Extended Abstract
Introduction: Ecosystem services, the direct and indirect benefits provided by natural systems, play an essential role in maintaining ecological balance, environmental sustainability, and effective natural resource management. Among these services, sediment retention is considered a crucial regulating function. It helps reduce soil loss, sustain agricultural productivity, protect water quality, extend the operational lifespan of reservoirs, and mitigate flood risks. However, land use change, driven by human activities such as urbanization, agricultural expansion, industrial development, and deforestation, has increasingly disrupted this service. In recent decades, factors like population growth, climate variability, and increasing demand for land and water have caused significant transformations in land cover, particularly in mountainous and erosion-prone watersheds. These changes have altered sediment production and transport processes, often resulting in increased erosion and downstream sedimentation. To assess and address these impacts, spatially explicit and process-based models have become essential tools. One such tool is the InVEST model, which allows for spatially distributed estimation of ecosystem services under different land use scenarios. This study aims to evaluate the effects of land use change on sediment retention in the Bazoft watershed, located in the upstream area of the Karun-4 Dam in southwestern Iran, using the sediment delivery ratio (SDR) module of the InVEST model. The results can offer useful insights into sediment dynamics and inform integrated watershed management and sustainable land use strategies.
Materials and Methods: The study was conducted in the Bazoft watershed, which is located in the upper part of the Northern Karun Basin and drains into the Karun-4 Dam. The area is mountainous, with steep slopes averaging around 45%, a cold and semi-humid climate, and annual precipitation of about 766 mm. The average annual discharge is approximately 57.8 m³/s. The natural land cover consists mainly of forests and rangelands, while cultivated and residential areas represent the dominant human-induced land uses. In this study, land use and land cover (LULC) maps were generated for the years 2001 and 2021 using Landsat satellite imagery (ETM+ and OLI sensors, respectively), classified using the Maximum Likelihood algorithm in ENVI and ArcGIS software. Future land use for the year 2041 was simulated using the Scenario Generator tool, incorporating land suitability analysis and expert knowledge. The InVEST SDR model was used to estimate soil loss, sediment export, and sediment retention capacity based on inputs including LULC maps, a 30-meter resolution DEM, rainfall erosivity (R), soil erodibility (K), slope length and steepness (LS), vegetation cover factor (C), and support practice factor (P). Additional ecological inputs such as NDVI and hydrological connectivity parameters were included.
Results and Discussion: The results of this study indicated that human land uses such as rainfed agriculture, irrigated agriculture, and residential areas have shown an increasing trend, while natural covers like dense forests and dense rangelands have declined. The greatest increases during the 2001–2021 period were observed in rainfed agriculture (25.9%) and residential areas (63.07%). These trends are projected to continue, with increases of 21.7% and 20.91% respectively for the 2021–2041 period. The most significant decreases were related to dense forests and dense rangelands, by −7.53% and −9.07%, respectively, in the 2001–2021 period. According to the results of the SDR (Sediment Delivery Ratio) model in the InVEST software, the potential soil loss, sediment export, and sediment deposition are expected to increase by 0.56%, 1.01%, and 0.16%, respectively, compared to the current conditions of the watershed due to land use changes. These findings suggest that the continued expansion of human land uses could lead to a long-term decline in the ecosystem’s capacity to control erosion and sedimentation. The results are consistent with similar studies and highlight the importance of sustainable land use management.
Conclusion: This study demonstrates that land use changes significantly reduce the sediment retention service in the Bazoft watershed. Continued expansion of agriculture at the expense of natural vegetation exacerbates soil erosion and sediment delivery, undermining watershed sustainability. The findings reinforce the necessity of conserving forests and rangelands, and support the use of scenario-based tools like InVEST for guiding land use planning and environmental policy in fragile mountainous ecosystems.