The Effect of Inflection Angle and Relative Location on Runoff Curve Numbers from Zero-Order Watersheds

For the past 40 years researchers have been questioning Horton's model of infiltration excess for surface runoff generation, especially in humid regions. Researchers have been studying the concept that storm runoff is often generated by only a portion of the watershed and that runoff occurs first around streams or areas where the water table and soil depth readily cause surface saturation. This study evaluated the effects of inflection angle (a measure of contour curvature; defined as the angle a contour line makes with itself when crossing a drainage-way) and the relative location (the position of a land area in relation to a drainage-way) on the rate and volume of runoff. In this study, zero-order watersheds were separated into four distinct location types, which were based on their relative location to larger order drainage-ways. Traditionally-computed curve numbers for hypothetical watersheds (land areas without distinct natural drainage-ways or outlets) were shown in this study to be typically from 10 to 40 curve number values too high when used in the NRCS CN runoff model. Minor draws off of the main watershed drainage-way were shown to have both runoff curve numbers and peak runoff rates that were highly dependent on the average watershed inflection angle. NRCS runoff model error estimates were shown to be distinctly correlated (r² = 0.64) to the inflection angle. Watersheds that were located upslope from the direct extension of a stream appeared to be dependent on the relative distance of the watershed outlet to an active stream.