Rates of evapotranspiration, recharge from precipitation beneath selected areas of native vegetation, and streamflow gain and loss in Carson Valley, Douglas County, Nevada, and Alpine County, California

Rapid growth and development in Carson Valley is causing concern over the continued availability of water resources to sustain such growth into the future. A study to address concerns over water resources and to update estimates of water-budget components in Carson Valley was begun in 2003 by the U.S. Geological Survey, in cooperation with Douglas County, Nevada. This report summarizes micrometeorologic, soil-chloride, and streambed-temperature data collected in Carson Valley from April 2003 through November 2004. Using these data, estimates of rates of discharge by evapotranspiration (ET), rates of recharge from precipitation in areas of native vegetation on the eastern and northern sides of the valley, and rates of recharge and discharge from streamflow infiltration and seepage on the valley floor were calculated. These rates can be used to develop updated water budgets for Carson Valley and to evaluate potential effects of land- and water-use changes on the valley's water budget.

Data from eight ET stations provided estimates of annual ET during water year 2004, the sixth consecutive year of a drought with average or below average precipitation since 1999. Estimated annual ET from flood-irrigated alfalfa where the water table was from 3 to 6 feet below land surface was 3.1 feet. A similar amount of ET, 3.0 feet, was estimated from flood-irrigated alfalfa where the water table was about 40 feet below land surface. Estimated annual ET from flood-irrigated pasture ranged from 2.8 to 3.2 feet where the water table ranged from 2 to 5 feet below land surface, and was 4.4 feet where the water table was within 2 feet from land surface. Annual ET estimated from nonirrigated pasture was 1.7 feet. Annual ET estimated from native vegetation was 1.9 feet from stands of rabbitbrush and greasewood near the northern end of the valley, and 1.5 feet from stands of native bitterbrush and sagebrush covering alluvial fans along the western side of the valley. Uncertainty in most ET estimates is about 12 percent, but ranged from +30 and +50 percent to -20 and -40 percent for nonirrigated pasture and native bitterbrush and sagebrush. Estimated rates for water year 2004 likely are less than those during years of average, or above average precipitation when the water table would be closer to land surface.

Test holes drilled in areas of native vegetation on the northern and eastern sides of Carson Valley had high concentrations of soil chloride at depths ranging from 4 to 18 feet below land surface at six locations on the eastern side of the valley. The high chloride concentrations indicate that modern-day precipitation at the six locations does not percolate deeper than the root zone of native vegetation. Estimates of the time required to accumulate the measured amount of chloride to depths of about 30 feet below land surface at the six test holes ranged from about 3,000 to 12,000 years.

Low concentrations of soil chloride in two test holes on the northern end of Carson Valley and in a test hole on the eastern side of Fish Spring Flat indicate that a small amount of recharge from modern-day precipitation is taking place. Estimated annual recharge from precipitation at the two locations was 0.03 and 0.04 foot on the northern end of the valley and 0.02 foot on the eastern side of Fish Spring Flat. Uncertainty in the estimated recharge rates was about ?0.01 foot. Estimates of the time required to accumulate the measured amount of chloride to depths of about 30 feet below land surface at the three test holes ranged from about 100 to 700 years. The two test holes near the northern end of the valley are in gravel and eolian sand deposits and recharge from precipitation may be taking place at similar rates in other areas with gravel and eolian sand deposits. Based on results from other test holes, recharge at the rate estimated for the test hole on the eastern side of Fish Spring Flat is not likely applicable to a large area.

Data from 37 site