Journal of Watershed Management Research

Extended Abstract
Background: At a global level, land degradation is a serious challenge to the productive capacity of water and land resources. Soil erosion, water loss, vegetation cover reduction, biodiversity loss, and nutrient depletion are all signs of land degradation. The most significant indicator of land degradation is soil erosion, which, due to the water and nutrient loss, is one of the most important indicators of soil degradation. Soil erosion and sediment production are influenced by the interplay of climate, land cover, and land use. Due to this complexity, models are often used to support soil and water management, such as the Soil and Water Assessment Tool (SWAT) and the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST). In the present study, the InVEST Sediment Delivery Ratio model was used for sediment modeling. Research on sediment modeling and soil retention in South Khorasan Province is essential due to the region's arid and semi-arid climate, which poses serious challenges of soil erosion and land degradation. Conducting these studies can help identify critical areas and provide effective management strategies to preserve natural resources and improve the region's environmental sustainability.
Estimating the economic value of functions and services of natural resources is a challenging and complex task, but essential for economic management. In recent years, the necessity of determining the value of ecosystem services and biological resources has been firmly established worldwide, and efforts have been made in this regard as well. The present study aims to quantitatively assess the ecosystem service of sediment retention using the InVEST-SDR model and its economic valuation in the watersheds of the eastern border regions of the country, located in South Khorasan Province. No research in the study area has utilized the InVEST model, highlighting a significant research gap.
Methods :The study area includes 10 watersheds in the eastern region of South Khorasan Province, covering an area of 2,008,357 hectares. Soil erosion and sediment yield modeling were initially conducted in this study. The InVEST Sediment Delivery Ratio (SDR) model, a spatial model operating at the resolution of the digital elevation model (30 m), was used to estimate soil erosion, retention, and sediment yield annually in the northeastern part of South Khorasan Province. The model first calculates the annual soil loss using the soil loss algorithm, then determines the SDR as a function of the hydrological connectivity of the basin, and finally performs the economic valuation. The cost replacement or restoration method was employed for the economic valuation of sediment retention. In this research, therefore, the economic value of preserving soil fertility was defined based on three elements: nitrogen (N), phosphorus (P), and potassium (K). Surface soil is the first part susceptible to soil erosion, and the nutrient content of the soil at this level decreases significantly. This decrease results in a reduction in organic nitrogen (N), phosphorus (P), potassium (K), and other elements.
Results :The most substantial sediment retention is observed in dense forests, with dense pastures ranking second within the region. This aligns with the conclusions drawn in similar research. The Esfadan and Shahrokht watersheds showed the highest sediment production with more than 13 and 12 million tons, respectively, among other basins. The highest sediment retention rate was in the Esfadan watershed, with 22626499.4 tons, and the lowest was in the Khaf salt marsh watershed, with 1316208.6 tons. The soil retention rate at the pixel level was also obtained in the range of 0-1058.3 tons. The lowest and highest soil loss rates were also obtained from 4845857.7 to 63854710.6 tons for the Khaf and Esfadan salt marsh basins, respectively. The range of 0-1489 tons per pixel was calculated for soil loss. On the other hand, areas without vegetation cover (especially built-up areas) have the lowest sediment retention capacity and the highest sediment export. The results of economic valuation in the present study indicate that the average value of sediment retention service per pixel (900 m²) is equivalent to 2731.48 Iranian Rials, which amounts to 30349.77 Iranian Rials per hectare.
Conclusion :The Esfadan and Shahrakht Watersheds, with over 13 and 12 million tons, respectively, had the highest sediment production among the other watersheds. The Namakzar Khaf Watershed showed the lowest sediment amount, with 737504 tons. The average slope in the Namakzar Khaf Watershed is 1.6%, which has the lowest sediment production and the lowest slope among the other watersheds. In contrast, the average slope in the Esfadan Watershed, which has the highest sediment production, is 14.7%, and it has the second-highest slope after the Ghaen Watershed. At the pixel level, sediment amounts in the eastern border watershed of the country ranged from 0 to 747.6 tons. The highest sediment retention was observed in the Esfadan Watershed, with 22626499 tons, and the lowest in the Namakzar Khaf Watershed, with 1316208.6 tons. The minimum and maximum soil loss rates were obtained for the Namakzar Khaf and Esfadan Watersheds, respectively, ranging from 4845857 to 6385471 tons.