Laboratory-Scale Simulation of Runoff Response from Pervious-Impervious Systems

We posit that a more complete understanding of runoff response from urban catchments requires an assessment of the hydrologic behavior of composite impervious-pervious patches. We therefore examined how the factors of impervious extent, connectivity, and antecedent moisture content of pervious areas might affect mechanisms of runoff production at small spatial scales in a laboratory setting. We used rainfall simulation (with a storm comprised of $20,30,40\mathrm{mm}{\mathrm{hr}}^{-1}$ rainfall rates for 48, 24, and $24\min$ , respectively) to observe surface runoff from $0.6{\mathrm{m}}^2$ boxes (impervious or pervious-soil) $0.2\mathrm{m}$ deep that were connected together in series to produce different arrangements of impervious and pervious surfaces (0, 25, 50% impervious) with different connectivity to the outlet (disconnected, connected), and under two different antecedent moisture conditions for pervious areas (drier, wetter). In general, an increase in percent impervious area led to fewer opportunities for infiltration, and a quicker onset of runoff, which was intensified by wetter antecedent moisture conditions and connectivity to the outlet. Runoff rate ratios were strongly affected by antecedent moisture condition and somewhat less significantly by an interaction between impervious area extent and its connectivity status. In each impervious treatment, we observed a decreased time to runoff initiation and higher final runoff rate ratio for wetter than drier treatments. Interestingly, we found that the connectivity of 25% impervious area accounted for differences in runoff rate ratio only early in the simulation. The patterns in runoff from connected and disconnected 25% treatments eventually converged, leaving antecedent moisture conditions the only relevant factor. As impervious area was increased to 50%, we noted a precipitous decline in infiltration rates due to a reduction in infiltration opportunities and infiltration behavior of the pervious surfaces in these treatments. Evidence of return flow in the 50% disconnected treatment is presented and discussed in the context of saturation-excess runoff mechanisms. These experimental results are then discussed in terms of their potential extension and application to better understand aspects of urban hydrology and models thereof.