Field experiments on the resistance to overland flow on desert hillslopes.

At-a-glance and downslope variations in resistance to overland flow on desert hillslopes were investigated by performing a series of experiments on six runoff plots in southern Arizona. The surfaces of these plots were irregular and covered with stones. As overland flow increased, the stones and microtopographic protuberances, which constituted the major roughness elements, were progressively inundated, thereby altering the flow resistance. Analyses of 14 cross sections revealed that the relation between the Darcy-Weisbach friction factor f and the Reynolds number Re has two basic shapes: convex-upward and negatively sloping. These findings have profound implications for the mathematical modelling of overland flow on desert hillslopes, as the computed overland flow hydrograph is very sensitive to the form of the f-Re relation. The downslope analyses indicated that there is a general tendency for f to decrease down the runoff plots, owing to the progressive downslope concentration of flow. This finding contrasts with Emmett's conclusion that f remains approximately constant downslope because of ponding behind microtopographic highs, notably vegetation mounds. (from authors' abstract)