Extreme Winter Precipitation Drives Recharge of Mountain Groundwater in the Sierra Nevada and Cascades of the Western United States

Groundwater flowing within mountainous regions is a critical water source for mountain and adjacent low-elevation communities and ecosystems. Despite their importance, quantifying storage changes within these groundwater systems remains a challenge. To evaluate the fate of mountain groundwater and the role of extremely wet winters in replenishing groundwater storage, we use high-precision Global Navigation Satellite Systems surface displacements to constrain changes in groundwater storage within the Sierra Nevada and Cascade mountain ranges, two important mountainous regions spanning much of the westernmost US. We find that groundwater storage has significantly declined over the past two decades, comprising over 90% of the total water storage lost in these regions, associated with recurrent, severe, multi-year droughts. However, extremely wet winters can replenish groundwater by more than twice the average annual flux over short periods, driving these systems from historical lows to above normal conditions. Furthermore, discharge from these groundwater systems is 2–3 times less variable than recharge, indicating they release a relatively constant amount of water year over year, allowing for surplus storage following extremely wet winter periods. These findings show that extreme precipitation (snow + rain) rapidly recharges mountain groundwater and can offset multi-year losses. Moreover, mountain groundwater systems can retain storage gains for at least 1 year post-event, providing a durable source of streamflow, agricultural, and municipal water supply. As extremely wet winters are predicted to become more intense and frequent, we hypothesize they may help offset groundwater loss due to aridification, sustaining ecosystems and buffering against a new, drier climate normal.