Improvement to the prediction of the USLE K factor

- There is a great need for accurate estimators of the K for general soil types.
- Lime in soil makes better aggregate stability and can increase infiltration rate.
- The inclusion of lime and slope to the USLE model resulted in a better estimate of K.
- A newly developed regression equation is recommended.

In the Universal Soil Loss Equation (USLE), the soil erodibility factor (K) corresponds to the collective effects of the detachment susceptibility of soil and the sediment transportability as well as the amount and rate of runoff under a given rainfall erosivity. Based on the USLE equation, K is sensitive to the particle size distribution (M), the percentage of organic matter (%OM), soil structure (Z), and soil permeability (perm). This study evaluated the sensitivity of K to lime content (%lime) in the soil and slope (%slope) of the site. Although the effects of the slope factor (S) on the amount of soil loss (A) have been independently taken into account in the USLE, our results and other studies showed that K is highly sensitive to other factors including %lime and %slope. To evaluate the appropriateness of the USLE nomograph and other methods for estimating K and to develop a K estimation method for limy soils, a set of K values were measured in northern Iran using standard plots and natural precipitation events, for four different land uses (forest, rangeland, irrigated farming, and dry farming) and three slope categories (3-8%, 8-18% and 18-40%). Results indicated that there was considerable association between K and soil properties including the contents of sand, silt, very fine sand, organic matter and particularly lime, as well as slope inclination. A strong linear relationship was observed between the K values estimated from our model and the measured K was observed (adjusted R2 = 0.89), indicating that considering lime and slope gives a better estimate of K.