Comparison Between USLE and USLE-M for Estimation of Erodibility for Six Soil Series with Variable Rock Fragments

Abstract

Soil loss is estimated by different models in which the soil erodibility factor, K, is one of the important parameters, especially in soils with rock fragments. The objective of this study was to determine the soil erodibility factor by the Universal Soil Loss Equation (USLE) and USLE-M models by direct soil loss measurements for six soil series (three of them contained high amounts of rock fragments) under a rainfall simulator with rainfall intensities of 26–55 mm h−1. The erodibility factor of the six soil series varied between 0.0053 and 0.0125 t ha h (ha MJ mm)−1, in which the three gravelly soils (Bamoo, Loamy-skeletal over fragmental, carbonatic, mesic, Typic Xerorthents; Kuye-asateed, Loamy-skeletal over fragmental, carbonatic, mesic, Typic Xerorthents; and Shekarbany, Fragmental, mixed, mesic, Typic Xerorthents) had medium K values [0.006 t ha h (ha MJ mm)−1]. Other soils (Pump-namazi, Fine, mixed, mesic, Fluventic Haploxerepts; Ramjerdi, Fine, mixed, mesic, Fluventic Haploxerepts; and Daneshkadeh, Fine, mixed, mesic, Typic Calcixerepts) had high K values [0.01 t ha h (ha MJ mm)−1]. Using Wischmeier–Smith nomograph, the estimated K factor (Ku) for the six soil series were by average 5.1 times that of measured K values by the USLE model. The K factor from Wischmeier–Smith nomograph (Ku) was modified for gravel contents for Bamoo, Shekarbany, and Kuye-asateed soil series, therefore, its average value was about 3.5 folds of the amount determined by the USLE model. This modification decreased the Ku/K ratio about 33%. The erodibility factor obtained by USLE-M (Kum) for the six soil series ranged from 0.0184 to 0.0509 t ha h (ha MJ mm)−1. Kum for the six soil series was on the average 5.7 times of that measured K values by USLE models. Ratio of the modified Ku to Kum for the six soil series was on the average 1.39. Therefore, it was concluded that the USLE-M model and the Wischmeier–Smith nomograph after modification for gravel content closely determined the soil erodibility factor.