Effect of snow-to-rain transition on precipitation partitioning in a mountainous catchment: insights from geophysics-informed hydrologic modeling

In snow-dominated regions, snowmelt water plays a critical role in recharging the subsurface and generating streamflow. With a changing climate, the fraction of annual precipitation that falls as snow will probably decline. Rainfall and snowmelt water have different interactions with the subsurface and potentially vegetation, thus affecting the partitioning of precipitation into subsurface storage and streamflow. Currently, our understanding of how snow-to-rain transition affects this hydrologic partitioning in mountainous catchments is still limited. To take the best management practices for climate change adaptation, it is of critical importance to study how a catchment responds to such environmental disturbances.

In this study, we use the geophysics-informed hydrologic modeling to study the effect of snow-to-rain transition on hydrologic partitioning in a snow-dominated mountainous catchment in Idaho, USA. In the modeling, the subsurface structure was extracted from velocity map obtained from seismic refraction tests. Many studies has highlighted the importance of the heterogeneous subsurface in water partitioning in catchments, but accurate characterizations with traditional field techniques such as drilling are challenging. The hydrologic model developed from geophysical results is then calibrated with historical hydrometeorological measurements. Two climate change scenarios are designed to study the impact of warming on streamflow generation and water storage. In Scenario 1, a uniform warming is considered throughout the year, and an air temperature increase (+2.5 °C) is applied to change the phase of precipitation. In scenario 2, warming is only applied to the snow season (i.e., from December to April). The numerical modeling results show that a uniform warming (scenario 1) significantly promotes evapotranspiration (ET), and streamflow becomes less productive. Warming in the snow season only (scenario 2) induces an earlier, flashier streamflow but the partitioning of precipitation between storage and streamflow is not significantly changed. Compared to simulation results from traditional hydrologic modeling (without the heterogeneous deep subsurface), geophysics-informed hydrologic modeling reveals the importance of water storage in the fractured bedrock in response to the climate change.