Detection and Measurement of Land Subsidence Using Global Positioning System Surveying and Interferometric Synthetic Aperture Radar, Coachella Valley, California, 1996-2005

Land subsidence associated with ground-water-level declines has been investigated by the U.S. Geological Survey in the Coachella Valley, California, since 1996. Ground water has been a major source of agricultural, municipal, and domestic supply in the valley since the early 1920s. Pumping of ground water resulted in water-level declines as large as 15 meters (50 feet) through the late 1940s. In 1949, the importation of Colorado River water to the southern Coachella Valley began, resulting in a reduction in ground-water pumping and a recovery of water levels during the 1950s through the 1970s. Since the late 1970s, demand for water in the valley has exceeded deliveries of imported surface water, resulting in increased pumping and associated ground-waterlevel declines and, consequently, an increase in the potential for land subsidence caused by aquifer-system compaction. Global Positioning System (GPS) surveying and interferometric synthetic aperture radar (InSAR) methods were used to determine the location, extent, and magnitude of the vertical land-surface changes in the southern Coachella Valley. GPS measurements made at 13 geodetic monuments in 1996 and in 2005 in the southern Coachella Valley indicate that the elevation of the land surface had a net decline of 124 to 9 mm ±54 mm (0.41 to 0.03 ft ±0.18 ft) during the 9-year period. Changes at 9 of the 13 monuments exceeded the maximum expected uncertainty of ±54 mm (±0.18 ft) at the 95-percent confidence level, indicating that subsidence occurred at these monuments between June 1996 and August 2005. GPS measurements made at 20 geodetic monuments in 2000 and in 2005 indicate that the elevation of the land surface changed –192 to +51 mm ±36 mm (–0.63 to +0.17 ft ±0.12 ft) during the 5-year period. Changes at 6 of the 20 monuments exceeded the maximum expected uncertainty of ±36 mm (±0.12 ft) at the 95-percent confidence level—subsidence occurred at five monuments and uplift occurred at one monument between August 2000 and August 2005. GPS measurements at two of the five subsiding monuments for which subsidence rates could be compared indicate that subsidence rates decreased during this period compared with subsidence rates before 2000. InSAR measurements made between May 7, 2003, and September 25, 2005, indicate that land subsidence, ranging from about 75 to 180 millimeters (0.25 to 0.59 foot), occurred in three areas of the Coachella Valley: near Palm Desert, Indian Wells, and La Quinta; the equivalent subsidence rates range from about 3 to more than 6 mm/month (0.01 to 0.02 ft/month). The subsiding areas near Palm Desert, Indian Wells, and La Quinta were previously identified using InSAR measurements for 1996–2000, which indicated that about 35 to 150 mm (0.11 to 0.49 ft) of subsidence occurred during the four-year period; the equivalent subsidence rates range from about 1 to 3 mm/month (0.003 to 0.01 ft/month). Comparison of the InSAR results indicates that subsidence rates have increased 2 to 4 times since 2000 in these three areas. Water-level measurements made at wells near the subsiding monuments and in the three subsiding areas generally indicated that the water levels fluctuated seasonally and declined annually between 1996 and 2005; some water levels in 2005 were at the lowest levels in their recorded histories. The coincident areas of subsidence and declining water levels suggest that aquifer-system compaction may be causing subsidence. If the stresses imposed by the historically lowest water levels exceeded the preconsolidation stress, the aquifer-system compaction and associated land subsidence may be permanent. Although the localized character of the subsidence signals is typical of the type of subsidence characteristically caused by localized ground-water pumping, the subsidence may also be related to tectonic activity in the valley. Detection and Measurement of Land Subsidence Using Global Positioning System Surveying and Interferometric Synthetic Aperture Radar, Coachella Valley, California, 1996–2005 By Michelle Sneed and Justin T. Brandt 2 Detection and Measurement of Land Subsidence Using GPS and InSAR, Coachella Valley, California, 1996–2005 Introduction Ground water has been a major source of agricultural, municipal, and domestic water supply in Coachella Valley, California (fig. 1), since the early 1920s. Pumping of ground water resulted in water-level declines as large as 15 m (50 ft) between the early 1920s and late 1940s. In 1949, the importation of Colorado River water through the Coachella Branch of the All-American Canal to the southern Coachella Valley began. As a result of the importation of surface water, pumping of ground water decreased in the southern Coachella Valley during the 1950s through the 1970s, and water levels in some wells in the lower valley recovered as much as 15 m (50 ft). Since the late 1970s, however, the demand for water in the southern Coachella Valley has exceeded the deliveries of imported surface water, pumping has increased, and water levels have again declined. By 2005, water levels in many wells in the southern Coachella Valley had declined 15 to 30 m (50 to 100 ft) and water levels in some wells were at their lowest recorded levels. Declining water levels can contribute to, or induce, land subsidence in aquifer systems that consist of a significant fraction of unconsolidated fine-grained sediments (silts and clays). Ikehara and others (1997) reported that as much as 150 mm ± 90 mm (0.5 ft ± 0.3 ft) of subsidence occurred in the southern parts of the Coachella Valley between 1930 and 1996. Land subsidence can disrupt surface drainage; cause earth fissures; and damage wells, buildings, roads, and utility infrastructure. A large earth fissure was discovered in 1948 about 3 km (2 mi) north of Lake Cahuilla in La Quinta. Because subsidence had not been documented in the southern parts of the Coachella Valley prior to the report by Ikehara and others (1997), it is not known if this fissure formed in response to differential land subsidence during the earlier period (early 1920s–late 1940s) of ground-water-level declines. However, fissuring is recurring in this area (Clay Stevens, TerraPacific Consultants, Inc., written commun., 2006). Subsidencerelated earth fissures and reactivated surface faults have been identified in many other ground-water basins in the western United States (Holzer, 1984). The Coachella Valley Water District (CVWD) works cooperatively with local stakeholders to manage the water supply for a large part of the Coachella Valley (fig. 1). Because of the potential for ground-water pumping to cause land subsidence, the CVWD entered into a cooperative agreement with the U.S. Geological Survey (USGS) to monitor vertical changes in land surface to determine if land subsidence was occurring in the Coachella Valley. In 1996, the USGS, in cooperation with CVWD, established a geodetic network of monuments to monitor vertical changes in land surface in the southern Coachella Valley using Global Positioning System (GPS) surveys and to establish baseline values for comparisons with results of future surveys. The geodetic network needs to be surveyed intermittently to determine the distribution and amount of land subsidence. In addition, interferometric synthetic aperture radar (InSAR) data collected since 1996 have been used to detect and quantify land subsidence in areas removed from the geodetic monuments. Purpose and Scope The objectives of this study are to detect and quantify land subsidence that has occurred in the Coachella Valley from 1996 through 2005 by completing GPS surveys at the established geodetic network of monuments and by using InSAR data. This report presents the results and interpretations of GPS data collected at the monuments in the monitoring network during surveys in 1996, 1998, 2000, and 2005, and also of spatially detailed maps of vertical land-surface changes generated using InSAR data collected between 1996 and 2005. The InSAR-generated maps extend from near Palm Springs to near the Salton Sea (fig. 1). Data showing ground-waterlevel change during 1996–2005 were examined and compared with the GPS measurements and the InSAR-generated maps to determine if the vertical changes in land surface may be related to the changes in ground-water levels. Description of Study Area The Coachella Valley is a 100-km (65 mi) long, northwest-trending valley in southeastern California (fig. 1). The valley covers about 1,000 km2 (400 mi2) (California Department of Water Resources, 1964) and includes the cities of Cathedral City, Coachella, Desert Hot Springs, Indio, La Quinta, Palm Desert, Palm Springs, and Rancho Mirage. The valley is bordered by the San Jacinto and Santa Rosa Mountains on the west, the San Bernardino and the Little San Bernardino Mountains on the north, the Cottonwood Mountains and the Mecca Hills on the east, and the Salton Sea on the south (fig. 1). The Coachella Valley is drained primarily by the Whitewater River, which flows into the Whitewater Stormwater Channel and eventually discharges into the Salton Sea (fig. 1). Land-surface elevations vary from more than 70 m (230 ft) below sea level at the Salton Sea to more than 3,000 m (10,000 ft) above sea level at the peaks of the surrounding mountains. The climate of the Coachella Valley floor is arid. Average annual rainfall ranges from 80 mm (3 in) on the valley floor to more than 760 mm (30 in) on the crests of the mountains to the west and north of the valley (California Department of Water Resources, 1964). Temperatures range from about 50oC (120oF) on the valley floor in the summer to below 0oC (32oF) in the surrounding mountains in the winter.