Scientific Investigations Report 2007–5205
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5205
Rapid population growth (+49 percent from 1990 to 2000; Economic Research Service, 2003) and changing land use in Carson Valley, Douglas County, Nevada, is creating an increasing demand for potable water and concern over the continued availability of water to sustain future growth. Water- and land-use changes may alter the distribution and magnitude of ground-water recharge and discharge and consequently may alter flows in the Carson River, affecting water users downstream of Carson Valley, who depend on sustained river flow (fig. 1). As competition grows for limited water resources, water managers increasingly rely on the ground-water system to supply future water demand. Management of ground-water resources relies on reasonably accurate recharge rates, an important component of the ground-water budget; however, the commonly used methods to estimate recharge are limited by the scale of application (Cherkauer, 2004). A watershed-scale method for estimating ground-water recharge and other ground-water budget components uses process-based models that compute distributed water budgets for individual watersheds—a scale useful and familiar to water managers (Ely, 2006).
The U.S. Geological Survey (USGS) recently made estimates of water-budget components for basin-fill aquifers beneath the floor of Carson Valley (Maurer and Berger, 2007). A major water-budget component included ground-water inflow to the basin-fill aquifers of Carson Valley from the Carson Range and Pine Nut Mountains. Ground-water inflow was estimated to range from 22,000 acre-ft/yr using a water-yield method to 40,000 acre-ft/yr using a chloride-balance method (Maurer and Berger, 2007, p. 38).
Because of the relatively large range in these estimates and uncertainties in each method noted by Maurer and Berger (2007, p. 55), the U.S. Geological Survey, in cooperation with Douglas County, Nevada, and the Carson Water Subconservancy District, began a study in 2006 to update ground-water-budget estimates for Carson Valley using watershed models. Results of the models provide independent estimates of ground-water inflow to basin-fill aquifers underlying Carson Valley, and ephemeral runoff tributary to Carson Valley, both major components of the ground-water budget for Carson Valley.
The estimates of ground-water inflow simulated by the models provide information to update estimates of mean annual ground-water recharge to basin-fill sediments of Carson Valley. The estimate of mean annual ground-water recharge provides an estimate of the perennial yield of the basin-fill aquifers, defined by the Nevada Division of Water Planning (1992, p. 73) as:
“The amount of usable water from a ground-water aquifer that can be economically withdrawn and consumed each year for an indefinite period of time. It can not exceed the natural recharge to that aquifer and ultimately is limited to the maximum amount of discharge that can be utilized for beneficial use.”
Perennial yield typically is used by the Nevada State Engineer to determine the maximum limit of ground-water pumping allowed in a ground-water basin. However, recent publications have noted the inadequacy of using perennial yield as a limit to protect water resources (Bredehoeft, 1997; Sophocleous, 1997). The authors of these publications point out that the ultimate results of ground-water pumping are to increase, or induce, ground-water recharge, decrease ground-water discharge, or some combination of the two. Additionally, they state that streams and wetlands may be affected by ground-water pumping long before pumping reaches the volume of perennial yield. Prudic and Wood (1995, p. 10) have shown that in Carson Valley, pumping increases recharge through water losses from channels of the Carson River and irrigation ditches, and decreases ground-water discharge to the Carson River. These effects are caused by the hydraulic connection between the aquifer and surface-water flow created by the permeable sediments and shallow depth to water beneath much of the valley floor.
Bredehoeft (1997) and Sophocleous (1997) both note the utility of ground-water flow models to quantify the changes in recharge and discharge caused by pumping. Such a model is currently being developed for the basin-fill aquifers of Carson Valley by the USGS in cooperation with the Carson Water Subconservancy District. Results of the watershed modeling also provide a means to spatially and temporally distribute estimates of ground-water recharge near the boundaries of the numerical ground-water flow model. The model will provide a useful tool for the State Engineer and water planners to evaluate the ultimate effects of different ground-water management options on the water resources of Carson Valley.
This report documents the development and calibration of precipitation-runoff models for watersheds with perennial streams, and for watersheds with ephemeral streams in the Carson Range and Pine Nut Mountains and presents estimates of ground-water inflow to basin-fill aquifers of Carson Valley based on the model results. The model results were compared with previous estimates of ground-water inflow to Carson Valley, and used to update the ground-water budget for basin-fill aquifers of Carson Valley.
The precipitation-runoff models were developed using the Precipitation-Runoff Modeling System (PRMS: Leavesley and others, 1983) within the Modular Modeling System (MMS: Leavesley and others, 1996). Input data used in the models were daily precipitation and daily minimum and maximum air temperature from four National Weather Service Stations and one Natural Resources Conservation Service high-altitude station, land-cover and soils data, and slope, aspect, and altitude. Model output consisted of runoff, evapotranspiration, and ground-water inflow.
The precipitation-runoff models were calibrated against measured runoff for water years 1990–2001 and 1990–2002 for two gaged watersheds with perennial streams in the Carson Range, and for water years 1981–97 for two gaged watersheds with perennial streams in the Pine Nut Mountains. Runoff and ground-water inflow from 10 ungaged watersheds with perennial streams in the Carson Range were simulated using the model parameters from the calibrated models having similar bedrock geology for water years 1990–2002. The models of watersheds with ungaged perennial streams were calibrated against runoff estimated in an earlier study. The precipitation-runoff models were calibrated to match the full period of record for each respective watershed rather than using separate calibration and verification periods. This allowed for a direct comparison to the mean annual water budget values estimated from earlier studies.
Additionally, 11 watersheds with ungaged ephemeral streams were modeled, 10 in the Carson Range and 1 large aggregated area on the east side of Carson Valley, using the model parameters from the calibrated models of watersheds with gaged perennial streams having similar bedrock geology. For brevity, in the remainder of the report, the modeled watersheds will be referred to as perennial and ephemeral watersheds, although the term actually applies to the streams themselves. Simulated ground-water inflow and ephemeral runoff were used to update estimates of ground-water inflow from the Carson Range and Pine Nut Mountains. The updated estimates of ground-water inflow were combined with previous estimates of other ground-water recharge components, to obtain an updated ground-water budget for basin-fill aquifers of Carson Valley.
Carson Valley is primarily in Douglas County, Nevada, about 4 mi south of Carson City, Nevada’s capital (figs. 1 and 2). The southern end of the valley extends about 3 mi into Alpine County, California (fig. 2). The floor of the valley is oval-shaped, about 20 mi long and 8 mi wide, and slopes from an altitude of about 5,000 ft at the southern end to about 4,600 ft at the northern end. The Carson Range on the western side of the Sierra Nevada rises abruptly from the valley floor with mountain peaks ranging in altitude from 9,000 to 11,000 ft, whereas, the Pine Nut Mountains on the eastern side rise more gradually to peaks ranging in altitude from 8,000 to 9,000 ft.
The major towns in the valley are Minden and Gardnerville (fig. 2) with populations of 2,800 and 3,400, respectively (U.S. Census Bureau, 2003). Three subdivisions, Gardnerville Ranchos south of Gardnerville, and Johnson Lane and Indian Hills north of Minden, are growing rapidly, with populations of 11,000, 4,800, and 4,400, respectively (U.S. Census Bureau, 2003). In addition, development is increasing along the eastern and western sides of the valley, and on the valley floor on land that historically has been agricultural. Douglas County has grown from a population of about 28,000 in 1990 to 41,000 in 2000, an increase of 46 percent (Economic Research Service, 2003).
The Carson Valley study area is a subarea of the entire Carson Valley Hydrographic Area and includes the portion of the hydrographic area underlain by permeable materials capable of transmitting ground water to aquifers beneath the floor of Carson Valley (figs. 1 and 2). Along the southern boundary, the headwaters of the West and East Forks of the Carson River were not included in the study area because bedrock underlies the points where the West and East Forks of the Carson River cross the study area boundary, restricting ground-water inflow. The study area boundary covers 253,570 acres, or about 396 mi2.
The valley floor is covered with native pasture grasses, croplands of primarily alfalfa, and near the northern end of the valley, phreatophytes such as greasewood, rabbitbrush, and big sage. On the western side of the valley, bitterbrush and sagebrush cover steep alluvial fans, and manzanita and ponderosa pine cover the slopes of the Carson Range. Alluvial fans and foothills of the Pine Nut Mountains on the eastern side of the valley are covered with sage and rabbitbrush, and pinyon and juniper grow at high altitudes on the Pine Nut Mountains.
The distribution of surficial geologic units in Carson Valley is shown in figure 3. The geologic units of Stewart and Carlson (1978) were grouped into unconsolidated alluvial fan, gravel, eolian sand, and basin-fill deposits of Quaternary age, volcanic rocks and semiconsolidated sediments of Tertiary age, granitic rocks of Cretaceous age, and metamorphic rocks of Triassic to Jurassic age.
During the Cretaceous Period, 63 to 138 million years (m.y.) ago, the granitic magma of the Sierra Nevada pluton intruded into sedimentary and volcanic rocks of the Triassic and Jurassic Periods (138 to 240 m.y. ago). The resulting granodioritic and metavolcanic and metasedimentary rocks form the bulk of the Carson Range of the Sierra Nevada and the Pine Nut Mountains (fig. 3), and underlie the floor of Carson Valley (Moore, 1969, p. 18; Pease, 1980, p. 2). The Tertiary semiconsolidated sediments are exposed on the eastern side of the valley and likely also underlie Quaternary basin-fill sediments beneath the valley floor. Basin and Range faulting took place from 10 to 7 m.y. ago, producing the present topography of Carson Valley by uplifting the Carson Range and Pine Nut Mountains and downdropping the floor of Carson Valley (Muntean, 2001, p. 9).
The mountain blocks bounding Carson Valley are west-tilted structural blocks (Stewart, 1980, p. 113), with the valley occupying the downdropped western edge of the Pine Nut Mountains block (Moore, 1969, p. 18). A steep, well-defined normal fault creates a 5,000 ft escarpment along the Carson Range on the west, whereas a diffuse fault zone is found on the eastern side of the valley, dividing the Pine Nut Mountains block into several smaller blocks (fig. 3). Evidence of continued westward tilting is demonstrated by recent faulting along the base of the Carson Range (Pease, 1980, p. 15) and by displacement of the Carson River to the extreme western side of the valley (Moore, 1969, p. 18). A gravity survey by Maurer (1984) indicates that the depth to consolidated bedrock beneath the western half of Carson Valley is as great as 5,000 ft.
Carson Valley lies in the rainshadow of the Carson Range, with annual precipitation at the town of Minden on the valley floor averaging 8.4 in/yr (period of record 1971–2000; National Oceanic and Atmospheric Administration, 2002, p. 12). In contrast, the top of the Carson Range receives about 40 in/yr and the top of the Pine Nut Mountains receives from 15 to 18 in/yr (Maurer and Halford, 2004, p. 35). From 1984 to 1992 and from 1999 to 2004, conditions were dry with annual precipitation less than average (fig. 4A). The Palmer Drought Severity Index (PDSI; National Oceanic and Atmospheric Administration, 2006) is based on long-term weather conditions and provides a cursory indication of regional meteorological wet or dry periods (fig. 4C). The PDSI indicates that the longest recorded period of severe to extreme drought conditions was from 1999 to 2004.
The hydrology of Carson Valley is dominated by flow of the Carson River. The East and West Forks of the Carson River enter from the southern parts of the valley and flow northward to join near Genoa (fig. 2). The combined flow continues north to leave the valley southeast of Carson City. Flow of the Carson River is diverted across the valley floor through a network of canals and ditches for flood irrigation of crops and native pasture grasses. Twelve perennial streams drain the Carson Range, their flow reaches the valley floor even during extended periods of drought (fig. 3; Maurer and Berger, 2007, p. 36). Only two perennial streams, Buckeye and Pine Nut Creeks, drain the Pine Nut Mountains and their flow only rarely reaches the valley floor, becoming ephemeral a short distance downstream of their gaging stations (fig. 3) Ten ephemeral watersheds also drain the Carson Range, and a large area of ephemeral runoff is present on the eastern side of the valley. However, runoff from these ephemeral watersheds has not been measured.
Infiltration of surface water through streambeds and ditches and beneath flood-irrigated fields maintains a shallow water table less than 5 ft below land surface beneath much of the valley floor (fig. 2). Depth to the water table beneath alluvial fans on the western side of the valley quickly increases to greater than 200 ft within 1 mi of the valley floor, whereas depth to the water table on the eastern side of the valley reaches 200 ft about 3 mi from the valley floor (fig. 2).
Ground water flows downgradient from the Carson Range on the west and the Pine Nut Mountains on the east towards the Carson River on the valley floor. Beneath alluvial fans on the western side of the valley, ground water flows eastward and the gradient is about 100 ft/mi, whereas on the eastern side of the valley, ground water flows westward and the gradient ranges from 20 to 100 ft/mi (Maurer, 1986, p. 18). Beneath the valley floor, ground water flows toward the north and gradients range from about 100 ft/mi in the southwestern part of the valley to about 5 ft/mi in the northern part of the valley (Berger and Medina, 1999). Maurer (1986, p. 18), Maurer (2002, p. 10), and Maurer and Berger (2007, p. 57) present water-level data that indicate ground water flows from semiconsolidated sediments on the eastern side of the valley towards the valley floor.
Unconsolidated sediments that form the alluvial fans surrounding the valley, and that underlie the flood plain of the Carson River are the principal aquifers in Carson Valley (Maurer, 1986, p. 17). In the semiconsolidated Tertiary sediments, lenses of sand and gravel are the primary water-bearing units, and probably transmit most ground-water flow through the units. The consolidated granitic and metamorphic rocks forming the bulk of the Carson Range and Pine Nut Mountains are much less permeable to ground-water flow than other geologic units in Carson Valley. However, numerous wells have been drilled in the consolidated rocks that provide sufficient water for domestic use from fractured or weathered zones.
The components of the ground-water budget for basin-fill sediments of Carson Valley were delineated by Maurer and Berger (2007, p. 20). The major component of ground-water recharge to basin-fill sediments is ground-water inflow from the Carson Range and Pine Nut Mountains (fig. 5). Ground-water inflow was estimated by Maurer and Berger (2007) from the perspective of the basin-fill aquifers beneath the floor of Carson Valley. For this reason, the term ground-water inflow was used to describe ground-water flow from watersheds in the Carson Range and Pine Nut Mountains. Ground-water inflow from the Carson Range includes inflow from perennial and ephemeral watersheds, and inflow from infiltration of precipitation and ephemeral runoff on the western alluvial fans. Maurer and Berger (2007, p. 36) concluded that infiltration of perennial runoff on the western alluvial fans was negligible. Ground-water inflow from the Pine Nut Mountains includes ground-water inflow from perennial and ephemeral watersheds, and ground-water inflow from infiltration of ephemeral runoff. The term ground-water inflow is used throughout this report to remain consistent with previous descriptions of ground-water movement into the basin-fill aquifers of Carson Valley.
Such ground-water inflow is not strictly considered ground-water recharge because the flow does not cross the water table (Freeze and Cherry, 1979, p. 211). However, for the purposes of this study, which is focused on the basin-fill aquifers of Carson Valley, ground-water inflow entering basin-fill aquifers from the mountain blocks was considered ground-water recharge.
Maurer and others (2006, p. 28) used soil-chloride data to show that recharge from precipitation does not take place at most locations on alluvial fans and semiconsolidated sediments on the eastern side of Carson Valley. The soil-chloride data showed that ground-water recharge from precipitation does take place on eolian sand and gravel deposits in the northern part of Carson Valley (fig. 3), although at relatively low rates.
Ground-water inflow to the basin-fill deposits of Carson Valley from the Carson Range and Pine Nut Mountains was estimated by Maurer and Berger (2007) using a water-yield method and a chloride-balance method. The water-yield method uses an equation developed for nearby Eagle Valley (Maurer and Berger, 1997, p. 34) that computes estimates of water yield, defined by Maurer and Berger (1997) as the sum of runoff and ground-water inflow, from annual precipitation. The equation was applied to precipitation estimates for each watershed made by Maurer and Berger (2007, p. 29) using the linear relations of Maurer and Halford (2004) to estimate water yield from the perennial watersheds. The estimates of runoff from the perennial watersheds by Maurer and others (2004, p. 14) were subtracted from the computed water yield to obtain estimates of ground-water inflow.
The estimates of ground-water inflow from the water-yield method were combined with estimates of inflow from infiltration of precipitation and ephemeral runoff on the western alluvial fans to obtain the total ground-water inflow to basin-fill aquifers in Carson Valley (Maurer and Berger, 2007, p. 38). Maurer and Berger (2007) assumed that ephemeral runoff is largely lost to infiltration and that infiltration of ephemeral runoff becomes ground-water inflow to the basin-fill aquifers of Carson Valley. The total ground-water inflow to the basin-fill deposits of Carson Valley estimated using the water-yield method was 22,000 acre-ft/yr (Maurer and Berger, 2007, p. 40).
The chloride-balance method uses a chloride mass-balance equation (Wilson and Guan, 2004, p. 122; Maurer and Berger, 2007, p. 34). The method assumes that chloride deposited from precipitation is concentrated in the subsurface as water is lost to evapotranspiration (ET) in the mountains and on alluvial fans. The masses of chloride deposited by precipitation and removed in runoff are determined from the chloride concentrations of precipitation and runoff multiplied by their mean annual volumes. The mass of chloride from precipitation minus the mass of chloride in runoff is divided by the chloride concentration of ground water in basin-fill aquifers to estimate ground-water inflow. The estimate includes ground-water inflow from perennial and ephemeral watersheds and from infiltration of runoff and precipitation on the alluvial fans when the chloride concentration of ground water near the toe of the alluvial fans is used (Wilson and Guan, 2004, p. 123). Ground-water inflow to Carson Valley estimated using the chloride-balance method was 40,000 acre-ft/yr (Maurer and Berger, 2007, p. 40).
Other components of ground-water recharge estimated by Maurer and Berger (2007) include ground-water inflow to the northern end of Carson Valley from Eagle Valley, ground-water recharge from precipitation on eolian sand and gravel deposits, ground-water recharge from streamflow of the Carson River and irrigation ditches, and secondary recharge of pumped ground water (fig. 5). Using streambed temperature data, and data on ground-water levels compared to stream stage, Maurer and Berger (2007, p. 51) showed that the Carson River and irrigation ditches generally lose flow to ground-water recharge on the southern and southeastern parts of Carson Valley, and gain flow from ground-water discharge on the western and northern parts of Carson Valley. Components of ground-water discharge estimated by Maurer and Berger (2007) include ground-water discharge by evapotranspiration (ET) of phreatophytes, ground-water discharge to streamflow, and net ground-water pumping.
Water-budget estimates by Maurer and Berger (2007) were representative of water years 1990–2005. The estimates of ground-water recharge from discharge to the Carson River, net ground-water pumping, and secondary recharge of pumped ground water were made from analyses of streamflow gains and losses, and data on annual pumping averaged over the period 1990–2005. The estimates of ET were based on a land-use map developed from imagery collected in 2004 and updated by field checks for 2005. Estimates of ground-water inflow used mean annual runoff for 1990–2002, and mean annual precipitation for 1971–2000.
In this report, water-budget components simulated by the watershed models are mean annual values representative of water years 1990–2002, the period for which measured and estimated daily mean runoff data were available for model calibration. The estimates were combined with other water-budget components estimated by Maurer and Berger (2007) for water years 1990–2005, under the assumption that the two periods have similar hydrologic conditions. Mean annual precipitation at Heavenly Valley, near the crest of the Carson Range averaged 33.29 in. for water years 1990–2002 and 33.26 in. for water years 1990–2005. Similarly, mean annual precipitation at Minden, Nev., on the floor of Carson Valley averaged 8.72 in. for water years 1990–2002 and 8.67 in. for water years 1990–2005; indicating only a small difference in mean annual precipitation during the two periods.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5205
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