Effect of variable roughness on runoff

Overland roughness is one of the key parameters in hydraulic modeling of watershed runoff. During the evolution of a runoff hydrograph the flow roughness is not constant. Hypothesizing a nonlinear relation between surface roughness and flow depth, this study incorporates variable roughness in a kinematic wave (KW) model and investigates its effect on runoff from four watersheds of varying sizes and hydroclimatic conditions. The hydrographs yielded by the KW model with varying overland roughness were in satisfactory agreement with observed runoff hydrographs and the hydrograph shape was well preserved. To quantify the effect of variable roughness on runoff, the hydrographs generated by the KW model with variable roughness were compared with those generated with constant overland roughness. Based on the study, it was observed that hydrograph shape is, in general, better preserved in case of variable roughness than the constant roughness.