Gully Erosion after Wildfire

Predicting runoff and erosion from watersheds burned by wildfires requires an understanding of the spatial structure of both hillslope and channel drainage networks. We investigate the small-scale and large-scale structures of drainage networks using field studies and computer analysis of 30-m digital elevation model. Topologic variables were derived from a composite 30-m DEM, which included 14 order 6 watersheds within the same geological terrain (Pikes Peak batholith). Both topologic and hydraulic variables were measured in the field in two burned watersheds (3.7 and 7.0 hectares) located within one of the order 6 watersheds burned by the 1996 Buffalo Creek Fire. Horton ratios of topologic variables (stream number, drainage area, stream length, and stream slope) for small-scale and large-scale watersheds are shown to scale geometrically with stream order (i.e., to be scale invariant). Hydraulic variables (width, depth, cross-sectional area, and bed roughness) for small-scale watersheds also were found to be scale invariant across 3 to 4 stream orders. Bed roughness and width-to-depth ratio were constant across all scales. Fewer order 1 and order 2 streams were observed in the field than predicted by theory. The different hillslope drainage network pattern, composed of multiple parallel rills or multiple converging rills, may replace some order 1 streams. This reduction in the number of order 1 and 2 streams appears to be a consequence of hillslope processes.