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Professional Paper 1805

Prepared in cooperation with the Bureau of Reclamation

Groundwater Discharge by Evapotranspiration, Dixie Valley, West-Central Nevada, March 2009–September 2011

By C. Amanda Garcia, Jena M. Huntington, Susan G. Buto, Michael T. Moreo, J. LaRue Smith, and Brian J. Andraski

Thumbnail of and link to report PDF (4.8 MB)Abstract

With increasing population growth and land-use change, urban communities in the desert Southwest are progressively looking toward remote basins to supplement existing water supplies. Pending applications by Churchill County for groundwater appropriations from Dixie Valley, Nevada, a primarily undeveloped basin east of the Carson Desert, have prompted a reevaluation of the quantity of naturally discharging groundwater. The objective of this study was to develop a revised, independent estimate of groundwater discharge by evapotranspiration (ETg) from Dixie Valley using a combination of eddy-covariance evapotranspiration (ET) measurements and multispectral satellite imagery. Mean annual ETg was estimated during water years 2010 and 2011 at four eddy-covariance sites. Two sites were in phreatophytic shrubland dominated by greasewood, and two sites were on a playa. Estimates of total ET and ETg were supported with vegetation cover mapping, soil physics considerations, water‑level measurements from wells, and isotopic water sourcing analyses to allow partitioning of ETg into evaporation and transpiration components. Site-based ETg estimates were scaled to the basin level by combining remotely sensed imagery with field reconnaissance. Enhanced vegetation index and brightness temperature data were compared with mapped vegetation cover to partition Dixie Valley into five discharging ET units and compute basin-scale ETg. Evapotranspiration units were defined within a delineated groundwater discharge area and were partitioned as (1) playa lake, (2) playa, (3) sparse shrubland, (4) moderate-to-dense shrubland, and (5) grassland.

Groundwater ET is influenced primarily by phreatophytic vegetative cover, salinity of soil and groundwater within the playa, depth to groundwater, solar radiation, and air temperature. The annual groundwater contribution to site‑scale ET ranged from 24 to 61 percent of total ET at vegetated sites and 4 to 15 percent of total ET at playa sites. Mean annual ETg from vegetated sites ranged from 53 millimeters (mm) (0.17 foot [ft], 7.3 percent vegetative cover) to 225 mm (0.74 ft, 24.8 percent vegetative cover). Cumulative liquid‑water fluxes in the unsaturated zone indicate that ETg at vegetated sites was influenced primarily by plant transpiration. Binary mixing analyses of oxygen-18 isotopes in groundwater and shallow soil water indicate that plants predominantly use groundwater throughout the year. Groundwater fractions in greasewood stem water varied seasonally and ranged from 0.63 to 1.0. Mean annual playa ETg ranged from about 11 mm (0.04 ft) at the inner playa site (near-surface volumetric water content of 37–53 percent) to about 20 mm (0.07 ft) at the outer playa site located within 2 kilometers of the playa edge (near-surface volumetric water content of 25–38 percent), but playa ETg estimates were within the probable error (plus or minus [±] 20–23 mm; 0.06–0.08 ft). Varying playa ETg was influenced predominantly by salinity rather than depth to groundwater. Osmotic resistance and physical impediments to ET (such as surface salt crusts and salt precipitate in the soil pore space) increased with increasing salinity toward the playa center, whereas vapor pressure decreased.

Mean annual basin-scale ETg totaled about 28 million cubic meters (Mm3) (23,000 acre-feet [acre-ft]), and represents the sum of ETg from all ET units. Annual groundwater ET from vegetated areas totaled about 26 Mm3 (21,000 acre-ft), and was dominated by the moderate-to-dense shrubland ET unit (54 percent), followed by sparse shrubland (37 percent) and grassland (9 percent) ET units. Senesced grasses observed in the northern most areas of the moderate-to-dense ET unit likely confounded the vegetation index and led to an overestimate of ETg for this ET unit. Therefore, mean annual ETg for moderate-to-dense shrubland presented here is likely an upper bound. Annual groundwater ET from the playa ET unit was 2.2 Mm3 (1,800 acre-ft), whereas groundwater ET from the playa lake ET unit was 0–0.1 Mm3 (0–100 acre-ft). Oxygen-18 and deuterium data indicate discharge from the playa center predominantly represents removal of local precipitation-derived recharge. The playa lake estimate, therefore, is considered an upper bound. Mean annual ETg estimates for Dixie Valley are assumed to represent the pre‑development, long-term ETg rates within the study area.

First posted October 2, 2014

Revised April 7, 2015

For additional information, contact:
Director, Nevada Water Science Center
U.S. Geological Survey
2730 N. Deer Run Rd.
Carson City, NV 89701
http://nevada.usgs.gov/water/

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Suggested citation:

Garcia, C.A., Huntington, J.M., Buto, S.G., Moreo, M.T., Smith, J.L., and Andraski, B.J., 2015, Groundwater discharge by evapotranspiration, Dixie Valley, west-central Nevada, March 2009–September 2011 (ver. 1.1, April 2015): U.S. Geological Survey Professional Paper 1805, 90 p., http://dx.doi.org/10.3133/pp1805.

ISSN 2330-7102 (online)



Contents

Abstract

Introduction

Description of Study Area

Groundwater Discharge by Evapotranspiration—Site Scale

Groundwater Discharge by Evapotranspiration—Basin Scale

Limitations of Methodology

Summary and Conclusions

Acknowledgments

References Cited

Appendix 1. Evapotranspiration and Micrometeorological Data for the Dixie Valley Study Area, Nevada, April 2009–September 2011

Appendix 2. Measured and Computed Soil Hydraulic Properties at Evapotranspiration Sites within the Dixie Valley Study Area, Nevada, and Unsaturated-Water Movement Equations

Appendix 3. Source Area Analysis for Evapotranspiration Sites within the Dixie Valley Study Area, Nevada, April 2009–September 2011

Appendix 4. Playa Groundwater-Level Data for the Dixie Valley Study Area, Nevada, April 2009–August 2011

Appendix 5. Playa Runoff Data for the Dixie Valley Study Area, Nevada

Appendix 6. Chamber Evaporation Data for the Dixie Valley Study Area, Nevada

Appendix 7. Description of Spatial Datasets Used to Calculate Basin-Scale Annual Groundwater Discharge Estimates by Evapotranspiration

Appendix 8. Playa Groundwater Discharge Determined from Analytical Hydraulic Calculations Based on Darcy’s Law in the Dixie Valley Study Area, Nevada


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