On determining resistance to interrill overland flow

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Abstract
Simulated rainfall was applied to runoff plots of four sizes at three sites to generate overland flow. The hydraulic resistance to the flow was assessed using the Darcy‐Weisbach friction factor which was calculated both from direct measurement of discharge and depth of flow and by fitting model hydrographs based on the kinematic wave equations to the rising limbs of the runoff hydrographs. Results show that there is no consistent variation in the friction factor as a function either of plot size or of the method used in its calculation. However, the direct measurement of depth and discharge appears to give more reliable results. Values for the Darcy‐Weisbach friction factor observed here are an order of magnitude greater than those obtained by Abrahams et al. (1994) in experiments on nearby plots but where runoff was generated by flow onto the upper end of the plots from a trickle pipe. It is concluded that determination of friction factors to be used in modeling runoff and soil erosion must be made by direct measurement of flow hydraulics of rain‐induced overland flow rather than from the runoff hydrograph or by using trickle‐induced flow.

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