Investigating indices of soil surface erosion and their relations to slope characteristics in semi-arid rangelands

Surface erosion is a one of the major factors of soil degradation and gradual decline in soil productivity. Little studies have been done on the surface erosion indices in the semi-arid rangelands. Therefore, this study was conducted to quantify surface soil erosion indices using the land characteristics in a semi-arid region. Five southern hillslopes with different slope gradients (9, 13, 17, 31 and 33%) which subjected to surface erosion were selected in Zanjan province. Soil samples were collected from two depths (0-5 and 5-15 cm) in four locations at a 2-m distance along the slope at two replications. A total of 80 soil samples were collected for analyzing particle size distribution. Soil surface erosion indices were determined using the proportion of particle diameter for given frequency in surface and sub-surface soils. Toward this, the proportion of particle diameter in the frequency of 40 (d401/d402), 50 (d501/d502), 60 (d601/d602) and 70 % (d701/d702) was computed. Additionally, proportions of mean weight diameter (MWDp1/MWDp2), geometric mean diameter (dg1/dg2) and geometric standard deviation (δg1/δg2) of surface and subsurface soils particles were determined. Based on the results, significant differences were found among the hillslopes in all soil surface erosion indices, whereas the differences for the locations along the hillslopes were not statistically significant. In the steep slopes, the frequency of fine particles (silt and clay) and organic matter content was very low in surface soil. Multiple linear regression analysis appeared a strong relationship between the MWDp1/MWDp2 and slope gradient (R2= 0.51, p