Scientific Investigations Report 2006–5305

Scientific Investigations Report 2006–5305

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Rapid population growth and development in Carson Valley, west-central Nevada, is causing concern over the continued availability of water resources to sustain such growth into the future. As population growth continues, ground-water pumping will increase, land presently used for agriculture will be urbanized, and the effects of these changes on ground-water recharge and discharge are uncertain. These changes may affect outflow of the Carson River and, in turn, water users downstream of Carson Valley, who depend on sustained river flow (fig. 1).

In the early 1980s, the U.S. Geological Survey (USGS) estimated water-budget components for Carson Valley (Maurer, 1986). Major water-budget components included inflow from precipitation and from infiltration of streamflow, and outflow from evapotranspiration (ET) by plants and from ground-water seepage to the Carson River. Since that time, additional data have been collected on precipitation at stations in the Pine Nut Mountains and the Carson Range, and on streamflow from perennial streams tributary to the valley floor. In addition, new methods and instrumentation have been developed to estimate ET using micrometeorological measurements (Duell, 1990), to estimate recharge from precipitation using soil-chloride data and the chloride-balance method (Allison and Hughes, 1983; Dettinger, 1989), and to estimate rates of streamflow loss to infiltration and gain from ground-water seepage using streambed-temperature data (Constantz and Stonestrom, 2003).

To address concerns over continued growth, in February 2003 the USGS, in cooperation with Douglas County, Nev., began a study to update estimates of water-budget components in Carson Valley. As part of the study, three reports have been published. The first two used precipitation and streamflow data to develop updated estimates of the distribution of precipitation in Carson Valley (Maurer and Halford, 2004), and updated estimates of streamflow tributary to the floor of Carson Valley (Maurer and others, 2004). The third report used ET, soil-chloride, and streambed-temperature data collected in 2003 and 2004 to provide estimates of ET from various types of vegetation and land use, estimates of recharge from precipitation on the northern and eastern sides of the valley, and estimates of the location and rates of streamflow losses and gains from streams and irrigation ditches on the valley floor (Maurer and others, 2006).

A final phase of work to address concerns over continued growth and to determine the potential effects of changes in land and water use involves the development of a numerical ground-water flow model of Carson Valley. This work is currently underway as a cooperative study between the USGS and the Carson Water Subconservancy District. The model is planned to be used to evaluate and potentially refine estimates of water-budget components presented in this report, and to estimate the effects of changes in land and water use.

Purpose and Scope

The purposes of this report are to present updated estimates of water-budget components for Carson Valley and to evaluate the potential effects of changes in land and water use on water-budget components. Estimates of water-budget components were updated using annual ET rates, rates of streamflow loss to infiltration and gain from ground-water seepage, rates of recharge from precipitation, estimates of streamflow from perennial streams, volumes of streamflow into and out of Carson Valley, volumes of effluent imported into Carson Valley, and annual ground-water pumping. Overall water budgets were developed for the area of basin-fill deposits of Carson Valley for water years 1941–70 and for 1990–2005. A ground-water budget was developed for the same area for water years 1990–2005. Estimates of water-budget components were compared with previous estimates and the uncertainty of the estimates was evaluated.

The potential effects of land-use changes on water-budget components were evaluated using examples of existing changes and application of ET rates to estimate changes in ET volumes and potential changes in Carson River outflow from Carson Valley.

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