Many agricultural regions around the world rely on melt from mountainous snowpacks for irrigation. As climate change-induced snow droughts intensify, water resource managers will need more efficient and accurate methods to characterize the snowmelt cycle and forecast water availability. Here, we integrate in-situ and remotely-sensed data to assess the relative contributions of groundwater and the current season’s snowmelt to irrigation water supply for water year 2023 (WY, Oct 1 – Sep 30) for a montane headwater catchment in southwestern Montana (423 km ², elevation ranges between 1465 m and 3270 m). We analyze Sentinel-1 Synthetic Aperture Radar (SAR) data to approximate dates of snowmelt runoff onset at 10 m resolution every twelve days. We find that the watershed’s median date of snowmelt runoff onset in WY 2023 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 against monthly samples of snow and groundwater. We find that stream water below the highest diversion point is predominantly composed of groundwater. The maximum contribution from snowmelt was measured in May at 22%. Results demonstrate alignment between two disparate approaches for estimating snowpack contribution to stream flow. While our work focuses on a catchment in Montana, the approaches used are potentially applicable globally for agricultural regions that rely on snowmelt for irrigation, particularly in poorly instrumented areas.