Ambiguity in the results of USLE K–factor obtained by nomograph and erodibility equation

Abstract

Universal Soil Loss Equation (USLE) is the most applied model for soil erosion assessment. It takes into consideration five soil erosion factors among which soil erodibilty factor (K) is a measure of inheritant reistance of soils to erosion. In the formulation of USLE model, K–factor was determined experimentally, and then the authors created USLE nomograph to facilitate the work, and in the next step, created the equation for faster K–factor determination. This paper aims to compare K–factor obtained by USLE nomograph and USLE equation. A total number of 108 soil samples have been collected in Western Serbia. For the determination of K–values, particle size distribution and soil organic matter were analyzed in the laboratory, whereas soil permeability and soil structure were assessed according to the USLE document procedure. The results have been compared using basic statistics. The average K–values obtained by nomograph and equation differ for almost 15%. There is statistically significant correlation between the two methods, but coefficient of determination amounts to 0.5802, which indicates pretty high deviation. Root Mean Square Error (RMSE) amounts to 0.008, which is quite high, whereas index of agreement amounts to 0.93, representing good agreement. These results are ambiguous. We think that 15% of absolute error is a threshold value between single samples, which occur in only 56% of the cases. Therefore, the obtained results indicate the gaps in the determination of K–factor and could contribute to further improvment of soil erodibilty determination and consequently, soil erosion assessment.