Effectiveness of watershed management measures on soil erosion and sediment yield reduction (Case study: Doholkooh Watershed, South Khorasan Province)

Decreased fertility, destruction of aggregates and erosion are the most common forms of soil degradation. Soil erosion is carried out by erosive agents, which remove the soil from its origin place. Among the measures to control unauthorized erosion are measures that prevent water erosion agents, which are called watershed management. Given the 60-year history of watershed management measures in the country, the evaluation of these measures has received less attention. The purpose of this study is to investigate watershed management measures to reduce erosion and sedimentation in Doholkooh Watershed.

Considering that before the implementation of watershed management measures, erosion and sedimentation of the study area was calculated using the Modified PSIAC (MPSIAC) model, in new studies for the accuracy of the work (evaluation of the executive effects on the rate of erosion and sedimentation), the same model (MPSIAC) was used. In this model, nine effective factors in erosion are considered, each of which has a score depending on its severity and weakness, and finally, by considering these scores, the amount of sediment in the area is considered.

The results showed that due to the implementation of watershed management plans, the average specific sedimentation of the watershed has increased from 6.7 to 5.18 t ha-1. As a result, the average of special sedimentation at the watershed level has decreased and the area covered by the upper classes of special sedimentation has also been reduced. During the 11-year statistical period after the implementation of the project, the total erosion of Doholkooh Watershed was reduced from 28393.7 t y-1 to 16679.5 t y-1 (40% reduction) and the total sediment of the watershed was also reduced from 15647.6 t y-1 to 12103.5 t y-1 (21% reduction). During the implementation process in some sub-watersheds, confinement operations were not performed properly, in which the amount of litter decreased and the amount of bare soil increased, which resulted in a high volume of runoff in these sub-watersheds. Surface erosion has increased compared to the past and other areas have decreased.

Given that the purpose of watershed management operations is to stabilize the soil and provide suitable conditions for plant growth, the implementation of extensive biological operations and exclosure management in the basin had a positive effect on reducing erosion and sedimentation of the watershed and was able to prevent soil particles from separating slowly. Also, the soil particles that were separated from their place, after a path, finally, were collected inside the canals behind the constructed dams, and by accumulating sediments behind the constructed structures, its transfer to the canals was prevented.