Complex Riparian Interactions Mediate Groundwater Storage and Runoff During Snow Drought

In seasonally snow-covered watersheds, snow and snowmelt play an outsized role in producing the streamflow and groundwater recharge that downstream communities rely on. As many mountainous headwater systems experience warmer winters and declining snowpacks, unravelling how low snow, or snow drought, years translate to changes in water availability is crucial for predicting the future of water resources across the region. Decades of research show that the propagation of precipitation deficits to streamflow and groundwater deficits depends on the interactions between soil moisture, evapotranspiration and plant response to water stress. However, very few empirical studies have compared how snow drought propagation differs from meteorological drought. This study presents new insights into how riparian-stream interactions dictate runoff generation and groundwater recharge in a semi-arid, non-perennial headwater stream during a dry snow drought (low annual precipitation, low snow), a warm snow drought (average annual precipitation, low snow), and a wet year (above-average precipitation and snow). Time series of shallow soil moisture, stream channel water level, riparian groundwater and stable water isotopes show that water in and below the stream channel is more responsive to summer and fall precipitation events during both low snow years compared to the wet year, which reflects less sustained groundwater inputs during both dry and warm snow droughts. We see the lowest contribution of snowmelt reflected in late-season groundwater during the dry snow drought, with a slightly higher contribution during the warm snow drought. Using convergent cross-mapping to detect causal interactions between riparian and stream processes reveals strong interactions during each year; however, the interactions driving stream corridor groundwater dynamics are the strongest and most complex during the warm snow drought year. With many regions expected to experience warmer summer temperatures and more winter precipitation falling as rain, this study highlights the increasing role that warm season climate and vegetation dynamics will play in mediating how snowmelt becomes runoff and recharge in mountain systems as riparian zones become more water-limited.