Scientific Investigations Report 2006–5305

Scientific Investigations Report 2006–5305

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Water Budgets

The estimates of water-budget components developed in the previous section were compiled into overall water budgets for water years 1941–70 and 1990–2005, and a ground-water budget for water years 1990–2005. Components of the ground-water budget were compared to previous estimates, and the relative uncertainty of the components was discussed.

Overall Water Budget

Sources of inflow in the overall water budget include streamflow tributary to the floor of Carson Valley, precipitation on Quaternary basin-fill sediments, ground-water inflow from the mountain blocks and alluvial fans, and effluent imported from outside the basin. Sources of outflow include streamflow of the Carson River that leaves the valley, ET, and net ground-water pumping. Differences in streamflow, precipitation, effluent imports, and ground-water pumping for two periods, water years 1941–70 and 1990–2005, were determined from the long-term records and estimates of such data. Differences in ET for the two periods were estimated from application of differing ET rates to areas where land use has changed from agricultural or from phreatophytic vegetation (rabbitbrush and greasewood) to residential or commercial land use between the two periods (see section titled “Evapotranspiration”). The volumes of ground-water inflow for the two periods likely are not significantly different and the same estimates of ground-water inflow were used for each period.

The combined estimates of inflow, including the range in estimated ground-water inflow, total from 432,000 to 450,000 acre-ft/yr for water years 1941–70 and 430,000 to 448,000 acre-ft/yr for water years 1990–2005. Estimated volumes of inflow were similar for the two periods because a decrease in streamflow was offset by an increase in imported effluent (table 21). The combined estimates of outflow total 446,000 acre-ft/yr for water years 1941–70, and 439,000 to 442,000 acre-ft/yr, for water years 1990–2005. Again, decreases in ET and outflow of the Carson River were offset by the increase in net ground-water pumping. The greater volume of ground-water inflow using the chloride-balance method was closest to estimates of outflow; less than 1 percent of outflow for both periods. However, the lesser volumes of ground-water inflow estimated using the water-yield method also were relatively close, within 2 to 3 percent of outflow, for both periods. The large volumes of inflow and outflow of the Carson River dominate the overall water budget.

The overall water budget illustrates that the major differences in the overall water budget for the two periods was the increased use of effluent for irrigation, increased net ground-water pumping, and changes in land use that replaced native phreatophytes and irrigated lands with residential or commercial land use. Application of 9,500 acre-ft/yr of effluent in water years 1990–2005 decreased the volume of streamflow and ground water applied for irrigation for that period compared to water years 1941–70 by 9,500 acre-ft/yr. Changes in land use for water years 1990–2005 reduced the annual volume of ET by about 5,000 acre-ft/yr. Combining that change with the application of 9,500 acre-ft/yr of effluent for irrigation resulted in an overall decrease of about 15,000 acre-ft/yr, approximately equal to the net ground-water pumping of 15,000 to 18,000 acre-ft/yr. The decrease in ET and in the use of streamflow and ground water for irrigation would tend to increase outflow of the Carson River from Carson Valley, offsetting the decrease in outflow caused by ground-water pumping without changes in land use predicted by Maurer (1986) and Prudic and Wood (1995).

Ground-Water Budget

Ground-water inflow from Eagle Valley and the Carson Range and Pine Nut Mountains was estimated to range from about 22,000 to 40,000 acre-ft/yr using average values for inflow estimates (table 15). The low-range estimate was obtained using the water-yield method and the high-range estimate was obtained using the chloride-balance method. Both volumes include an average estimate of 250 acre-ft/yr of recharge from precipitation on Quaternary eolian and gravel deposits in the northern part of the valley (table 6). However, the small volume of recharge is essentially lost in the rounding required to present values that include the uncertainty in the estimates. A minimum estimate of ground-water recharge from streamflow losses, 10,000 acre-ft/yr, was obtained from the difference between daily mean streamflow losses during summer months, and the volume of ET from the combined sources of streamflow and ground water. Estimates of secondary recharge of pumped ground water range from 3,000 to 6,000 acre-ft/yr. Estimates of total ground-water recharge to basin-fill sediments in Carson Valley, for water years 1990–2005, range from 35,000 to 56,000 acre-ft/yr (table 22).

Components of ground-water discharge include ground-water ET from native phreatophytes, riparian vegetation, and non-irrigated pasture grasses totaling 11,000 acre-ft/yr (table 20); ground-water discharge to streamflow of the Carson River of 15,000 acre-ft/yr, and net ground-water pumping of 15,000 to 18,000 acre-ft/yr (table 18). Estimates of total ground-water discharge from basin-fill sediments in Carson Valley, for water years 1990–2005, range from 41,000 to 44,000 acre ft/ yr (table 22).

The average low-range estimate for ground-water recharge was about 15 percent less than the low-range estimate of ground-water discharge, and the average high-range estimate was about 25 percent greater than the high-range estimate of ground-water discharge. Inclusion of the total range in uncertainty for the estimates of ground-water recharge estimated from the water-yield (15,000 to 29,000 acre-ft/yr) and chloride-balance methods (17,000 to 58,000 acre-ft/yr) resulted in estimates of ground-water recharge from 32 percent less than the estimates of ground-water discharge to 68 percent greater (table 22). For this reason, the average estimates of ground-water inflow were considered to provide a more reasonable range for estimates of ground-water recharge.

As stated previously, the ground-water budget summarizes sources of ground-water recharge and discharge and provides an estimate of the perennial yield of Carson Valley. The perennial yield of an aquifer is defined 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.” (Nevada Division of Water Planning, 1992, p. 73).

Perennial yield is typically 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 publications point out that the ultimate results of ground-water pumping are to increase, or induce, additional recharge, to 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. In Carson Valley this is especially true because of the close hydraulic link between the aquifer and surface-water flow created by the permeable sediments and shallow depth to water beneath much of the valley floor. Pumping causes both additional recharge to be induced through the channels of the Carson River and irrigation ditches, and a decrease in ground-water discharge to the Carson River (Prudic and Wood, 1995, p. 10.). Sophocleous (1997) and Bredehoeft (1997) both note the utility of ground-water flow models to quantify the changes in recharge and discharge caused by pumping.

Comparison with Previous Water-Budget Estimates

Comparison of water-budget components with previous estimates is somewhat hampered by the different areas included in the estimates. Estimates of ET for the overall budget, 146,000 acre-ft/yr (table 21), are quite similar to that determined using a steady-state numerical model, 149,000 acre-ft/yr, by Prudic and Wood (1995, p. 9; table 2), but was greater than previous estimates ranging from 80,000 acre-ft/yr (Glancy and Katzer, 1976, p. 66) to 134,000 acre-ft/yr (Walters and others, 1970), and less than the estimate of 235,000 acre-ft/yr by Spane (1977, p. 91). All previous estimates of ET and the estimate of ET in the overall budget were made for ET supplied by ground water, surface water, and precipitation. The estimate of ground-water ET from phreatophytes, riparian vegetation, and non-irrigated pasture grasses, 11,000 acre-ft/yr, (table 22) is considerably less than any previous estimate.

Estimates of ground-water recharge, including secondary recharge, total from 35,000 to 56,000 acre-ft/yr, and are greater than previous estimates of 27,400 acre-ft/yr (Vasey-Scott Engineering, 1974, p. 10–11), 28,000 acre-ft/yr (Nevada State Engineer, 1971, p. 5), similar to estimates of 41,000 acre-ft/yr (Glancy and Katzer, 1976, p. 48), 47,000 acre-ft/yr (Maurer, 1986, p. 35 and 36), and 51,000 acre-ft/yr (Spane, 1977, p. 143). Previous studies did not consider secondary recharge. Ground-water recharge simulated from precipitation and subsurface flow by a steady-state numerical model totaled 102,000 acre-ft/yr (Prudic and Wood, 1995, p. 9; table 2).

The volume of 10,000 acre-ft/yr (table 22) estimated for ground-water recharge from streamflow was a minimum value, assuming no contribution of ground water to ET, and was considerably less than estimates of streamflow loss of 52,000 to 58,000 acre-ft/yr (Spane, 1977, p. 14) the net infiltration of surface water, 44,000 acre-ft/yr (Maurer, 1986, p. 59 and 60) and the average annual leakage from the Carson River and irrigation ditches, 105,000 acre-ft/yr simulated by a steady-state numerical model (Prudic and Wood, 1995, p. 9; table 2). However, the estimate of 10,000 acre-ft/yr represents only that part of streamflow loss that contributes ground-water recharge and does not supply ET. The volume of streamflow loss estimated from application of infiltration rates to the areas of irrigation ditches on the southeastern part of the valley, 18,000 to 26,000 acre-ft/yr was less but of a similar magnitude to previous estimates of streamflow loss. Estimates of streamflow loss during summer months, 89,000 acre-ft/yr, (fig. 11) determined from the difference between mean daily inflow to and outflow from Carson Valley, was similar to that simulated by the numerical model.

Similarly, estimates of ground-water seepage to streamflow, 15,000 acre-ft/yr (table 22) was considerably less than that estimated by the steady-state numerical model, 58,000 acre-ft/yr (Prudic and Wood, 1995, p. 9). The estimate of 15,000 acre-ft/yr may represent a minimum value because it was calculated only for the main stem and West Fork of the Carson River downstream of Muller Lane. Streamflow gains in the remainder of the valley were assumed to be lost to ET from downstream application of the water for irrigation.

Estimates of streamflow loss and gain presented in this report are considered to be approximations only because the application of appropriate rates of loss and gain is uncertain over large parts of the valley floor. A more appropriate tool for refining estimates of streamflow loss and gain is a numerical ground-water flow model using accurate altitudes for stream stage relative to ground-water levels adjacent to the streams and reasonable estimates for the hydraulic conductivity of the streambed materials.

Uncertainty of Water-Budget Components

The largest components of the overall water budget were the main-stem river flows of the East and West Forks of the Carson River and outflow of the Carson River, which are gaged near the study area boundaries. Uncertainties in the volumes of streamflow diversions and return flows across the study area boundary on the West Fork of the Carson River were small relative to the volume of streamflow in the Carson River.

The uncertainty of the gaged mainstem flows may be evaluated from the accuracy attributed to the records published for each water year. Records described as “excellent” means that 95 percent of the daily discharges are within 5 percent of their actual values; “good” within 10 percent; and “fair” within 15 percent. Records that do not meet these criteria are rated “poor.” Record descriptions for the mainstem gages range from “excellent” to “fair” from water years 1940 to 2005. However, Burkham and Dawdy (1968, p. 8–9) note that the uncertainty in annual flows may be less than that of the daily flows because of the compensating effects of errors in the daily flows. Anning (2002) presented methods for calculating standard errors of annual discharge, however, application of the methods is complex and beyond the scope of this study. Anning (2002, p. 37) estimated uncertainties ranging from about 8 to 14 percent for annual flows from a desilting basin on the Colorado River with annual flows ranging from about 200,000 to more than 300,000 acre-ft/yr, similar to the annual flows of the East Fork Carson River and the Carson River near Carson City (table 1). Assuming uncertainties are similar for the gaged Carson River flows, the uncertainty in annual flows may be from about 20,000 to 40,000 acre-ft/yr. Such volumes are of a similar, or greater, magnitude than many of the other water-budget components.

The next largest component of the overall water budget was ET. The accuracy of ET rates used in this report were estimated to be about 12 percent for irrigated pasture grasses and alfalfa and 20 to 30 percent for non-irrigated pasture grasses (Maurer and others, 2006, p. 23). Inaccuracies in the areas of land use to which the ET rates were applied may be present because it was not possible to field check all digitized polygons, however, these errors were considered to be small.

Uncertainty in the volume of precipitation on basin-fill sediments may be about 15 percent (Maurer and Halford, 2004, p. 37), or about 6,000 acre-ft/yr. The volumes of imported effluent are closely measured by the importing agencies. The volumes of ground-water pumping determined by the State Engineer’s Office from 1987 to 2005 were considered to be the best available estimates. However, estimates of pumping prior to the 1980s and for water years 1941–70 were considered approximations only. Estimates of the volumes of return flow from irrigation pumping and secondary recharge from lawn watering may be considerably in error.

The range for estimates of ground-water inflow indicates an uncertainty of 40 to almost 70 percent. The uncertainty includes the differences between estimates of subsurface inflow from perennial drainages, calculated using the water-yield method, combined with estimates of ephemeral streamflow lost to infiltration, and ground-water inflow calculated from the chloride-balance method. The estimates of subsurface inflow from perennial stream drainages appear to provide reasonable volumes compared to those determined for Eagle Valley where the method was developed, and provide estimates of ET that compare well with those reported in the literature (table 11). Estimates of ephemeral streamflow have an uncertainty of 50 percent and the uncertainty of chloride concentrations used in the estimates from the chloride-balance method results in a range of more than 50 percent (tables 12 and 13). However, inclusion of these uncertainties results in estimates of ground-water recharge that appear unreasonable (table 22). Development of watershed models for the Carson Range and Pine Nut Mountains could provide a more rigorous analysis of subsurface inflow and an independent check on the estimates of ground-water inflow. Such models use data on daily temperature and precipitation combined with vegetation cover, soils, altitude, slope, and aspect within the watershed to simulate runoff and provide an estimate of excess water that would become subsurface flow from the watershed. A cooperative study between the USGS and the Carson Water Subconservancy District to develop a ground-water flow model for Carson Valley includes the development of watershed models for the Carson Range and the Pine Nut Mountains. Work on the watershed models is planned to be completed in 2007.

The lack of data on the volumes of surface-water return flow from irrigation and the relative contribution of ground water and streamflow applied for irrigation to ET in Carson Valley make for considerable uncertainty in the estimate of ground-water recharge from streamflow. The estimate of ground-water recharge from streamflow represents a minimum value, assuming that ground-water contribution to ET from irrigated lands was minimal. Studies in the 1970s by Guitjens and others (1976 and 1978) indicate that ground water does supply water for ET to irrigated crops where the water table is shallow, however, the locations and rates were uncertain. The studies further indicate that ground-water recharge from flood irrigation could be significant where the water table is relatively deep and soils are sandy. Additional study that includes direct measurement of the volumes of streamflow or ground water applied for irrigation and the volumes of return flow from fields in areas having different soil types and depth to water is needed. Such data would allow refinement of the estimates of secondary recharge and net ground-water pumping for irrigation, and ground-water recharge from streamflow.

The estimate of ground-water discharge to streamflow was considerably less than previous estimates and is only approximate. Development of a numerical ground-water flow model for Carson Valley is planned and should provide a better tool to estimate the volumes of water exchanged between the surface-water and ground-water systems in Carson Valley.

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