Total Water Storage Change Over the San Joaquin and Sacramento River Basins Comparing GRACE and Observational Data

In recent years, the state of California has experienced drought conditions that have not significantly improved. Of particular concern are the major sources for California’s developed water system, the Sacramento and San Joaquin River basins, which lie in the semi-arid Central Valley. Recent GRACE satellite data show a pronounced decrease in water storage in the basins over the past several years. The goal of this study is to use a combination of the most recent remote sensing products to calculate the water balance of the Sacramento and San Joaquin River basins in order to determine whether the GRACE data are accurate; and if so, the underlying causes for the decrease in water storage. Precipitation, evapotranspiration and streamflow data were assembled and compared to GRACE observations of storage change. Additionally, snow water equivalent data were compared to GRACE storage anomalies. Results show that the observed water balance (precipitation minus evapotranspiration and streamflow) agrees well with the storage changes observed from GRACE, giving confidence to the GRACE-based estimates of declining water storage. Additionally, results also indicate that the trend of decreasing water storage seen in the GRACE data may be due to decreasing groundwater supplies, which may well be the result of excessive groundwater pumping in the Central Valley. Further research will be required to better understand the forces driving decreasing water storage in the Sacramento and San Joaquin River basins.