Identifying Subsidence using Interferometric Synthetic Aperture Radar (InSAR) in Coachella Valley, California

InSAR techniques are used to measure land subsidence associated with ground-water-level declines in the Coachella Valley, California. Since the early 1920s, ground water has been a major source of agricultural, municipal, and domestic supply in the valley, resulting in water-level declines of more than 30 m. InSAR measurements from ENVISAT show two areas in Palm Desert and La Quinta with at least 85 mm of subsidence between February 2003 and September 2004 and two areas in Indian Wells with at least 40 mm of subsidence between September 2003 and November 2004. All four of these features are located in urban areas along the mountain front of the San Jacinto and Santa Rosa Mountains, where sediments susceptible to subsidence (primarily clays) are not expected. The InSAR technique is adversely impacted in the central part of the Coachella Valley because of agricultural activity. In Palm Desert, the area of maximum subsidence is concentric. Although water-level data are not available immediately within the InSAR signal, the surrounding region exhibits fairly uniform water-level declines. These data suggest that the aquifer system is homogenous in the area of maximum subsidence. In La Quinta, the northwestern part of the subsiding area has an irregular shape that appears to be related to the contact of the alluvial aquifer and the bedrock basement complex that defines the extent of the aquifer system. The InSAR signal indicates that subsidence continues along the mountain front to the southeast, but the extent and shape are poorly defined because of agricultural activity. In Indian Wells, the areas of subsidence form two separate elongate shapes-one parallel to and the second perpendicular to the mountain front. Available water-level data indicate uniform water-level declines in the area, suggesting that the aquifer system is heterogeneous in this part of the valley. The measured subsidence pattern also is influenced by the bedrock basement complex of the San Jacinto and Santa Rosa Mountains to the south. Distinct linearity shown on the northern margin of the subsiding areas suggests the presence of a barrier to ground-water flow or an abrupt change in lithology controls the northern extent of subsidence. Recently collected and repeated Global Positioning System (GPS) observations (August 2000, August 2005), GPS time series from the Southern California Integrated GPS Network array, additional ENVISAT InSAR imagery, and detailed geological and water-level information may provide further insight into the physical controls of the processes of subsidence in the Coachella Valley.