GROUND-WATER DEPLETION AND SPACED-BASED MONITORING OF AQUIFER-SYSTEM COMPACTION IN THE WESTERN USA

Throughout many basins in the western USA, agricultural and municipal-industrial demands for ground water deplete ground-water resources. A hundred meters or more of ground-water level declines have been observed and in many places the declining trends continue at rates of 300 mm or more per year. Aquifer-system compaction and land subsidence have accompanied ground-water depletion in many areas where unconsolidated basin-fill deposits constitute the principal aquifer systems. Nearly 10 m of subsidence has been observed since 1925 in the San Joaquin Valley, CA; subsidence is an ongoing concern in numerous other areas of California, the Houston-Galveston, TX area, Las Vegas Valley, NV, and throughout south-central Arizona. Satellite-borne interferometric synthetic aperture radar (InSAR), combined with other geodetic techniques are proving invaluable in mapping and monitoring land subsidence in many of these basins.

Spatially-detailed (20m x 20m pixels) patterns of land subsidence at a resolution of <10 mm of vertical displacement afforded by InSAR, enable detection of areas that are more susceptible to aquifer-system compaction, especially in the southwestern USA where the SAR coverage is good. This information is valuable to resource managers who attempt to mitigate subsidence by altering the magnitude, distribution and timing of ground-water extractions. In coastal basins where subsidence is aggressively managed and contributes to a relative sea-level rise as well as problems and expenses associated with coastal flooding and loss of sensitive coastal ecosystems this information can be crucial. InSAR also contributes to our understanding of the hydrogeology of these affected basins by observing 1) the patterns of subsidence and thereby discern the location of susceptible areas within the basin; 2) differential subsidence that reveals local heterogeneities related to facies changes, faults, or transitions from over-to-under-consolidated basin-fill deposits; 3) seasonal patterns of elastic (recoverable) deformation. This information is used to improve conceptual and numerical hydrogeologic models, and when combined with ground-water level transients, to compute spatially-detailed estimates of the aquifer-system storage coefficients.