When and where does water originate? Leveraging stable water isotopes and Synthetic Aperture Radar to assess the hydrology of a snow-dominated watershed in Southwestern Montana

Many regions around the world rely on melt from mountain snow as a water resource, including for irrigation. Under current climate trends, snow droughts are anticipated to intensify. Hence, water managers will need more efficient and accurate methods to characterize the timing and magnitude of the snowmelt cycle and water availability. This is especially true in data-scarce regions and watersheds. Here, we integrate in-situ and remotely sensed data to assess the relative contributions of groundwater and the current season’s snowmelt to the water supply for water year 2023 in a montane headwater watershed (Montana, USA; 45.27°, -110.47°) that provides critical water resources for downstream agriculture and irrigation. We analyze Sentinel-1 Synthetic Aperture Radar to approximate the dates of snowmelt runoff onset at a 10 m and 12-day resolution. For our study year, the median date of snowmelt runoff onset for the watershed was April 20. To assess relative contributions to streamflow, we compare stable water isotope ratios (δH 2 , δO 18 ) from biweekly stream water samples at low elevations with monthly samples of snow (event) and groundwater (pre-event). We find that stream water below the highest diversion point is predominantly composed of groundwater (pre-event). The maximum contribution from snowmelt (event) was measured in May at 22%. Results indicate the onset of melt in late-April and demonstrate general alignment within the temporal resolution of the Sentinel-1 and isotopic datasets. While our work focuses on a watershed in Montana and is limited to one water year, the approaches used are applicable globally and especially relevant in regions that are poorly instrumented and rely on snowmelt as a water resource.