Extreme Winter Precipitation Drives Recharge of Deep Mountain Groundwater

Extreme winter precipitation events, associated with frequent and intense atmospheric rivers, deposit significant quantities of water in mountain regions over short periods of time. Precipitation is forecast to become more variable as climate change intensifies; however, it is unclear how that will affect mountain aquifer recharge. Here we use high-precision Global Navigation Satellite Systems (GNSS) surface displacements and elastic deformation models to surface loading to estimate total water storage changes.  Using independent estimates of water stored within shallow subsurface and surface reservoirs, we isolate changes in mountain groundwater storage in two important mountain regions of the western US at high spatial (~30km) and temporal (~ 1 week) resolution. We find that groundwater storage is the dominant component of long-term total water loss within the Sierra Nevada and Cascades, composing up to 95% of the total water lost over the past two decades. However, extremely wet winters, such as that of 2023, can recharge groundwater storage by more than twice the average annual amount, driving the state of groundwater storage from historical lows to above or near-normal conditions over relatively short periods. Further, we find gains in groundwater storage associated with these events are relatively durable, persisting over several proceeding years following the extreme recharge event. Mountain aquifers have been increasingly recognized as a dynamic and critical source of water storage and release to adjacent low-elevation communities; however, persistent declines in mountain aquifer storage have been observed across the western US over the past two decades. In a future with increasingly variable precipitation, the strong influence of extreme events may act to maintain mountain groundwater, sustaining ecosystem health and buffering adjacent areas against drought conditions in between events.