WRIR 01-4195:
Ground-Water Discharge Determined from Estimates of Evapotranspiration,
Death Valley Regional Flow System, Nevada and California
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The DVRFS lies within the southern Great Basin region, an internally drained subdivision of the Basin and Range physiographic province. The region is characterized by low rainfall, intermittent streams, large internal surface drainages, and sparsely distributed spring-fed oases. The dominant physiographic features are linear, north trending mountain ranges separating broad, elongated valleys, formed in response to a long and ongoing period of crustal extension (Stewart, 1980, p. 110). Large vertical displacements along faults offset bedrock blocks separating north-trending mountain ranges from similar trending alluvial-filled valleys. The region includes several large, prominent valleys (Amargosa Desert, and Pahrump and Death Valleys; fig. 1) and major mountain ranges (Amargosa, Panamint, Kawich, and Sheep Ranges, and Spring Mountains; fig. 1).
Major mountain ranges rimming the larger valleys typically rise more than 5,000 ft above valley floors. Higher peaks typically crest at more than 8,000 ft above sea level. The highest peaks, reaching more than 11,000 ft above sea level, are found in the Spring Mountains and Panamint Range. One of the more striking elevation contrasts occurs in Death Valley where peaks in the Panamint Range rise as much as 11,000 ft above the valley floor. Valley floors are typified by a gently sloping terrain and range in altitude from about 200 ft below sea level in Death Valley to about 5,000 ft above sea level in the northernmost valleys.
Most of the major mountain ranges in the general region are composed of pre-Cenozoic rocks of diverse age and lithology (Stewart, 1980). Paleozoic carbonate rock and Paleozoic and Proterozoic siliciclastic rock, and Tertiary volcanic rock constitute the primary rock types of the hills, ridges, and mountain ranges. The intervening valleys are filled primarily with alluvium, lacustrine and palustrine deposits, and volcanic rock.
The climate of the DVRFS is arid in the south to semiarid in the north. The southern part of the flow system lies within the Mojave Desert, an area characterized by short mild winters, long hot summers, and low annual rainfall and humidity. The northern part of the flow system lies within the Great Basin Desert, an area characterized by cold winters, warm summers, and low-to-moderate annual rainfall and humidity. The wide ranges in altitude and latitude over the DVRFS contribute to climatic conditions that vary dramatically on both seasonal and daily time scales. Temperatures, higher at lower altitudes and more southern latitudes of the DVRFS, range from winter lows below zero in mountainous areas to summer highs that exceed 120°F in Death Valley. In many valleys, the daily temperature range exceeds 30°F. Precipitation also ranges widely over the DVRFS and is dependent on altitude, latitude, and location relative to surrounding mountain peaks. Mean annual precipitation is lowest on the valley floors at the more southern latitudes and ranges from less than 2 in. in Death Valley to more than 25 in. in the Spring Mountains (Daly and others, 1994).
A great diversity of plants, fish, and local wildlife are found throughout the area. Much of the biota relies on water originating from mountain runoff and from mountain and valley springs scattered about the area for survival. Spring pools and their associated drainages and wetlands provide habitat for numerous species of endemic and rare fish, aquatic insects, and plants. Ash Meadows, a major ground-water discharge area (fig. 1), has the highest concentration of endemic species in the continental United States (Chaplin and others, 2000, p. 198). A notable example of one of these endemic species is the endangered Devils Hole pupfish (Cyprinodon diabolis) whose only natural habitat is limited to a small ground-water pool known as Devils Hole in Ash Meadows. Other prominent species prevalent throughout the area are the Desert Bighorn Sheep (Ovis canadensis) and the Desert Tortoise (Gopherus agassizii).
Vegetation throughout the area is diverse and exhibits a strong dependence on climate, water availability, and soil chemistry. More dense growths are concentrated near springs, on poorly drained bottomland, and at higher altitudes in the major mountain ranges. This vegetation provides food and shelter to numerous birds, insects, reptiles, and small mammals. Although the area is characterized as desert, plant assemblages and species are numerous and include many varieties of grasses, reeds, shrubs, and trees. Vegetation supported by local springflow includes groves of ash, cottonwood, willow, and mesquite; thick stands of saltcedar; expansive meadows of salt, wire, and bunch grasses; and open marshland of cattails, reeds, and bulrush. Extensive stands of phreatophytic shrubs, including greasewood, rabbitbrush, seepweed, seep willow, and wolfberry, are found in many valleys lowlands. Upland areas beyond the influence of local drainages support more classic desert flora including sparse covers of healthy creosote bush, saltbush, and desert holly. Mountainous areas and highlands are dominated by sparse to dense woodlands that primarily include juniper, piņon pine, and various species of conifer.
The area occupied by the DVRFS is drained internally with Death Valley serving as the terminal drainage of the flow system. Drainage features consist primarily of intermittent streams fed by spring snowmelt or infrequent, major storms. Only a few short reaches, located downgradient of major springs, flow year round. Perennial flows are greatest in winter, when cooler temperatures and dormant vegetation result in reduced ET rates. Many of the individual intermountain basins are themselves internally drained with only a few having any surface outlet. The Amargosa River, the largest and most continuous drainage within the flow system, drains about 5,800 mi2 and is the only major drainage into Death Valley originating outside the valley proper (fig. 1).
A total of 27 hydrographic areas1 (fig. 3) are recognized within the DVRFS. Hydrographic areas are delineated primarily from topography and geologic structures, and generally correspond to major surface drainages. These areas serve as the basic units used by State and local agencies for water-resources planning, and in combination, form the regional flow systems delineated throughout the area (Harrill and others, 1988).
Ground water in the DVRFS passes through a diverse assemblage of rocks. These rocks differ substantially in terms of age, composition, and water-transmitting properties. The northwestern part of the flow system is predominantly volcanic rock, the eastern and southern parts are predominantly carbonate and siliciclastic rocks, and most major valleys are filled with unconsolidated deposits derived from the surrounding highlands (Laczniak and others, 1996, fig. 4).
Ground water originates from precipitation falling on the higher mountain ranges and mesas in the northern and eastern parts of the region (Winograd and Thordarson, 1975; Waddell and others, 1984; Laczniak and others, 1996; Harrill and Prudic, 1998). Regional flow generally is southward and westward away from the major recharge areas toward Death Valley and other intermediate areas of ground-water discharge (fig. 1). Ground water flows primarily through faults, fractures, and joints in consolidated rock, and through the coarser grained sand and gravel deposits in unconsolidated sediment. Beneath the major valleys, ground water flows primarily through unconsolidated valley-fill deposits and, where present, through underlying fractured and faulted volcanic and carbonate rocks. Beneath highland areas, ground-water flows through fractured and faulted volcanic and carbonate rock. Low permeability siliciclastic (quartzite and siltsone) and crystalline rocks, fine-grained valley-fill deposits (clay), and non-fractured volcanic rock form the major confining units of the flow system. Total ground-water flow is estimated at about 70,000 acre-ft/yr with the largest proportion moving through highly transmissive, fractured and faulted carbonate rock in the eastern and southern parts of the flow system (Winograd and Pearson, 1976; Harrill and others, 1988, sheet 2; Dettinger and others, 1995).
Ground-water flow rates and directions are influenced by abrupt changes in the water-transmitting properties of the rocks. Tectonic movements along major faults often juxtapose rocks of differing permeability and can intensely fracture the nearby host rock. Permeability contrasts caused by these faults and other lithologic controls create local pathways by which some ground water exits the regional flow system and discharges to land surface. Ground water, some of which originates at or flows beneath the NTS and Yucca Mountain area, emerges as seeps and springs in the alluvium, and as springs in fractured and faulted volcanic and carbonate rock. Most local springflow evaporates or is transpired by the local vegetation. Springflow, while relatively constant throughout recent history (Tim Mayer, U.S. Fish and Wildlife Service, written commun., 1997), varies substantially across the flow system. Measured springflow ranges from less than 1 gal/min at numerous small springs scattered about the area to nearly 3,000 gal/min at the largest spring in Ash Meadows (fig. 1). Ground water not discharged to the surface, exits the discharge area and continues flowing down gradient toward the next area of discharge. This underflow differs from area to area and can be a major component of the water budget.
The few communities, Federal facilities, and agricultural, mining, ranching, and recreational interests within the area of the DVRFS rely almost entirely on ground water for their water supply. A large part of this need is fulfilled by spring-flow diversions, but the demands of larger users and those more distant from springs are met by pumping ground water. Most wells produce water from valley-fill deposits, but some larger supply wells produce from fractured and faulted carbonate and volcanic rock. The withdrawal of ground water from much of the area is limited by Federal mandates instituted to protect rare and endangered species dependent for their survival on regional springflow in Ash Meadows and Death Valley.
1 Formal hydrographic areas in Nevada were delineated systematically in the late 1960's by the U.S. Geological Survey and Nevada Division of Water Resources for scientific and administrative purposes (Cardinalli and others, 1968; Rush, 1968). The official hydrographic-area names, numbers, and geographic boundaries continue to be used in Geological Survey scientific reports and Division of Water Resources administrative activities.
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