Analysis of the relationship between land use contributions to sediment yield with landscape metrics and soil erosion factors in the Kasilian watershed

Introduction

Soil erosion and sedimentation are integral components of natural geomorphological cycles, playing a pivotal role in regulating watershed functionality, preserving water quality, maintaining agricultural productivity, and ensuring the long-term sustainability of ecosystems. Under natural conditions, these processes contribute to landscape evolution and nutrient redistribution. However, human activities combined with climate change have greatly accelerated erosion and sedimentation rates, causing severe environmental damage and substantial economic losses worldwide. Factors such as land use changes, deforestation, overgrazing, and improper land management have intensified soil degradation and sediment transport, seriously threatening both terrestrial and aquatic ecosystems across diverse regions. In response to these growing challenges, understanding sediment source dynamics has become critically important. Identifying main sediment contributors and accurately measuring their relative impacts are vital for developing effective conservation and management strategies. Additionally, analyzing the landscape structure through landscape metrics which assess spatial patterns, connectivity, and fragmentation of land uses offers important insights into how landscape features directly affect erosion and sediment movement processes. Integrating such comprehensive landscape level analyses significantly improves our ability to predict erosion risks and design targeted interventions to effectively protect vital soil and water resources, ultimately supporting environmental health and promoting sustainable land management practices for future generations.

Materials and Methods

This study aimed to examine the effects of landscape metrics and erosion-related factors on suspended sediment generation across different land use types in the Kasilian Watershed, located in Mazandaran Province, northern Iran. The area lies on the southern slopes of the Alborz Mountains and drains into the Caspian Sea. It features a complex land use mosaic including natural forest, agriculture, rangeland, and plantation forest, each with distinct structural and ecological characteristics affecting erosion potential. A sediment fingerprinting approach using 59 geochemical tracers was employed. These tracers were analyzed in 36 source samples and 8 suspended sediment samples collected at the watershed outlet. Discriminant function analysis was performed using the FingerPro package in R. Potassium, sodium, and lead were identified as the most effective tracers for source discrimination. These elements were then applied in a multivariate model to estimate the proportional contributions of each land use type. Simultaneously, spatial analysis using the Revised Universal Soil Loss Equation (RUSLE) was conducted. RUSLE factors included soil erodibility, rainfall erosivity, topographic slope-length and steepness, cover management, and conservation practices. These were mapped with spatial data and overlaid with land use distribution. Landscape structure was also quantified using FRAGSTATS 4.2 to calculate metrics such as patch density, mean patch size, edge contrast, and nearest neighbor distance.

Results and Discussion

The sediment source apportionment results revealed a striking dominance of rangelands, which contributed an estimated 76% of the suspended sediment load. In contrast, natural forests, agricultural lands, and plantation forests were responsible for only 9%, 6%, and 6% of sediment contributions, respectively. This disproportionate contribution from rangelands is particularly noteworthy given their relatively remote position from the watershed outlet. Several factors account for this observation: (1) Rangelands exhibited the highest values for the cover-management factor (C), indicating sparse and degraded vegetation cover; (2) The slope-length and steepness factor (LS) was also elevated in rangeland areas, signaling steep terrain prone to rapid runoff and detachment; (3) Soil erodibility (K) was greater in these zones, further exacerbating erosion potential. Additionally, a high mean Euclidean nearest neighbor distance (ENN_MN) in rangeland parcels suggests that these patches are more isolated and less buffered by adjacent vegetative covers, reducing their resilience to erosive forces. These results underscore the complexity of sediment production processes and highlight the importance of considering both biophysical and spatial variables in watershed-scale erosion studies. Despite the geographical distance of rangelands from the watershed outlet, their structural fragility, topographic exposure, and low vegetative cover make them particularly susceptible to erosion and major contributors to downstream sediment loads.

Conclusion

From a management perspective, the integration of landscape metrics with erosion related biophysical parameters provides a comprehensive framework for identifying critical sediment source areas. This approach goes beyond traditional point-based monitoring and enables a broader understanding of sediment dynamics across the landscape. The insights gained from this study offer a solid scientific basis for prioritizing conservation actions such as reforestation, rangeland restoration, slope stabilization, and the implementation of soil conservation practices tailored to specific local conditions. Moreover, this research contributes meaningfully to the broader conversation on sustainable watershed management in the face of climatic uncertainty and increasing human pressures. By combining sediment fingerprinting techniques, geospatial analysis, and landscape ecology principles, this study presents a replicable methodology that can be applied in other watersheds facing similar challenges. In conclusion, the findings emphasize the urgent need for integrated assessment frameworks that link land use, topography, and landscape structure with sediment generation and transport processes. These approaches are essential for guiding targeted, evidence-based policies and building ecological resilience. As climate change continues to intensify hydrological extremes and land degradation, proactive sediment management informed by science and supported by spatial tools will be vital to maintaining the productivity, sustainability, and environmental health of watersheds.