The Big Soak: Change in Water in 2023 in North America's Pacific Mountain System

We are strengthening the application of GPS's capability to estimate change in total water using measurements of elastic displacements of Earth's surface; breaking down total water into its components such as snow, soil moisture, and groundwater; and integrating GRACE gravity data to infer change in total water in groundwater basins.

In California's Sierra Nevada, GPS each day tracks the dumping and dissipation of storm water.  In Water Year 2023, total water increased abruptly during each of two sequences of snow-dominated atmospheric rivers.  Subsurface water, which we take to be total water inferred from GPS minus snow water equivalent, to rise in early January at the time of the first AR sequence, remain constant from late Jan through March (with no increase during the second AR sequence), and rise from April to June as the snowpack melts.  Subsurface water increases in the Sierra Nevada by 0.6 m from Oct 2022 to Jun 2023, 45 per cent of cumulative precipitation of 1.4 m.  Such a big rise in subsurface water begins to rejuvenate the Sierra Nevada critical zone (Earth's living outer layer between the top of the trees and the bottom of groundwater) and to replenish subsurface water lost during the prior 3 years of drought from 2020 to 2022.

Change in total water in California's Central Valley can be determined neither by GRACE alone nor GPS alone.  There GPS records primarily Earth's poroelastic response, from which water change is difficult to infer.  GRACE cannot distinguish water change in Central Valley from water change in the Sierra Nevada without assuming a hydrology model.  We integrate GPS elastic displacements and GRACE gravity to estimate water change in the Central Valley.  In the rigorous inversion, GPS determines water change in the Sierra Nevada and Coast Ranges and the remaining water change from GRACE is placed in the Central Valley.  We find Central Valley groundwater increased by 0.75 m in the first nine months of Water Year 2023 (the biggest gain ever recorded), replenishing more groundwater than lost during the prior 3 years of drought.