Modeling the USLE K-factor for calcareous soils in northwestern Iran

Soil erodibility defines the resistance of soil to detachment by rainfall impact and/or surface flow force. In the Universal Soil Loss Equation (USLE), the soil erodibility (K) is estimated using the texture, organic matter content, permeability and structure of a soil. The USLE was originally developed for non-calcareous soils in the USA. However, in calcareous soils, calcium is an important factor affecting soil structure and hence may influence soil erodibility. The application of the USLE to calcareous soils therefore requires a reassessment of K. The present study evaluates K and identifies factors affecting K for calcareous soils in Hashtrood City, northwestern Iran. The soils contain 13% lime and 1% organic matter, and are mainly utilized for wheat dry farming. A square agriculture area of 900 km2 was selected and then divided into 36 grids of 5 × 5 km. The erosion unit plots at three replicates with 1.2 m spacing were installed in each grid. K was measured based on soil loss and the rainfall erosivity index from March 2005 to March 2006. The rate of soil loss resulting from 23 natural rainfall events during the study period was measured at the unit plot scale. Various soil properties including the contents of sand, silt, silt + very fine sand, clay, gravel, organic matter, lime, and potassium as well as aggregate stability and permeability were measured in the vicinity of each plot. The results show that K significantly correlates with the contents of sand, silt, silt + very fine sand, organic matter, and lime as well as water-aggregate stability and permeability. The application of principal component analysis (PCA) also indicates that the contents of clay and lime as well as permeability strongly control K. The contents of clay and lime, which have not been well considered in USLE studies, significantly decrease K due to their strong effects on aggregate stability and water infiltration into soil. K can be estimated using a linear regression equation based on the contents of sand, clay and lime.