Comparison between estimated annual soil lossusing RUSLE model with data from the erosion pins and plots in Khamsan representative watershed

The presentstudy aimed to compare the annual soil loss prediction of RUSLE model with the soil erosion measurements using erosion pins and plots in Khamesan representative watershed, Kurdistan Province. For this purpose, the distributed annual soil loss was estimated by RUSLE model. The suspended sediment samples were then collected daily for one year (2015/7 to 2016/6) in hydrometry station at the watershed outlet. Soil erosion was also measured in pins and plot located in North, West and East aspects of control subwatershed at the same period. The sediment delivery ratio (SDR) was then calculated through dividing total sediment load and erosion of the watershed resulted from three methods of RUSLE, erosion pins and plots. Results indicated that in plot method, the erosion generalized to the whole watershed (0.06 t ha-1 y-1) was much lower than reality and therefore, SDR was overestimated (655%). In erosion pin method, the erosion generalized to the whole watershed (76.79 t ha-1 y-1) was much more than reality and therefore, SDR was underestimated (0.51%). Whereas in RUSLE method, SDR was estimated more acceptably (2.21%) and estimated soil erosion by model (18.53 t ha-1 y-1) was clearly closer to reality. Therefore, generalizing the results of erosion pins and plots considering only the area ratio, can not be a suitable estimate of erosion to the whole watershed. Investigating watershed topography showed that low-slope area in the middle and downstream probably is the main factor of sediment trapping and decreasing sediment transport ratio to the watershed outlet.