Evaluation of Relationship Between Soil Erodibility and Sediment Connectivity and its Application in Sustainable Management of Chehl -Chay Watershed in Golestan Province

 
Excessive use of natural resources has led to soil erosion and sediment production aggravation. Knowledge of areas prone to sediment transfer is essential for effective strategic planning in the sustainable management of natural resources. One of the key concepts in this field is the investigation of sediment connectivity, which examines the transfer of sediment from one area to another and the migration potential of sediment particles. The selection of the weighting factor is one of the most challenging aspects in the calculation of sediment connectivity, as it is chosen based on the conditions of each region, the characteristics of the soil surface, and the available data. The purpose of this research is to evaluate the soil erodibility factor as a weighting factor in the calculation of the sediment connectivity index and to investigate the spatial distribution of sediment connectivity. In this study, the flow direction and cumulative flow were determined using a digital elevation model (DEM) of the area and slope layers. The vegetation factor was derived from Landsat images using the NDVI index. To prepare a soil erodibility map, 40 topsoil samples were collected, and key parameters, including texture, excellent sand content, and organic carbon, were measured. The results showed that the average erodibility in forested and pasture areas had the lowest and highest rates, at 0.014 and 0.026, respectively. Additionally, based on the model implementation, sediment connectivity ranged from -1.9 to 5.3. A linear regression equation was calculated, and the coefficient of determination (R² = 0.74) was estimated. The findings of this research can be useful for watershed management and subsequent studies related to sediment connectivity.
 
Extended Abstract
1-Introduction
The increase in sediment yield within watersheds poses significant challenges, including the loss of arable soil, reduced soil productivity, increased sediment accumulation in water systems, damage to water infrastructure, declining water quality, and decreased storage capacity of reservoirs in dams. Sediment connectivity refers to the transfer of sediment across different parts of a watershed and the potential for sediment particles to move through the system. This concept encompasses processes occurring on hillslopes, interactions between hillslopes and watercourses, and sediment dynamics within the watercourses themselves. Hydrological connectivity, defined as the linkage between runoff and sediment sources in the upstream areas of a catchment and the sediment deposition in downstream areas, is a critical concept for evaluating erosion and hydrological processes in watersheds on both temporal and spatial scales. According to the sediment connectivity formula, the characteristics of sediment pathways, factors influencing cumulative runoff flow, and local conditions are integrated as a weighting factors. A review of research conducted between 2008 and 2024 reveals variations in the variables selected for the weighting factor. These include vegetation characteristics, physical soil properties (such as infiltration capacity and soil aggregate stability), land use practices, hydraulic roughness indices, thresholds for rill and gully formation, and erosion control factors like the Cover Factor from the Revised Universal Soil Loss Equation (RUSLE). This study aims to evaluate soil erodibility as a weighting factor for calculating the sediment connectivity index and to analyze the spatial distribution of sediment connectivity across the study area. By integrating soil erodibility into the sediment connectivity model, this research seeks to provide insights into sustainable watershed management and contribute to mitigating the adverse effects of sediment yield in vulnerable regions.
 
2-Materials and Methods
To evaluate soil erodibility parameters, field and laboratory analyses were conducted, focusing on soil texture and organic carbon content. Organic carbon was measured using the titration method (Walkley & Black, 1934), while soil texture was determined through the hydrometer method (Kroetsch, 2011). A digital elevation model (DEM) with a resolution of 30 meters was preprocessed to enhance hydrological accuracy. Vegetation mapping was performed using the NDVI index, derived from Landsat satellite imagery dated June 8, 2023. The sediment connectivity index and its corresponding map were calculated using the Connectivity Toolbox for ArcGIS 10.7.1 (Cavalli et al., 2013). Land use classifications and quantitative evaluations of the sediment connectivity index were conducted using the Zonal Statistics tool within ArcGIS 10.7.1. These methodologies provided a comprehensive framework for analyzing sediment dynamics and spatial distribution across the study area.
 
3- Results and Discussion
The slope gradient of the watershed ranged from a minimum of 0.5 to a maximum of 1. Vegetation in the watershed was predominantly composed of native forests and man-made plantations, particularly concentrated in the northwest and northeast regions. Elevation within the watershed varied significantly, with the highest point reaching 2,454 meters at the southern boundary adjacent to Semnan Province and the lowest point at 188 meters near the watershed outlet. Spatial patterns of soil erosion were clearly observed. The central areas, primarily used for agriculture and rangelands, exhibited the highest soil erodibility values, reaching 0.026 t ha⁻¹ MJ⁻¹ mm⁻¹. In contrast, forested areas showed the lowest erodibility rates, at 0.014 MJ/mm per year. Sediment binding rates varied across different land uses: forests recorded -1.9, orchards 1.3, rangelands 4.6, and agricultural lands 5.9. Positive sediment connectivity indices indicate stronger functional connectivity and higher sediment transport potential, while negative values suggest weaker connectivity and reduced transport potential.   The spatial distribution of sediment connectivity revealed that areas with higher soil erodibility also exhibited greater sediment transport potential. Waterways draining agricultural lands and rangelands showed high sediment connectivity, whereas forested areas demonstrated low connectivity. These findings underscore the significance of soil erodibility as a key factor influencing sediment connectivity. Linear regression analysis of the sediment binding index and erodibility factor yielded a coefficient of determination (R² = 0.74), confirming a strong relationship between the two variables. These results are consistent with the findings of Najafi et al. (2018), who analyzed sediment connectivity patterns in the Tehmcha watershed (Zanjan Province) across three periods (1990, 2001, and 2014). Their study reported a decrease in connectivity indices (from -5 to -7) due to the implementation of effective watershed management practices aimed at controlling erosion and sedimentation.
 
4- Conclusion
This study simulated the impact of soil erodibility on runoff resistance and sediment transport in relation to sediment connectivity. The findings highlight the suitability of soil erodibility as a weighting factor for sediment binding analyses, as it incorporates key parameters derived from field and laboratory studies—specifically, organic matter content and soil texture—which influence soil strength and erosion resistance. Forests and rangelands were identified as the land uses with the highest organic matter content, contributing to lower soil erodibility. The results demonstrate the critical role of soil erodibility in determining sediment connectivity, underscoring its importance in watershed management and erosion control strategies. By integrating soil erodibility into sediment connectivity models, this research provides valuable insights for sustainable land use planning and the development of effective measures to mitigate sediment-related challenges in vulnerable watersheds.