Sediment Connectivity Index Zoning in the Shazand Watershed, Markazi Province, Iran

Extended Abstract Background: Soil erosion is the land that separates soil particles from their original location, then changes to other places through various processes, and then deposits. Soil erosion and sediment production are among the most important problems in most regions of the world, causing a decrease in soil productivity, an increase in flood flows, a decrease in the health of water systems, and an increase in sediments in dam reservoirs. In addition to identifying the sources of sediment production, a model of potential sediment delivery to the waterway network is made to manage soil erosion and sediment production. As a result, the sedimentation of watersheds and sediment production areas, its delivery patterns from production sources to the network, and finally, depositing it in reservoirs, are needed for soil erosion management. One of the most important methods and characteristics of watershed sediment production is the sediment index of connectivity (IC). The connection process is an emerging concept to better understand the processes occurring at the surface of the watershed that affect water flow and sediment movement in different ways. This index explains the degree of connection of the sediment flow throughout the watershed, especially between the sediment source and the downstream area. Sediment management includes all measures that affect erosion, transportation, redistribution, and deposition of sediments in the direction of sediment diversion or trapping in waterway systems. Accordingly, sediment connectivity as an emerging concept in sediment management has been considered in recent years to investigate sediment transport in different parts of watersheds. The concept of connectivity is related to the state of a system, such as a watershed, which determines how and the degree of ease of material and energy transfer throughout the watershed system. In other words, connectivity indicates the continuity or discontinuity of runoff and sediment paths at a specific time and place. In this regard, the assessment of the sediment connectivity index in the Shazand Watershed was mentioned in the present study. ods: This study was conducted in the Shazand Watershed located in Markazi Province. The spatial pattern of sediment yield in the watershed and the sediment connectivity map of the watershed were investigated using the Sediment Delivery Ratio (SDR) model, the method presented by Borselli et al., and the definition of the connectivity index. IC consists of two parts, including upstream and downstream. This index was calculated by incorporating the distance from the sediment sink (i.e., the end of the slope or the connection to the main waterway), the characteristics of the sediment movement path, factors affecting the cumulative flow of runoff, and local conditions in each part considering as a weighting factor (W). The more positive IC and tending to the positive infinity, the higher the probability of structural connection for sediment transport. The probability of sediment connectivity will decrease with more tendency to the negative infinity. Therefore, the sediment IC was estimated using the corrected digital elevation model and the cover management factor with a spatial accuracy of 30 m. Next, spatial changes and values of the sediment IC were calculated for 24 sub-watersheds of the study. Results: The changes in the sediment IC in different sub-watersheds ranged from -7 to 1.4. The slope, land use, and land cover maps were used for this purpose. Comparing the land use map with the sediment IC shows the role of human disturbance in the natural functioning of ecosystems and natural systems. According to these maps, mountainous and high areas and rainfed lands transfer sediments and hydrological and sediment connectivity to the downstream area due to the naturalness of the areas and human interventions. Moreover, the lowland and steep highland areas mainly have the minimum and the maximum values of this index, respectively. One of the main factors affecting connectivity is the type of vegetation that affects the movement of particles so that connectivity increases and leads to more runoff production in areas without vegetation or with little coverage, but less runoff is produced, penetration increases, and connectivity decreases in areas with vegetation. Based on the resulting map and the concept of the sediment IC, the smaller its value is, the lower the probability of sediment transfer. In general, considering this index and its distribution can be important for prioritizing areas in terms of soil erosion and sediment management. Conclusion: Considering the importance of preserving Iran's water and soil resources and providing suitable solutions for managing sediments in watersheds, the integrated management of watersheds in the country requires a specific framework in planning related to sediment monitoring and control. Recently, the concept of sediment connectivity has been more frequently used in the studies of soil erosion and sediment yield of watersheds. In this connection, the spatial variability and zoning of the sediment IC in the Shazand Watershed were evaluated in the present study for the management of sediment resources. The results of the present study can, therefore, be employed to plan management measures to control sediments in the Shazand Watershed, Markazi Province, Iran. Nevertheless, conducting more detailed and extensive studies, especially in different geomorphological conditions, will provide the necessary platforms for comprehensive conclusions.