REGIONAL WATER TABLE (2002) AND WATER-LEVEL CHANGES IN THE MOJAVE RIVER AND MORONGO GROUNDWATER BASINS, SOUTHWESTERN MOJAVE DESERT, CALIFORNIA by Gregory A. Smith, Christina L. Stamos and Steven K. Predmore
Static water-level measurements collected during the spring of 2002 by the U.S. Geological Survey (USGS), the Mojave Water Agency (MWA), and local water districts from about 660 wells were used to construct a regional water-table map, which shows the altitude of the water table and general direction of groundwater movement in the Mojave River and Morongo groundwater basins. Water-level contours from the 2000 water-level map (Smith 2002) were used as a guide to interpret the 2002 water-level contours in areas where 2002 water-level data were not available. In addition to being available on the interactive data map, the 2002 water-level data and contours are shown on a PDF formatted plate. Historical water-level data were used in conjunction with data collected from this study to construct 37 water-level hydrographs to show both long-term (as early as 1930) and short-term (beginning in 1990) water-level changes in the Mojave River and Morongo groundwater basins. Water-level changes between spring 2000 and spring 2002 were determined by comparing water levels measured in the same well during both periods.
Historical water-level data were used in conjunction with data collected during this study to determine both long-term (1930–2002) and short-term (1997–2002) water-level changes in the Mojave River and Morongo groundwater basins. Long-term water-level changes are depicted by 28 water-level hydrographs (PDF formatted plate, hydrographs). Some hydrographs combine data from more than one well to show water-level changes over a greater period of time in a particular area. The long-term hydrographs in the Mojave River groundwater basin show that water levels have declined between 50 and 75 ft in the Alto subarea since the mid 1940s, about 100 ft in the Harper Lake region of the Centro subarea since the 1960s, and as much as 100 ft in the area northwest of Newberry Springs in the Baja subarea since the early 1940s. In the Morongo groundwater basin, the long-term hydrographs show little or no change in most of the subbasins, but there have been significant water-level declines in five of the subbasins due to pumpage. Water levels have declined about 40 ft in the Joshua Tree subbasin since the early 1960s, about 50 ft in the Reche subbasin since the early 1960s, about 100 ft in the Lucerne subbasin since the mid 1950s, about 150 ft in the Surprise Spring subbasin since the early 1950s, and over 300 ft in the Warren subbasin since the mid 1940s (location map, PDF formatted plate).
Nine short-term hydrographs were constructed from data collected between 1997 and 2002 in the Mojave River groundwater basin (hydrographs) to record the effects of seasonal recharge and discharge along the river, and the effects of evapotranspiration of riparian vegetation, which is minimal during winter. These short-term hydrographs show that, since 1997, there has been some recharge to the floodplain aquifer from stormflows in the Mojave River in the Alto and Centro subareas, but that there has been minimal recharge from stormflows in the Baja subarea. In the Transition zone, the groundwater levels in the vicinity of well 7N/5W-23R3 (PDF formatted plate) have been maintained by recharge from treated wastewater that is discharged about 4 mi upstream by the Victor Valley Wastewater Reclamation Authority (location map).
A water-level change map was prepared by comparing water levels from spring 2000 and spring 2002 in the same wells. Of the 474 wells for which water-level data had been collected for both years, about 24 percent had water levels in 2002 that were within 0.5 ft of the water levels in 2000, and about 56 percent had water-level declines of 0.5 ft or more. About 21 percent of the wells had declines greater than 5 ft. In the Mojave River groundwater basin, about 66 percent of the wells had water-level declines of 0.5 ft or more and 27 percent of the wells had declines greater than 5 ft. In the Morongo groundwater basin, 36 percent of the wells had water-level declines of 0.5 ft or more and 10 percent of the wells had declines greater than 5 ft
The water-level change data also show that about 19 percent of the 474 wells had water-level increases. These increases occurred near Victorville, near Harper Lake (dry), and in Yucca Valley (Warren subbasin) (location map, PDF formatted plate). Near Victorville, pumpage from the numerous municipal wells caused fluctuations in water levels and affected static water-level measurements in the wells in that area. In the Harper Lake (dry) area, there has been a significant reduction in pumpage during the last decade (Stamos and others, 2001), resulting in steadily increasing water levels since the early 1990s. Well 11N/4W-30N1 is the only well in the Mojave River Basin that had a water-level increase greater than 5 ft that was not due to fluctuations in nearby pumpage. The only area in the Morongo groundwater basin that had water level increases greater than 5 ft was the Warren subbasin, where artificial-recharge operations in Yucca Valley (PDF formatted plate, location map, Warren Subbasin map) have caused some water levels to rise more than 160 ft.