Comparing the Performance of Several Erodibility Indices' Equations of USLE Model at Laboratory Condition

The performance of four erodibility indices' equations including K-USLE, K-Vaezi, K-EPIC and K-Dg were evaluated by measuring soil loss at four dryfarming soil samples in the Rainfall and Erosion Simulation Laboratory of Soil Conservation and Watershed Management Research Institute. For each soil sample, several experiments (33, 64 and 110 mm/h rainfall intensities, RI, at 12% slope and 64 mm/h RI at 6%, 12% and 25% slopes) were conducted for 30 min in two replicates and the soil loss were measured. In addition, the volume of rills was measured at the end of each experiment. The observed erodibility (K-Obs.) was obtained by dividing the soil loss to RLSCP factors product of USLE in each treatment which showed a high scatter. The 110 mm/h RI on 12% slope was selected as the reference treatment due to complete developed rill erosion network. Then, the estimated K value was compared to it. The results indicate an underestimation of K-Vaezi (and a negative value in one case) and an overestimation of the other three indices. The lowest difference is for K-Dg with 3-5 times overestimation. In the Next step, for each treatment, a ratio was calculated by dividing the K-Obs. of each pair soils and compared with the corresponding ratios of estimated erodibility indices using t-student statistic. The results of ratios analysis showed that the K-Dg and K-EPIC indices had better agreement with K-Obs. at 5% probability level. As a conclusion, it is recommended to use K-Dg by applying a reduction coefficient for similar soils.