Scientific Investigations Report 2006–5305
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5305
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The most recent study that delineates the major water-budget components for Carson Valley was Maurer (1986), which includes estimates of ET, leakage from the Carson River and irrigation ditches to the water table and seepage from the water table back to the Carson River and irrigation ditches, ground-water recharge, and subsurface inflow. Estimates of the annual volumes of these components on the basis of a best-fit steady-state numerical model simulation are most clearly reported by Prudic and Wood (1995, p. 9) and summarized in table 2. In comparison, ground-water pumping in the early 1980s was relatively small and ranged from about 15,000 acre-ft/yr in dry years to about 7,000 acre-ft/yr in wet years (Maurer, 1986, p. 62–63).
The largest water-budget component is ET, or the discharge of ground water by native plants and irrigated crops, and evaporation from bare soil. Early studies used the term consumptive use which usually did not include estimates of evaporation. The earliest estimate of consumptive use in Carson Valley was made by Piper (1969, p. 7–8). He used estimates of surface-water runoff and gaged outflow of the Carson River to derive a volume of 77,200 acre-ft/yr for the depletion of surface water from consumptive use by plants. To this volume, he added 41,000 acre-ft/yr of annual precipitation on the area in Carson Valley at altitudes less than 4,800 ft, to obtain a total consumptive use of 118,000 acre-ft/yr. From estimates of the areas of native meadows and xerophytic vegetation prior to large-scale irrigation in Carson Valley, Piper (1969) estimated the increased consumptive use from irrigation to total 45,000 acre-ft/yr. Rabbitbrush, greasewood, pasture grasses, alfalfa, willow, and cottonwood are considered to be phreatophytic plants, meaning that their roots tap the water table, whereas plants such as bitterbrush and sagebrush are xerophytic, meaning that their roots do not tap the water table.
Walters and others (1970) applied an annual estimate of 2.5 ft for consumptive use by irrigated plants to derive a total of 110,000 acre-ft/yr. For native vegetation, consumptive use was estimated to be 24,000 acre-ft/yr, for a total consumptive use of 134,000 acre-ft/yr.
For the Nevada portion of Carson Valley, Glancy and Katzer (1976, p. 66) estimated 2,800 acre-ft of annual evaporation from surface-water bodies, and 80,000 acre-ft that included annual ET by crops and phreatophytes along with water consumptively used by ground-water pumping. The estimate of 80,000 acre-ft/yr was derived as the amount required to balance estimates of other components of inflow and outflow.
Spane (1977, p. 91) estimated annual ET to range from about 209,000 acre-ft in 1973, and 235,000 acre-ft in 1974. The estimates were based on mapped acreages of irrigated pasture and alfalfa, natural flood plain and phreatophytic vegetation, free-water surfaces, residential, and xerophytic vegetation. He used modified monthly pan-evaporation data and crop coefficients to obtain the total values.
Maurer (1986, p. 60) reported the total outflow from evaporation and ET was about 170,000 acre-ft/yr. This volume included 148,000 acre-ft/yr from phreatophytes and croplands, 2,800 acre-ft/yr from open-water evaporation, and 23,000 acre-ft/yr from xerophytic plants. The volume of 148,000 acre-ft/yr from phreatophytes and croplands was derived from a numerical ground-water flow model. The model applied maximum rates of ET ranging from 0.4 ft/yr in areas of sparse rabbitbrush, to 4.0 ft/yr in areas of irrigated pasture and alfalfa (Prudic and Wood, 1995, p. 7). The rates applied in the numerical model are a maximum where the water table is near land surface and decrease linearly to zero where the depth to water is equal to or greater than 35 ft (Maurer, 1986, p. 54).
Prudic and Wood (1995, p. 9), using the steady-state numerical ground-water flow model developed by Maurer (1986), reported annual ET from phreatophytes and irrigated crops to be 149,000 acre-ft/yr; slightly greater than that reported by Maurer (1986) due to differences in numerical rounding.
The interactions between ground water and surface water represent the next largest water-budget component. Previous estimates of leakage to ground water from the Carson River and irrigation ditches are sparse. Estimates of ground-water discharge by seepage to the Carson River and ditches are limited to those obtained from numerical modeling. Glancy and Katzer (1976, p. 34–35) discuss and present a hydrograph showing the monthly difference between surface-water inflow and outflow of the Carson River. The hydrograph shows that river outflow from Carson Valley is greater than inflow from November to March when ET is minimal, whereas inflow is greater than outflow from March to September when ET is greatest and when streamflow is applied for irrigation recharges the ground-water system. However, no volumetric estimates of net flow losses or gains were made. Spane (1977, p. 32–34) noted similar variations and calculated net losses of 52,000 and 58,000 acre-ft/yr in 1973 and 1974, respectively. Based on shallow water-level altitudes, Spane (1977, p. 144) stated that the Carson River gains flow along its entire length through Carson Valley, with the exception of the East Fork Carson River above Gardnerville, where streamflow is lost to infiltration.
Estimates of infiltration of surface water were made by Maurer (1986) and Prudic and Wood (1995) using a numerical ground-water flow model. Maurer (1986, p. 59–60) reported that the net annual infiltration of surface water from the Carson River was 44,000 acre-ft, with flow lost on the eastern and southern parts of the valley, and flow gained over the remainder of the valley floor. Using the same model, Prudic and Wood (1995, p. 9) reported average annual leakage from the Carson River and irrigation ditches was 105,000 acre-ft/yr, and average annual seepage from ground water back to the Carson River was 58,000 acre-ft/yr. The difference, 47,000 acre-ft/yr, is slightly greater than the net loss reported by Maurer (1986, p. 60), due to differences in numerical rounding. Prudic and Wood (1997, p. 10–11) describe losing streams and ditches near the southern end of the valley, and gaining streams and ditches at the northern end of the valley.
Estimates of recharge have been made by many studies in Nevada using the empirical method described by Maxey and Eakin (1949, p. 40). The method assumes that increasing percentages of precipitation becomes recharge for increasing amounts of annual precipitation, and was developed initially for 13 closed basins in eastern Nevada where precipitation is the sole source of water for recharge. The distribution of precipitation used was that of Hardman (1936). The method was later modified (Eakin, 1960, p. 12) by relating precipitation to altitude, and using altitude zones in place of the original precipitation zones. Subsurface inflow from the mountain blocks is assumed to be included in the estimate of recharge (Glancy and Katzer, 1976, p. 49).
Application of the Maxey-Eakin method involves many uncertainties. Descriptions of the method do not clearly state if recharge from infiltration of streamflow is included in the resulting estimate of recharge. However, inclusion of recharge from streamflow is implied when Eakin and Maxey (1951, p. 81) justify a larger amount of recharge to Ruby Valley, Nev., because the steep slopes in the basin “favor a high percentage of runoff to the area of recharge.” The exact “area of recharge” is not explicitly stated in descriptions of the method, and because the method was developed for entire basins, the accuracy of the method when applied to smaller portions of a basin is uncertain. The original recharge percentages have been modified by many workers for various reasons, including Glancy and Katzer (1976, p. 47–48) for Carson Valley, to account for greater amounts of precipitation. Glancy and Katzer (1976, p. 47) also state that the method only provides an estimate of potential recharge, because in areas like Carson Valley, where the water table is shallow and runoff from tributary streams joins the Carson River, not all estimated recharge reaches the ground-water reservoir. The accuracy of the method is uncertain when applied to precipitation distributions different than that originally used to develop the method (Maurer, 1997, p. 23). In addition, the method only provides an estimate of the total ground-water recharge to a basin, with no information on the areal distribution of recharge. The areal distribution of recharge is needed to evaluate the potential effects of ground-water pumping and changes in land use, and for development of numerical ground-water flow models.
Recharge from precipitation in Carson Valley has been estimated by several studies using the Maxey-Eakin method. All studies applied different recharge percentages and none explicitly stated if recharge included infiltration of streamflow. Recharge was first estimated (Nevada State Engineer, 1971, table 3, p. 5) to be 25,000 acre-ft/yr for the Nevada portion of the valley, and 3,000 acre-ft/yr was estimated for inflow from the California portion of the valley. The estimate included a note that part of the estimated recharge may be rejected with actual recharge being “somewhat” smaller (Nevada State Engineer, 1971, p. 40). Vasey-Scott Engineering (1974, p. 10–11) estimated that recharge was 27,400 acre-ft/yr for that part of the valley downstream of the Woodfords gaging station on the West Fork Carson River, and the Gardnerville gaging station on the East Fork Carson River (fig. 3). Glancy and Katzer (1976, p. 48) estimated 41,000 acre-ft/yr, and included the area downstream of the Markleeville gaging station on the East Fork Carson River (fig. 3). Using a precipitation distribution derived from 43 stations in eastern California and western Nevada, and including the Clear Creek drainage (fig. 3), Spane (1977, p. 143) obtained an estimate of 51,000 acre-ft/yr. Using Spane’s (1977) distribution of precipitation and not including the Clear Creek drainage, Maurer (1986, p. 35–36) obtained an estimate of 47,000 acre-ft/yr, and cautioned that the value was only a crude estimate of recharge from the mountain blocks surrounding the valley.
Estimates of subsurface inflow to Carson Valley are sparse. Vasey-Scott Engineering (1974, p. 11) cited a volume of 3,000 acre-ft/yr, reported by the Carson River Basin Council of Governments, for subsurface inflow to Carson Valley from “upstream areas”; the exact location of these areas is unclear. Glancy and Katzer (1976, p. 51) estimated subsurface inflow across the State line from the portion of Carson Valley in Alpine County to be 7,000 acre-ft/yr, and estimated inflow beneath the channel of the East Fork Carson River at the State line to be 150 acre-ft/yr. Maurer and Thodal (2000, p. 33) estimated that 400 acre-ft/yr of subsurface flow enters Carson Valley beneath Clear Creek on the northern end of the valley. Maurer and Thodal (2000, p. 34) also estimated that 2,500 acre-ft/yr could flow beneath the ridge separating the upper part of the Clear Creek drainage and Jacks Valley into Carson Valley, but note that such flow is not confirmed. Maurer (1986, p. 60) estimated subsurface inflow from the mountain blocks surrounding Carson Valley to range from 37,000 to 57,000 acre-ft/yr, but considered 37,000 acre-ft/yr to be the more reasonable volume. The volume of subsurface flow (55,000 acre-ft/yr) reported by Prudic and Wood (1995, p. 9) includes leakage from small perennial and ephemeral streams.
Subsurface outflow from Carson Valley to the Dayton Valley Hydrographic Area likely is small. Glancy and Katzer (1976, p. 51) estimated 15 acre-ft/yr of subsurface flow beneath the channel of the Carson River at the gaging station near Carson City (fig. 2), a negligible volume for purposes of evaluating the water-budget components of Carson Valley. Maurer (1997, p. 31) noted a lack of water-level data east of Hot Springs Mountain that would confirm a hydraulic gradient from that area towards the Dayton Valley Hydrographic Area, but also noted that water levels in wells near the Carson River were lower than river stage; indicating subsurface flow across the divide and toward the river is minimal.
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