerical model of net infiltration. The model was developed in studies at Yucca Mountain, Nevada, which is located in the approximate center of the Death Valley ground-water flow system. The conceptual model describes the effects of precipitation, runoff, evapotranspiration, and redistribution of water in the shallow unsaturated zone on predicted 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. Daily precipitation for 1980 through 1995, averaging 202 millimeters per year over the 39,556 square kilometers area of the ground-water flow model, was input to the numerical model to simulate 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 overlying permeable bedrock. Estimated average net infiltration over the entire ground-water flow model domain is 7.8 millimeters per year.
To evaluate the application of the net-infiltration model developed on a local scale at Yucca Mountain, to net-infiltration estimates representing the magnitude and distribution of recharge on a regional scale, the net-infiltration results were compared with recharge estimates obtained using empirical methods. Comparison of model results with previous estimates of basinwide recharge suggests that the net-infiltration estimates obtained using this model may overestimate recharge because of uncertainty in modeled precipitation, bedrock permeability, and soil properties for locations such as the Spring Mountains. Although this model is preliminary and uncalibrated, it provides a first approximation of the spatial distribution of net infiltration for the Death Valley region under current climatic conditions.
Abstract
Introduction
Study Area Description
Geographic Setting
Climate
Hydrogeology
Surface Water
Ground Water
Soils
Vegetation
Previous Work
Acknowledgments
Conceptual Model of Net Infiltration for the Death Valley Region
Processes Controlling Net Infiltration
Development of a Conceptual Model of Net Infiltration
Precipitation
Evapotranspiration
Surface Water and Infiltration
Shallow Infiltration
Surface Water (run-on/runoff)
Redistribution and Net Infiltration
Processes at the Soil-Bedrock Interface
Estimation of Net Infiltration
Overview of Preliminary Net Infiltration Model (INFIL)
Model Grid
Model Inputs
Precipitation
Spatially Distributed Basin Properties
Soils
Rocks
Model Calculations
Energy Balance Calculations
Solar Radiation
Air Temperature
Calculation of Evapotranspiration
Calculation of Net Infiltration Using a Root-Zone Water Balance Model
Overview of the Root-Zone Water Balance
Daily Water-Balance Calculations
Soil and Bedrock Properties Used to Calculate Net Infiltration
Assumptions and Model Limitations
Model Results
Description of Preliminary Net Infiltration Model Results
Comparison of Preliminary Net Infiltration Model Results with Estimates of Spatially Distributed Recharge
Comparison of Preliminary Net Infiltration Model Results with Previous Basinwide Estimates of Recharge
Summary
References Cited
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