Did the Record-Breaking Rains of 2023 Revive Central Valley's Deep Aquifers?
Authors: Sadhasivam Nitheshnirmal , Werth Susanna , Carlson Grace , Shirzaei Manoochehr 
Published: September 2024

After enduring severe drought conditions from 2020 to 2022, California experienced an exceptionally wet 2023 water year, driven by a series of atmospheric rivers, unusually frequent tropical storms, and extreme snowfall during the winter months. Statewide precipitation levels significantly exceeded normal averages, with some areas reporting increases of over 100%. Notably, the Sierra Nevada mountains witnessed unprecedented snowfall, with averages reaching approximately 200%, leading to a surge in surface water levels above the average level statewide, potentially reducing the need for groundwater overdrafts for agricultural purposes and allowing for the recharge of aquifers. The Central Valley of California, a major agricultural hub responsible for producing more than a quarter of the food consumed in the US, heavily depends on groundwater from deep confined aquifers for the water used in farming. Years of excessive groundwater extraction have led to significant land subsidence, primarily impacting the San Joaquin Valley and Tulare Basin in the southern part of the Central Valley. Following the extremely wet water year of 2023, questions persist about whether the increased availability of surface water has positively impacted the deep confined aquifers of the Central Valley, which have suffered from severe overdraft for decades. To address this pressing question and decipher the complex recharge dynamics of the Central Valley's deep confined aquifers, we combine water storage datasets from GRACE-Follow On (GRACE-FO), Interferometric Synthetic Aperture Radar (InSAR) based vertical land motion (VLM), measurements of groundwater level, and surface hydrological data. We estimate the recharge volume of both shallow (unconfined) and deep (confined) aquifers during the wet water year of 2023 (Sep 2022 &#8211; Aug 2023). We observed a significant rise in groundwater head levels across most wells in the Central Valley, with approximately 599 wells (89%) recording positive rates. A seamless, high-resolution (~75m) spatiotemporal map of vertical land motion (VLM) for the Central Valley has been generated using a mosaic of three overlapping SAR frames from Sentinel-1 satellite images. The VLM map reveals maximum uplift and subsidence rates of 10.4 cm/year and -29.3 cm/year, respectively. We noted that about 58.83% of Central Valley experienced subsidence, while the rest (41.17%) uplifted. By integrating GRACE-FO and hydrological data, we have identified a net gain in groundwater storage amounting to approximately 19.4 &#177; 4.1 km&#179; during the 2023 water year. We use head level changes from well measurements and aquifer properties to quantify groundwater storage changes in Central Valley's unconfined (shallow) and confined (deep) aquifers. In addition, we apply VLM data in a 1D poroelastic model to independently estimate groundwater storage change in deep aquifers of Central Valley during the post-drought wet year and compare the results with those from GRACE-FO observations and groundwater well measurements. Our widely applicable approach combines multiple remote sensing technologies with ground-based observations to monitor groundwater dynamics accurately. The findings of this research enhance our understanding of how deep confined aquifers respond to transient, extreme weather phenomena like atmospheric rivers and unusually severe winter events, which are expected to become more common with climate change.