Assessing uncertainty of erodibility factor in national cooperative soil surveys: a case study at Fort Hood, Texas

Soil erodibility is the inherent susceptibility of soil to be lost to erosion. The Universal Soil Loss Equation (USLE) is an erosion model that predicts soil loss as a function of soil erodibility (known as K-factor), as well as topographic, rainfall/runoff, cover, and support management factors. The National Cooperative Soil Survey (NCSS) has assigned each soil series (minimum mapping unit) one value of K representing classes of soil erodibility. Information contained in those surveys implies that K-factor values are free of estimation errors, and remain unchanged not only across whole soil series but also over time. However, evidence provided by soil science literature challenge those assumptions. The objective of this study was to evaluate variability and uncertainty in K-factor as reported by the NCSS at Fort Hood, Texas. We obtained one K value from each of 524 soil samples collected in the late summer of 1998 following a 10 m square grid. For each soil series, sampled K means were significantly different than K values proposed by NCSS, and intraseries coefficients of variation showed variability of up to about 20%. Sampled K values disregarded NCSS soil series boundaries and departed from NCSS K values as the distance from the soil series boundary increased. The USLE estimates of soil loss using sampled K versus NCSS K were also significantly different. Limitations in K estimation from typical pedons of original soil surveys, soil misclassification, as well as changes in surface soil structure may have contributed to the differences found.