California's arid Central Valley (CV) relies on groundwater pumped from deep aquifers (i.e., >50m) and surface water transported from the Sierra Nevada to produce a quarter of the United States’ food demand. Similar to other basin aquifers adjacent to high mountains, the natural recharge to CV’s deep aquifers is thought to be regulated by the adjacent high mountains of the Sierra Nevada, but the underlying mechanisms remain elusive. We investigate large sets of geodetic remote sensing, hydrologic, and climate data and employ process-based models at annual time scales to investigate possible recharge mechanisms. Peak annual groundwater storage in the CV lags several months behind groundwater levels, suggesting a longer transmission time for water flow than pressure propagation. We further find that peak groundwater levels lag the Sierra Nevada snowmelt by about one month, consistent with an ideal fluid pressure diffusion time in the Sierra’s fractured crystalline body. Our results suggest that high mountain snowpack changes likely impact freshwater availability in the basin aquifers. Our analysis and process-based models link the current precipitation and meltwater in the high mountain Sierra to deep CV aquifers through mountain block recharge process, highlighting the importance of longer groundwater flow paths through bedrocks for recharging deep aquifers in CV and other basin aquifer systems adjacent to mountains globally. This underscores the need for new hydroclimate models to fully account for the role of high mountains in lowland water cycles by including mountain block recharge, and revision of current management and drought resiliency plans in California.

Note: This document has been revised and resubmitted to WRR. Reviewer responses and revised manuscript are included below.