The soil erodibility factor is supposed as a key parameter in predicting soil erosion. To evaluate the applicability of incorporating soil surface stoniness in estimating K-factor this research was conducted in Khorasan-e-Razavi province in the northeast of Iran. The values of the K-factor varied between 0.018 and 0.053 by an average of
0.038 t ha h ha−1 Mj−1mm−1 by Wischmeier’s nomograph equation and those by incorporating surface stoniness ranged from 0.006 to 0.053 with an average of 0.033 t ha h ha−1Mj−1mm−1. It was demonstrated that the most important parameters affecting soil erodibility were very fine sand and silt particles, as well as surface stoniness. The zonation of K-factor by both approaches revealed that the plains with mostly fine textural classes with a high percentage of very fine sand and silt particles along with lower percentage surface stoniness exposed to high soil erodibility compared to high lands in mountainous regions. It was revealed that the surface stoniness affects soil erodibility, where the values of estimated K-factor by incorporating surface stoniness showed 0 to 77.23 % with an average of 18.06 % reduction in soil erodibility compared with predicting soil erodibility by Wischmeier’s nomograph equation. Incorporation of the surface stoniness into the K-factor resulted in a mean 27.92 % reduction of soil erodibility in the upland mountainous regions (>1500 m asl), and 13.75 % reduction in the flat plain areas (≤ 1500 m asl). It was revealed that the study area is greatly vulnerable to erosion due to high levels of soil erodibility, hence precise soil conservational and management practices in rehabilitating programs are of great importance in the study area.
Key Words: Erosion, Soil erodibility, Kst-factor, K-factor, Surface stoniness, Khorasan-e-Razavi