Do Upslope Impervious Surfaces Impact the Run-on/Runoff Relationship?

As a watershed is urbanized, characteristics of runoff from new upslope impervious surfaces may differ from runoff generated on the predevelopment soil surface in quantity, time of concentration, and sediment load. This may cause changes to the erosion regime on downslope soil surfaces. We simulated rainfall at three rates (20, 30, 40 mm/h) to generate runoff from 0.6 m2 boxes. Boxes were either treated with an impervious surface or filled with soil 0.2 m deep and were connected together in series of four boxes along the 4-m slope to produce different arrangements of impervious and pervious soil surfaces (0, 25, 50% impervious) and under different antecedent soil moisture conditions. Results indicate that previously established numerical models predicting runoff characteristics as a function of run-on characteristics generate good correlations at 0% imperviousness, but these correlations become insignificant as imperviousness increases. Imperviousness significantly influenced sediment regime, suggesting that some previously established equations relating soil erosion to run-on characteristics cannot be simply applied to areas where runoff production occurs on surfaces having an impervious component.