59th EASTERN SNOW CONFERENCE Stowe, Vermont USA 2002 1 Northwest Watershed Research Center, United States Department of Agriculture-Agricultural Research Service, 800 Park Blvd., Suite 105, Boise, ID 83712. 161 Simulating Wind Fields and Snow Redistribution Using Terrain-Based Parameters to Model Snow Accumulation and Melt Over a Semi-Arid Mountain Catchment ADAM WINSTRAL 1 AND DANNY MARKS 1 ABSTRACT. In mountainous regions, wind plays a prominent role in determining snow accumulation patterns and turbulent heat exchanges strongly affecting the timing and magnitude of snowmelt runoff. In this study, digital terrain analysis was employed to quantify aspects of the upwind topography related to wind shelter and exposure, to efficiently develop a distributed time- series of snow accumulation rates and wind speeds to force a distributed snow model. Parameters are presented that determined each grid cell’s topographic exposure and potential for drift development relative to observed winds. Using meteorological data taken from both an exposed and a sheltered site in the Reynolds Mountain East watershed (0.38 km 2 ) in southwestern Idaho, the terrain parameters were used to distribute rates of snow accumulation and wind speeds at an hourly time-step for input to ISNOBAL, an energy and mass-balance snow model. Model runs were initiated prior to the development of the seasonal snow cover and continued through complete meltout for the 1986 (precipitation 128% of average), 1987 (66%), and 1989 (108%) water years. A comprehensive dataset consisting of a time-series of aerial photographs taken during meltout, measured runoff, and snow data from the sheltered meteorological site were used to validate the simulations. ISNOBAL forced with accumulation rates and wind fields generated from the applied terrain parameterizations accurately modeled the observed snow distribution including the formation of drifts and scoured wind-exposed ridges, and snowmelt runoff for all three years of study. By contrast, ISNOBAL forced with spatially constant accumulation rates and wind speeds taken from the sheltered meteorological site, a typical snow-monitoring site, overestimated peak snowmelt inputs and tended to underestimate snowmelt inputs prior to the runoff peak. Keywords: distributed modeling, snow redistribution, digital terrain analysis, runoff 1. INTRODUCTION In mountainous basins, snow distribution exhibits tremendous spatial heterogeneity (Elder et al., 1991; Doesken and Judson, 1996; Luce et al., 1998; Balk and Elder, 2000). The disparate nature of snow distribution, an effect of both accumulation and melt differences, consequently affects snowmelt runoff patterns. Snow accumulation disparities in alpine basins are largely a function of wind redistribution (Elder et al., 1991; Blöschl and Kirnbauer, 1992; Luce et al., 1998; Prasad et al., 2001; Winstral et al., in press), while snowmelt contributions are additionally affected by spatially varying energy fluxes (Elder et al., 1991; Marks et al., 1998, 2001; Marks and Winstral, 2001). Spatially and temporally varying meltwater production can strongly influence discharge (Seyfried and Wilcox, 1995; Luce et al., 1998), plant communities and ecology (Barron et al., 1993, Flerchinger and Cooley, 2000), water chemistry (Woolford et al., 1996), and hillslope erosion (Tarboton et al., 1991) Though wind induced snow redistribution has often been cited as one of the strongest influences on snow accumulation patterns, very few studies have directly examined the relative impact of