Scientific Investigations Report 2005–5288
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005–5288
Rapid growth and development in Carson Valley is causing concern over the continued availability of water resources as land presently used for agriculture is converted to residential and commercial use. Demand for ground water likely will increase in order to supply these areas. The effects of these changes on ground-water flow and flow in the Carson River are uncertain. The flow of the Carson River downstream of Carson Valley is important to water users dependent on water in the river for many varied uses.
The U.S. Geological Survey, in cooperation with Douglas County, Nevada, began a study in February 2003 to update estimates of Carson Valley’s water-budget components. This report presents and summarizes micrometeorologic, soil-chloride, and streambed-temperature data collected as part of the study and presents updated estimates of the rate of evapotranspiration (ET), recharge from precipitation, and streamflow infiltration and seepage. Micrometeorologic data were used to estimate annual and monthly ET rates for water year 2004 from irrigated pasture and alfalfa, and stands of native vegetation including greasewood, rabbitbrush, bitterbrush, and sagebrush. Soil-chloride data were used to estimate rates of recharge from precipitation on the northern and eastern sides of the valley. Streambed-temperature data were used to identify gaining and losing stream reaches and estimate rates of gain and loss for selected sites on the Carson River and irrigation canals and ditches. This information can be used to update estimates of the major water-budget components in Carson Valley, and to evaluate the effects of land- and water-use changes on the water budget.
ET rates from native vegetation and nonirrigated land during water year 2004 were considerably less than ET rates on land that was regularly irrigated. A stand of bitterbrush and sagebrush on an alluvial fan on the western side of Carson Valley where the depth to water is about 60 ft had the lowest ET rate of 1.5 ft/yr. Estimated ET from nonirrigated pasture was only slightly higher at 1.7 ft/yr. A stand of rabbitbrush and greasewood on the northern end of the valley had an estimated ET rate of 1.9 ft/yr. For comparison, the ET rate from most irrigated lands, both alfalfa and pasture, was about 3 ft/yr, but was greater than 4 ft/yr from irrigated pasture where the water table was 2 ft or less from land surface.
ET rates measured in water year 2004 probably are less than what would be measured in a year with average, or above average precipitation because water year 2004 was considerably drier than average. Water year 2004 was the sixth consecutive year of a drought with average or below average precipitation. During average or wet years, the water table beneath the sites likely would be shallower than the water table during 2004. The greatest ET rates were measured at site ET-8, where the depth to water was less than at the other sites; thus, the depth to water is an important factor controlling the ET rate.
The estimated uncertainty in ET rates was about 12 percent for sites ET-1, ET-2, ET-3, ET-5, and ET-8 where measured daily ET was available for most of water year 2004. The estimated uncertainty in the ET rates for sites ET-4 and ET-6 were +30 percent and –20 percent, and for site ET-7 was +50 percent and –40 percent.
Data from six soil-chloride test holes in areas of native vegetation on the northern and eastern sides of Carson Valley indicate that recharge from modern-day precipitation in these areas is not taking place because the precipitation that infiltrates below land surface is lost to ET by native plants. High concentrations of soil chloride at depths ranging from 4 to 18 ft below land surface in six test holes on the eastern side of Carson Valley indicate that modern-day precipitation does not percolate deeper than the roots of native vegetation. The presence of fine-grained semiconsolidated sediments at some test holes appears to limit the depth to which plant roots may penetrate. Estimates of the time required to accumulate the amount of chloride to depths of about 30 ft below land surface at the six test holes range from 3,000 to 12,000 years.
Data from two soil-chloride test holes near the northern end of the valley and one test hole on the eastern side of Fish Spring Flat indicate that a small amount of recharge from modern-day precipitation is taking place. Low concentrations of soil chloride in the three test holes indicate annual recharge from precipitation is 0.03 and 0.04 ft at the northern test holes, and is 0.02 ft on the eastern side of Fish Spring Flat. Estimates of the time required to accumulate the amount of chloride to depths of about 30 ft below land surface at the three test holes range from about 100 to 700 years. The uncertainty in the estimated recharge rates is about ±0.01 ft.
The two test holes near the northern end of the valley are in gravel and eolian sand deposits where precipitation infiltrates through the coarse-grained and well sorted sediments to depths below the root zone. Recharge from modern-day precipitation may be taking place at similar rates in other areas where gravel and eolian sand deposits are mapped. Based on results from the other test holes, it is unlikely that the recharge rate estimated for the test hole on the eastern side of Fish Spring Flat is applicable to a large area.
Data from 37 streambed-temperature sites indicate that the Carson River and irrigation ditches generally gain flow from ground water on the extreme western side of the valley and north of Muller Lane, and generally lose flow over the remainder of valley. Estimated infiltration rates at losing sites ranged from 1 to 4 ft/d, and estimated seepage rates at gaining sites may be about 1 ft/d. Extrapolating the estimated loss rates for a site to reaches more than 1 mi should be made with caution. An estimated seepage rate of 0.3 ft/d for the Carson River in the northern part of Carson Valley indicates a gain in streamflow from ground water that compares well with measured gain in streamflow during November 2003.
U.S.
Department of the Interior | U.S. Geological
Survey
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