Snow Distribution, Melt and Surface Water Inputs to the Soil in the Mountain Rain-Snow Transition Zone

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The timing, magnitude, and spatial distribution of snow cover and the resulting surface water inputs (SWI) are simulated at a small catchment located in the rain–snow transition zone of southwest Idaho, USA. A physically based snow model is run on this 1.5 ha study catchment, which has an elevation range of 1600–1645 masl. The catchment is divided into relatively steep (mean slope angle of 21 degrees) northeast and southwest facing hill slopes by an ephemeral stream that drains to the southeast. SWI are fundamental controls on soil moisture, streamflow generation, groundwater recharge, and nutrient cycling. Although the timing of melt events is similar across the basin, southwest facing slopes receive smaller magnitude and more frequent SWI from mid winter snow melt, while the northeast facing slope receives greater SWI during the spring. Three spatial patterns are observed in the modeled SWI time series: (1) equal between slopes, (2) majority of SWI on southwest facing slopes, and (3) majority of SWI on northeast facing slopes. Although any of these three spatial patterns can occur during the snow season, four emergent SWI patterns emerge through the melt season: (1) near uniform, (2) controlled by topographic differences in energy fluxes, (3) transitional, and (4) controlled by snow distribution. Rain on snow (ROS) events produce similar SWI between the northeast and southwest facing slopes, with the difference being attributed primarily to snow distribution. Turbulent fluxes dominate the snowpack energetics in four of the five rain on snow events, and advective fluxes from precipitation are greater than 17% during the 2 rain on snow events in December and January. Net radiation fluxes dominate spring melt events. Variations in the method used to distribute precipitation may result in large differences in total precipitation to the basin.