WRIR 01-4195:
Ground-Water Discharge Determined from Estimates of Evapotranspiration,
Death Valley Regional Flow System, Nevada and California
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The Death Valley regional flow system (DVRFS) is one of the larger ground-water flow systems in the southwest United States and includes much of southern Nevada and the Death Valley region of eastern California. Although situated in an arid region, large quantities of ground water discharge from a few localized areas where geologic and hydrologic conditions are such that ground water is pushed upward to the surface and is discharged from springs and seeps. The water emerging from these sources supports a great diversity of vegetation and wildlife and provides habitat for a variety of endangered plant and animal species. Some water flowing from these sources evaporates shortly after emerging, some water flows to pools and reservoirs where it too evaporates, and the remainder of the water infiltrates downward from drainage channels to recharge the underlying shallow flow system. Moisture held in the local soils and water contained in the shallow flow system sustains thriving populations of phreatophytes year round. Together these water sources and local plant communities create distinct oases within an expansive, generally barren desert region.
The Nevada Test Site (NTS), with an area of about 1,375 mi2, is centrally located within the DVRFS. Some of the ground water emerging from the major discharge areas of the DVRFS originates or flows beneath the NTS. The NTS historically has been used for testing nuclear devices and currently is a potential location for the permanent disposal of high-level nuclear waste in United States. The U.S. Department of Energy, as mandated by Federal and State regulators, is evaluating the risk associated with contaminants that have been or may be introduced into the subsurface as a consequence of any past or future activities at the NTS. To assess risk, the potential for contaminant transport away from the site must be determined. Because subsurface contaminants can be transported away from the NTS by ground water, components of the ground-water budget that provide information on the flow of ground water are of extreme interest. One such component influencing ground-water flow is regional ground-water discharge. Because some uncertainty exists as to the amount of ground water that discharges from the DVRFS, studies were initiated to re-evaluate and better quantify estimates of ground-water discharge. This report documents the result of a previous study that applies a technique to estimate ground-water discharge at Ash Meadows to other major discharge areas of the DVRFS. The discharge areas evaluated as part of this effort include Chicago Valley, the Franklin Well area, Franklin Lake, Sarcobatus Flat, the Shoshone area, Stewart Valley, and the Tecopa/California Valley area. Although Ash Meadows and Oasis Valley were studied as part of independent efforts, estimates for these two areas are included in this report for purposes of completeness. This effort does not include Death Valley, which currently is being evaluated as part of a separate study by the U.S. Geological Survey in cooperation with the National Park Service.
Ground-water discharge is estimated throughout much of the DVRFS from a rigorous quantification of evapotranspiration. This approach assumes that all ground-water discharge is evaporated or transpired from within the discharge area. Although the approach does not account for springflow directly, it assumes that all springflow is evaporated or recycled back into the shallow flow system where later it is transpired or evaporated. Any recycled water not locally evaporated or transpired is not accounted for in the discharge estimate. Mean annual evapotranspiration (ET) from each discharge area is calculated as the sum of mean annual ET estimates determined for each of the ET units (an area of similar vegetation and moisture conditions) present within the discharge area. Mean annual ET from an ET unit is computed as the product of the unit's acreage and its estimated annual ET rate.
ET units are defined on the basis of differences in spectral-reflectance characteristics. Spectral differences are determined from thematic mapper (TM) imagery acquired June 21, 1989, and June 13, 1992. Except for Sarcobatus Flat, all discharge areas are classified from the June 13, 1992, imagery. TM imagery acquired June 21, 1989, is used to classify ET units in Sarcobatus Flat because excessive cloud cover over this area diminished the overall quality of the 1992 imagery. Ten unique ET units are identified that include areas of open playa, sparse to dense vegetation, moist bare soil, and open water. Sarcobatus Flat has the largest ET unit acreage of any discharge area at 34,250 acres. Of this total, 19,372 acres are classified as sparse to moderately dense shrubs and 10,817 acres as open playa. The dominant phreatophyte found in Sarcobatus Flat is greasewood (Sarcobatus vermiculatus). The discharge area having the next largest acreage is Ash Meadows at 12,467 acres. The largest ET unit classified within Ash Meadows is sparse grassland vegetation at 7,059 acres and is dominated by expansive meadows of saltgrass (Distichlis spicata var. stricata). The smallest discharge area is the Franklin Well area with 297 acres.
Mean annual ET rates for seven ET units are estimated from daily ET rates computed primarily from micrometeorological data collected at 15 sites instrumented in Ash Meadows and Oasis Valley. Sites were instrumented for periods of 1 to 3 years as part of separate studies of Ash Meadows and Oasis Valley and are located within 7 of the 10 ET units identified. In the three ET units not instrumented, annual ET rates were estimated from rates reported in the literature. Together, these values established a range of ET rates for each ET unit. Estimated mean annual ET rates range from 0.5 ft/yr for open playa to nearly 9 ft/yr for open water. Mean annual ET estimates for discharge areas range from 450 acre-ft in the Franklin Well area to 30,000 acre-ft in Sarcobatus Flat.
Ground-water discharge for each ET unit is estimated as the product of an adjusted ET rate and the ET unit acreage. The ET rate is adjusted to remove local precipitation components from the ET estimate. Precipitation ranges from 3.5 in. in the Shoshone and Tecopa discharge areas to 6 in. in the Sarcobatus Flat and Oasis Valley discharge areas. Estimates of mean annual ground-water discharge range from 350 acre-ft in the Franklin Well area to 18,000 acre-ft in Ash Meadows. Ground-water discharge estimates generally are greater for the northern discharge areas (Sarcobatus Flat and Oasis Valley) than previous estimates reported in the literature and less for the southern discharge areas (Franklin Lake, Shoshone, and Tecopa/California Valley).
The accuracy of ground-water discharge estimates is limited primarily by the assumptions inherent in the classification procedure and in the energy-budget methods (primarily Bowen ratio) used to compute daily ET. Other factors potentially affecting the accuracy of the ground-water discharge estimates include (1) the assumption that all springflow ultimately is evaporated or transpired from within the discharge area; (2) the assumption that no external surface-water inflow contributes to local ET measurements; (3) the short period of record used to compute mean annual ET rates; (4) the limited number of local sites used to estimate mean annual ET rates; (5) the uncertainty associated with computing local ET estimates on computed relative density differences; and (6) the uncertainty in the adjustment applied to remove precipitation from ET estimates. Multiyear classifications, longer-term data acquisition, and a greater number of local ET-site installations would help refine, improve, and provide more confidence in estimates of mean annual ground-water discharge. Additional measurements of ET rates in areas such as open playa where estimates of annual ET rates are based on values reported in the literature for vegetation and soil conditions outside the study area or on limited data, and a better understanding of the contribution of precipitation and surface-water inflow to measured ET rates are vital to establishing a higher degree of confidence in estimates of ET and ground-water discharge. The stated conjecture that these data would improve confidence is supported by results of the uncertainty analysis presented in the appendix. Results indicate that the largest uncertainties are most often coincident with discharge areas dominated by open playa and that the adjustment made to remove the nonground-water component (precipitation and surface-water inflow) from the measurements of ET rates is an extremely sensitive parameter.
The estimate of ground-water discharge presented in this report includes water lost through evaporation and transpiration but does not include water that may exit discharge areas as subsurface flow. Absent better estimates of subsurface flow, annual estimates of ground-water discharge presented here should be considered a minimum value of total outflow from a discharge area.
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