THE EFFECT OF VARIABLE PATTERNS OF SNOW DEPOSITION AND DRIFTING ON SNOWMELT, RUNOFF, AND STREAM DISCHARGE IN A SEMI-ARID MOUNTAIN BASIN

In semi-arid mountainous regions, local topography and canopy cover strongly affect wind patterns during storms which alters snow distribution and causes the development of hydrologically significant drifts. While large drifts may cover only 5—15% of the basin area, they can hold 50% or more of the basin SWE at peak accumulation and 75—100% of the basin SWE in late spring. Snowmelt from the drifts provides essentially the only source of water to the basin in late spring and early summer, and therefore directly affects basin ecology, runoff response, and the basin hydrograph. To understand how snow distribution and drifting affect the timing and magnitude of snowmelt and the delivery of melt- water to the stream, we simulate both the patterns of snow deposition and melt over a small headwater basin in the Owyhee Mts. (the Reynolds Mountain East basin (0.36 kin2)) using the energy balance snowmelt model ISNOBAL. Simulations were run for several snow seasons in the 1980’s for which time-series aerial photos monitoring of the location and depletion of snow drifts during melt-out were available. Precipitation input was modeled as a function of topographic exposure relative to storm-event winds and the difference in storm-event catch between a sheltered site and an exposed site. Snowcover in the modeled drift zones lasted well into the spring and showed good agreement with drift assessments made from time-series late-season aerial photography. This experiment shows that disparate patterns of snow deposition and melt, including the effect of drifting, which is typical of semi-arid mountain basins, can be modeled as a function of terrain and vegetation.