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Single, Undrained, Closed Basin

Antelope Valley, Calif., which is an example of a single, undrained, closed basin, is a large topographic and ground-water basin in the western part of the Mojave Desert in southern California. Antelope Valley occupies part of a structural depression that has been downfaulted between the Garlock and the Cottonwood­Rosamond Faults and the San Andreas Fault Zone (fig. 26).

Consolidated rocks that yield virtually no water underlie the basin and crop out in the highlands that surround the basin. They consist of igneous and metamorphic rocks of pre-Tertiary age that are overlain by indurated continental rocks of Tertiary age interbedded with lava flows. Alluvium and interbedded lacustrine deposits of Quaternary age are the important aquifers within the closed basin and have accumulated to a thickness of as much as 1,600 feet (fig. 27).

The alluvium is unconsolidated to moderately consolidated, poorly sorted gravel, sand, silt, and clay. Older units of the alluvium are more compact and consolidated, somewhat coarser grained, more weathered, and more poorly sorted than the younger units. The rate at which water moves through the alluvium (the hydraulic conductivity of the alluvium) decreases with increasing depth.

During the depositional history of Antelope Valley, a large intermittent lake occupied the central part of the basin and was the site of accumulation of fine-grained material. The rates of deposition varied with the rates of precipitation. During periods of relatively heavy precipitation, massive beds of blue clay formed in a deep perennial lake. During periods of light precipitation, thin beds of clay and evaporative salt deposits formed in playas or in shallow intermittent lakes. Individual beds of the massive blue clay can be as much as 100 feet thick and are interbedded with lenses of coarser material as much as 20 feet thick. The clay yields virtually no water to wells, but the interbedded coarser material can yield considerable volumes of water. During deposition of the lacustrine deposits, alluvial material that was supplied from the San Gabriel Mountains encroached upon the lake and forced it northward, which resulted in a northward transgression of alluvium over lacustrine deposits. The subsurface extent of the buried lacustrine deposits is shown in figure 27. The lacustrine deposits underlie the central part of the basin and have a somewhat lenticular shape. The thickest section is near the center of the basin, and the deposits thin towards the edges of the basin. Near Little Buttes and near the east and north edges of Rogers Lake, the deposits pinch out (fig. 27, section B­B¹).

Along the northern and southern boundaries of the basin, the lacustrine deposits are about 100 and 400 feet thick, respectively, where they abut buried escarpments of consolidated rocks (fig. 27, section A­A¹).

Near the southern limit of the basin, southeast of Lancaster, the lacustrine deposits are buried beneath about 800 feet of alluvium, but near Rosamond Lake, they are exposed at the surface (fig. 27, section A­A¹).

Two aquifers, which are separated by the lacustrine deposits, are in the alluvial material (fig. 27).

The upper aquifer is the principal and most used aquifer and contains water under unconfined, or water table, conditions. Where the lower, or deep, aquifer underlies lacustrine deposits, it contains water under confined, or artesian, conditions; elsewhere, unconfined conditions prevail. Transmissivity values for the principal aquifer (fig. 28) are estimated to range from less than 1,000 to more than 10,000 feet squared per day.

The transmissivity of an aquifer is a measure of how rapidly water will pass through the aquifer; the greater the transmissivity, the faster the movement of the water and the more water the aquifer will yield to wells. Where the principal aquifer is thin, either near its boundaries or on the uplifted parts of fault blocks, its transmissivity is low; where the aquifer is thick or consists of coarse-grained deposits, or both, the transmissivity is high. The estimated transmissivity of the deep aquifer (fig. 29) ranges from about 2,000 to 10,000 feet squared per day and is greatest where the aquifer is thick.

The transmissivity of the deep aquifer varies less than that of the principal aquifer (compare figs. 28 and 29) probably because the thickness of the deep aquifer is more uniform than that of the principal aquifer. Ground water in the Antelope Valley Basin moves from the base of the San Gabriel and the Tehachapi Mountains toward Rosamond Lake in the north-central part of the basin (fig. 30).

As ground water moves eastward across the western limit of the lacustrine deposits, part of the water moves above the lacustrine deposits to recharge the principal aquifer and part moves below the lacustrine deposits to recharge the deep aquifer. Major faults that cut the alluvial deposits in Antelope Valley, especially the Randsburg­Mojave Fault (fig. 30), act as partial barriers to the movement of ground water. Water-level differences of more than 300 feet in the same aquifer are present across the Randsburg­Mojave Fault. Along several other faults, the water table is several tens of feet higher on the upgradient side of the fault than on the downgradient side. An estimate of the shape of the predevelopment potentiometric surface of the principal aquifer in 1915 (fig. 30) shows that before extensive pumping began, the water table was near the land surface in the central part of the basin; ground water moved northward and northeastward, and discharged by evapotranspiration at Rosamond Lake, which was dry. Withdrawal of ground water from the principal aquifer and the subsequent lowering of the water table reduced this natural discharge. By 1961, the direction of ground-water movement in the principal aquifer had been reversed from northeastward to southward and southeastward, toward the center of the basin in the area immediately southeast of Rosamond Lake (fig. 31).

The main change in the potentiometric surface was the development of areas of low water levels near the withdrawal centers and the resultant reversal in the direction of ground-water flow near these areas. Ground water leaks through the lacustrine deposits between the principal and deep aquifers even though the lacustrine deposits do not readily yield water to wells. Based on the hydraulic heads for the two aquifers, water leaks downward along the western and southern periphery of the lacustrine deposits. In the north-central part of the area underlain by the lacustrine deposits, water leaks upward. Because of the large withdrawals from the principal aquifer, the area of upward leakage has expanded toward the areas of concentrated withdrawal (fig. 31). The aquifers in Antelope Valley are recharged primarily by infiltration of streamflow that originates in the mountainous areas that surround the valley. The average annual precipitation on the valley floor is less than 10 inches, and runoff is minor. For the most part, streamflow that enters the valley is intermittent. During storm periods, streamflow enters the valley along its perimeter and moves across the surface of the alluvial fans toward the playas at Rosamond and Rogers Lakes. As the streams flow across the alluvial fans, all the streamflow generally infiltrates the permeable surficial deposits on the fans. Because of the desert conditions, much of the infiltrating water is quickly lost by evaporation or as transpiration by riparian vegetation. The remainder of the water infiltrates downward through the alluvial deposits until it reaches the water table. The drainage area tributary to Antelope Valley is about 385 square miles. Runoff from about 20 percent of this area is measured and the collective average annual discharge at the measured points is about 24,300 acre-feet. By calculating the measured runoff per unit area and extrapolating this value to unmeasured areas, the total runoff that enters the valley was estimated to be 40,700 acre-feet. Evapotranspiration is the major natural discharge of ground water in Antelope Valley. Ground water generally discharges by evaporation from the water table where the water table is within 10 feet of the land surface, and, where vegetation is present, transpiration may also occur. Evaporation from an open body of water in Antelope Valley was measured at about 114 inches per year, which is an upper limit for evaporation of ground water. Because evapotranspiration from the ground-water system is complex, exact values cannot be determined. The use of ground water for agriculture in Antelope Valley began about 1880, when wells were drilled near the center of the valley and yielded flowing water in quantities sufficient for irrigation. In 1891, more than 100 wells were in use, but most had stopped flowing. About 1915, intense use of ground water began when a large number of wells were drilled and equipped with pumps. An estimate of annual withdrawal rates from 1915 to 1975 is shown in figure 32.

The maximum rate of withdrawal of about 400,000 acre-feet per year is about 10 times the estimated annual recharge to the basin. Water removed from storage in the aquifers was a major part of the ground-water withdrawals, and severe water-level declines resulted. By about 1950, studies showed that ground-water withdrawals in the valley were greatly in excess of natural recharge and withdrawals were curtailed. The geographic distribution of withdrawals was generally unchanged between 1915 and 1960. After 1960, withdrawals were redistributed by abandoning some wells and adding some new wells. With the new distribution, the center of withdrawal was split into two areas; one was approximately 5 miles southeast, and the other approximately 10 miles southwest of Rosamond Lake (fig. 31).

Withdrawals from the deep, or confined, aquifer in Antelope Valley have caused an increase in leakage to the deep aquifer from the principal aquifer along the western and southern peripheries of the lacustrine deposits. This leakage has locally lowered the water table in the principal aquifer and has resulted in the reduction of natural discharge from the aquifer. Most of the declines in the principal aquifer, however, are the consequence of withdrawals from that aquifer. Field data are not available to show the effects of water-level declines on the amount of natural discharge, but the results of a digital flow model indicate that most of the natural evapotranspiration from the center of the valley might have ceased by 1950 (fig. 33) because water levels in the principal aquifer were too deep to allow evaporation or transpiration.

Ground water in closed basins is commonly highly mineralized because discharge by evapotranspiration increases the concentrations of minerals in the water. Some of the minerals might precipitate at or near the center of the basin. However, dissolved-solids concentrations in ground water remained practically the same or decreased slightly in Antelope Valley between 1908 and 1955 (fig. 34); this was probably caused by the reduced evapotranspiration that resulted from declining water levels in the principal aquifer.