The Big Soak Part 2:California's Big 2023 Storms Replenish Groundwater in California's Central Valley

GPS measurements of solid Earth's displacements are bringing a better understanding of the water cycle in the Pacific Mountain system of the western U.S.in particular on how water processes transfer water between storage reservoirs through the season.  In this study, we estimate change in water in the mountains of California, Oregon and Washington every 10 days with an accuracy of 0.1 mm and a spatial resolution of about 75 km.

In the 2023 rainy season, big storms dumped 1.5 m of snow and water on the Sierra Nevada.  During the 1st sequence of atmospheric rivers in Jan 2023, subsurface water increased by 0.2 m and 0.25 m of snowpack formed, accounting for most of 0.5 of water dumped. During the 2nd sequence of atmospheric rivers in Mar 2023, snowpack increased by 0.25 m but subsurface water held constant, because Earth's surface was either frozen or saturated. As the snow melted from Apr to Jun, subsurface water increased by 0.3 m.  During the 12 months of the water year from Oct 2022 to Sep 2023, subsurface water increased by 0.5 m, 1/3 of total precipitation for the year.

To estimate water change in the southern Central Valley, where there are few GPS sites recording elastic deformation, we added GRACE gravity data to execute a joint inversion.  GRACE gravity resolves water change with coarse spatial resolution, but GRACE resolves change in water in southern Central Valley given that GPS determines water change in the mountains surrounding the Valley.  We estimate that Central Valley groundwater increases slowly by 0.5 m from Jan 2022, the time of the 1st atmospheric river to Jul 2023, the time Sierra Nevada snow has melted.  We find that in the southern Central Valley, groundwater increase slightly exceeds cumulative precipitation. We postulate that this is because significant water is moving from the Sierra Nevada to the Central Valley deep underground (mountain block recharge).  It is believed to take decades to centuries for groundwater to flow from the Sierra Nevada to the Central Valley, but an increase in fluid pressure in the Sierra Nevada could move groundwater underground perhaps over tens of days.

In conclusion, GPS and GRACE data are characterizing spatial and temporal fluctuations in water storage and how the water cycle transfers water between different reservoirs.