Extreme Winter Precipitation Drives Recharge of Deep Mountain Groundwater

Persistent declines in groundwater storage observed in mountainous regions of the western US over the past two decades are expected to continue, driven by increasingly variable winter temperatures and snowpack accumulation (Carroll et al., 2024; Hall et al., 2024), threatening human and ecosystem health. However, brief but extreme periods of precipitation associated with frequent and intense atmospheric river events deposit significant amounts of water in the mountains of the western US, acting as potentially significant sources of groundwater recharge in an increasingly arid environment. Here, we provide high-resolution estimates of groundwater storage within the western US by removing estimates of water stored in winter snowpack, the soil column, and artificial reservoirs from Global Navigation Satellite Systems (GNSS) inferred estimates of terrestrial water storage (TWS) between January 2006 and June 2024. We find long-term declines in water storage within mountainous regions of the western US such as the Sierra Nevada and Cascades (approx. 355 mmand 105 mm of equivalent water thickness, respectively) align with estimates derived from GRACE/GRACE-FO and watershed mass balance models, corroborating observed aridification within mountainous regions over the past two decades. Despite these declines, we find periods of extreme precipitation, such as winters 2011, 2017, and 2023, can provide more than twice the average annual recharge of mountain groundwater (Fig.1). Furthermore, we find the state of groundwater in many mountainous regions of the west following winter 2023 were driven from record lows in autumn 2022 to above or near normal conditions and have been maintained over the past year despite moderate winter conditions in 2024, indicating that extreme precipitation events can maintain mountain groundwater storage over prolonged periods. As the strength and frequency of atmospheric river events are predicted to increase due to anthropogenic warming (Gershunov et al., 2019; Nellikkattil et al., 2023), we hypothesize that mountain groundwater storage may be maintained by extreme precipitation events in the coming decades.