THE MEASUREMENT OF 2 SIMPLE TOPOGRAPHIC INDEXES OF WIND SHELTERING-EXPOSURE FROM RASTER DIGITAL ELEVATION MODELS

A FORTRAN-77 computer program, FETCHR, is presented which measures two simple topographic indices of wind sheltering from a raster digital elevation model. These indices are fetch and directional relief. Fetch is defined as the distance in a specified direction from a point in the landscape to a topographic obstacle. The definition of topographic obstacles incorporates the notion that the effect of barriers to wind flow decrease in the downwind direction. Directional relief is the relative position of a point above or below the average topographic relief in a specified direction. These are general geomorphometric variables that can be measured for any rough continuous surface. The topographic sheltering indices were applied to examine and empirically model the association between topography and snow cover patterns in a Prairie agricultural landscape. Some illustrative results of this application are presented.