USGS

Estimates of Deep Percolation beneath Native Vegetation, Irrigated Fields, and the Amargosa-River Channel, Amargosa Desert, Nye County, Nevada

Open - File Report 03 - 104

By David A. Stonestrom, David E. Prudic, Randell J. Laczniak, Katherine C. Akstin, Robert A. Boyd, and Katherine K. Henkelman

Prepared in cooperation with the Nevada Operations Office, U.S. Department of Energy, under Interagency Agreement DE - AI08 - 96NV11967

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Abstract

The presence and approximate rates of deep percolation beneath areas of native vegetation, irrigated fields, and the Amargosa-River channel in the Amargosa Desert of southern Nevada were evaluated using the chloride mass-balance method and inferred downward velocities of chloride and nitrate peaks. Estimates of deep-percolation rates in the Amargosa Desert are needed for the analysis of regional ground-water flow and transport. An understanding of regional flow patterns is important because ground water originating on the Nevada Test Site may pass through the area before discharging from springs at lower elevations in the Amargosa Desert and in Death Valley. Nine boreholes 10 to 16 meters deep were cored nearly continuously using a hollow-stem auger designed for gravelly sediments. Two boreholes were drilled in each of three irrigated fields in the Amargosa-Farms area, two in the Amargosa-River channel, and one in an undisturbed area of native vegetation. Data from previously cored boreholes beneath undisturbed, native vegetation were compared with the new data to further assess deep percolation under current climatic conditions and provide information on spatial variability.

The profiles beneath native vegetation were characterized by large amounts of accumulated chloride just below the root zone with almost no further accumulation at greater depths. This pattern is typical of profiles beneath interfluvial areas in arid alluvial basins of the southwestern United States, where salts have been accumulating since the end of the Pleistocene. The profiles beneath irrigated fields and the Amargosa-River channel contained more than twice the volume of water compared to profiles beneath native vegetation, consistent with active deep percolation beneath these sites. Chloride profiles beneath two older fields (cultivated since the 1960’s) as well as the upstream Amargosa-River site were indicative of long-term, quasi-steady deep percolation. Chloride profiles beneath the newest field (cultivated since 1993), the downstream Amargosa-River site, and the edge of an older field were indicative of recently active deep percolation moving previously accumulated salts from the upper profile to greater depths.

Results clearly indicate that deep percolation and ground-water recharge occur not only beneath areas of irrigation but also beneath ephemeral stream channels, despite the arid climate and infrequency of runoff. Rates of deep percolation beneath irrigated fields ranged from 0.1 to 0.5 m/yr. Estimated rates of deep percolation beneath the Amargosa-River channel ranged from 0.02 to 0.15 m/yr. Only a few decades are needed for excess irrigation water to move through the unsaturated zone and recharge ground water. Assuming vertical, one-dimensional flow, the estimated time for irrigation-return flow to reach the water table beneath the irrigated fields ranged from about 10 to 70 years. In contrast, infiltration from present-day runoff takes centuries to move through the unsaturated zone and reach the water table. The estimated time for water to reach the water table beneath the channel ranged from 140 to 1000 years. These values represent minimum times, as they do not take lateral flow into account. The estimated fraction of irrigation water becoming deep percolation averaged 8 to 16 percent. Similar fractions of infiltration from ephemeral flow events were estimated to become deep percolation beneath the normally dry Amargosa-River channel. In areas where flood-induced channel migration occurs at sub-centennial frequencies, residence times in the unsaturated zone beneath the Amargosa channel could be longer. Estimates of deep percolation presented herein provide a basis for evaluating the importance of recharge from irrigation and channel infiltration in models of ground-water flow from the Nevada Test Site.

Contents

Abstract

Introduction

Purpose and Scope

Acknowledgments

Methods of Estimating Deep Percolation

Theory and Assumptions

Applicability of Assumptions

Collection and Analysis of Sediments

Analysis of Depositional Sources and Uncertainty

Atmospheric Deposition

Irrigation Water

Amargosa-River Water

Uncertainty in Deep-Percolation Estimates

Uncertainty in Atmospheric Deposition

Uncertainty in Agricultural Deposition

Uncertainty in Channel Deposition

Analysis of Unsaturated-Zone Profiles

Vertical Profiles of Sediments, Water Status, and Selected Anions

Sediments

Water Potential and Water Content

Chloride, Nitrate, and Sulfate

Cumulative Water and Cumulative Chloride

Cumulative Water

Cumulative Chloride

Cumulative Chloride versus Cumulative Water

Estimated Rates of Deep Percolation

Beneath Native Vegetation

Beneath Fields

Beneath the Amargosa-River Channel

As a Percentage of Irrigation

As a Percentage of Channel Infiltration

Summary and Conclusions

References Cited

Appendix A—Description of sediments

Appendix B—Water content and water potential of sediments, and selected chemical properties of water extracts

Appendix C—Chloride, sulfate, and nitrate concentrations in pore water


For additional information contact:

 

District Chief

U.S. Geological Survey

333 W. Nye Lane, Suite 203

Carson City, NV 89706-3810

 

Email: GS-W-NVpublic-info@usgs.gov

 

Copies of this report can be purchased from:

 

U.S. Geological Survey,

Information Services

Box 25286

Federal Center

Denver, CO 80225-0286

 


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