CHANNEL NETWORK SOURCE REPRESENTATION USING DIGITAL ELEVATION MODELS

Methods for identifying the size, or scale, of hillslopes and the exte nt of channel networks from digital elevation models (DEMs) are examin ed critically. We show that a constant critical support area, the meth od most commonly used at present for channel network extraction from D EMs, is more appropriate for depicting the hillslope/valley transition than for identifying channel heads. Analysis of high-resolution DEMs confirms that a constant contributing area per unit contour length def ines the extent of divergent topography, or the hillslope scale, altho ugh there is considerable variance about the average value. In even mo derately steep topography, however, a DEM resolution finer than the ty pical 30 m by 30 m grid size is required to accurately resolve the hil lslope/valley transition. For many soil-mantled landscapes, a slope-de pendent critical support area is both theoretically and empirically mo re appropriate for defining the extent of channel networks. Implementi ng this method for overland flow erosion requires knowledge of an appr opriate proportionality constant for the drainage area-slope threshold controlling channel initiation. Several methods for estimating this c onstant from DEM data are examined, but acquisition of even limited fi eld data is recommended. Finally, the hypothesis is proposed that an i nflection in the drainage area-slope relation for mountain drainage ba sins reflects a transition from steep debris flow-dominated channels t o lower-gradient alluvial channels.