Sustained Water Loss in California's Mountain Ranges During Severe Drought From 2012 to 2015 Inferred From GPS
Drought struck California during 7 of the 9 years from 2007 to 2015, reducing the state's available water resources. Pumping of Central Valley groundwater has produced spectacular land subsidence. Uplift of the adjacent Sierra Nevada mountains has been proposed to be either tectonic uplift or solid Earth's elastic response to unloading of Central Valley groundwater. We find that of the 24 mm of uplift of the Sierra Nevada from October 2011 to October 2015, just 5 mm is produced by Central Valley groundwater loss, less than 2 mm is tectonic uplift, and 17 mm is solid Earth's elastic response to water loss in the Sierra Nevada. We invert GPS vertical displacements recording solid Earth's elastic response to infer changes in water storage across the western U.S. from January 2006 to October 2017. We find water changes to be sustained over periods of drought or heavy precipitation: the Sierra Nevada lost 15 ± 19 km³ of water during drought from October 2006 to October 2009, gained 18 ± 14 km³ of water during heavy precipitation from October 2009 to October 2011, and lost 45 ± 21 km³ of water during severe drought from October 2011 to October 2015 (95% confidence limits). Such large changes are not in hydrology models: snow accumulation in October is negligible and long-term soil moisture change is small. We infer that there must be large loss of either deep soil moisture or groundwater in river alluvium and in crystalline basement in the Sierra Nevada. The results suggest there to be parching of water in the ground during the summer of years of drought and seeping of melting snow into the Sierra Nevada in the spring of years of heavy precipitation.
Total water storage inferred from GPS in this study as a function of location each month from January 2006 to October 2017 are available by anonymous ftp at sideshow.jpl.nasa.gov in the directory /home/ftp/pub/usrs/argus/water/west.us; please contact firstname.lastname@example.org for information. We are grateful to two anonymous reviewers, editor Paul Tregoning, Roland Burgmann, Kristel Chanard, and Manoo Shirzaei for constructive edits, suggestions, and criticism. We thank Claudia Faunt for the USGS CVHM model and Mu Xiao and Dennis Lettenmaier for the UCLA model of Central Valley groundwater change. NASA ROSES ESI 2013 supported this project. JPL's GPS team determined most of the position-time series in this study; Geoff Blewitt and Corne Kreemer at Nevada Geodetic Laboratory determined the remainder of the series. We are grateful to Greg Deemer at the National Snow and Ice Data Center and to Karl Tarbet at the U.S. Bureau of Reclamation for technical help on obtaining hydrologic data. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.
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