Investigating Groundwater Depletion and Aquifer Degradation in Central Valley California from Space

The Central Valley in California includes one of the world's largest and yet most stressed aquifer systems. The large demand for groundwater, accelerated by population growth and extreme droughts, has been depleting the region's groundwater resources for decades. However, the lack of dense monitoring networks and inaccurate information on geophysical aquifer response pose serious challenges to water management efforts in the area and put the groundwater at high risk. Here, we performed a joint analysis of large SAR interferometric data sets acquired by ALOS L-band satellite in conjunction with the groundwater level observations across the Central Valley. We used 420 L-band SAR images acquired on the ascending orbit track during period Dec 24, 2006 - Jan 1, 2010, and generated more than 1600 interferograms with a pixel size of 100 m × 100 m. We also use data from 1600 observational wells providing continuous measurements of groundwater level within the study period for our analysis. We find that in the south and near Tulare Lake, north of Tule and south of Kaweah basin in San Joaquin valley, the subsidence rate is greatest at up to 20-25 cm/yr, while in Sacramento Valley the subsidence rate is lower at 1-3 cm/yr. From the characterization of the elastic and inelastic storage coefficients, we find that Kern, Tule, Tulare, Kaweah and Merced basins in the San Joaquin Valley are more susceptible to permanent compaction and aquifer storage loss. Kern County shows 0.23%-1.8% of aquifer storage loss during the study period, and has higher percentage loss than adjacent basins such as Tule and Tulare Lake with 0.15%-1.2% and 0.2 %-1.5% loss, respectively. Overall, we estimate that the aquifers across the valley lost a total of 28 km3 of groundwater and 2% of their storage capacity during the study period. Our unique observational evidence including valley-wide estimate of mechanical properties of aquifers and model results will not only facilitate monitoring water deficits and estimating water storage variations, but is also useful to water managers, policy makers, and the communities living in water-stressed regions for development and management plans. Ongoing analysis using data from Envisat, Sentinel-1 and Radarasat-1 satellites will further play a key role is characterizing the evolution of groundwater resources.