Effect of Slope Steepness Factor on Soil Loss Estimation Accuracy of the USLE Model Versions in Semi-arid Rainfed Lands, Zanjan Province

Slope steepness is one of the very important factors in soil erosion which has been defined as slope steepness factor (S) in soil erosion models such as the Universal Soil Loss Equation (USLE) and its versions. Sufficient information is not available on the accuracy of the erosion models in estimating the S-factor in semi-arid rainfed lands. Therefore, in this study, six rainfed lands with 8, 11, 15, 18, 20 and 24% slopes were selected in a semi-arid region of Zanjan and soil loss was measured at 1.5 m × 2.5 m plots under natural rainfalls during a-year period from early October 2015 to end of September 2016. Based on the obtained results, eight rainfalls among 82 rainfall events caused soil loss at the plots during the study period. The S-factor was measured using the ratio of soil loss from the plots to the soil loss from a land with 9% slope and estimated using the USLE, RUSLE, USLE-MM and AUSLE models. A strong relationship was found between soil loss and slope steepness of rainfed land (R2= 0.92). Soil loss increased strongly with increasing the slope steepness and the S-factor values estimated using the USLE and USLE-MM were about 4% and 15% higher than the measured value, respectively. The AUSLE and RUSLE showed about 13% and 10% lower estimations than the measured ones, respectively. This study revealed that the USLE is more sutible model than the other models to estimate the S-factor in the semi-arid rainfed lands.