The Importance of Interflow to Groundwater Recharge in a Snowmelt‐Dominated Headwater Basin

Understanding the sensitivity of groundwater generation to climate in a mountain system is complicated by the tight coupling of snow dynamics to vegetation and topography. To address these feedbacks, we combine light detection and ranging (LiDAR)‐derived snow observations with an integrated hydrologic model to quantify spatially and temporally distributed water fluxes across varying climate conditions in a Colorado River headwater basin. Results indicate that annual groundwater flow is an important and stable source of stream water. However, interflow decreases during drought as a function increased plant water use and the relative fraction of groundwater to streams increases. Seasonal snowmelt and vegetation water use regulate small recharge rates in the lower portions of the basin, but snowmelt transported via interflow from high mountain ridges toward convergent topographic zones defines preferential recharge in the upper subalpine. Recharge in this zone appears decoupled from annual climate variability and resilient to drought. Plain Language Summary Accumulated snow in mountain basins is a critical water source, but little is known about how groundwater is influenced by changing snowpack. We combine airborne snow observations with a physically based hydrologic model to better understand how snowmelt is partitioned across the landscape and routed to streams. Results indicate that annual groundwater is an important and stable source of water to a mountain stream with the relative fraction of groundwater increasing during drought as a function of increased plant water use and decreased soil water flow (interflow). We find that the dominant mechanism generating groundwater is topography. Specifically, snowmelt is focused via interflow from steep, mountain ridges into the upper subalpine where slopes flatten and sparse conifer forests begin to grow. This mechanism of recharge appears resilient to drought and may buffer recharge under climate change. Lower in the basin, snowmelt occurs before peak vegetation water use to allow for some groundwater generation. Interflow and monsoon rains then subsidize plant water use once snowmelt ceases but do not generate substantive recharge. Understanding the key mechanisms of groundwater recharge in headwater basins allows us to better predict a mountain stream's response to change for improved water and environmental management. Key Points Annual groundwater flow is an important and stable source to a mountain stream