Scientific Investigations Report 2007–5251
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5251
Version 2.0, June 2013
Continued monitoring in the southern Coachella Valley is warranted because ground-water levels continue to decline to record-low levels in some areas of the valley and, therefore, the significant amounts and rising rates of land subsidence documented by this study are expected to increase. Most of the changes in the vertical positions of the GPS network monuments measured in 1996, 1998, and 2000 were within the measurement errors, indicating that a longer period of time would have been needed to see significant changes. Comparing the land-surface-elevation changes of the monuments in the GPS network in 2005 with those of the previous three surveys did show changes greater than the anticipated GPS measurement errors. As a result, GPS surveys completed every 3-5 years would be sufficient to monitor subsidence rates. Spatially detailed InSAR-derived maps of ground displacements, however, could continue to be processed annually or more frequently depending on data availability, because InSAR can detect relative changes in vertical position as small as 5 mm (0.02 ft) (Hoffmann and others, 2001). Because InSAR-detected areas of subsidence spatially overlap the GPS network, future monitoring of the GPS network could provide ground truth for the more spatially detailed and higher resolution InSAR measurements, as was done during this study. Furthermore, as InSAR technology matures, it is likely that some InSAR measurements made in agriculturally active areas, such as those in the southern Coachella Valley, could result in improved spatial detail of displacement in these areas.
The frequency of water-level measurements in wells in the Coachella Valley is too low to permit meaningful interpretations of the aquifer-system response to water-level changes, particularly when paired with the more frequent InSAR measurements. Furthermore, the wells generally have long or multiple screens such that the water-level measurement is a composite measurement representing a large thickness of the aquifer system. Increasing the measurement frequency of ground-water levels in just a few piezometers (wells constructed with short screens and small diameters for monitoring purposes)—both in areas of known subsidence and in areas of relative stability—would significantly improve analysis of the relationship between changes in water levels and in land-surface elevations. In concert with more frequent water-level measurements, more frequent and high-resolution measurements of aquifer-system compaction, such as those derived from a borehole extensometer, would improve the analysis of aquifer-system response. The information derived from such a monitoring site(s) would be useful in estimating aquifer-system hydraulic parameters that govern ground-water flow and land subsidence. This information could be used to construct a numerical model of ground-water flow and aquifer-system compaction to refine estimates of governing parameters and to predict potential aquifer-system compaction which could be used by CVWD to manage water resources while considering land subsidence.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5251
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