Seasonal Variations in Groundwater Storage and Well Levels: Implications for Groundwater Recharge in the Central Valley

Abstract:

Data from both GRACE missions continue to provide insight into the alarming rates of groundwater depletion in large aquifers worldwide. Recent works show that GRACE-based estimates of groundwater storage change in the Central Valley, California, are consistent with those obtained by utilizing measurements of surface deformation and poroelastic modeling approaches. The compatibility of these different geodetic approaches provides further confidence in the usefulness of hydrogeodetic approaches to quantify long-term changes in aquifer stocks and their applicability for regional groundwater management.

Here, we further investigate seasonal variations in groundwater storage by identifying how their effect is manifested in remotely sensed GRACE and terrestrial well datasets. We perform a Wavelet multi-resolution analysis of GRACE TWS variations as well as of hundreds of groundwater well time series distributed across the Central Valley to isolate their seasonal signal component. We show that most groundwater well observations in the Central Valley indicate maximum groundwater levels at the beginning of the year between February to April and lowest water levels in the middle of the year about July to October. Meanwhile, GRACE groundwater storage estimates peak about four months later. We investigate complementary GRACE groundwater datasets, including snowcap, soil moisture, and reservoir level variations to get insight into the mechanisms leading to the temporal discrepancy. We further perform experiments with models of fluid pressure front propagation for deriving a hypothesis explaining the delay of the GRACE groundwater peak. This study raises and also addresses the question of how well different hydrological and geodetic signals that reflect groundwater discharge and recharge processes agree with one another and what are the possible causes of disagreements. We emphasize the need for interdisciplinary efforts for the successful integration of geodetic and hydrological datasets of different scales to improve our ability to utilizing geodetic sensors such as GRACE and GRACE-FO for climate research and water resources management.