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.

In the Indus River Basin, groundwater plays a key role in mitigating the water storage fluctuations due to climate variation and meeting the rapidly increasing water demand in agriculture dominated basins. A comprehensive understanding of groundwater dynamics is essential for a transition to more efficient and sustainable water resources management. To gain detailed insight response of water flows and storage in the Indus aquifers to agricultural activities, we build a high resolution 3D regional groundwater flow model for the entire basin. However, in practice, regional flow models, as they are most widely used, suffer from calibration challenges. To address the sparsity of in-situ groundwater data in the region and to acquire a realistic reproduction of flow dynamics, we calibrate the model using both in-situ and satellite-based estimates of ground states. We test the advantage of such a multi-objective approach by comparing its results with a single-objective approach in which we constraint the model parameter only against in-situ data. We examine and discuss the model results for flow and storage conditions, which reveal: 1) depth to water table has decreased (1998-2007) almost exclusively in urban areas (1 m), and 2) groundwater storage depletion averaged ~5cm in equivalent water thickness basin-wide over 20 years of simulations. Groundwater storage depletion results primarily from intensive groundwater withdrawal to meet extensive irrigation demands. Optimizing crop patterns and associated groundwater extraction in space and time could improve groundwater conditions.