Influences of spatially heterogeneous roughness on flow hydrographs

Spatially uniform roughness, which although may not be adequate in all cases, is often conveniently adopted for rainfall-runoff simulations in conventional watershed hydrology. In this study, effects of spatially heterogeneous roughness on hydrological response were investigated systematically. Three types of surface roughness scenarios, including: roughness decreasing in a downstream direction (Scenario 1), roughness increasing in a downstream direction (Scenario 2) and roughness distributed at random (Scenario 3), were assigned to an overland plane. A non-inertia wave model was developed to generate hydrographs at the end of the overland plane for certain rainstorms. The hydrographs in the three scenarios showed that when rainfall duration was less than the time of concentration of the overland plane, peak discharges generated from the spatially heterogeneous roughness scenarios significantly exceeded those from the spatially uniform roughness condition. The temporal equilibrium discharge which was usually observed on an overland plane under the spatially uniform roughness condition was not found in the present study. The flow peak in Scenario 1 occurred before that in Scenario 2. Nevertheless, the runoff peak could occur at the preceding, the middle or the later part of the hydrograph in Scenario 3, and this depended on the spatial distribution of the roughness. Reasons for the variation of peak discharge and the time to peak discharge were analyzed in detail based on water depth and flow velocity. Spatially heterogeneous roughness had an obviously influence on runoff generation, which ought to be handled with care in hydrological simulations.