Quantifying Snowmelt-Driven Runoff and Estimating the Regional Water Balance of the Smith River Watershed, Montana, USA

Abstract. Seasonal snowpacks in snow-dominated watersheds across the western United States play a critical role in sustaining regional water resources by delaying runoff and regulating streamflow. Understanding snowmelt contribution to runoff is essential for quantifying regional water resources. While prior studies have utilized proportional approaches to estimate this contribution, a lack of incorporating the physical processes simulating snowpack evolution yields some uncertainty regarding the fate of deposited snow. This study explored water balance approaches and estimates snowmelt-induced runoff in the Smith River Watershed (SRW) located in Montana for a five-year period between the water years of 2017 and 2021. Annual watershed-averaged water balance analyses were carried out using a Noah Multiparameterization land surface model (LSM) dataset and hybrid observation-model-based approaches. While the hybrid approach provides more accurate estimates for water budget variables, the LSM-based approach was more suitable for the annual water balance analysis due to its ability to close the water budget and simulate relevant hydrological processes. The analysis also revealed that underestimations of evapotranspiration and sublimation in Noah-MP result in an overestimation of runoff. Snow-induced runoff was quantified by separating rain- and snow-generated surface and subsurface runoff using outputs from the Noah-MP dataset and hybrid mass balance approaches. The analysis yielded a value of ~32 % (85 mm) of runoff originating from snowmelt during the study period—substantially lower than previous regional estimates (~65 %) within the SRW reported in the literature. The study’s results highlight the value of utilizing land surface models to accurately quantify snowmelt contributions to runoff in mountain watersheds, and underscore the importance of continued refinement of multi-method approaches for representing watershed hydrology and supporting water resource management in snow-dominated regions of the western United States.