Static water-level measurements collected between February and May 2006 by the U.S. Geological Survey (USGS), the Mojave Water Agency (MWA), and local water districts from about 680 wells were used to construct a regional water-table map, which shows the altitude of the water table and general direction of ground-water movement in the Mojave River and Morongo ground-water basins. Water-level contours from the 2004 water-level map (Stamos and others, 2004) were used as a guide to interpret the 2006 water-level contours in areas where 2006 water-level data were not available. In addition to being available on the interactive map (2006 water-level contours on interactive map), the 2006 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 34 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 ground-water basins. Water-level changes between spring 2004 and spring 2006 (on interactive map) were determined by comparing water levels measured in the same well during both periods.
Long-term (1930–2006) water-level changes are depicted by 25 water-level hydrographs (hydrographs). Some hydrographs combine data from more than one well to show water-level changes over a greater period of time for a particular area. The long-term hydrographs for the Mojave River ground-water basin show that water levels have declined between 50 and 75 ft in the Alto subarea (location map, water-level hydrographs ) since the mid-1940s (wells 5N/5W-22E1, -22E2, and -22E6), about 75 ft in the Harper Lake region of the Centro subarea since the 1960s (wells 11N/4W-29R1), and more than 100 ft in the Baja subarea since the early 1950s (wells 9N/2E-20Q1, and -20K1). The long-term hydrographs for the Morongo ground-water basin 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 (well 1N/7E-32C1), about 50 ft in the Reche subbasin since the early 1960s (well 2N/6E-18B1), about 100 ft in the Lucerne subbasin since the mid 1950s (well 5N/1W-25G1), about 150 ft in the Surprise Spring subbasin since the early 1950s (well 2N/7E-2C1), and more than 300 ft in the Warren subbasin between the mid 1940s and 1994 (well 1N/5E-36K1, -36K2). The rapid decline in some wells in the Warren subbasin has been reversed since 1995 because of artificial recharge to ponds at the Hi-Desert recharge sites (recharge sites on interactive map).
Nine short-term hydrographs were constructed from data collected between 1992 and 2006 in the Mojave River ground-water 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 1992, 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 ground-water levels in the vicinity of well 7N/5W-23R3 remained stable owing to recharge from treated wastewater that is discharged about 4 mi upstream by the Victor Valley Wastewater Reclamation Authority (location map).
Water-level changes between spring 2004 and February through March 2006 can be viewed by selecting those years on the water-level change map (water-level change on interactive map); 611 wells had water-level data for both years in the study area. One quarter (155) of the water levels in 2006 were within 0.5 ft of the water levels in 2004. Almost one third (188) of the wells had water-level declines greater than 0.5 ft; 41 wells had declines greater than 5 ft. Almost half (268) of the wells had a water-level increase of 0.5 ft or more.
Of the 393 wells compared within the Mojave River ground-water basin, about one third (120) of the wells had water-level declines of 0.5 ft or more; 26 of the wells had declines greater than 5 ft. The water-level change data also show that more than half (212) of the wells compared in the Mojave River ground-water basin had water level increases of 0.5 ft or more. Most of these increases were the result of stormflow in the Mojave River during the winter and spring of 2005, which resulted in recharge to wells in the floodplain aquifer mainly along the river in the Alto subarea and the Transition zone and slight increases along the river east of Barstow (water-level change on interactive map). Some increases also occurred near Harper Lake (dry) where 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.
Of the 198 wells compared within the Morongo ground-water basin, about one third (62) of the wells had water-level declines of 0.5 ft or more; 16 of the wells had declines greater than 5 ft. The water-level change data also show that about one quarter (48) of the wells compared had water level increases of 0.5 ft or more. These increases occurred mainly in the Warren subbasin, where artificial-recharge operations in Yucca Valley have caused water levels to rise more than 100 ft since 2004 (hydrographs, water-level change on interactive map).