Water-Resources Investigations Report 01-4239
Ground-Water Discharge Determined from Measurements of Evapotranspiration, Other Available Hydrologic Components, and Shallow Water-Level Changes, Oasis Valley, Nye County, Nevada
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Oasis Valley is one of four major areas of natural ground-water discharge within the Death Valley regional ground-water flow system of southern Nevada and adjacent California. Ground water beneath Oasis Valley is recharged from an extensive area to the north and northeast that includes much of Pahute Mesa in the northwestern part of the Nevada Test Site (NTS). Currently, contaminants generated at the NTS by past nuclear testing are the subject of the U.S. Department of Energy's Environmental Restoration Program. In support of this program, the amount of ground water discharging from Oasis Valley was quantified to provide information to better evaluate the potential transport of radionuclides away from the NTS. Ground-water discharge was estimated by quantifying evapotranspiration (ET), estimating subsurface outflow, and compiling ground-water withdrawal data. Spring discharge and ground-water levels were measured to help evaluate ET and characterize hydrologic conditions.
ET was quantified by identifying areas of ongoing ground-water ET, delineating unique areas of ET defined on the basis of similarities in vegetation and soil-moisture conditions (referred to as ET units), and computing ET rates for each of these ET units using micrometeorological data. Mean annual ET for each ET unit was calculated as the product of the unit's acreage and annual ET rate. Mean annual evapotranspiration from the Oasis Valley area was calculated as the sum of mean annual ET determined for each ET unit.
Eight ET units were delineated within Oasis Valley on the basis of spectral-reflectance characteri-stics derived from satellite imagery acquired in 1992. Together these ET units encompassed about 3,426 acres of sparse to densely vegetated grassland, shrubland, and wetland. About 35 percent of this acreage is sparse to moderately dense grassland (SGV) and 26 percent (892 acres) is sparse to moderately dense shrubland (SSV). Denser vegetation types, such as dense meadow and woodland vegetation (DMV), moderately dense to dense grassland vegetation (DGV), and dense wetland vegetation (DWV), make up about 35 percent of the total area. About 4 percent of the area is moist bare soil (MBS), submerged and sparse emergent aquatic vegetation (SAV), or open water body (OWB).
The Bowen-ratio method, based on balancing the energy budget, was used to compute ET rates at 5 sites within the 3 largest ET units in Oasis Valley. ET rates were computed from micrometeorological data collected from 1996 through 2000. Annual ET at these sites ranged from 3.14 ft over dense meadow vegetation to 0.62 ft over sparse shrubland vegetation. Differences in ET rates computed for sites within an ET unit are attributed to spatial changes in the density of local vegetation.
An annual ET rate for each of the eight ET units was estimated by averaging all ET rates calculated for sites located within the unit. Averages were determined from ET rates computed at 5 ET sites in Oasis Valley and 9 similar ET sites in nearby Ash Meadows. Average annual ET rates range from 1.2 ft/yr for SSV to 8.6 ft/yr for OWB and SAV.
An estimate of the mean annual ET from Oasis Valley was computed by summing estimates of the mean annual ET from each ET unit. Estimates of mean annual ET range from 8.6 acre-ft at OWB to 2,700 acre-ft at DMV. The estimate of the mean annual ET from Oasis Valley is 7,800 acre-ft.
Mean annual ground-water ET was calculated by removing water from the estimate of mean annual ET contributed by local precipitation. The local precipitation component was assumed to be equal to the mean, annual, long-term precipitation of 0.5 ft. Estimates of mean annual ground-water ET from each ET unit range from 8.1 acre-ft at OWB to 2,300 acre-ft at DMV. Mean annual ground-water ET from Oasis Valley is estimated at 6,000 acre-ft.
Subsurface outflow from Oasis Valley to the Amargosa Desert occurs through alluvium at the Amargosa Narrows in southernmost Oasis Valley. Subsurface outflow through the alluvium was estimated using Darcy's Law and average values determined for the hydraulic gradient, cross-sectional area, and hydraulic conductivity of the alluvium. Substituting an average hydraulic gradient of 0.017 ft/ft, a cross-sectional area of 88,000 ft2, and the range of 2 to 10 ft/day for hydraulic conductivity into Darcy's law resulted in a computed annual subsurface outflow that averages about 80 acre-ft/yr.
Ground water is withdrawn in Oasis Valley from municipal water supply wells owned and operated by the Beatty Water and Sanitation District, from some non-municipal wells, and from a few springs. Annual ground-water withdrawal in Oasis Valley has declined from 440 acre-ft in 1996 to 210 acre-ft in 1999. To compensate for this decrease in withdrawal from within the valley, ground water was withdrawn from a well drilled in the Amargosa Desert south of Oasis Valley.
Spring discharge measured at spring and channel sites ranged from less than 1 gal/min to about 250 gal/min. Annual maximum discharge at channel sites occurred in winter or early spring (January to April), coincident with minimum ET, while annual minimum discharge occurred in late spring through summer and early fall (May to September), coincident with increasing or maximum ET. In general, the annual maximum and minimum measurements at spring sites were not seasonally dependent. The annual fluctuations in discharge at channel sites were larger and more variable than at spring sites. The larger fluctuations were attributed primarily to seasonal changes in ET and not to changes in springflow.
Ground-water discharge was calculated by summing estimates of mean annual ground-water ET, subsurface outflow, and ground-water withdrawal. Based on these individual estimates, natural ground-water discharge from Oasis Valley is about 6,100 acre-ft/yr. Total discharge was about 6,500 acre-ft in 1996 and 6,300 acre-ft in 1999. Ground-water ET accounted for more than 90 percent of the total ground-water discharge from Oasis Valley, and subsurface outflow and ground-water withdrawal accounted for the remainder. These ground-water discharge estimates include spring and seep discharge as this flow evaporates or infiltrates the subsurface, where it recharges the alluvial aquifer and subsequently undergoes ET, subsurface outflow, or ground-water withdrawal.
The estimates of mean annual ground-water discharge by ET and of annual natural ground-water discharge in Oasis Valley are about 2.5 times greater than those previously reported in 1962. The primary discrepancy between these estimates is the result of differences in the approach used to estimate average ET rates. Although the accuracy of one rate estimate versus another is difficult to evaluate, the more localized nature of the data and more rigorous methods used in this study are likely to result in a more accurate quantification of ET rates for the Oasis Valley area. The larger annual estimate of ground-water discharge agrees with that previously reported in 1973.
To gain additional insight into the ET process, ground-water levels were measured in Oasis Valley during the ground-water discharge investigation. Depth-to-water measurements in shallow wells showed a wide range in annual and daily fluctuations. The amount of annual fluctuation varied between and within ET units, ranging from 0.8 ft to 7.7 ft. These variations would be expected considering that each unit includes areas of different vegetation, varying vegetation density, and varying soil and moisture conditions. In general, annual minimum depth to water in shallow wells occurred in winter or early spring, shortly after daily ET rates reached a minimum value, while annual maximum depth to water occurred in late summer or fall, shortly after daily ET rates maximized. The magnitude of daily water-level fluctuations in the shallow wells measured at ET sites ranged from less than 0.05 ft to 0.2 ft. The magnitude of daily fluctuations in the shallow wells decreased as water level declined and increased during periods of larger ET rates when the water table was near the surface.
Although the annual and daily fluctuations in the water table may be good indicators of ongoing ET, the magnitude of the changes were not always indicative of the rate of ET. Water-level fluctuations result from many factors, including the depth of the water table, distance from a surface-water source, aquifer and soil properties, soil-moisture conditions, and precipitation.
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