California's arid Central Valley relies on groundwater pumped from deep
aquifers and surface water transported from the Sierra Nevada to produce
a quarter of the United States' food demand. The natural recharge to
deep aquifers is thought to be regulated by the adjacent high Sierra
Nevada mountains, 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 mechanism. Peak annual groundwater storage
in the Central Valley lags several months behind that of groundwater
levels, which suggests 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. This suggests that Sierra Nevada snowpack changes likely impact
freshwater availability in the Central Valley aquifers. Our datasets,
analysis and process-based models link the current precipitation and
meltwater in the high mountain Sierra to deep Central Valley aquifers
through the mountain block recharge process. We call for new
hydroclimate models to account for the role of the Sierra in
California's water cycle and for revision of the current management and
drought resiliency plans.