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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The vegetation thriving in Oasis Valley requires more water than is provided by local rainfall, and must rely on local ground water. The uptake by local phreatophytes of ground water from the alluvial aquifer and losses through the evaporative process often are reflected by concurrent changes (fluctuations) in the water table. Water levels were measured in a network of wells to assess daily, seasonal, and annual fluctuations in the water table and to gain greater insight into the ET process at Oasis Valley.
Water levels were measured in 27 wells located throughout Oasis Valley from 1996 through 2000 (table 11; pl. 2). Manual measurements were made on a periodic basis, and, in selected wells, electronic pressure-transducer measurements were made and recorded hourly. The wells were selected to obtain water-level measurements from areas representing most major vegetation types, soil moisture conditions, and the range of depths to water below land surface.
Ten shallow wells (table 11) were installed during the study to assess the possible effect of ET on water-table fluctuations. These wells ranged in depth from 5.6 to 16.9 ft below land surface. Shallow wells were located at instrumented ET sites and at representative sites within each of the four largest ET units. Three shallow wells were located in the DMV ET unit, three in the SGV ET unit, two in the DGV ET unit, and two in the SSV ET unit.
Water levels also were measured in seven other existing wells in Oasis Valley (table 11). These wells, referred to as deep wells, are located where the water table is relatively shallow (less than 25 ft) and have depths less than or equal to 120 ft below land surface. Three of these wells are located within classified ET units -- one in DMV; one in SGV; and one in SSV (table 11); the other four wells are in the unclassified (UCL) ET unit.
Ten wells within the study area were installed by the USGS during August through October 1997 as part of the USDOE-ERP long-term ground-water monitoring network (Robledo and others, 1998). These wells, referred to as ER-OV wells, range in depth from 65 to 642 ft below land surface (table 11). Most of these wells were used to measure water levels in the deeper zones (i.e., more than 100 ft below land surface) of the ground-water flow system. None of these wells were located within classified ET units.
Water levels were measured in shallow wells each month from the date of installation through September 2000. On occasion, a monthly measurement could not be obtained due to difficulties in accessing the site. Water levels also were periodically measured in deep and ER-OV wells throughout the area during the study, but less frequently than in the shallow wells. The frequency of measurements in these wells varied, but was sufficient to qualitatively evaluate possible seasonal fluctuations. All water levels were measured with steel or calibrated electric tapes.
Water levels in selected wells were measured once every hour to evaluate the response of the water table to daily changes in hydrologic stress. Pressure transducers were installed in seven shallow wells and the ER-OV-06a well, and measurements were recorded on data loggers. Pressure transducers were installed in shallow wells at each ET site except the site at UOVUP. A pressure transducer was installed at OVU-Dune well to represent water-table conditions similar to those at UOVUP. The data-collection period at each site differed in accordance with the well installation and the period of interest. A barometer was installed at the OVU-Lower ET well to record and document changes in local barometric pressure. Pressure transducers were checked for accuracy by periodically measuring water levels with a steel tape.
The shallow water table, as determined from depth-to-water measurements made in shallow wells throughout the Oasis Valley discharge area, fluctuated both annually and daily. Fluctuations are primarily a response to local ET and precipitation. The magnitude and timing of the fluctuations differ with well depth, vegetation and soil conditions, climate, and distance from a spring or spring-channel source. Other less- significant factors affecting the shallow water table are changes in atmospheric (barometric) pressure and earth tides. Fluctuations also were noted in the deep and ER-OV wells; these can be attributed primarily to barometric-pressure and earth-tide responses. Local ET also may influence water-level fluctuations in deep and ER-OV wells with depths to water of less than 25 ft.
Annual changes in water level measured in each of the shallow and deep wells are summarized in tables 12 and 13 and shown on plates 1 and 2. Annual changes are based on periodic or hourly measurements. Maximum and minimum values determined from periodic measurements may not be indicative of the actual high and low water level because of the time periods between measurements (monthly or greater). Water-table fluctuations formulated from hourly measurements in shallow wells are shown in figure 19 and compared with calculated daily evapotranspiration on plate 1.
Depth-to-water measurements made in the shallow wells (table 12) show a wide range in the annual fluctuation of the water table. The amount of annual fluctuation varied between and within ET units. The measured within-unit variation ranged from 2.5 to 5.7 ft in DMV, from 1.9 to 7.7 ft in DGV, from 2.4 to 3.9 ft in SGV, and from 0.8 to 3.6 ft in SSV. Variations measured between and within these ET units were expected considering that each unit includes areas of different vegetation, varying vegetation density, and varying soils and soil-moisture conditions.
The annual minimum depth to water in shallow wells typically occurred in winter or early spring, while annual maximum depth to water occurred in late summer or fall (table 12; figs. 19, 20, and 21; pls. 1 and 2). As was true of the annual water-table fluctuation, the annual minimum and maximum depth to water varied among wells between and within the same ET unit. The smallest minimum depths to water (highest water table) were measured in wells near perennial springs or spring-channel sources or in areas flooded during the cooler periods of the year. The largest maximum depths to water (deepest water table) were measured in wells located most distant from spring or spring-channel sources.
Minimum depth to water in some shallow wells stabilizes at a peak level near land surface (figs. 19, 20; pls. 1 and 2). These peaks typically occur in winter through spring (December through April) and likely are the result of localized surface flow. The periods of stabilized minimum depth-to-water generally coincide with periods of minimum ET (pl. 1).
Fluctuations in the depth to water at a given ET site generally lag daily ET such that the annual maximum occurs shortly after daily ET reaches a maximum and the annual minimum shortly after ET reaches a minimum (pl. 1). This delay indicates that the fluctuation in the water table is largely a response to a change in ET rate. Somewhat contrary to this conclusion is the observation that larger changes in water level may occur at sites of low to moderate ET than at sites of higher ET (pl. 1). For example, annual water-table fluctuations at ET sites MOVAL, UOVLO, and UOVMD are all larger than at SDALE, which has a higher ET rate (pl. 1; tables 4 and 12; fig. 22). This observation may be explained by the presence of a spring or spring-channel source near sites of higher ET. At SDALE, a nearby spring source provides sufficient water to replace much of the water lost through local ET, thus helping maintain the level of the water table and the local vegetation.
Although a decline in the water table is a good qualitative indicator of ongoing local ET within an area (pl. 1), the magnitude of the annual decline is not necessarily indicative of the rate of ET. The annual decline of the water table is dependent on many factors -- including the depth to the water table and the distance to a spring or spring-channel source. Aquifer characteristics, soil properties, and soil moisture conditions also influence the magnitude and timing of the response of the water table to changes in ET.
Annual changes in water levels measured in the deep and ER-OV wells are summarized in 13. The general differences between measured annual water-level fluctuations in shallow, deep and ER-OV wells are evident by comparing tables 12 and 13 and are illustrated on plate 2 and in figures 20, 21, 22, 23, and 24. The annual fluctuations measured in deep and ER-OV wells generally are smaller (less than 1 ft) and more subdued than those measured in shallow wells. The larger annual fluctuations (greater than 1 ft) noted in shallow wells (figs. 20, 21, 22, 23, and 24) imply that the net loss of ground water by ET is greater in those areas in which the water table is relatively close to land surface. Larger fluctuations were also noted in deep wells whose production zone or open interval may be affected by evapotranspiration and/or intermittent surface water flow (Beatty Wash Terrace Well, BGC-2, Central Beatty Well, Narrows South Well).
The water table shows only a minimal response to measured annual precipitation. Springdale Lower Well (pl. 2; table 12; fig. 22) showed the most correlation between annual water-table fluctuations and precipitation. The response in this well to increased precipitation in 1998 (fig. 14) was a shallower maximum depth to water and a smaller annual fluctuation. These responses were approximately 1 ft different from years of normal precipitation. Although fluctuations measured in other shallow wells show little correlation to measured changes in precipitation, any response may have been masked by other factors potentially affecting the water table.
In general, water-level elevations in Oasis Valley increase with well depth, indicating an upward gradient (figs. 22 and 23). Upward flow is consistent with the concept of flow from the underlying welded-tuff aquifer into the overlying alluvial aquifer. Also, in general, water-level elevations in the alluvial aquifer decrease with land-surface altitude, indicating regional flow toward the south and the Amargosa Narrows.
The water table, as measured in shallow wells throughout the area, also fluctuates on a daily basis. The shape, magnitude, and phase of daily fluctuations varied between wells and over time, and are typified in figures 25, 26, 27, 28, 29, and 30. Reasons for observed differences in daily fluctuations are many and complex, but most likely are caused by differences in ET rate, depth to water, distance from a spring or spring-channel source, confinement of the aquifer system, or some combination thereof. The purpose of evaluating these daily fluctuations is not to explain or rationalize every difference but rather to help validate concepts of where and how much ET occurs in Oasis Valley.
In general, the magnitude of the daily fluctuation of water levels measured at each ET site is largest during periods of high ET when the water table is near the surface and generally increases as daily ET increases and often decreases as depth to water increases (fig. 29). The largest daily fluctuations, nearly 0.2 ft, were measured in the Springdale ET Shallow well at the SDALE ET site during periods of high daily ET (fig. 25). Small daily fluctuations (less than 0.05 ft) were measured in wells at nearly every ET site.
Daily fluctuations in the water table and ET measured at the SDALE, UOVLO, and UOVMD ET sites for 10-day periods in late spring/early summer are shown in figures 25, 26, and 27. The overall water-level trend is downward at all three ET sites over these periods. At the SDALE and UOVLO ET sites (figs. 25 and 26) the daily fluctuation is opposite and nearly in phase with that of calculated ET. At the UOVMD ET site (fig. 27), the magnitude of the daily fluctuation is much smaller and the phase is shifted from that of ET. Magnitude and phase differences of daily fluctuations are likely related to differences in depth to the water table. Laczniak and others (1999) attribute these fluctuation differences to the relative amounts of water being removed from the saturated and unsaturated zones.
Daily fluctuations also were measured in well ER-OV-06a (figs. 29 and 30). Within this well, daily fluctuations differ substantially in magnitude, character, and phase from those measured in shallow wells. Daily fluctuations such as those noted in well ER-OV-06a are documented in other wells throughout the region that tap confined, partly confined, or thick water-table aquifers (Galloway and Rojstaczer, 1988; Laczniak and others, 1999). Fluctuations of this type are unlikely to be responses to daily ET, but rather are a reflection of water-level disturbances caused by changes in the aquifer system resulting from atmospheric loading (fig. 30) and earth tides (Galloway and Wilcoxon, 1993; Galloway and others, 1994).
Short-term responses to precipitation also are evident in the water-table record of many shallow wells measured throughout the area (figs. 20, 21, and 30). A rise in the water table coincides with periods of precipitation but varies among wells in magnitude and duration. Differences in responses to precipitation are most certainly related to the amount of precipitation falling at a site, but also are likely related to many other factors including differences in the local vegetation, soil properties, and water-table conditions.
Daily and annual fluctuations in the water table can be a good indicator of ongoing ET, but their magnitude is not necessarily a reliable method of quantifying ET rates. Quantifying ET rates on the basis of water- table fluctuations was considered in previous studies (Laczniak and others, 1999), but was not attempted in Oasis Valley. Any attempt to calculate ET on the basis of water-level changes would require a better understanding of all the inflow and outflow components contributing to the local water budget, as well as additional knowledge of the hydrologic and physical properties of the soil and aquifer system that govern the movement and storage of water.
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