Scientific Investigations Report 2006–5305
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5305
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To address concerns over continued growth in Carson Valley, the USGS, in cooperation with Douglas County, began a study in February 2003 to update estimates of water-budget components in Carson Valley, Nevada and California. The estimates of water-budget components were updated using annual ET rates, rates of streamflow loss to infiltration and gain from ground-water seepage, and rates of recharge from precipitation, determined from data collected in 2003 and 2004 for this study and reported in the literature. Overall water budgets were developed for the area of basin-fill deposits in 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.
Annual ET rates were applied to areas of rabbitbrush and greasewood, bitterbrush, irrigated pasture grasses and alfalfa, non-irrigated pasture grasses, wetlands, open-water, and cottonwood and willow to determine the volumes of water discharged to the atmosphere. The areas covered by these types of vegetation were delineated from a land-use map of Carson Valley developed from imagery collected in July 2004 and used to estimate ET volumes for water years 1990–2005. Aerial photography from 1979 was used to estimate changes in land use between water years 1941–70 and 1990–2005 and estimate ET volumes for water years 1941–70.
Rates of streamflow loss and gain on the valley floor were applied to selected stream reaches to estimate ground-water recharge and discharge. The estimates of ground-water recharge and discharge were compared with the annual volumes of streamflow loss and gain determined from the long-term annual difference between mean daily inflow and outflow for water years 1990–2005.
Rates of recharge from precipitation were used to estimate recharge from precipitation on Quaternary gravel and eolian sand deposits, and the western alluvial fans. Estimates of ground-water inflow from the Carson Range and Pine Nut Mountains were derived from estimates of ephemeral streamflow, subsurface flow from perennial stream drainages estimated using a water-yield equation, referred to as the water-yield method, and the chloride-balance method.
The estimates of average inflow in the overall water budget total from 432,000 to 450,000 acre-ft/yr for water years 1941–70 and 430,000 to 448,000 acre-ft/yr for water years 1990–2005. The volumes were relatively similar, because variations in streamflow and precipitation are offset by imported effluent. Components of inflow included precipitation on basin-fill deposits of 38,000 for water years 1941–70, streamflow of the Carson River and tributaries to the valley floor of 372,000 acre-ft/yr for water years 1941–70 and 360,000 acre-ft/yr for water years 1990–2005, ground-water inflow ranging from 22,000 to 40,000 acre-ft/yr for both periods, and imported effluent of 9,800 acre-ft/yr for water years 1990–2005 with none imported for water years 1941–70. The flow of perennial streams tributary to the valley floor averaged about 32,000 acre-ft/yr for water years 1990–2005, but varies considerably, from 16,000 acre-ft/yr during dry years to 51,000 acre-ft/yr during wet years. Estimates of ground-water inflow from the California portion of Carson Valley average about 6,000 acre-ft/yr and range from 4,000 to 8,000 acre-ft/yr, comparing well with a previous estimate of ground-water inflow across the State line.
The estimates of outflow in the overall water budget total 446,000 acre-ft/yr for water years 1941–70, and 439,000 to 442,000 acre-ft/yr, for water years 1990–2005. Variations in ET and outflow of the Carson River were offset by the increase in net ground-water pumping for water years 1990–2005. Components of outflow include ET of 151,000 acre-ft/yr for water years 1941–70 and 146,000 acre‑ft/yr for water years 1990–2005, streamflow of the Carson River of 293,000 acre‑ft/yr for water years 1941–70 and 278,000 acre-ft/yr for water years 1990–2005, and net ground-water pumping of 2,000 acre-ft/yr for water years 1941–70, and 15,000 to 18,000 acre-ft/yr for water years 1990–2005. The decreased average flows for water years 1990–2005 compared to water years 1940–71 were likely the result of dry conditions from 1987 to 1992 and 1999 to 2005. The large volumes of inflow and outflow of the Carson River dominate the overall water budget.
Water levels in wells near the valley floor show little long-term rise or decline from 1977 to 2006, suggesting that this part of Carson Valley is in a state of approximate dynamic equilibrium. However, on the eastern side of Carson Valley in areas of increased growth and where recharge is limited to ground-water inflow from the Pine Nut Mountains, water levels show long-term water-level declines of 5 to 10 ft lower than during the previous dry period in early 1990s. Wet conditions during water year 2006 may cause these water levels to rise as increased recharge from the Pine Nut Mountains moves westward.
Analyses of precipitation and ET for the overall water budget have shown that precipitation estimated using the adjusted PRISM distribution underestimates precipitation on the mountain blocks, and that the volume of precipitation estimated from the linear relations distribution, 270,000 acre-ft/yr, provides a reasonable estimate.
Estimates of ground-water recharge for water years 1990–2005 range from 35,000 to 56,000 acre-ft/yr, and sources of ground-water discharge range from 41,000 to 44,000 acre-ft/yr. Components of ground-water recharge include ground-water inflow from the Carson Range and Pine Nut Mountains ranging from 22,000 to 40,000 acre-ft/yr, ground-water recharge from streamflow, a minimum value of 10,000 acre-ft/yr, and secondary recharge of pumped ground water that returns to the aquifer of 3,000 to 6,000 acre-ft/yr. Components of ground-water discharge include ground-water ET from native phreatophytes, riparian vegetation, and non-irrigated pasture grasses totaling 11,000 acre-ft/yr; ground-water discharge to streamflow of the Carson River, 15,000 acre-ft/yr, and net ground-water pumping, 15,000 to 18,000 acre-ft/yr.
Changes in land use between water years 1941–70 and 1990–2005 have decreased ET by about 5,000 acre-ft/yr. The increased application of effluent for irrigation between those years has decreased the use of surface water and ground water for irrigation by about 9,500 acre-ft/yr. The total decrease, about 15,000 acre-ft/yr, was approximately equal to the net ground-water pumping of 15,000 to 18,000 acre-ft/yr. The reduction in ET and in the use of streamflow and ground water for irrigation would tend to increase outflow of the Carson River from Carson Valley, offsetting the decrease in outflow caused by ground-water pumping without changes in land use predicted by Maurer (1986) and Prudic and Wood (1995).
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