Evaluation of soil loss tolerance via soil productivity and quality at a watershed scale: Haji-Ghushan watershed, Golestan province

Erosion has been known as an important factor in soil resources degradation and non-point pollutions. A wide range of damaging effects involve social, economic and environmental problems occur when soil erosion rate go beyond the allowable value. There are several method to determine soil loss tolerance that each has specific advantage and limitations. In this paper, two widely used methods have been compared in determining tolerable erosion on a watershed scale. The first approach is based on assessment of the productivity index (PI) and the second approach is based on soil depth and soil quality index. A particular minimum data set of soil properties include infiltration, water holding capacity, organic carbon, aggregate stability, bulk density, and fertility status (nitrogen, phosphorus, potassium) was used to calculate criteria. The results showed the calculated tolerable erosion by two methods are closely related. The average tolerable soil loss for the study area was determined 9.2 and 10.2 ton/ha/year based on soil productivity method and soil depth-quality approach, respectively. The PI-based approach is preferred over the soil depth-quality approach for two reasons: First, the PI-based approach is a depth-wise and chemo-physical properties of topsoil are compared with subsoil layers. Second, the soil depth-quality based approach is a general guide that cannot take the differences between soils into account in details. As both model take into account just onsite effects of erosion, these values can be used for managers and decision-makers to soil conservation regardless off offsite impacts.