Incorporating subsurface-flow mechanism into geomorphology-based IUH modeling

Surface- and subsurface-runoff processes on hillslopes are quite different in nature; nevertheless, the geomorphological instantaneous unit hydrograph (GIUH) simulates runoff using only the surface-flow mechanism. In this study, the GIUH model was revised to consider both the surface- and subsurface-flow processes. Kinematic-wave approximation was used to estimate the mean value of the travel-time probability distributions for runoff in surface-flow regions and channels, and Darcy's law was adopted to estimate the runoff travel time in subsurface-flow regions. Based on the linear system assumption, the hydrologic response of a watershed is the superposition of the hydrologic responses resulting from the surface flow and subsurface flow of the watershed. The IUH can adequately reflect the variation of surface-flow roughness and hydraulic conductivity. An example watershed was selected to test the capability of the revised model. The simulated hydrographs obtained using the original and revised models were basically in good agreement with the observed hydrographs. However, significant improvement of the simulation was found in the recession limb of the simulated hydrograph because flood recession was dominated by the subsurface-flow process.