SUMMARY

Ash Meadows is a major area of regional discharge for ground water flowing within Death Valley ground-water flow system of southern Nevada and adjacent California. Although Ash Meadows is situated in an arid region, large quantities of ground water discharge from more than 30 known springs and seeps aligned linearly along the trend of a major fault system. The water emerging from these springs supports a large diversity of vegetation and wildlife and provides habitat for a variety of endangered plant and animal species. Some water flowing from these springs evaporates shortly after emerging, some water flows to pools and reservoirs where it too is evaporated, and the remainder of the water originating as springflow infiltrates downward from drainage channels to recharge the underlying shallow valley-fill aquifer. Moisture held in the local soils and water contained in the shallow valley-fill aquifer sustain thriving populations of local phreatophytes year round. Together these spring ­features and plant communities create a unique oasis within the expansive, generally barren Mojave Desert.

Ground water discharging at Ash Meadows originates from areas to the north and east and is transported into the area through the regional carbonate-rock aquifer. Discharge at Ash Meadows is continually replenished by ground water derived from an extensive area that includes the eastern part of the Nevada Test Site (NTS). Currently, contaminants introduced into the subsurface by past nuclear testing at the NTS are the subject of the U.S. Department of Energy's Environmental Restoration Program. One requirement of this program is to evaluate the risk that these contaminants pose to the general public. To assess risk, the potential for contaminant transport must be determined. The transport of contaminants residing within the ground water is controlled, in part, by the rate and direction of ground-water flow. The amount of ground water that moves through the subsurface is controlled, in part, by the amount of ground water that leaves the flow system. Because some uncertainty exists as to the amount of ground water discharging downgradient from the NTS, studies have been initiated to re-evaluate and more thoroughly quantify current estimates. This report documents the result of a study to estimate ground-water discharge at Ash Meadows.

Ground-water discharge is estimated through a rigorous quantification of evapotranspiration. This approach assumes that all ground water discharging from the aquifer system beneath Ash Meadows evaporates or transpires locally. Although the approach does not account for springflow directly, it assumes that all springflow evaporates or recycles back into the shallow valley-fill aquifer where later it is transpired or evaporated. Mean annual evapotranspiration from the Ash Meadows area is calculated as the sum of mean annual ET determined for areas of similar vegetation and moisture conditions (referred to as ET units). Mean annual ET for each ET unit is calculated as the product of the unit's acreage and annual ET rate.

Seven unique ET units are defined for the Ash Meadows area on the basis of spectral-reflectance characteristics derived from satellite images recorded in 1992. Six units were delineated by a procedure that combined separate classifications of a June and of a September thematic mapper (TM) image to form a generalized ET-unit map. A third satellite image, an August 1993, SPOT scene, was used to delineate the seventh ET unit by refining identified areas of open water. Together these units encompass about 10,350 acres of sparsely to densely vegetated grassland and wetland. The largest of the seven units, about 7,160 acres, is dominated by sparse, relatively dry grassland, which generally is mixed with shrubs and small trees; and the smallest unit, 81 acres, by submerged aquatic vegetation growing in the shallows and along the shoreline of a few larger open-water bodies.

A mean annual rate of ET is computed for each ET unit from annual ET rates calculated by energy-budget methods (primarily Bowen ratio) at 10 sites instrumented to collect micrometeorological data. Sites are located within six of the seven ET units; 9 of the 10 sites are over land and 1 is over open water. Daily ET rates are computed from micrometeorological measurements averaged for 20-minute periods. Annual ET computed from daily rates determined over a minimum of 1 year ranged from 8.60 ft over an open water site to 0.62 ft over a sparse saltgrass site. Some sites having multiple years of data showed variations in annual ET with precipitation. ET units having the greatest diversity of vegetation and largest contrast in soil-moisture conditions showed the greatest variation in annual ET. Mean annual ET estimates ranged from 8.6 ft for the unit delineating areas of open water to 1.3 ft for the unit delineating areas dominated by sparse grassland vegetation.

Water levels measured in shallow wells within the different ET units show significant annual and daily fluctuations in the water table that are attributed to local water losses associated with evapotranspiration. The largest measured annual water-table fluctuation was 10.2 ft, and the smallest was about 0.4 ft. Smaller annual fluctuations were measured at sites near a constant surface-water source (usually sustained by springflow), whereas, the larger fluctuations were measured at sites in densely vegetated areas most distant from any surface-water source.

In general, measured annual water-table fluctuations are consistent but slightly shifted in time from annual changes in daily ET. The shift is such that the maximum depth to water occurs shortly after ET reaches its maximum rate. Although measured declines are good indicators of ongoing ET, the magnitude of the decline is not always indicative of the rate of ET. Annual water-table declines depend on many other local factors--including the depth of the water table, distance from a surface-water source, aquifer and soil properties, soil-moisture conditions, and precipitation.

The largest measured daily water-table fluctuation is about 0.3 ft at a site dominated by marsh vegetation and standing water. Daily fluctuations typically are larger and in-phase with changing ET rates during periods when the water level is nearest the surface. As water levels drop in response to ET losses, the magnitude of the daily fluctuation attenuates and the daily water-level peak and trough shift from the daily high and low in the ET rate. Water levels measured in deeper (more than 90 ft) wells within ET units and in wells outside ET units generally show smaller (less than 2 ft) responses in the annual water-level fluctuation. These smaller magnitude fluctuations are attributed to processes other than ET, primarily atmospheric loading and earth tides.

Mean annual ET is estimated at 21,000 acre-ft. An estimate of the mean annual ground-water discharge, based solely on ET, is presented as a range to account for uncertainties in the contribution of local precipitation. Annual ET rates determined for each ET unit were adjusted to remove any contribution by local precipitation from the ET estimate. Adjustments of 0, 2.5, and 4.25 inches are applied to span the range of the potential precipitation contribution. Mean annual precipitation is estimated between 2.5 and 4.25 inches. Assuming a zero adjustment (no local precipitation contribution), the estimate of the mean annual discharge is 21,000 acre-ft. Assuming a 4.25-inch adjustment (the maximum precipitation contribution), the estimate of mean annual ground-water discharge is 18,000 acre-ft.

Estimates of mean annual ET fall near the middle of the range defined by previous estimates--11,000 acre-ft (Walker and Eakin, 1963, p. 24; Winograd and Thordarson, 1975, p. 84) to about 35,500 acre-ft (D'Agnese and others, 1997, p. 46). Although the accuracy of one method over another is difficult to evaluate, the more local and rigorous character of the techniques used in this study suggest a more accurate quantification of ET acreage and rates upon which the mean annual estimate is based. The range given for ground-water discharge is slightly greater than that estimated previously for Ash Meadows primarily on the basis of springflow measurements. The small difference might be the result of errors, either in springflow measurements or in measurements of the micrometeorological data required to compute ET, or the result of erroneous assumptions in the classification procedure or in the Bowen ratio solution. But if both estimates are assumed reasonably accurate, the higher ET-based estimate can be attributed to another water source helping sustain the shallow water table. Likely sources for the additional inflow are diffuse upflow from the underlying regional carbonate aquifer or discharge from subsurface seeps.

The accuracy of the estimate of ground-water ­discharge is limited by the assumptions inherent in the classification procedure, the energy-budget methods (primarily Bowen ratio) used to compute daily ET, and the averaging techniques applied to estimate mean annual ET. Other limitations include (1) the assumption that all springflow is ultimately evaporated or transpired from within the bounds of one of the delineated ET units; (2) the short-term data used to compute mean values; (3) the limited number of sites used to estimate ET for each ET unit; and (4) the uncertainty in the amount of local precipitation included in the computed ET rates. Multiyear classifications, longer term data acquisition, and a greater number of local ET-site installations would help refine, improve, and provide more confidence in the estimate of mean annual ground-water discharge.

The estimate of ground-water discharge presented includes only water lost through evaporation and transpiration and does not include any water that may be leaving the Ash Meadows area through subsurface means. Assuming some potential for subsurface outflow, the range given for annual ground-water discharge should be considered a minimum value of total outflow from the Ash Meadows area.