Variability of Snow and Rainfall Partitioning Into Evapotranspiration and Summer Runoff Across Nine Mountainous Catchments

Understanding the partitioning of snow and rain contributing to either catchment streamflow or evapotranspiration (ET) is of critical relevance for water management in response to climate change. To investigate this partitioning, we use endmember splitting and mixing analyses based on stable isotope (18O) data from nine headwater catchments in the East River, Colorado. Our results show that one third of the snow partitions to ET and 13% of the snowmelt sustains summer streamflow. Only 8% of the rainfall contributes to the summer streamflow, because most of the rain (67%) partitions to ET. The spatial variability of precipitation partitioning is mainly driven by aspect and tree cover across the sub‐catchments. Catchments with higher tree cover have a higher share of snow becoming ET, resulting in less snow in summer streamflow. Summer streamflow did not contain more rain with higher rainfall sums, but more rain was taken up in ET. Plain Language Summary Snowmelt from the Rocky Mountains is crucial for the water supply in the Upper Colorado River Basin (UCRB). With reduced snowpack and earlier snowmelt due to climate change, it is important to understand how much of the snow directly contributes to streamflow and how much returns directly to the atmosphere via evaporation and vegetation use, called evapotranspiration (ET). We applied a stable isotope mass balance approach to investigate this for nine catchments in the UCRB. We found that snow sustains not only most the streamflow but also ¾ of the ET. Rainfall was mostly (2/3) lost to the atmosphere through ET. The variation of the snow and rain contributions to streamflow and ET were mainly driven by the catchment aspect and tree cover. The findings show that the timing of snowmelt (influenced by aspect) and plant water use (influenced by tree cover) determined how much snow became streamflow and ET. Key Points For the mountainous catchments in the Upper Colorado River, the fate of snow (rain) is 33% (67%) evapotranspiration (ET) and 13% (8%) summer streamflow In catchments with relatively higher tree cover, snow was more likely to evapotranspire with less rain and snow sustaining streamflow Increased rainfall led to greater share of rain in ET rather than streamflow, while snowfall variation had little effect