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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5205

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Updated Ground-Water Budget for Basin-fill Aquifers of Carson Valley

An updated water budget for the basin-fill aquifers of Carson Valley was calculated using estimates of ground-water inflow and ephemeral runoff from the Carson Range and Pine Nut Mountains simulated from the watershed models, combined with estimates of other ground-water budget components made by Maurer and Berger (2007, p. 53). Estimates of mean annual ground-water inflow for water years 1990–2002 were assumed to be representative of water years 1990–2005, the period for which the other water-budget components were derived.

Mean annual ground-water recharge using ground-water inflow simulated from the watershed models ranged from 51,000 to 54,000 acre-ft (table 9). Estimates of mean annual ground-water discharge ranged from 44,000 to 47,000 acre-ft, including an increase of 3,000 acre-ft/yr from the previous estimate of net ground-water pumping by Maurer and Berger (2007, p. 42). Maurer and Berger (2007, p. 43) subtracted estimates of 3,000 acre-ft for return flow from ground water pumped for irrigation and for a U.S. Fish and Wildlife fish hatchery from total pumping for water years 1990–2005 to obtain estimates of net ground-water pumping. The return flow to irrigation ditches, however, is likely to be diverted further downstream in Carson Valley for irrigation. Thus, the return flow may effectively be removed from the hydrologic system prior to leaving Carson Valley and not be available for ground-water recharge. This is especially true during dry years when ground-water pumping for irrigation is greatest. For these reasons, estimates of net ground-water pumping for water years 1990–2005 were increased from the estimate made by Maurer and Berger (2007, p. 43; 15,000–18,000) by 3,000 acre-ft/yr, to range from 18,000 to 21,000 acre-ft.

Estimates of ground-water recharge using results of the watershed models were somewhat greater than estimates of ground-water discharge. The difference is likely because of the uncertainties in both estimates, rather than an actual difference between recharge and discharge. Estimates of ground-water inflow simulated from the watershed models were in close agreement with those obtained from the chloride-balance method (table 8). This provides more confidence that the relatively independent estimates from the chloride-balance method and simulated values from the watershed models provide a reasonably accurate volume for estimates of ground-water recharge components. The two estimates are not completely independent, however, because they use the same distribution of annual precipitation. The chloride-balance method uses physical data on runoff and the chloride concentration of precipitation, runoff, and ground water; whereas the watershed models use physical data on vegetation, soils, and topography, and daily data on runoff and climate.

The estimate for mean annual ground-water recharge at the low end of the range, 51,000 acre-ft, is similar to the estimate for mean ground-water discharge at the high end of the range, 47,000 acre-ft (table 9). Thus, a mean annual volume of about 50,000 acre-ft is a reasonable estimate for ground-water recharge to and discharge from the basin-fill aquifers of Carson Valley.

Estimates of infiltrated ephemeral runoff and mean annual ground-water inflow from the mountains simulated using the watershed models vary over an order of magnitude from dry to wet conditions from 7,800 to 76,000 acre-ft, respectively (table 6). Ground-water inflow is only one component of recharge for the ground-water budget, although it is the largest in magnitude. Ground-water recharge from runoff of the Carson River would likely be greater in wet years and lesser in dry years, but the magnitude of change is uncertain. Secondary recharge or return flows of pumped ground water from municipal and domestic pumping would likely not change significantly from wet to dry years. During multi-year drought conditions, the watershed simulations indicate that ground-water recharge could be as much as 80 percent less than the mean annual volume of 50,000 acre-ft.

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