Simulating snowmelt processes during rain-on-snow over a semi-arid mountain basin

In the Pacific Northwest of North America, significant flooding can occur during mid-winter rain-on-snow events. Warm.. wet Pacific storms caused significant floods in the Pacific Northwest in February 1996, January 1997 and January 1998. Rapid melting of the mountain snow cover substantially augmented discharge during these flood events. An energy-balance snowmelt model is used to simulate snowmelt processes during the January 1997 event over a small headwater basin within the Reynolds Creek Experimental Watershed located in the Owyhee Mountains of southwestern Idaho, U.S.A. This sub-basin is 34% forested (12% fir, 22% aspen and 66% mixed sagebrush (primarily mountain big sagebrush)), Data from paired open and forested experimental sites were used to drive the model. Model-forcing data were corrected for topographic and vegetation canopy effects. The event was preceded by cold, stormy conditions that developed a significant snow cover over the subbasin. The snow cover at sites protected by forest cover was slightly reduced, while at open sites significant snowmelt occurred. The warm, moist, windy conditions during the flooding event produced substantially higher melt rates in exposed areas, where sensible- and latent-heat exchanges contributed 60-90% of the energy for snowmelt. Simulated snow-cover development and ablation during the model run closely matched measured conditions at the two experimental sites. This experiment shows the sensitivity of snowmelt processes to both climate and land cover, and illustrates how the forest canopy is coupled to the hydrologic cycle in mountainous areas.