FLUCTUATION IN SUBSURFACE WATER INFERRED FROM GPS ELASTIC DISPLACEMENTS:INSIGHT ON WATER CYCLE PROCESSES AND THE CRITICAL ZONE

We have, over the past eight years, developed the capability to apply GPS positioning to understanding water processes. We first carefully distinguish between different phenomena deforming solid Earth and next invert elastic vertical displacements for change in total water, snow, and ice at Earth's surface as a function of location at quarter-degree intervals of latitude and longitude and time each month from Jan 2006 to Present. We are finding:

1. Not all rain and melting snow runs off into rivers. In California's Sacramento-San Joaquin-Tulare (SST) River basin, the mean seasonal oscillation in subsurface water is half yearly cumulative precipitation. Snow and lake surface water accounts for 1/3 of the seasonal oscillation in stored water, while subsurface water in the critical zone, consisting of soil moisture and groundwater, accounts for 2/3.

2. In California's mountains, more subsurface water is lost during drought and gained during heavy precipitation than in the land surface hydrology models. During harsh drought from October 2011 to October 2015, the SST River basin lost a total of about 100 cubic kilometers of water, about six times that lost in the hydrology models.

3. By combining GPS positioning and GRACE gravity, we estimate, that California's fertile Central Valley has lost groundwater at a mean rate of 2 cubic kilometers per year, with 2/3 of the loss coming from the southern 1/3 (Tulare River) part of the Central Valley.

4. Solid Earth subsided as much as 24 mm in elastic response to the roughly 1 m rise in Great Lakes surface water from 2012 to 2020. In the seasons, total water in the Great Lakes watershed is maximum in March (at the time of maximum snowpack), six months before Great Lakes surface water peaks in September.

In conclusion, GPS and GRACE are strongly constraining variations in water in the critical zone that contribute to understanding the ecosystem in Earth's thin living skin. There are significant seasonal oscillations and interannual variations stored in the 'world water towers', that is, in the mountains.