Impact of decling snow fraction on runoff efficiency in the rain-snow transition zone.

The proportion of annual precipitation that falls as snow, called snow fraction (S_f), in the western United States is declining. The impact of this transition on streamflow timing and magnitude is unclear.  Some studies report declining runoff efficiency (streamflow/precipitation, RE) with declining S_f , while others report no change and even increases. The causes of variability in the S_f-RErelationship involve complex interactions between climate and landscape properties. To understand and perhaps mitigate the impact of declining S_f on water resources, it is essential to be able to represent the physical processes and properties controlling that variability in predictive models. While significant insights in S_fRErelationships across catchments have been revealed in recent years, few have investigated the variability within a catchment over time. Here, we report S_f-RE relationships from two long-term, highly instrumented catchments in the rain-to-snow transition zone in southwest Idaho, USA. The Dry Creek Experimental Watershed (DCEW) and the Reynolds Creek Experimental Watershed (RCEW) have been monitoring hydrometeorological variables for approximately 25 and 60 years, respectively. Analyzing these long-term records allows us to identify potential physical mechanisms controlling the S_f-RE relationship that short-term, spatially-focused studies cannot. Preliminary results suggest that variability in the alignment of energy and water availability across the rain-dominated to snow-dominated elevation gradient controls how snow fraction impacts streamflow response.