Spatial Variability of Soil Erodibility Factor (K) of the USLE in North West of Iran

The soil erodibility factor varies spatially according to variations of some soil properties on the surface. This study was carried out to compare spatial variability of the soil erodibility factor as estimated and measured using the USLE. The study was conducted in an agricultural zone with an area of in 900 km2 in Hashtrood, northwestern Iran. In the study area, 36 square grids with a dimension of 5 km were considered. In each grid, three unit plots were installed on the southern aspect with a slope of 9%. The soil erodibility factor was estimated using the USLE nomograph and measured as mean rate of soil loss from the unit plots per unit rainfall erosivity factor on an annual basis. The results indicated that the difference between the measured and estimated soil erodibility factor was significant (P 0.001) and correlation between the two was very poor with r2= 0.21. The spherical simulations were the best models to explain spatial variations of both the estimated and measured erodibility factors. The effective range of the spatial variations of the measured soil erodibility factor (2.43 km) was smaller than that in the estimated value (11.51 km). There was a considerable difference in the effective range (P 0.001) of spatial variations between the estimated and measured soil erodibility factor on the study area. The map of the proportion of the estimated values to measured values of the soil erodibility factor was nearly uniform (between 7.4 and 9.6) on the study area. The study indicated that use of the USLE nomograph would considerably lead to over-estimation of the soil erodibility in the entire the study area.