Evaluation and comparison of representative hill slope and raster based hill slope methods for computation of topography factor in USLE

In watershed management and conservation programs, hill slope form (slope length and gradient) is the most important indicator of erosion intensity. In the Universal Soil Loss Equation (USLE), combined effects of slope length and gradient has been introduced as the topography factor (LS). Computation of this factor is difficult and ambiguous one among other factor of the USLE. Considering the importance of the LS factor, in the present study some of the equations for LS computation including representative hill slope and raster based hill slope methods has first been explained. Then LS factor was computed using four equations, six conditions of these methods in the Kamal Saleh dam watershed in the Markazi Province. Then performance of values using precision index has been evaluated. The results showed that precision index value in representative hill slope method for USDA diagram is 0.883 and for Mishra equation is 0.860. These results were under-estimation for two equations. In raster hill slope method with resolutions of 30, 50 and 100 meters; precision index values were 1.031, 1.326 and 1.837 respectively. These results showed over-estimation for theses three conditions. According to the results, LS factor value of Moore and Wilson equation with resolutions of 30 m showed the highest precision. Therefore this model can be introduced as optimal model in Kamal Saleh watershed.