Among the various processes that destroy lands, soil erosion is a complex and hazardous environmental process that constitutes a serious threat to preservation of soil and water resources in Iran and has inflicted its negative effects on the vital ecosystem with the intensified utilization of nature by human beings. The present descriptive and analytic research employed the PSIAC model to study erosion and compare it with other empirical methods and models by investigating the effects of nine important and influential factors in soil erosion and sediment production in various parts of the Sefidrood Basin through giving intra-layer numerical scores to them. Erosion and landform zoning maps and also zoning maps for the nine factors were then extracted. Results indicated that 77 percent of the lands in the plain landforms faced very little erosion and 23 percent little erosion, 14 percent of the lands in the mountainside landforms were exposed to very high erosion rates, 55 percent with high erosion rates, and 31 percent with average and low erosion rates, and 17 percent of the lands in the mountain landforms experienced very high erosion rates, 58 percent high erosion rates, and 25 percent average erosion rates. In other words, the rates of erosion changed from low in the plain landforms to very high in the mountain landforms. This implies that pastures and natural resources are confronted with numerous problems. In general, considering the study carried out on the nine influential factors in the soil erosion model (the PSIAC model), we notice that the mountainside and mountainous regions are more sensitive to erosion and influential factors in erosion are more active in these regions.

In order to protect soil and water resources should be identified erodible areas of watersheds, to be able to prevent land degradation and to control erosion in the form of soil conservation planning or watershed management. Therefore, the present study aimed to classification and delineation of susceptible areas to water erosion in the Hervi watershed using Hjulstrom curve, has been conducted. Increasing of accuracy, speed and facility of spatial achievement, using GIS technology, are advantages of the present study. In this study, were separated 10 sub-watersheds based on the feeding levels of streams. Spatial variability of soil properties in sub-watersheds, such as texture and the mean diameter size of the particles using hydrometer method were measured; also the maximum potential velocity of outlet in sub-watersheds based on the characteristics of each sub-watershed were calculated. Then, status of erosion and sediment in watersheds and sub-watersheds was studied using Hjulstrom curve (based on the mean diameter size of sediment particles and the maximum potential velocity of outlet), and was comparatively classified in term of erodibility. Finally, delineation map of susceptible regions to water erosion in the study area were obtained. The results revealed that the upstream and side areas of the watershed including: Sub-watershed No.4 (8.94% of the land), has very high erodibility, and Sub-watersheds No.1 and No.9 (36.94% of the land) have high erodibility. These areas are steep and often devoid of vegetation or have poor vegetation (such as poor pastures and rain-fed farming); so, it is necessary a proper strategy to prevent further erosion.