Uncertainty of the Soil Erodibility Factor (K), Estimated Using the USLE Nomograph in Calcareous Soils of Hashtrood, (North-West of Iran)



In the Universal Soil Loss Equation (USLE), Soil erodibility factor (K) is estimated through nomograph. Using the nomograph in calcareous soils may lead to uncertainty in estimating the erodibility factor. This study was conducted in Hashtrud, (north west of Iran) from March 2005 to March 2006. Field measurements were carried out in an agricultural area of 30 km × 30 km. Thirty six land grids were considered in the study area and three standard plots installed in a dry-farming land of each grid. K value was assessed based on soil loss per unit of the rainfall erosivity factor (R) and estimated using the USLE nomograph. Results indicated that the measured K value (0.0042 t h MJ-1 mm-1) was on the average 10.98 times lower than the estimated value (0.0360 t h MJ-1 mm-1). Mean Percentage Error of the estimation was 974.6. These results reveal that using the nomograph leads to uncertainty of the soil erodibility factor in the study area. Regression analysis showed that the soil erodibility factor is affected by mineral soil particles, organic matter, lime, and gravel (R2=0.85, p<0.001). Lime is a main factor in causing the uncertainty in the estimation of K.