Scientific Investigations Report 2006–5305
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5305
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Two types of water budgets were developed for Carson Valley. The first type, an overall water budget, summarizes sources of inflow to and outflow from the valley and represents the available water resources of Carson Valley. The second type, a ground-water budget, summarizes sources of ground-water recharge and discharge and provides an estimate of the perennial yield of Carson Valley. Both types of water budgets were estimated for that part of Carson Valley underlain by Quaternary basin-fill deposits, the principal source of water for irrigation, municipal, and domestic water use (fig. 5). The area underlain by basin-fill deposits is the location of discharge of water by ET and the exchange of water between the surface-water irrigation system and basin-fill aquifers.
Overall water budgets were developed for two time periods, water years 1941–70, and water years 1990–2005. Water years 1941–70 represent conditions prior to increased population growth and ground-water pumping, and the importation of effluent. Water years 1990–2005 represent conditions under increased population growth that has caused changes in land and water use, increased ground-water pumping, and the application of effluent for irrigation. A ground-water budget was developed for water years 1990–2005, representing conditions under increased growth. Estimates for the ground-water budget components used an analysis of mean daily surface-water inflow to Carson Valley, including perennial streams tributary to the valley floor, and surface-water outflow from Carson Valley for water years 1990–2005. Mean daily surface-water data were not available for perennial streams tributary to the valley floor for water years 1941–70 and a ground-water budget was not developed for that period.
Components of the overall water budget and the ground-water budget are shown graphically in figures 9 and 10. Components of the overall water budget supplying inflow to basin-fill sediments in Carson Valley include streamflow of the East and West Forks of the Carson River and perennial streams tributary to the valley floor, precipitation on basin-fill deposits; ground-water inflow from the Carson Range, the Pine Nut Mountains, and beneath Clear Creek; and effluent imported from the Lake Tahoe basin (fig. 9). Components of outflow in the overall budget include streamflow of the Carson River, ET, and net ground-water pumping.
The Maxey-Eakin method was not used to estimate ground-water recharge in this study because of the many uncertainties in application of the method, as discussed previously. Instead, the ground-water budget was divided into separate components that could be estimated with varying amounts of uncertainty. A ground-water budget often contains large inherent uncertainties because ground-water recharge cannot be directly measured.
Components of the ground-water budget supplying recharge to basin-fill sediments include ground-water inflow from the Carson Range and western alluvial fans, the Pine Nut Mountains and eastern alluvial fans, and beneath Clear Creek; recharge from precipitation on Quaternary eolian sand and gravel deposits and on the western alluvial fans; recharge from streamflow losses, and secondary recharge of pumped ground water that percolates to the water table (fig. 10). Ground-water inflow is not strictly defined as recharge because such flow does not cross the water table (Freeze and Cherry, 1979, p. 211). However, for purposes of this report, which is focused on water budgets for the basin-fill aquifers in Carson Valley, ground-water inflow entering basin-fill aquifers beneath Clear Creek and from the mountain blocks and alluvial fans were considered ground-water recharge. Sources of ground-water discharge from basin-fill aquifers include ground-water ET from non-irrigated phreatophytes: rabbitbrush and greasewood, riparian vegetation, and non-irrigated pasture grasses; ground-water discharge as seepage into the Carson River; and net ground-water pumping.
Streamflow of the East and West Forks of the Carson River is gaged upstream of the Carson Valley boundary (fig. 2). Flow losses or gains likely are small along the East Fork Carson River between the gage and the Carson Valley boundary and there are no diversions of streamflow. Streamflow from the West Fork Carson River is diverted between the gage and the Carson Valley boundary to the Snowshoe Thompson Ditches 1 and 2 for irrigation in Diamond Valley (fig. 2). The diversions have been recorded by the Federal Watermaster’s Office since about 1993. Total diversions to the Snowshoe Thompson ditches averaged about 5,900 acre-ft/yr for the 11-year period for water years 1993–2003, with 3,600 acre-ft/yr diverted to Ditch 1 and 2,300 acre-ft/yr diverted to Ditch 2 (David Waltham, Federal Watermaster’s Office, written commun., 2004).
Return flow from Snowshoe Thompson Ditch 1 drains to Indian Creek, which enters Carson Valley about 1 mi west of the East Fork Carson River near the southeastern boundary of the study area. About one-half of the return flow from Snowshoe Thompson Ditch 2 also may drain to Indian Creek and about one-half may enter the study area’s southwestern boundary downstream of Diamond Valley near the California State line (Donald Callahan, Federal Watermaster’s Office, oral commun., December 2005). The volume of return flow entering Carson Valley from Diamond Valley is not known. Streamflow of Indian Creek was gaged near the downstream end of Diamond Valley and averaged about 7,500 acre-ft/yr for water years 1987–91 (station 10309030). This volume is greater than the average flow diverted to Snowshoe Thompson Ditches 1 and 2 for water years 1993–2003. Although data are not available for diversions to the Snowshoe Thompson ditches for 1987–91, this indicates that streamflow losses through Diamond Valley likely are minimal.
Streamflow also is diverted for irrigation of land near the West Fork between the gage and the study area boundary, and return flows enter the river upstream of the boundary. Consumptive use of these diversions is likely offset by inflow from unmeasured springs located near the West Fork about 0.5 mi downstream of Woodfords. In addition, the springs suggest that the nearby reach of the West Fork may gain flow. The volume of streamflow gain and spring flow is unknown (Donald Callahan, Federal Watermaster’s Office, oral commun., December 2005), but, along with return flow from Snowshoe Thompson Ditch 2, the volume of streamflow would tend to offset reductions in flow from the diversions. For purposes of the overall water budget, the decrease in inflow to Carson Valley from West Fork diversions between the gage and the boundary is assumed to be negligible relative to the total volume of inflow.
In addition to flow in the East and West Forks of the Carson River, two perennial streams cross the study area boundary, Indian and Clear Creeks (sites 16 and 1, respectively, table 3, fig. 5). Indian Creek was gaged near the study area boundary for 4 complete water years from 1995 through 1998 (station 10309035), a period of above average streamflow. Average flow for that period was 8,480 acre-ft/yr (Preissler and others, 1999, p. 143). The average flow entering Carson Valley from Indian Creek for water years 1940–2005 was estimated by multiplying 8,480 acre-ft/yr by 0.6, the ratio of average annual streamflow in the West Fork Carson River for water years 1940–2005 to that for water years 1995–98. The resulting flow for Indian Creek for water years 1940–2005 is about 5,100 acre-ft/yr. Streamflow of the West Fork Carson River for water years 1941–70 and 1990–2002 is only 0.2 to 0.6 percent less than that for water years 1940–2005 (table 1). Thus, the volume of 5,100 acre-ft/yr is a reasonable estimate for inflow of Indian Creek during those periods as well.
Streamflow from Clear Creek enters Carson Valley near the northwestern boundary and is gaged about 2 mi upstream of the boundary. Flow at the gage averaged 4,210 acre-ft/yr for water years 1990–2002 (Maurer and others, 2004, p. 14). Maurer and Thodal (2000, p. 13) developed a relation between streamflow at the gage and flow at the Carson Valley boundary, based on measurements of flow losses from 1996 to 1998. Applying that relation to flow of Clear Creek for water years 1990–2002, flow from Clear Creek that enters Carson Valley is about 3,400 acre-ft/yr. Clear Creek also drains the Carson Range and because of the similarity of average annual streamflow for the West Fork Carson River for water years 1940–2005, 1941–70, and 1990–2002, 3,400 acre-ft/yr is a reasonable average annual volume for streamflow of Clear Creek for those periods.
Maurer and others (2004) estimated daily mean flow of perennial streams in the study area draining the Carson Range and Pine Nut Mountains (fig. 5, table 3, drainages 2–15, 17–18) for water years 1990–2002. Flow of perennial streams was estimated using the gaged flow of Daggett (site 6), Fredericksburg Canyon (site 15), Pine Nut (site 17), and Buckeye (site 18) Creeks, and Miller Spring (site 12). Flow from 10 other perennial but ungaged creeks was estimated using multivariate regressions of more than 400 individual discharge measurements against selected continuously gaged streams in and near Carson Valley (Maurer and others, 2004, p. 8).
Springs along Foothill Road also are tributary to the valley floor, however, data on their flow are sparse. Springs about 1 mi south of Jobs Canyon Creek (site 11) were called Benson Springs (site 13) by Maurer (1986, p. 16) and Jackson Springs by Nevada Division of Water Resources (Beutner and Squatrito, 1998). Maurer (1986, p. 16) estimated their combined flow to be 2,400 acre-ft/yr for water years 1981–83, and the average flow from five measurements by Beutner and Squatrito (1998) during water year 1997 totaled about 1,900 acre-ft/yr, about 1.55 times that of Miller Spring in water year 1997. Assuming that the flow of Benson/Jackson Springs varies similarly to that of Miller Spring, the average flow of Miller Spring for water years 1990–2002, 630 acre-ft/yr was multiplied by 1.55 to obtain an average flow for Benson/Jackson Springs of about 1,000 acre-ft/yr for 1990–2002. The flow of Walleys Hot Spring was estimated to be about 700 acre-ft/yr by Maurer (1986, p. 16) and was assumed to vary little. The total ungaged flow of Benson/Jackson and Walleys Hot Springs is estimated to be about 1,700 acre-ft/yr for water years 1990–2002 (table 3).
For water years 1990–2002, flow of perennial streams from the Carson Range to the valley floor, including Clear and Indian Creeks, totaled about 31,000 acre-ft/yr (table 3). As for Indian and Clear Creeks, flow of tributary streams from the Carson Range was assumed to vary similarly to that of the West Fork Carson River and estimates for water years 1990–2002 also were reasonable volumes of inflow for water years 1940–2005 and 1941–70. However, flow of the West Fork Carson River was about 3 percent less for water years 1990–2005 than for water years 1940–2005 and 1990–2002. For this reason, total tributary inflow from the Carson Range estimated for water years 1990–2002 was decreased by 3 percent for water years 1990–2005, to obtain a representative volume of tributary inflow to the valley floor for that period (table 4). Flow of perennial streams from the Pine Nut Mountains in Buckeye and Pine Nut Creeks for water years 1990–92 totaled 1,360 acre-ft/yr, however, streamflow from the Pine Nut Mountains generally does not extend to the valley floor.
Flow of perennial streams tributary to the floor of Carson Valley varies considerably during extremely wet and dry periods. To illustrate the range in variability of this water resource, average flow of perennial streams was estimated for water years 1995–97 and 1990–92, representing wet and dry periods, respectively. The total flow volume of perennial streams tributary to the valley floor increased to about 51,000 acre-ft/yr during extremely wet periods and decreased to about 16,000 acre-ft/yr during extremely dry periods (table 3).
Flow estimates of streams tributary to the valley floor combined with flow of the East and West Forks of the Carson River results in a total surface-water inflow to Carson Valley of about 372,000 acre-ft/yr for water years 1940–71, and about 360,000 acre-ft/yr for water years 1990–2005 (table 4). The total surface-water outflow from Carson Valley is about 293,000 acre-ft/yr for water years 1940–71, and about 278,000 acre-ft/yr for water years 1990–2005. The decreased average flows for water years 1990–2005 compared to water years 1940–71 are likely the result of dry conditions from 1987 to 1992 and from 1999 to 2005.
Maurer and others (2006) showed that streamflow was lost to infiltration to the water table on the southern and eastern parts of the valley, whereas streamflow gains from seepage of ground water into the streambed of the Carson River and irrigation ditches takes place in the northwestern part of the valley. Streamflow gains and losses in the Carson Valley study area can be estimated using the difference between mean daily surface-water inflow to and outflow from Carson Valley. The total volume of the mean daily streamflow entering Carson Valley in the East and West Forks of the Carson River, Indian and Clear Creeks, and perennial streams and springs tributary to the valley floor was subtracted from mean daily streamflow leaving Carson Valley in the Carson River near Carson City for water years 1990–2002. The difference between surface-water inflow to and outflow from Carson Valley provides estimates of the volumes of streamflow gains and losses for the valley as a whole, and the periods of gains and losses during the year.
Gaged streamflow for the East and West Forks of the Carson River and the Carson River near Carson City, Daggett, Fredericksburg Canyon, and Clear Creeks, and Miller Spring are available, along with mean daily flows for ungaged perennial streams estimated by Maurer and others (2004) for water years 1990–2002. The mean daily flow for Benson/Jackson Springs was adjusted to water years 1990–2002 by multiplying mean daily flow of Miller Spring for water years 1990–2002 by 1.55, the ratio of flow recorded at Miller Spring in water year 1997 to flow estimated for Benson/Jackson Springs in water year 1997. The mean daily flow of Indian Creek for 1995–98 was adjusted by the ratio 0.6 of flow in the West Fork Carson River during 1995–98 to flow during 1990–2002, discussed previously. The mean daily flow of Clear Creek also was adjusted to provide estimates of flow at the Carson Valley boundary using the relation provided by Maurer and Thodal (2000, p. 13). Because streamflow from Pine Nut and Buckeye Creeks is lost to infiltration prior to reaching the valley floor, their streamflow was not included as flow tributary to the valley floor.
Close inspection of hourly flows recorded at the gaging stations on the Carson River shows that peak flows of the Carson River near Carson City lag from 16 to over 20 hours behind peak flows of the East and West Forks of the Carson River. Because mean daily flows were used to estimate streamflow gains and losses, the effect of the time lag depends on the time of day when peak flows occurred. Peaks flows on the East and West Forks of the Carson River that occurred from midnight to about 8 AM would be recorded at the gage near Carson City on the same day. In addition, the time lags between peak flows of ungaged perennial streams tributary to Carson Valley and the gage near Carson City are not known, but are likely less than 16 to 20 hours. For these reasons, unlagged mean daily inflow was subtracted from outflow, with the assumption that apparent streamflow losses caused by not considering a lag time were largely offset by streamflow gains on the following day.
Inflow to Carson Valley generally was greater than outflow from the valley from mid-March to mid-November (fig. 11). Outflow from Carson Valley was greater than inflow to the valley from mid-November through mid-March, except for short periods in December and January when the calculated mean daily flow was affected by large mean daily flows caused by floods. For water years 1990–2002, the volume of streamflow loss during summer months was about 89,000 acre-ft, and the volume of streamflow gain during winter months was about 16,000 acre-ft. For purposes of the ground-water budget, these volumes were assumed to be representative of water years 1990–2005.
The distribution of average annual precipitation for Carson Valley was estimated by Maurer and Halford (2004, p. 35) using two independent methods, resulting in a range of 250,000 to 270,000 acre-ft/yr for the study area. Both methods used data collected from 14 stations in and near Carson Valley adjusted to a common period, 1971–2000. The 14 stations were located on the valley floor, on the western and eastern alluvial fans, and at higher altitudes in the Pine Nut Mountains and near the crest of the Carson Range. The estimate of 250,000 acre-ft/yr was obtained using a distribution of precipitation developed by Daly and others (1994) for the Western United States called Precipitation-elevation Regressions on Independent Slopes Model (PRISM). The PRISM distribution overestimated precipitation on the eastern side of the valley by 80 to 90 percent, and so, was adjusted to match measured average annual precipitation at the 14 stations (Maurer and Halford, 2004, p. 32). The estimate of 270,000 acre-ft/yr was obtained using two linear relations between annual precipitation and altitude, one for the western and one for the eastern side of Carson Valley. The distribution of annual precipitation in Carson Valley obtained using the two methods are applied elsewhere in this report and will be referred to as the linear relations and adjusted PRISM precipitation distributions. Estimates of annual precipitation on selected areas of Carson Valley obtained from the two methods and used in calculations in this report are shown in table 5. The volumes of precipitation on selected areas estimated from both methods were used in this report for comparison with other water-budget components to evaluate which method may provide the most reasonable estimate of annual precipitation.
Inflow from precipitation to basin-fill sediments in Carson Valley from the Carson Range and the Pine Nut Mountains, is included in estimates of streamflow from those areas. Similarly, inflow from precipitation is accounted for in estimates of ground-water recharge from precipitation on Quaternary gravel and eolian sand deposits and the western alluvial fans. Maurer and others (2006, p. 32) concluded that in areas of alluvial fans and Tertiary sediments on the eastern side of Carson Valley, precipitation does not percolate to the water table to become recharge. Thus, inflow from precipitation is limited to that which falls directly on the basin-fill deposits.
Average annual precipitation on Quaternary basin-fill deposits, including areas of pasture grasses in Jacks Valley, ranges from 37,000 to 39,000 acre-ft/yr from the linear relations and the adjusted PRISM distributions, respectively, for water years 1971–2000 (table 5). The volume of 38,000 acre-ft/yr represents an average for precipitation on Quaternary basin-fill deposits estimated using the two methods. Average annual precipitation at Minden was 8.4 in. for water years 1971–2000, 8.3 in. for water years 1941–70, and 8.7 in. for water years 1990–2005 (National Oceanic and Atmospheric Administration, written commun., 2005). The differences in average annual precipitation at Minden represent a decrease of about 1 percent for water years 1941–70 and an increase of about 5 percent for water years 1990–2005. Given the relatively small difference between the two periods, the volume of 38,000 acre-ft/yr was assumed to reasonably represent precipitation on basin-fill deposits for both periods.
Maurer and others (2006, p. 32) estimated recharge from precipitation at nine sites on the eastern side of Carson Valley using soil-chloride data collected from boreholes (fig. 5). Results showed that recharge from infiltration of precipitation at two sites near the northern end of Carson Valley was 0.04 ± 0.01 ft/yr on Quaternary gravel deposits capping Indian Hills, and 0.03 ± 0.01 ft/yr on Quaternary eolian sand deposits generally north and east of Johnson Lane (fig. 5; Maurer and others, 2006, p. 32). Recharge from infiltration of precipitation in these areas can be calculated from the estimated rates and the mapped areas of Quaternary gravel and eolian sand deposits, assuming that recharge rates estimated at the soil-chloride sites are applicable over their entire areal extent (table 6). The total estimated recharge from precipitation over the combined areas of gravel and eolian sand deposits is relatively small, ranging from about 200 to 300 acre-ft/yr (table 6).
The potential for recharge from infiltration of precipitation on the western alluvial fans may be evaluated using annual ET rates estimated by Maurer and others (2006) for the stands of bitterbrush and sagebrush that cover the fans, and the annual precipitation on the fans estimated by the linear relations and adjusted PRISM distributions (Maurer and Halford, 2004). ET from bitterbrush and sagebrush in water year 2004 was estimated to be 1.5 ft/yr at a site near the western end of Centerville Lane (fig. 2; Maurer and others, 2006, table 2). Assuming ET rates were similar for the entire area of the alluvial fans, a rate of 1.5 ft/yr results in an ET volume of about 16,500 acre-ft/yr. Estimates of annual precipitation on the western fans from both precipitation distributions were less than 16,500 acre-ft/yr (table 7). Streamflow may supply water for ET by vegetation near the stream channels, but for most of the fan area, precipitation is the only available source of water for ET. This suggests that recharge from precipitation may not take place on the fans, the volume of ET is overestimated, the volumes of precipitation are underestimated, or some combination of the latter two.
Precipitation estimated from the adjusted PRISM distribution is significantly less than that estimated from the linear relations on the western alluvial fans (table 7). As will be discussed in the following section, precipitation estimated using the adjusted PRISM distribution appears to underestimate precipitation on the Carson Range. The adjusted PRISM distribution likely underestimates precipitation on the western alluvial fans as well. For this reason, estimates of recharge from precipitation on the western alluvial fans will be made using precipitation estimated from the linear relations.
One possible cause for overestimation of ET is that vegetation at the site where ET was estimated may not be representative of the entire area of the western alluvial fans. Bitterbrush at the site is 6 to 7 ft tall and quite vigorous, with plant density estimated to be about 35 percent (Maurer and others, 2006, p. 10). Plant vigor and density in other areas of the western alluvial fans may be less than that at the ET site. The ET rate of 1.5 ft/yr is near the high range of ET rates reported for xerophytic vegetation elsewhere in Nevada (table 8).
It is reasonable to assume that some recharge from precipitation takes place on the western alluvial fans because precipitation is greater on the fans than at the soil-chloride sites. Bitterbrush and sagebrush also are the predominant types of vegetation at the soil-chloride sites (Maurer and others, 2006, p. 24), and soils on the western alluvial fans also consist of coarse-grained sand and gravel. Assuming that recharge from precipitation on the western alluvial fans does take place, recharge was estimated using rates determined from the soil-chloride sites on Quaternary gravel and eolian sand deposits and, for comparison, using recharge rates as a percentage of precipitation.
Estimated recharge from precipitation on the western alluvial fans may range 300 to 400 acre-ft/yr from application of rates estimated for the gravel and eolian sand deposits, 0.03 to 0.04 ft/yr, to the area of the western alluvial fans (table 7). The volumes of 300 to 400 acre-ft/yr range from 2 to 3 percent of the precipitation on the fans estimated using the linear relations distribution. ET rates for bitterbrush and sagebrush, calculated as the difference between precipitation and recharge and divided by the area of the fans, is about 1.3 ft/yr, assuming no runoff from the fans. These rates are somewhat less than the rate of 1.5 ft/yr estimated by Maurer and others (2006), but may be more reasonable average ET rates for the entire area of the alluvial fans. However, the volumes of 300 to 400 acre-ft/ yr may be underestimated because precipitation rates are greater on the western alluvial fans than at the soil-chloride sites farther east.
Estimates of recharge on the western alluvial fans based on their greater rates of precipitation may be calculated using the percentage of precipitation that becomes recharge at the soil-chloride sites, from 4 to 5 percent (table 7). If 4 to 5 percent of the precipitation on the western alluvial fans is assumed to become recharge, recharge may range from 600 to 700 acre-ft/yr. The resulting ET rates for the fans calculated using these volumes is about 1.2 ft/yr, similar to those calculated using the direct application of the soil-chloride recharge rates. Because it is uncertain which method of estimating recharge on the western alluvial fans may be more reasonable, recharge from precipitation on the western alluvial fans is estimated to range from 300 to 700 acre-ft/yr.
Ground-water inflow to Carson Valley was estimated for the Carson Range and western alluvial fans, and the Pine Nut Mountains and eastern alluvial fans. Ground-water inflow from the area beneath Clear Creek was estimated previously by Maurer and Thodal (2000, p. 33 and 34). For the water budgets, ground-water inflow to Carson Valley beneath Clear Creek was assumed to range from 400 to 2,500 acre-ft/yr and averaged about 1,400 acre-ft/yr.
Estimates of ground-water inflow from the Carson Range include subsurface inflow from the perennial drainages, infiltration of precipitation on the western alluvial fans, and infiltration of streamflow across the western alluvial fans. Estimates of ground-water inflow from the Pine Nut Mountains were limited to subsurface inflow from the drainages of Buckeye and Pine Nut Creeks and infiltration of streamflow on the eastern alluvial fans.
Ground-water inflow was estimated using two independent methods. The first method combines estimates of subsurface inflow from perennial stream drainages of the Carson Range and Pine Nut Mountains using a water-yield equation derived by Maurer and Berger (1997), with estimates of recharge from infiltration of precipitation and streamflow. This method will be referred to as the water-yield method. The second method is referred to as the chloride-balance method; discussed by Wilson and Guan (2004) and used in other locations throughout Nevada (Dettinger, 1989; Berger, 2005). Subsurface inflow estimated using the chloride balance method incorporates recharge from precipitation and infiltration of streamflow on the alluvial fans.
The high rate of precipitation on the crest of the Carson Range provides a likely source for subsurface inflow from the mountain blocks to basin-fill aquifers on the floor of Carson Valley. Maurer and Berger (1997, p. 32) estimated more than 3,000 acre-ft/yr of subsurface inflow from similar granitic, metamorphic, and volcanic rocks in mountain blocks surrounding the much smaller area of Eagle Valley (fig. 1). Wilson and Guan (2004, p. 113–115) note the high potential for mountain block recharge to basin-fill aquifers in other semiarid settings.
The potential for subsurface inflow from the mountain blocks may be estimated by comparing the annual runoff from perennial stream drainages determined by Maurer and others (2004, p. 14), to the annual precipitation that falls on the drainages. Runoff as a percentage of precipitation estimated from linear relations ranges from about 20 to 75 percent for the perennial stream drainages on the western side of Carson Valley with the exceptions of the Stutler Canyon and Sheridan Creeks (sites 9 and 10, respectively, table 9, fig. 5). The low percentage of runoff from Stutler Canyon Creek, 11 percent, compared to the high percentage of runoff from Sheridan Creek, 93 percent, suggests that subsurface flow may take place from the upper part of the Stutler Canyon Creek drainage to the Sheridan Creek drainage. The Sheridan Creek drainage is small and its source of flow is a series of springs that issue from the base of a ridge between the two drainages (fig. 5). Because the upper part of the Stutler Canyon drainage bends to the west of and lies higher than Sheridan Creek, subsurface flow between the two drainages appears likely. For this reason, these two drainages were combined for estimates of subsurface inflow.
Runoff as a percentage of precipitation estimated from the adjusted PRISM distribution ranges from about 30 to more than 100 percent for drainages on the western side of Carson Valley (table 9). The volume of runoff is greater than the volume of precipitation for the Monument Creek (site 8) drainage, suggesting that either runoff is overestimated or precipitation is underestimated. Because the estimate of runoff is based on many individual measurements of streamflow, the precipitation volume from the adjusted PRISM distribution is most likely underestimated.
Runoff from gaged perennial stream drainages in Eagle Valley underlain by similar metamorphic and granitic rocks was reported by Maurer and Berger (1997, p. 32) to range from about 20 to 30 percent of precipitation. Runoff as a percentage of precipitation estimated from the linear relations was close to this range for many of the perennial stream drainages in Carson Valley but ranges from 45 to 74 percent for Water Canyon (site 2), Mott Canyon (site 7), Monument (site 8), and Fredericksburg Canyon Creeks (site 15). The drainages of Water Canyon and Fredericksburg Canyon Creeks are underlain by mixtures of metamorphic and granitic rocks. However, the drainages of Mott Canyon and Monument Creeks are underlain entirely by granitic rocks, as are other drainages with lower percentages of runoff. Thus, the type of bedrock does not appear to explain the differences in the percentage of runoff from precipitation for drainages in the Carson Range.
The drainages of Mott Canyon and Monument Creeks have significantly less conifer cover than the other drainages, 36 and 18 percent, respectively (table 9). Less conifer cover likely reduces the amount of ET from those drainages, increasing the amount of runoff relative to the other drainages. Less conifer cover in these two drainage may be caused, in part, by the Autumn Hills fire which burned the lower half of the drainages in June, 1996 (Michael Wilde, U.S. Forest Service, written commun., 2006). However, streamflow measurements of Mott Canyon Creek, on which the estimate of annual runoff was based, were made prior to the fire from the late 1980s to 1996 (Maurer and others, 2004, p. 11).
Runoff is 2 and 8 percent of precipitation estimated from the linear relations for Buckeye (site 18) and Pine Nut (site 17) Creeks, respectively, on the eastern side of Carson Valley. These lower percentages may be caused by the low amounts of precipitation in the Pine Nut Mountains. The low amounts of annual precipitation do not provide sufficient water in excess of ET to produce runoff comparable to the western side of Carson Valley. Runoff as a percentage of precipitation estimated from the adjusted PRISM distribution for Buckeye and Pine Nut Creeks was similar to that estimated using the linear relations because the volumes of estimated precipitation were similar.
Maurer and Berger (1997, p. 31 and 34) derived equations that predict runoff and water yield (the sum of runoff and subsurface flow) from annual precipitation for Eagle Valley, immediately north of Carson Valley and having a similar geologic setting and distribution of precipitation. Those equations were applied to drainages in Carson Valley to evaluate the estimates of precipitation and runoff, and estimate water yield and subsurface flow. Application of the equation predicting runoff from precipitation can be used to evaluate the precipitation estimates by solving the equation for precipitation using runoff determined for water years 1990–2002. This assumes that runoff estimates are more accurate than the estimates of precipitation.
Application of the equation produces annual precipitation rates comparable to those estimated from the linear relations distribution, with the exception of Mott Canyon and Monument Creeks (table 10). For those drainages, precipitation estimated using the equation was greater than that estimated from the linear relations distribution. This indicates that runoff from those drainages is a greater proportion of precipitation than for the drainages used to derive the equation in Eagle Valley. For the remaining drainages, the precipitation rates estimated using runoff were similar to those estimated from the linear relations. This indicates that precipitation estimated from the linear relations was consistent with the distribution of precipitation used to derive the equation and estimate water yield and subsurface flow in Eagle Valley. Annual precipitation rates were all greater than those estimated using the adjusted PRISM precipitation distribution, further indication that the distribution underestimates precipitation in the Carson Range. The adjusted PRISM distribution may underestimate precipitation on the steep eastern slope of the Carson Range because of the relatively large 1.9 mi grid used to develop the original PRISM distribution (Daly and others, 1994).
Application of the equation used to estimate runoff from precipitation that was estimated from the linear relations distribution produces runoff rates that were comparable to those determined for the drainages in Carson Valley, again with the exceptions of Mott Canyon and Monument Creeks. The estimated runoff was considerably lower for Mott Canyon and Monument Creeks (table 10), indicating that runoff from Mott Canyon and Monument Creeks was greater than runoff from the drainages in Eagle Valley used to derive the equation. In part, this may be caused by the relatively small amount of conifer cover for those drainages compared to the other drainages in Carson Valley (fig. 12). However, some drainages in Eagle Valley used to derive the equations had conifer cover of 20 to 30 percent (Maurer and Berger, 1997, p. 34). Runoff was relatively greater from the Mott Canyon and Monument Creek drainages likely because the underlying bedrock is less permeable or fractured. The Mott Canyon and Monument Creek drainages are not incised as greatly into the mountain block as the other drainages (fig. 12) further suggesting more competent and less fractured bedrock underlying the drainages.
The equation used to estimate water yield from precipitation was applied to precipitation estimated from the linear relations distribution, The resulting estimates of water yield were less than the runoff for Water Canyon, Mott Canyon, Monument, and Fredericksburg Canyon Creeks (table 11). Runoff was greater than 45 percent of precipitation from these drainages (table 9), and subsurface flow is assumed to be negligible, likely because of relatively impermeable bedrock underlying the drainages. Ephemeral stream drainages generally are not deeply incised into the mountain blocks, similar to Mott Canyon and Monument Creeks. For this reason, subsurface flow from the ephemeral stream drainages also was assumed to be negligible.
Subsurface flow from the remaining drainages was calculated by subtracting runoff from the estimated water yield. In the Carson Range, estimates of subsurface flow totaled about 2,300 acre-ft/yr (table 11). In the Pine Nut Mountains, estimates of subsurface flow from Pine Nut and Buckeye Creeks totaled about 4,300 acre-ft/yr. The total estimated subsurface inflow to the floor of Carson Valley is about 6,600 acre-ft/yr. The general agreement between runoff and precipitation rates estimated using the equation from Maurer and Berger (1997) for most of the drainages indicates that the resulting estimates of subsurface flow are consistent with estimates made for Eagle Valley and are considered to be reasonable approximations for the perennial stream drainages in Carson Valley.
The estimates of subsurface flow calculated using the water-yield equation from Maurer and Berger (1997) may be evaluated by combining the estimates with runoff, and subtracting the total from estimated precipitation to obtain a volume of water lost to ET. The ET volume divided by the precipitation volume and multiplied by 100 provides ET as percentage of precipitation. The volume of ET divided by the area of the drainage, provides an ET rate. The resulting rates and percentages can be compared with values reported in the literature.
ET rates were lowest, 0.6 to 1.2 ft/yr for Monument and Mott Canyon Creeks, respectively, but range from 1.4 to 1.6 ft/yr at the remaining drainages in the Carson Range (table 11). ET as a percentage of precipitation ranges from 25 to 48 percent at Monument and Mott Canyon Creeks, and from 55 to 67 percent at the remaining drainages in the Carson Range. The lower ET for Monument and Mott Canyon Creeks corresponds to the lower amount of conifer cover in the drainages. ET rates for Pine Nut and Buckeye Creeks were about 1 ft/yr and ET as a percentage of precipitation was 85 and 87 percent, respectively.
ET rates reported for Ponderosa Pine and pinyon and juniper in other areas are about 1.5 and 1.0 ft/yr, respectively (table 8). ET as a percentage of precipitation also is similar to studies reviewed by Wilson and Guan (2004, p. 120) in the Wasatch Mountains of Utah where ET was estimated to range from 44 to 53 percent of precipitation. However, ET averaged 83 percent of precipitation over 3 years where ET was estimated by micrometeorological measurements (Bossong and others, 2003, p. 37) for a forested watershed in Colorado. Brandes and Wilcox (2000, p. 966) listed ET calculated as the residual of precipitation minus runoff to range from about 80 to 95 percent of precipitation for three studies of watersheds vegetated by Ponderosa Pine in Colorado and Arizona.
Thus, ET rates calculated using the estimates of subsurface flow compare well with those reported in the literature, but ET as a percentage of precipitation appears to be somewhat lower for the Carson Range than found elsewhere. Uncertainty in estimates of ET as a percent of precipitation when calculated indirectly, and variation in ET over widespread locations may cause the differences.
An alternative method of estimating subsurface inflow from the mountain blocks is application of the chloride-balance method recently summarized by Wilson and Guan (2004, p. 122) and used elsewhere in Nevada (Dettinger, 1989; Berger and others, 2005). This method assumes that chloride in precipitation is concentrated as water is lost to ET in the mountain block, and that mountain block recharge may be estimated using the following equation (Wilson and Guan, 2004, p. 122):
R = (Cp P - Cr R)/Cg (1)
is annual recharge, in acre-ft/yr,
is the chloride concentration of precipitation, in mg/L, including wet fall (precipitation) and dry fall (dust),
is the annual precipitation, in acre-ft/yr,
is the chloride concentration of runoff from the mountain block, in mg/L,
is the annual runoff from the mountain block, in acre-ft/yr, and
is the chloride concentration of ground water near the mountain front, in mg/L.
Assumptions in the method are that precipitation and dry fall are the only source of chloride and that chloride is conservative, the chloride deposition and precipitation rate have been constant over the period of ground-water residence time within the mountain block, and that the chloride concentration of ground water represents the mean value of ground water that has been recharged from the mountain block (Wilson and Guan, 2004, p. 123).
For the Carson Range, ground water near the toe of the western alluvial fans has been recharged by a combination of subsurface inflow from the Carson Range, and infiltration of streamflow and precipitation on the fans. Application of equation 1 using the chloride concentration of ground water near the toe of the fans provides an estimate of recharge from all three sources. Precipitation estimated from the linear relations distribution was used because precipitation estimated from the adjusted PRISM distribution likely was underestimated for the Carson Range. The chloride concentration of precipitation was estimated to be0.5 mg/L based on analysis of 79 snow samples from the Sierra Nevada (Feth and others, 1964, p. 35). Data on the chloride concentration of runoff and ground water near the toe of the western alluvial fans were obtained from the water-quality database of the USGS (http://nwis.waterdata.usgs.gov/nv/nwis/qwdata).
The chloride concentration of ground water sampled from 23 wells near the toe of the western alluvial fans (fig. 5) ranges from 0.1 to 3.0 mg/L and averages 1.1 mg/L with a standard deviation of 0.7 mg/L. An additional potential source of water and chloride along the fans is leachate from septic tanks. The use of septic tanks is thought to have caused nitrate dissolved-solids concentrations to increase from 1985 to 2001 along the western side of Carson Valley (Rosen, 2003, p. 4). Along with nitrate, recharge of leachate from the septic tanks may supply an additional source of chloride to ground water at the toe of the fans. For many of the ground-water samples from the toe of the fans, nitrate concentrations also were analyzed and samples with nitrate concentrations greater than 1.0 mg/L were assumed to represent ground water recharged in part by septic tank leachate (Rosen, 2003, p. 2). Nitrate concentrations from a total of 75 samples from 23 wells were 1.0 mg/L or less and were used to calculate the average concentration of ground water near the toe of the western alluvial fans.
The chloride concentration of streamflow of the West Fork Carson River likely is representative of perennial streams draining the Carson Range. Eighty-five samples from the West Fork Carson River near Woodfords, collected from 1960 to 1994 ranged from < 0.1 mg/L (reporting limit) to 2.5 mg/L, and average 0.9 mg/L, with a standard deviation of 0.6 mg/L
A volume of 27,000 acre-ft/yr was obtained for subsurface inflow from the Carson Range using equation 1, and average chloride concentrations of precipitation, runoff, and ground water near the toe of the western alluvial fans (table 12). The chloride concentrations for runoff, ground water, and precipitation were varied over a reasonable range for measured values to assess the uncertainty in the estimate of subsurface inflow. A reasonable range for the chloride concentration of precipitation was determined by Maurer and others (2006, p. 29) to be 0.3 to 0.6 mg/L. Ranges for the chloride concentration of runoff and ground water of ±1 standard deviation were used, 0.6 and 0.7 mg/L, respectively. The volumes of precipitation estimated using the linear relations distribution and runoff are assumed to be reasonable, based on the application of equation for runoff from precipitation and water yield, as discussed previously. The resulting estimates of recharge from subsurface inflow have a considerable range; from about 9,600 acre-ft/yr for the low range of chloride concentration in precipitation to 73,000 acre-ft/yr for the low range in chloride concentration in ground water.
ET rates calculated from the difference in total precipitation minus runoff and estimated subsurface inflow, divided by the area of the Carson Range and western alluvial fans were used to evaluate the estimates of subsurface inflow from the chloride-balance method (table 12). ET calculated from estimates of subsurface inflow from 27,000 to 37,000 acre-ft/yr ranges from 1 to 0.8 ft/yr, near the low end reported in the literature for Ponderosa Pine and bitterbrush (table 8). ET from subsurface inflow of 73,000 acre-ft/yr was 0.02 ft/yr, indicating that the value was greatly overestimated. ET from subsurface inflow estimates of 9,600 to 16,000 acre-ft/yr were 1.4 and 1.3 ft/yr, respectively, comparing well with reported values. Assuming ET rates for the Carson Range may range from 0.8 to 1.4 ft/yr, a reasonable range for ground-water inflow from the Carson Range and western alluvial fans estimated using the chloride-balance method and constrained by ET rates was from about 10,000 to 40,000 acre-ft/yr.
The uncertainty in estimates for recharge from the Carson Range was compounded by results of Feth and others (1964, p. 43) who noted that the chloride concentration of some springs in the Sierra Nevada were lower than the average concentration of precipitation. They suggest that chloride may be removed from solution by adsorption, limiting the use of chloride as a geochemical tracer (Feth and others, 1964, p. 67). However, they further point out that the chloride concentration of snow ranged from approximately 0 to 1.6 mg/L. Thus, the low chloride concentration of some springs may be explained by recharge of snow with low chloride concentration directly to the aquifer, with no concentration by ET (Feth and others, 1964, p. 45). Such a process may explain some of the low concentrations measured in both ground-water and surface-water samples in Carson Valley. The use of average chloride concentrations of runoff and ground water along the toe of the western alluvial fans was assumed to account for differences caused by direct recharge along localized flow paths.
The chloride balance method also was applied to the Pine Nut Mountains where ground water on the eastern side of the valley has been recharged by a combination of subsurface inflow from Pine Nut and Buckeye Creek drainages, and infiltration of ephemeral streamflow. Recharge from infiltration of precipitation was thought to be minimal on the eastern side of the valley.
The average chloride concentration of ground water near the eastern side of the valley floor was determined from a total of 89 samples collected from 21 wells during 1983–2006. As for the western side of the valley, samples with nitrate concentration greater than about 1 mg/L were not included in the average. Welch (1994, p. 41) notes a difference in ground-water chemistry for wells near the Johnson Lane area compared with other parts of Carson Valley, likely caused by differences in underlying bedrock. For this reason, samples from within about 2 mi of the Johnson Lane area also were not included in the average. The average chloride concentration of ground water was 5.9 mg/L, ranging from a minimum of 3.7 mg/L to a maximum of 10.0 mg/L, with a standard deviation of 1.4 mg/L (table 13). Streamflow from the Pine Nut Mountains is lost to infiltration before reaching the valley floor, so the term for the volume of runoff is reduced to zero in equation 1. Precipitation from both the linear relations and adjusted PRISM distribution were used to estimate the volume of precipitation. As for the Carson Range, the chloride concentration of precipitation was varied from 0.3 to 0.6 mg/L and the chloride concentration of ground water was varied by ± 1 standard deviation to evaluate the uncertainty of the recharge estimates.
The resulting estimates of ground-water inflow from the Pine Nut Mountains using average chloride concentrations were the same, 11,000 acre-ft/yr, respectively, from precipitation estimated by the adjusted PRISM and linear relations distributions. Varying the chloride concentration of precipitation resulted in a range of estimated recharge from 6,700 to 13,000 acre-ft/yr, whereas varying the chloride concentration of ground water resulted in a range from 9,000 to 15,000 acre-ft/yr. All estimates resulted in an ET rate of about 0.9 ft/yr, slightly less than that estimated for stands of pinyon and juniper and sage brush near Tracy, Nevada by Thodal and Tumbusch (table 8; Thodal and Tumbusch, 2006).
Streams draining the mountain blocks may lose flow to infiltration as they cross the coarse-grained alluvial fans. Such losses are an important part of the water budget in many closed basins of Nevada (Meinzer, 1917, p. 78; Cohen, 1964, p. 44; Cooley, 1968). However, in many of those basins, the alluvial fans are much broader than those on the western side of Carson Valley and often streams become ephemeral before reaching the valley floor. On the western side of Carson Valley, most fans extend for distances of less than 1 mi, many streams are diverted near the top or middle of the alluvial fans into pipelines for irrigation application on the valley floor, and streamflow remaining after diversion continues across the alluvial fans to join the irrigation distribution system on the valley floor.
Available streamflow measurements indicate that streamflow losses to infiltration beneath perennial streams on the western side of the valley are small. Measurements were made to determine streamflow losses using standard flow-tracker and pygmy meters in the spring of 2005 on Jobs Canyon (site 11), Sheridan (site 10), and Barber Creeks (flow included with adjacent Sheridan Creek in table 3), which cross the longest reach of alluvial fan on the western side of Carson Valley, about 1.1 mi (fig. 5). Two repeat measurements were made in quick succession at two times during the day at sites near the head, middle, and toe of the fan, with flow rates of about 2 ft3/s at Jobs Canyon Creek and 1 ft3/s for the combined flow of Sheridan and Barber Creeks. The average flow difference for all measurements showed that Jobs Canyon Creek gained about 10 percent from the head to mid-fan, and lost about 5 percent from mid-fan to the toe of the fan, for an overall gain of about 5 percent. Similarly, the average flow measurements for the combined flow of Sheridan and Barber Creeks showed a gain of about 38 percent from the head to mid-fan, and a loss of about 3 percent from mid-fan to the toe, for an overall gain of about 35 percent. The accuracy normally applied to flow-tracker and pygmy meter measurements is 5 percent, so the small measured gain for Jobs Canyon Creek may not be meaningful. The measured gain for the combined flow of Sheridan and Barber Creeks likely is meaningful. However, the magnitude of the gain, about 0.3 ft3/s, is small and if it takes place year round, amounts to only about 200 acre-ft/yr.
Conditions on the Jobs Canyon Creek alluvial fan may not be representative of streamflow losses or gains across other alluvial fans on the western side of Carson Valley. However, measurements of streamflow losses or gains on other fans are difficult to accomplish because the streamflow of other perennial streams are all or partly diverted into pipelines relatively short distances downstream of the bedrock contact. For the purposes of this report, streamflow losses to infiltration and recharge to basin-fill aquifers beneath perennial streams on the western side of Carson Valley were assumed to be negligible. This is likely because, over time, infiltration losses have established a shallow water table beneath the streambed that limits infiltration losses. The gaged flow and individual measurements of perennial streams made since the early 1980s show that streamflow is maintained across the entire length of the alluvial fans even during extended droughts. This is not the case for ephemeral streams which flow for only short periods, and for perennial streamflow of Buckeye and Pine Nut Creeks draining the Pine Nut Mountains, which is lost to infiltration prior to reaching the valley floor.
Ephemeral streamflow is largely lost to infiltration during spring runoff and during large precipitation events. Such loss is supported by observations of a local rancher, who reported rapid infiltration losses from Water Canyon Creek. In 2004, streamflow of about 2 ft3/s from Water Canyon, which had been completely diverted to a pipeline for more than a year, was temporarily diverted back to the stream channel. Streamflow in the channel was completely lost to infiltration within a few hundred feet, and after two weeks, flow did not extend more than about 1,000 ft from the point of diversion (Loren Mernock, Manager, Ascuaga Ranch, oral commun., 2004). Thus, ephemeral streamflow likely infiltrates to the water table to become ground-water inflow to Carson Valley. Inspection of stream channels in ephemeral stream drainages supports this conclusion, in that most do not have active channels. Their channels often are vegetated with stands of bitterbrush and sage with an understory of grasses (fig. 13).
Based on unit-area runoff from perennial stream drainages, estimated ephemeral streamflow from the Carson Range is about 8,000 acre-ft/yr (Maurer and others, 2004, p. 14). Maurer and others (2004, p. 18) reported that the uncertainty associated with estimates of ephemeral streamflow is large, about 50 percent, based on application of the range in unit-area runoff from perennial drainages. Thus, ephemeral streamflow from the Carson Range may range from about 4,000 to 12,000 acre-ft/yr. Estimated in a similar manner, ephemeral streamflow from the Pine Nut Mountains is about 3,600 acre-ft/yr and may range from about 1,800 to 5,400 acre-ft/yr (Maurer and others, 2004, p. 14). The total ephemeral streamflow from the Pine Nut Mountains including the flow of Pine Nut and Buckeye Creeks was about 5,000 acre-ft/yr, ranging from 3,200 to 6,800 acre-ft/yr.
Estimates of ephemeral streamflow to Carson Valley totaled about 13,000 acre-ft/yr and may range from 7,000 to 19,000 acre-ft/yr (table 14). Flow in ephemeral channels during extreme storms may reach the valley floor and join flow of the Carson River. Flow during such storms was assumed to be small because the flow takes place for very short periods. During most periods of ephemeral streamflow, the flow infiltrates to recharge alluvial fan sediments and becomes ground-water inflow to basin-fill sediments.
The estimates of recharge from infiltration of streamflow and precipitation were combined with estimates of subsurface inflow from the perennial stream drainages to obtain the total ground-water inflow estimated using the water-yield method for comparison with that obtained using the chloride-balance method. The average estimate of ground-water inflow from the water-yield method was considerably less than that estimated using average chloride concentrations and the chloride-balance method for the Carson Range and western alluvial fans (table 15). Average estimates of ground-water inflow from both methods were similar for the Pine Nut Mountains and eastern alluvial fans. Differences in the low- and high-range estimates from the water-yield method largely were caused by the uncertainty in estimates of ephemeral streamflow. Total ground-water inflow to basin-fill sediments averaged 22,000 and 40,000 acre-ft/yr using the water-yield and chloride-balance methods, respectively, and low- and high-range estimates were from 15,000 to 58,000 acre-ft/yr including uncertainties.
The relative amounts of ground-water inflow from the California and Nevada portions of Carson Valley also is of interest to water planners in Douglas County (Daniel Holler, Douglas County Manager, oral commun., 2005). Estimates of ground-water inflow from the California portion of Carson Valley were made using estimates of streamflow from ephemeral stream drainages, precipitation on alluvial fans, and subsurface inflow from perennial stream drainages that lie within, or largely within, California (fig. 5).
Ephemeral stream drainages in the California portion of Carson Valley cover about 7,100 acres. The unit-area runoff from ephemeral stream drainages of the Carson Range was estimated to be 0.57 ft/yr by Maurer and others (2004, p. 17), resulting in runoff of about 4,000 acre-ft/yr from ephemeral stream drainages in California (table 16). The uncertainty associated with the unit-area runoff is about 50 percent (Maurer and others, 2004, p. 18) resulting in a range of 2,000 to 6,000 acre-ft/yr for ephemeral streamflow in California. Precipitation on alluvial fans in California from the linear relations distribution was about 6,200 acre-ft/yr (table 16). Applying rates of recharge from precipitation ranging from 0.03 to 0.05 ft/yr (see table 7) results in recharge estimates ranging from about 200 to 300 acre-ft/yr. Subsurface inflow estimated from Stutler Canyon, Sheridan, Jobs Canyon, Luther, and Fredericksburg Canyon Creeks totals about 1,300 acre-ft/yr (table 11). The combined 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 (table 16). These estimates compare well with the estimate of 7,000 acre-ft/yr for ground-water inflow across the State line made by Glancy and Katzer (1976, p. 51).
Effluent is imported to Carson Valley from Carson City and from three sources in the Lake Tahoe basin; the South Tahoe Public Utility District (STPUD) beginning in 1968, the Douglas County Sewer Improvement District #1 (DCSID), beginning in 1969, and Incline Village General Improvement District (IVGID) beginning in 1971. Beginning in 1988, Carson City began exporting effluent to Carson Valley for irrigation at the northern end of the valley. Total volumes imported have increased over time from about 3,000 acre-ft in the early 1970s to about 11,000 acre-ft/yr during the wet years of the mid-1900s, and decreased to about 8,600 acre-ft/yr in 2005 during dry years from 1999 to 2005. Average inflow of imported effluent for water years 1990–2005 is about 9,800 acre-ft/yr (table 17). For water years 1941–70, effluent imports to Carson Valley were negligible.
Effluent from the STPUD has been applied for irrigation in the Alpine County portion of southern Carson Valley since 1972 (fig. 14; Hal Bird, STPUD, written commun., 2005). Effluent from the DCSID (located at Zepher Cove, Nev.) has been imported to Carson Valley since 1968 (JWA Consulting Engineers, Inc. 2004, p. II-I). Effluent was discharged into Daggett Creek from 1968 to 1971 and into the Carson River immediately upstream of an irrigation diversion near Genoa Road from 1971 to 1981. Effluent was applied directly for irrigation along Genoa Road from 1981 to 1993. Beginning in 1993, the effluent was stored in a reservoir near the mouth of Buckeye Creek on the eastern side of Carson Valley and used for irrigation along Genoa Road, Muller Lane, and Stockyard Road (fig. 14). Effluent from the IVGID at Incline Village, Nev., has been exported to Carson Valley since 1971 (Harvey Johnson, IVGID, written commun., 2006). Effluent was discharged to the Carson River from 1971 to 1984 and used for irrigation in Jacks Valley. Beginning in 1984, wetlands were constructed north of Johnson Lane where effluent is stored and largely lost to evaporation with some used for irrigation near the wetlands and in Jacks Valley (fig. 14).
Based on data from the late 1990s up to 2004, about 80 percent of the imported effluent was applied for irrigation in Carson Valley, the remainder being lost to evaporation or infiltration beneath holding ponds (Hal Bird, STPUD, Harvey Johnson, IVGID; Kyle Menath, Carson City Utilities Department; Cindy Neissess, JWA Consulting Engineers, Inc., oral and written commun., 2006). Assuming this holds true for water years 1990–2005, the average volume applied for irrigation during that period was about 7,800 acre-ft/yr. In addition to imported effluent, about 1,700 acre-ft/yr of effluent generated within Carson Valley from the Minden-Gardnerville Sanitation District (MGSD) was applied for irrigation in 2005, however, records of the volumes applied prior to 2004 are not available (fig. 14; Frank Johnson, Minden-Gardnerville Sanitation District, written and oral commun., 2006). About 180 acre-ft/yr of effluent generated from Indian Hills was applied for irrigation of a golf course in 2005 (Andy Joyner, Indian Hills General Improvement District, written commun., 2006), and about 200 acre-ft/yr of effluent generated in the northern part of the valley was applied to the Incline Valley wetlands (Cathe Pool, Douglas County, written commun., 2006). Both of these sources of effluent were not applied for irrigation of pasture grasses or alfalfa. Thus, for water years 1990–2005, about 9,500 acre-ft/yr of effluent was applied for irrigation of pasture grasses or alfalfa and other crops in Carson Valley.
Ground-water pumping in Carson Valley has increased 3-fold from the early 1970s as development has increased the demand for water (fig. 15). Estimates of annual ground-water pumping in Carson Valley have been made for only 4 years prior to 1981; 1965 by Harrill and Worts (1968, p. 14, 18, 24, and 26), 1968 and 1969 by Walters and others (1970, p. 42), and 1971 by Glancy and Katzer (1976, p. 56 and 59). The estimates made by Harrill and Worts (1968) were for all of Douglas County, and may be reasonable for Carson Valley, assuming pumping in the Topaz Lake and Lake Tahoe areas of Douglas County was minimal in 1965. From 1981 to 1986, estimates of annual pumping were made by USGS studies (Maurer, 1986, p. 62–63; Berger, 1987, p. 14; Berger, 1990, p. 9). These estimates were made using power consumption records and measurements of volume pumped per kilowatt/hour for irrigation pumping, pumping reported by municipalities, and domestic house counts. Since 1987, annual pumping estimates have been made by the Nevada Division of Water Resources and Water Planning in a publication titled “Carson Valley Groundwater Pumping Inventory.” These estimates are made using data similar to USGS estimates for irrigation pumping including irrigated acreages, meters for municipal and other types of pumping where available, and well inventories for domestic pumping.
In the late 1960s and early 1970s, annual pumping ranged from less than 5,000 acre-ft/yr during years of above average precipitation (1969 and 1971) to about 10,000 acre-ft/yr during years of below average precipitation (1968) when ground water was pumped to supplement surface water for irrigation. In the late 1980s, annual pumping increased to about 20,000 acre-ft/yr during extended drought conditions, decreased to less than 20,000 acre-ft/yr during wet years from 1995 to 1998, and increased to greater than 30,000 acre-ft/ yr in the dry year of 2004. Total pumping for water years 1990–2005 averaged about 24,000 acre-ft/yr.
As reported by Clark (2005, p. 2), pumping by manner of use is divided into irrigation, municipal, domestic, “other,” commercial and stock water. Irrigation pumping has varied similarly to total pumping, increasing in dry years and decreasing in wet years, averaging about 9,100 acre-ft/yr for water years 1990–2005 (table 18). Municipal pumping has steadily increased from about 5,000 acre-ft/yr in the late 1980s to about 10,000 acre-ft/yr in 2004 and 2005. Similarly, domestic pumping has increased from about 1,400 acre-ft/yr in the mid-1980s to about 4,000 acre-ft/yr in 2005. Pumping in the “other” category has not changed significantly from 1987 to 2005 and averaged about 3,400 acre-ft/yr for water years 1990–2005. Pumping for the combined categories of commercial and stock use has decreased from about 500 acre-ft/yr in the late 1980s to about 150 acre-ft/yr in 2004 and 2005.
Ground water pumped for municipal use is partly removed from the hydrologic system by ET from lawn watering in summer months and by evaporation of effluent from holding ponds or by ET in areas where effluent is applied for irrigation. Ground water pumped for irrigation is partly consumed by ET, part may percolate to the water table, and part becomes return flow to the surface-water irrigation system. Ground water pumped for domestic use is partly lost to ET from lawn watering, part may percolate to the water table beneath lawns, and part returns to the water table by percolation beneath septic tanks. A large part of the ground water pumped for “other” use is for the U.S. Fish and Wildlife fish hatchery where no consumptive use takes place; the water passes through the facility to return to the irrigation system. The remaining water pumped for “other” uses, commercial use, and stock water is assumed to be lost to the hydrologic system.
The net volume of ground-water pumping was estimated from secondary recharge and return-flow rates reported in the literature. Using soil-chloride data, Maurer and Thodal (2000, p. 21 and 27) reported that secondary recharge (pumped ground water that percolates back to the water table) from lawn watering in Eagle Valley ranged from 0.4 to 1.0 ft/yr and that lawns cover about 40 percent of residential land. The land-use map for Carson Valley shows about 12,000 acres of residential use. Application of secondary recharge rates determined for Eagle Valley to 4,800 acres (40 percent of 12,000 acres) results in estimates of secondary recharge from municipal and domestic pumping ranging from 2,000 to 4,800 acre-ft/yr. Maurer (1997, p. 26) estimated that secondary recharge from septic tanks was about 0.15 acre-ft/yr per tank in the Dayton Hydrographic area, and data from Douglas County indicate effluent volumes of about 0.14 acre-ft/yr per lot in the northern part of the valley (Cathe Poole, Douglas County Utilities, written commun., 2006). A generalized estimate for effluent volume per domestic unit is 250 gal/d, or about 0.28 acre-ft/yr (Frank Johnson, Minden-Gardnerville Sanitation District, oral commun., 2006). A conservative value of 0.15 acre-ft/yr per septic tank was used to estimate secondary recharge from domestic pumping. The number of septic tanks in Carson Valley totals about 4,400 (Dawn Patterson, Douglas County Multi-Agency Geographic Information Center [MAGIC], written commun., 2006), thus, about 700 acre-ft/yr of the water pumped for domestic use may percolate back to the basin-fill aquifer. The combined secondary recharge from lawn watering in areas of residential use and from septic tanks ranges from about 3,000 to 6,000 acre-ft/yr (table 18).
Studies by Guitjens and others (1978, p. 14) in the 1970s indicate that from 30 to 50 percent of water applied for irrigation became return flow back to the surface-water irrigation system. Laser-leveling of fields and borders, begun in the 1980s, increased the efficiency of flood irrigation and likely reduced the amounts of return flow to 20 to 30 percent (Arlan Neil, Vada Hubbard, Natural Resources Conservation Service, oral commun., 2006). In addition, increasing costs for pumping of ground water also likely reduced the volumes of return flow from ground water pumped for irrigation. Because recent data are not available, return flow from application of ground water pumped for flood irrigation was assumed to be about 10 percent. Ground water pumped for sprinkler irrigation totaled about 1,200 acre-ft in 2004 (James Usher, Bently Agrodynamics, oral commun., 2006) with likely no return flow. Assuming this volume was similar for water years 1990–2005, ground water pumped for flood irrigation was about 8,000 acre-ft/yr. Applying a rate of 10 percent return flow to 8,000 acre-ft/yr pumped for flood irrigation results in a volume of about 800 acre-ft/yr (table 18). About 70 percent of the pumping in the “other” category is for the fish hatchery (Clark, 2005, p. 7), thus, about 2,400 acre-ft/yr of the volume pumped in the “other” category also becomes return flow. The average return flow for water years 1990–2005 totals about 3,200 acre-ft/yr.
The total volume of pumped ground water not consumptively used is from about 6,000 to 9,000 acre-ft/yr (table 18). Subtracting this from average annual pumping of about 24,000 acre-ft/yr leaves from 15,000 to 18,000 acre-ft/yr of net ground-water pumping for water years 1990–2005.
Net ground-water pumping for water years 1941–70 was difficult to accurately determine because data on ground-water pumping prior to 1970 are sparse. Pumping estimates for water years 1965, 1968, and 1969 averaged about 6,500 acre-ft/yr (table 18), however, this volume likely is greater than pumping in the 1940s and 1950s. The Nevada Division of Water Resources Driller’s log database shows only four irrigation wells drilled prior to the 1950s, and 14 drilled from 1950 to 1960. Glancy and Katzer (1976, p. 59) estimate pumping for irrigation in Carson Valley in the 1970s ranged from 10,000 acre-ft/yr in dry years to 3,000 acre-ft/yr in wet years, and was about 5,000 acre-ft/yr in average years. Assuming pumping for irrigation was about 2,500 acre-ft/yr for water years 1941–70 and that about 40 percent became return flow, net irrigation pumping was about 1,500 acre-ft/yr. Harrill and Worts (1968, p. 7) and Glancy and Katzer (1976, p. 56) show the population of Carson Valley to be about 3,000 in 1965 and 1971, but estimates of pumping for municipal and domestic use for those years were considerably different; about 1,200 acre-ft/yr in 1965 (Harrill and Worts (1968, p. 18) and about 600 acre-ft in 1971 (Glancy and Katzer, 1976, p. 56). Assuming municipal and domestic pumping was considerably less from 1941 to the late 1960s, it may have averaged about 500 acre-ft/yr for water years 1941–70. Assuming that most of the volume pumped for municipal and domestic use was lost to the hydrologic system, and including net irrigation pumping of 1,500 acre-ft/yr, the total net pumping from 1941 to 1970 was estimated to be about 2,000 acre-ft/yr.
Annual ET rates were estimated by Maurer and others (2006) using micrometeorologic measurements for vegetation types that included rabbitbrush and greasewood, flood-irrigated pasture grasses and alfalfa, and non-irrigated pasture grasses. ET rates were applied to mapped acreages of these vegetation types on the floor of Carson Valley (fig. 4) to obtain estimates of the annual volumes of ET.
The largest sources of ET in Carson Valley are from areas of pasture grasses, alfalfa, and rabbitbrush and greasewood (table 19). ET rates estimated by Maurer and others (2006, p. 22) for the water year 2004 include: 2.8, 3.2, and 4.4 ft/yr for three different stands of flood-irrigated pasture grasses; 1.7 ft/yr for non-irrigated pasture grasses; 3.1 ft/yr for flood-irrigated alfalfa where the depth to water was 3 to 6 ft below land surface; 3.0 ft/yr for flood-irrigated alfalfa where the depth to water was about 40 ft below land surface; and 1.9 ft/yr for rabbitbrush and greasewood. The highest rates for pasture grasses were obtained for a site where the depth to water ranged from 0 to 2 ft below land surface, compared to depths to water ranging from 2 to 5 ft below land surface at the two other stands of flood-irrigated pasture grasses, and 6 to 7 ft below land surface at the non-irrigated stand of pasture grasses (Maurer and others (2006, p. 9 and 22). Thus, the ET rate for pasture grasses likely is a function of the depth to water as previously noted by Nichols (2000, p. A10) for native phreatophytic shrubs and grasses.
The area of Carson Valley covered by pasture grasses generally corresponds to that part of the valley where depth to water is less than 5 ft below land surface (figs. 2 and 8). However, the depth to water changes seasonally and annually (fig. 8B), and the areal variation in depth to water is not known in sufficient detail to allow application of variable ET rates as a function of depth to water. For this reason, an annual ET rate of 3.0 ft/yr was applied to the entire area of pasture grasses, with the assumption that ET may be somewhat greater in parts of the valley with a shallow water table and somewhat less in parts of the valley with a deeper water table.
The land-use map also included areas described as wetlands, riparian vegetation of cottonwood and willow, and open water (table 19). In areas classified as wetlands, the water table likely is within about 2 ft below land surface, and an ET rate of 4.4 ft/yr was applied. The rate of 4.4 ft/yr is similar to a rate of 4.2 ft/yr estimated for bulrush marshes in Ruby Valley, northeastern Nevada by Berger and others (2001, p. 16). An ET rate of 3.5 ft/yr was estimated for willow using micrometeorological measurements near Weber Reservoir, about 40 mi east of Carson Valley (U.S. Geological Survey, 2006, station 390653118583901). Reported ET rates for cottonwood and willow determined largely in Arizona and New Mexico range from about 4 to greater than 5 ft/yr (Unland and others, 1998, p. 541; Scott and others, 2000, p. 244; and Dahm and others, 2002, p. 837). ET rates for cottonwood and willow likely are less in northern Nevada than these reported rates because of the shorter growing season. For this reason, the rate estimated for willow near Weber Reservoir, 3.5 ft/yr was applied to areas of cottonwood and willow in Carson Valley. For open water, an estimated evaporation rate of 5.0 ft/yr was applied as determined by Huntington (2003, p. 55) for Washoe Valley, Nev. about 12 mi north of Carson Valley. The area of open water, as determined from the imagery collected in July 2004, likely changes during the year, greatly increasing during spring runoff when large areas of the valley floor are flooded, and greatly decreasing during winter months when evaporation rates are low and areas of open water are limited to the major ditches that provide stock water. The area of open water determined for July of a dry year was assumed to represent an approximate average annual area.
The resulting volumes of ET total about 146,000 acre-ft/yr from areas of native phreatophytes, irrigated alfalfa, irrigated and non-irrigated pasture grasses, wetlands, cottonwood/willow, and open-water bodies. This volume of ET is derived from precipitation, streamflow of the Carson River and streams tributary to the valley floor, shallow ground water, ground water pumped for irrigation, and effluent applied for irrigation.
The volume of ET for water years 1941–70 was estimated from changes in the acreage of irrigated lands and native phreatophytes determined from a comparison of color infrared aerial photography taken in 1979 with the imagery collected in 2004 and land-use map updated to 2005 (fig. 16A and 16B). For the comparison, geo-rectified digital images were overlain on the computer screen to delineate areas of land-use change. The color infrared photography is the earliest available imagery that could be used to determine actively irrigated lands. Information on the date and year of well constructions in Carson Valley (Mimi Moss, Douglas County MAGIC, written commun., 2006) shows about 800 parcels were developed from 1970 through 1978. Thus, the estimated changes in acreages represent minimum values.
Areas of rabbitbrush and greasewood in 1979 that had been removed for agricultural, residential, and commercial use in 2005 totaled about 2,650 acres reducing ET by about 5,000 acre-ft/yr (fig. 17; table 19). About 2,200 acres of irrigated alfalfa and pasture grasses in 1979 were replaced by residential or commercial use in 2005. However, the decrease in ET from irrigated land was mostly offset by an increase in irrigated lands south and east of the Douglas County airport, and near the northern end of the valley, totaling about 2,100 acres (fig. 17). The additional irrigated land generally was sprinkler irrigated rather than flood irrigated as was acreage removed from irrigation from 1979 to 2005. Application rates using sprinklers were about 3.5 ft/yr, somewhat greater than the ET rates determined from micrometeorologic measurements (James Usher, Bently Agrodynamics, oral commun., 2006). The additional 0.5 ft/yr likely is lost to evaporation to the atmosphere during sprinkler irrigation.
Other land-use changes include about 900 acres of non-irrigated pasture grasses in 2005 that were irrigated in 1979, about 500 acres of non-irrigated pasture grasses in 2005 replacing what was rabbitbrush and greasewood in 1979, and about 250 acres of wetland areas in 2005 that were covered by rabbitbrush and greasewood in 1979. Changes in open-water areas include a reservoir of about 60 acres that was present along Muller Lane in 2005 and not in 1979, however, two reservoirs of about 130 acres on the eastern side of the valley that were used in 1979 were not used in 2005. In addition, in 2005 numerous ponds were present on residential areas scattered across the valley floor that were not present in 1979. These changes in open-water areas were assumed to result in similar areas of open water for 1979 and 2005 because the aerial photography from 1979 is not of sufficient detail to discern small open-water areas. Similarly, detail is lacking to discern areas of riparian vegetation in 1979. Thus, areas of cottonwood and willow also were assumed to be approximately the same in 1979 and 2005.
Application of ET rates to the areas of vegetation in 1979 results in a somewhat greater volume of ET, about 151,000 acre-ft/yr. The difference in ET between the two periods was relatively small because the decrease in irrigated pasture grasses and alfalfa cause by land-use change was offset by the increase in irrigated alfalfa near the airport and the northern part of the valley. The greatest change in ET between 1979 and 2005 was not the overall volume, but the source of water that supplied ET. From 1990–2005, ET was supplemented by application of about 9,500 acre-ft/yr of imported effluent and effluent generated within Carson Valley, rather than streamflow of the Carson River or ground water pumped for irrigation.
ET from non-irrigated vegetation including rabbitbrush and greasewood, riparian vegetation, and non-irrigated pasture grasses is supplied only from precipitation and ground water. ET from irrigated pasture grasses and crops in Carson Valley is supplied by a combination of precipitation, ground water, and surface water applied for irrigation. The shallow depth to water over much of the valley floor makes it difficult to determine whether the plants use streamflow applied for irrigation as it percolates through the unsaturated zone, or the streamflow infiltrates to the water table, recharging the basin-fill aquifers, and the plant roots tap the ground-water system.
Data on the relative contribution of ground water and streamflow applied for irrigation to ET in Carson Valley consist solely of that collected during studies by the Department of Agriculture of the University of Nevada, Reno, in the 1970s (Guitjens and others, 1976; 1978). The studies included measurements of the volumes of water applied for irrigation of pasture grasses and alfalfa fields and the volumes of surface-water return flow, or runoff from the fields, at three locations in Carson Valley (fig. 18).
The studies showed that the net water lost to infiltration during the 1974 and 1975 irrigation seasons totaled 3.1 and 3.6 ft for alfalfa and pasture grass near the northern end of the valley, respectively, where the water table ranged from 4 to 7 ft below land surface. Net water lost to infiltration was 2.4 ft for pasture grass near the west-central part of the valley where the water table ranged from 0.2 to 3 ft below land surface, and was 8.5 ft for alfalfa and pasture grass near the southern end of the valley where the water table ranged from 4 to 20 ft below land surface and soils are sandy (Guitjens and others, 1978, p. 14). ET rates for alfalfa and pasture grasses used in this report range from about 3 to as much as 4.4 ft/yr where the water table is shallow, less than about 2 ft below land surface (table 17).
The difference between water lost to infiltration determined by Guitjens and others (1978) and ET rates used in this report was small for the fields near the northern end of the valley, indicating that most water applied for irrigation likely was consumed by ET. Near the west-central part of the valley, water lost to infiltration was less than ET, indicating that the crops were supported by shallow ground water (Guitjens and Mahannah, 1972, p. 14). However, the areas and rates of ground-water contribution to ET are not known. Near the southern end of the valley, water lost to infiltration is 3-5 ft/yr greater than that required for ET. This indicates that considerable volumes of water applied for irrigation may be lost to infiltration and supply ground-water recharge in flood-irrigated areas where the water table is relatively deep and soils are sandy. Irrigated areas where the water table is from 5 to 20 ft below land surface are relatively small near the southern end of the valley and mostly lie on the eastern side of the valley (figs. 2 and 4). Presently, land on the southern end of the valley where the study by Guitjens and others (1978) was conducted, and much land on the eastern side of the valley is irrigated by sprinkler application rather than flood irrigation, so recharge from infiltration of water applied for irrigation in these areas likely is small. Recharge from flood-irrigation on the eastern side of the valley may take place, but data are not available to make estimates of this potential source of recharge.
Because of the uncertainty in the relative contributions of surface water and ground water to ET, ET derived from sources known with reasonable accuracy were used to estimate ET from the combined sources of streamflow and ground water (table 20). All sources of ET except those derived from a combination of streamflow and ground water (ET from wetlands and irrigated crops and pasture grasses) total about 67,000 acre-ft/yr (table 20). Subtracting this volume from the total ET of 146,000 acre-ft/yr, results in a volume of about 79,000 acre-ft/yr for ET derived from a combination of streamflow and ground water.
The locations of streamflow losses and gains from the Carson River and irrigation ditches and estimates of infiltration loss rates and seepage gain rates were made by Maurer and others (2006) using streambed-temperature data. That study showed that gaining and neutral reaches were found on the westernmost side of Carson Valley generally north of Muller Lane (Maurer and others, 2004, p. 34), South of Muller Lane, gaining sites generally were west of the West Fork Carson River. Losing reaches were found on the eastern side and southern end of the valley.
Measurements of ground-water levels in wells adjacent to stream channels relative to stream stage also provide data on the locations and potential rates of streamflow losses. These measurements were made at 23 sites in Carson Valley in the mid-1980s and in 2003–06 (fig. 18). However, many wells where measurements were made in the mid-1980s have been destroyed. For wells where data were collected only in the mid-1980s, the water-level difference for May 1985 is posted in figure 18. For wells where data were collected in the mid-1980s and in 2003–06, the average water-level difference for all measurements is posted in figure 18. By convention, a negative difference in water levels indicates stream stage is higher than the adjacent water table and streamflow loss to infiltration may take place. A positive difference indicates stream stage is lower than the adjacent water table and streamflow gain may take place from ground-water seepage into the streambed. The distribution of gaining and losing conditions shown by water-level differences is in general agreement with the distribution of gaining and losing stream sites determined from streambed-temperature measurements (fig. 18; Maurer and others, 2006, p. 34).
As shown in figure 18, stream stage ranges from 15 to 41 ft greater in altitude than adjacent ground-water levels in the southeastern part of Carson Valley. In the southeastern part of the valley, streamflow likely was lost to infiltration at greater rates than at sites near the center of the valley where water-level differences were negative (stream stage higher than ground-water level) and less than 1 ft. This is because the hydraulic gradient between the stream’s stage and ground water that drives flow between the two was small. The remaining factor that controls the rate of infiltration is the hydraulic conductivity of the streambed materials, which may vary across the valley floor. Infiltration rates calculated from temperature data ranged from about 2 to 3 ft/d for sites on the eastern side of Carson Valley (Maurer and others, 2006, p. 40). Stream lengths and widths determined from the imagery used to develop the land-use map were used to calculate the surface area of major irrigation ditches where the depth to water was greater than about 20 ft (fig. 18) in the southeastern part of the valley. The total surface area of the ditches is about 1.6 million ft2.
Application of infiltration rates from 2 to 3 ft/d to the area of 1.6 million ft2 results in estimates of streamflow infiltration losses ranging from 18,000 to 26,000 acre-ft/yr. This calculation includes the assumption that losses take place only during the irrigation season from March through September, a period of 210 days. Although streamflow often is present in these ditches during winter months for stock water, stream stage is lower than during the irrigation season, and infiltration rates would accordingly be less. The infiltration losses of 18,000 to 26,000 acre-ft/yr represent a minimum range because infiltration losses in other ditches in Carson Valley also may take place. However, the loss rates likely are less because of the relatively small hydraulic gradient in other areas. In addition, the location of such losses is uncertain and likely change during the irrigation season and from wet years to dry years. For these reasons, estimation of streamflow losses to infiltration in the remainder of the valley was not attempted. The streamflow losses to infiltration through irrigation ditches in part supplies water for ET, and in part may supply recharge to basin-fill aquifers.
An estimate of ground-water recharge from streamflow can be made by subtracting the volume of ET from the combined sources of streamflow and ground water (79,000 acre-ft/yr, table 19), from the volume of streamflow loss during summer months estimated from the difference between mean daily inflow and outflow for water years 1990–2005 (fig. 11; 89,000 acre-ft/yr). The difference between the volume of streamflow loss during summer months and the volume of ET derived from the combined sources of streamflow and ground water is 10,000 acre-ft/yr. This volume provides an estimate of streamflow losses that do not supply ET and is a minimum estimate of ground-water recharge from streamflow losses, assuming that the contribution of ET from ground water is minimal.
Rates of streamflow gain based on streambed temperature data were reported by Maurer and others (2006, p. 43) to range from 0.1 to 1.0 ft/d for strongly gaining reaches. A gain rate of 0.3 ft/d was estimated for the Carson River in the northern part of Carson Valley based on streamflow measurements (Maurer and others, p. 43). Gaining sites along the southwestern part of the valley may receive flow from the ground-water system, but this flow may be lost to ET after downstream application for irrigation. Streamflow gains from ground-water discharge that actually leave Carson Valley likely are limited to those downstream from Muller Lane on the main stem and the West Fork Carson River. The East Fork of the Carson River north of Muller Lane was not considered to be gaining because there are no data to support gaining conditions, and gaining sites south of Muller Lane generally are found within about 1 mi from the mountain front.
Stream lengths and widths determined from the imagery used to develop the land-use map were used to calculate the area of the Carson River and the West Fork Carson River for reaches north of Muller Lane (fig. 18), resulting in an area of 5.65 million ft2. Application of a gain rate of 0.3 ft/d to the stream area, and assuming that the rate of streamflow gain is constant throughout the year, results in a volume of about 14,000 acre-ft/yr.
As discussed previously, the outflow of the Carson River is a total of about 16,000 acre-ft/yr greater than inflow during the period from mid-November through mid-March (fig. 11). This volume represents the net streamflow gain for the valley as a whole during periods when precipitation rates are high and ET rates are low. However, temperature data collected by Maurer and others (2006) showed strongly gaining conditions in July, indicating that the northern reach of the Carson River may gain flow throughout the year. Despite such gains, the high rate of ET during summer months causes an overall loss of streamflow through the valley. Both methods of estimating streamflow gains by ground-water discharge produce similar volumes, from 14,000 to 16,000 acre-ft/yr. For purposes of the ground-water budget, 15,000 acre-ft/yr is assumed to represent a reasonable estimate for streamflow gains from ground-water discharge.
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