The first study, published recently in the journal Scientific Reports, looks at monthly GRACE data to measure changes in the water stored on the ground within California’s Central Valley. The authors found that the increase in the region’s water reserves between January and March 2017 – following several years of drought – accounted for 30 percent of the total storage gain seen in the U.S. over that timeframe. The study also found that about 45 percent of the water accumulated during the wet months was lost during the following dry season.
The second study, published recently in the journal Water Resources Research, combines monthly GRACE data with other data sources, including measurements from the Soil Moisture Active Passive (SMAP) satellite and model-based estimates of water storage in the Sacramento and San Joaquin River basins. This allowed researchers to separate out the amount of water stored on the surface (rivers and lakes) versus the amount stored beneath the surface (groundwater). Over the study period, approximately one-third of the water was on the surface and two-thirds below.
The authors of this second study also noted that the water was concentrated in the northern and southern extremes of the state and relatively low in the central area, which corresponds to the region that had been most severely affected by the previous years of drought.
“The large water storage recovery we saw in California during the winter of 2016-2017 was a dramatic reversal from the large storage deficits that persisted during California’s recent prolonged drought,” said study co-author Thomas Scanlon, a senior research scientist at the University of Texas at Austin. “Although this wet winter brought substantial relief, it only partially recovered the large storage deficits that had accrued over several years of drought, and many parts of the state still face storage deficits.”
GRACE measures tiny variations in the pull of Earth's gravity resulting from the movement of water, ice and solid Earth material. One-half of the GRACE mission is led by NASA's Jet Propulsion Laboratory in Pasadena, California, and the other half by the German Research Centre for Geosciences (GFZ) in Potsdam. The satellites were built by a joint team at JPL and GFZ and are monitored from the GRACE Science Data System office at JPL.
JPL manages the SMAP mission for NASA’s Science Mission Directorate in Washington. SMAP is led by Principal Investigator Dara Entekhabi from MIT’s Earth, Atmospheric and Planetary Sciences Department in Cambridge, Massachusetts. JPL is responsible for the design and development of the SMAP mission, with day-to-day mission execution by the SMAP Mission Operations team.
The University of Texas at Austin research team is supported by NASA grants.
