Modeling the USLE K-factor for calcareous soils in northwestern Iran A.R. Vaezi a , S.H.R. Sadeghi b, ⁎ , H.A. Bahrami a , M.H. Mahdian c a Department of Soil Science, Tarbiat Modares University, Tehran 14115-336, Iran b Department of Watershed Management Engineering, Tarbiat Modares University, Noor 46417-76489, Iran c Institute of Soil Conservation and Watershed Management, Tehran, 13445-1136, Iran Received 11 March 2007; received in revised form 25 August 2007; accepted 30 August 2007 Available online 9 September 2007 Abstract 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 km 2 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. © 2007 Elsevier B.V. All rights reserved. Keywords: USLE; Erodibility factor; Calcareous soil; Erosion modelling; Iran 1. Introduction Soil erosion is one of the most important envi- ronmental problems in the world, causing great eco- nomical losses every year and threatening sustainable development (Jianping, 1999). About 85k of global land degradation is associated with soil erosion, most of which occurred after World War II, causing a 17% reduction in crop productivity (Oldeman et al., 1990; Biot and Lu, 1995; Bruce et al., 1995) and environ- mental damage. For this reason, prevention of soil erosion is of paramount importance in the management and conservation of natural resources (Morgan, 1995; Available online at www.sciencedirect.com Geomorphology 97 (2008) 414 – 423 www.elsevier.com/locate/geomorph ⁎ Corresponding author. E-mail address: shrsadeghi@yahoo.com (S.H.R. Sadeghi). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.08.017