THE INFLUENCE OF THE SPATIAL-DISTRIBUTION OF SNOW ON BASIN-AVERAGED SNOWMELT

Spatial variability in snow accumulation and melt owing to topographic effects on solar radiation, snow drifting, air temperature and precip itation is important in determining the timing of snowmelt releases, P recipitation and temperature effects related to topography affect snow pack variability at large scales and are generally included in models of hydrology in mountainous terrain. The effects of spatial variabilit y in drifting and solar input are generally included only in distribut ed models at small scales. Previous research has demonstrated that sno wpack patterns are not well reproduced when topography and drifting ar e ignored, implying that larger scale representations that ignore drif ting could be in error. Detailed measurements of the spatial distribut ion of snow water equivalence within a small, intensively studied, 26- ha watershed were used to validate a spatially distributed snowmelt mo del. These observations and model output were then compared to basin-a veraged snowmelt rates from a single-point representation of the basin , a two-region representation that captures some of the variability in drifting and aspect and a model with distributed terrain but uniform drift. The model comparisons demonstrate that the lumped, single-point representation and distributed terrain with uniform drift both yielde d poor simulations of the basin-averaged surface water input rate. The two-point representation was a slight improvement, but the late seaso n melt required for the observed stream-flow was not simulated because the deepest drifts were not represented. These results imply that rep resenting the effects of subgrid variability of snow drifting is equal ly or more important than representing subgrid variability in solar ra diation. (C) 1998 John Wiley & Sons, Ltd.