Scientific Investigations Report 2006–5145
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5145
Ground water flowing from major springs and seeps along the eastern margin of Death Valley supplies most of the water consumed locally, and also sustains much of the unique habitat in Death Valley National Park. Together these spring complexes constitute the terminus of the DVRFS—one of the larger flow systems in the Southwestern United States. The Grapevine Springs complex is the least exploited for water supply and consequently contains the largest area of undisturbed riparian habitat in the park.
More accurate and reliable estimates of ground-water discharge are needed to document the water requirements of sensitive spring-fed habitats and to develop a better understanding of the long-term sustainability of the ground-water resource. The USGS, in cooperation with the National Park Service, began a 3-year study in 2000 to develop an estimate of mean annual ground-water discharge from the Grapevine Springs area and estimates of the amount of ground water transpired by the local riparian vegetation within each of the major spring discharge areas along the eastern margin of Death Valley. Results of the study are intended to establish a sound basis for estimating water rights, and also provide baseline information for determining and documenting any future changes in ground-water discharge in the park.
The assumption used to estimate local discharge from the spring-fed riparian areas along the eastern margin of Death Valley is that ET rates vary with the health, density, and type of the vegetation and wetness of the soil. These variations can be adequately characterized by a finite number of generalized spatial groupings (ET units) delineated using multi-spectral imagery. The approach first computes ET for each ET unit as the product of its acreage and a representative ET rate, and then calculates ET for a discharge area by summing ET computed for each of its component ET units.
High-resolution multi-spectral imagery was used to group riparian vegetation in each major spring discharge area into two unique ET units discriminated on the basis of vegetation density. This high-resolution imagery was acquired by the IKONOS satellite. This satellite is equipped with multi-spectral sensors that measure reflected solar radiation within four bands, each of which spans a discrete wavelength within the visible or near-infrared regions of the electromagnetic spectrum. The satellite also acquires a panchromatic band that spans a broader portion of the electromagnetic spectrum. The imagery resolution is 13.1 feet for multi-spectral bands and 3.28 feet for the panchromatic band.
Two areas, each encompassing about 39 square miles, were imaged within Death Valley. Imagery of the Furnace Creek area was acquired on June 10, 2001, and imagery of the Grapevine Springs area was acquired on July 16, 2001. Each image was orthorectified to improve its spatial accuracy and calibrated to reduce astronomic and atmospheric effects. The imagery was calibrated to field-acquired spectral data measured over a black tarp (dark reflector) and white tarp (bright reflector) on or near the day the imagery was acquired.
Two different procedures were applied to delineate ET units using IKONOS imagery. Both procedures delineated the two ET units in each discharge area—one unit representing high-density and the other moderate-density vegetation. One procedure developed pixel groupings on the basis of differences in a vegetation index, while the other used a land-cover classification. Because the primary focus of the study was Grapevine Springs, the criteria used to delineate ET units was developed in the Grapevine Springs area and applied to the other major spring discharge areas along the eastern margin of Death Valley.
ET units first were delineated on the basis of the modified soil-adjusted vegetation index (MSAVI). This index was selected because it best reduces soil influences. The MSAVI, as is typical of most vegetation indices, is calculated using the percent reflectance of the red (band 3) and near-infrared (band 4) wavelengths. ET-unit acreage in the Grapevine Springs area determined from MSAVI values totaled about 192 acres—of which 80 acres were moderate-density vegetation and 112 acres were high-density vegetation. ET-unit acreage for all major discharge areas in the Grapevine Springs imagery (Grapevine Springs, Staininger Springs, and Surprise Springs) totaled 239 acres, and ET-unit acreage for all major spring discharge areas in the Furnace Creek imagery (Nevares Springs, Cow Creek–Salt Springs, Texas Spring, and Travertine Springs) totaled about 74 acres. Springflow diversions and turf watering have altered the distribution of the natural vegetation in discharge areas other than Grapevine Springs.
ET units also were delineated using a land-cover classification. The land-cover classification grouped pixels into two ET units on the basis of similarities between reflectance values in all four multi-spectral bands of the IKONOS imagery. Although the ET-unit acreages delineated within the Grapevine Springs imagery by land-cover classification were nearly equivalent to those delineated from MSAVI values, delineations in major discharge areas in the Furnace Creek imagery differed by as much as 50 percent. Based on comparisons of ET-unit delineations with aerial photographs and high-resolution infrared images, the distributions and acreages delineated from MSAVI values were considered more accurate.
A ground-water ET rate for high-density vegetation was estimated from micrometeorologic data collected at a site in the Grapevine Springs area. The site was in a dense vegetation cluster dominated by desert wild grape. ET at the site was computed using the Bowen ratio to solve the energy budget. Instrumentation included paired temperature and humidity probes, multiple soil heat-flux plates, multiple soil temperature and moisture probes, a net radiometer, and bulk rain gage. In addition, a pressure sensor was set in a nearby shallow well to acquire information on the daily and annual water-table fluctuation. Micrometeorologic data were collected at 20‑minute intervals and water levels were collected at hourly intervals. Bulk precipitation was measured about monthly during each field visit. The site was selected specifically to allow year-round access and to meet the fetch criteria required by the Bowen ratio solution. Two surface-flow sites located along primary drainage channels were established in the vicinity of the ET site. Flow measurements were made during each site visit using a portable Parshall flume.
Data collection started in September 27, 2000, and ended in November 4, 2002. Precipitation data indicate that about 90 percent of the nearly 7.5 inches of measured rainfall fell during the first one-half of the 2-year study period. Trends in water-level, surface-water-discharge, temperature, and humidity data all show responses to the greater precipitation period. Water levels fluctuated about 1.2 feet annually over the 2-year collection period and were a few tenths of a foot higher in water year 2001 than in 2002. Surface-water discharge ranged from a high of about 25 gallons per minute in late winter to a low of less than 1 gallon per minute in late summer. The seasonal decrease in surface-water discharge was attributed to increasing evapotranspiration by the local riparian vegetation.
ET at the Grapevine Springs ET site generally begins increasing in late spring and peaks in the early through mid-summer period (June and July). During this peak period, daily ET ranged from about 0.18 to 0.25 inch, and monthly ET ranged from about 5.7 to 6.2 inches. ET totaled about 2.7 feet in water year 2001 and about 2.3 feet in water year 2002. The difference in precipitation between the two water years is nearly equivalent to the difference in annual ET. Annual trends in daily ET show an inverse relation with water levels—as ET begins increasing in April, water levels begin declining, and as ET begins decreasing in September, water levels begin rising. The slightly greater ET and higher water levels in water year 2001 compared with water year 2002 are assumed to be a response to greater precipitation.
Daily water-level fluctuation varied from near zero in the mid-winter months to a maximum of about 2.3 inches in the mid- and late-summer months. During the active ET period (late spring through summer), the daily water-level change exceeded the daily ET. Similarities between ET and water-level trends indicate that ET is the primary process removing water from the water table.
The rate at which ground water is transpired at the Grapevine Springs ET site was calculated by subtracting measured precipitation from computed ET. The mean annual ground-water ET rate was calculated by averaging values computed for three different 365-day periods within the 2-year period of record. The ground-water component of ET at the Grapevine Springs ET site ranged from 2.1 to 2.3 feet, with the mean annual ground-water ET from high-density vegetation being 2.2 feet. A value of 2.0 feet was used to represent the mean annual discharge of ground water by moderate-density vegetation.
The mean annual discharge of ground water by ET for discharge areas in the Grapevine Springs imagery ranged from 9 to 405 acre-feet, and in the Furnace Creek imagery ranged from 21 to 61 acre-feet. The highest estimate, 405 acre-feet, is from the Grapevine Springs discharge area, and the lowest estimate, 9 acre-feet, is from the Surprise Springs discharge area.
The estimate of ground-water discharge by ET given for the Grapevine Springs discharge area also represents the amount of ground water discharged annually from the area’s local springs and seeps and the ground-water requirement of the area’s natural riparian vegetation. Estimates of the total volume of ground water discharged from other major spring discharge areas were not attempted primarily because of uncertainties associated with ongoing diversions. Acknowledging some uncertainty in the estimate of the ET rate, a range bracketing a reasonable estimate for ground-water discharge by riparian vegetation at Grapevine Springs is 400–550 acre-feet.
The overall accuracy of the estimates given for ground-water discharge by ET depends on the validity of the assumptions made to calculate volumetric discharge and errors in acreages, ET rates, and ground-water ET rates estimated for delineated ET units. Assumptions that affect the overall accuracy of discharge estimates are: (1) the precipitation component of ET can be removed by subtracting measured precipitation at the site, (2) no sources other than precipitation and ground water contribute to ET, (3) ground water is evaporated and transpired only from those areas delineated as part of an ET unit, (4) the variation in evapotranspiration within a discharge area can be described using only two ET units, and (5) ET rates assigned to each ET unit adequately represent the average values for the area. The estimate of ground-water discharge depends on the validity of the assumption that all springflow and seepflow is evaporated internally or transpired. The potential error resulting from these assumptions is not expected to greatly alter any of the estimates given in this report.
Estimates of ground-water discharge given in this report account only for ground water locally lost to the atmosphere by evapotranspiration and are not inclusive of any springflow diverted, evaporated, or transpired outside the discharge area. Absent knowledge of estimates of other outflow, values given in this report should be considered minimum estimates of the volume of ground water discharged at major spring complexes along the eastern margin of Death Valley.
U.S.
Department of the Interior | U.S. Geological
Survey
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