Effects of Topography on Runoff Coefficient and Flood of hillslopes Watershed Using TOPMODEL

Runoff coefficient (RC) is included in the rational method for computation of runoff, and is used in water balance to measure infiltration and the excess rainfall. The RC is the percentage of rainfall which is made runoff, and depends on topography, slope, soil texture, plant coverage, and rainfall. These features alter temporally and spatially along the watershed. A semi-distributive model, TOPMODEL is such that the topography of the hillslope and the contributing levels play the main role in runoff generation. It is also capable of computing soil moisture deficit at each point along the hillslope. In the current research, the effects of the hillslope topography on infiltration, temporally and spatially, and its spatial changes are examined using TOPMODEL and according to the topography. To this end, first, the parameters of TOPMODEL and its equations for complex hillslopes were developed. By combining with SCS-CN model, the impact of geometry of complex hillslopes on RC was investigated. The runoff hydrograph of the hillslopes was scutinized using the rational method combined with the complex time-area model. According to the results, the divergent hillslopes have a smaller RC, as well as flood, than the parallel and convergent ones. Also, the convex hillslopes have smaller RC, as well as flood, with respect to the straight and concave ones. As for the convergent-concave hillslopes, the runoff is 80% more than that for the divergent-convex.