Effects of Changing Winter Precipitation Phase on Streamflow Magnitude in Mountainous Regions

Snowmelt-produced streamflow (Q) is the dominant water source for a large portion of the global population. This is especially true in higher elevation regions in the Western United States. Precipitation is highly variable and seasonal in this region of the world. Generally, most precipitation occurs in winter and comes as snow in higher elevations. However, due to climate change, precipitation and temperature patterns are changing. It has been observed in many areas across the Western United States that the fraction of precipitation falling as snow (Sf) is declining because of a warming climate. In addition, warmer temperatures cause snow to melt earlier in the season. These changes are causing unforeseen changes to the snowmelt-driven Q relationship. Predictions on how a warming climate will affect Q response vary and often conflict. In some studies, a reduction in Sf and earlier melt-off results in less runoff generation; in others, a decrease in Sf results in more runoff generation. The relationship between changing temperatures, precipitation patterns, and the resulting runoff magnitude is poorly understood. To better understand the relationship between precipitation, precipitation phase, and the resulting Q, this study uses meteorological data and hydrological data from multiple sub-basins within the Dry Creek Experimental Watershed (DCEW) near Boise, Idaho, to identify years in each basin where most of the precipitation fell as either snow or rain. We compared the resulting streamflows by calculating runoff ratios (Ro) for each year and sub-basin to determine if Q changes with a change in the precipitation phase. In addition to finding the trend between the precipitation phase and the resulting Ro, correlation analysis was done on various other predictor variables such as soil moisture, SWE, Sf, snow persistence (Sp), and other meteorological variables to determine why Q magnitude may be changing. We found that for years where more of the precipitation fell as snow, the resulting runoff ratios were higher, and evapotranspiration, spring precipitation, soil moisture, and snow persistence were important controlling variables. We must understand how this change will affect Q magnitude and timing to better predict water availability in snow-dominated regions, which will be primarily affected by changing precipitation patterns.