Assessing the Influence of Land Use Change Pattern and Soil Erosion on Sediment Delivery Ratio in the Upper Mahaweli River Catchment of Sri Lanka

The Mahaweli River takes in the largest volume of water amounting to one seventh of Sri Lanka’s total runoff and has a catchment area that covers 15 % of the land in the country. A number of multipurpose reservoirs have been stored in the Upper Mahaweli River Catchment (UMRC) mainly to generate hydropower, which contributes to about 40-50% of the total hydropower production in the country. This study investigates the impacts of land use change patterns and soil erosion on the sediment delivery ratio (SDR) within the UMRC in Sri Lanka. Utilizing Geographic Information Systems (GIS) and Remote Sensing (RS) tools, the land use and land cover (LULC) changes were analyzed over a period from 2007 to 2023, focusing on soil erosion's contribution to sediment dynamics in the basin. By employing the Universal Soil Loss Equation (USLE) model, factors such as rainfall erosivity, soil erodibility, slope characteristics, crop cover, and land management practices were assessed to estimate soil loss and map erosion hotspots. The InVEST SDR model was then applied to quantify sediment transport and deposition patterns, helping to identify priority areas for soil conservation. Results revealed an average annual soil loss ranging from 0 to 95.015 t ha⁻¹ yr⁻¹ in the catchment and a mean annual sediment export of 5.56 t ha⁻¹ yr⁻¹. It shows mean sediment deposition rate as 2.4 t ha⁻¹ yr⁻¹. Erosion severity was classified into five hazard classes, very low (0 - 5 t ha-1 yr-1), low (5 - 10 t ha-1 yr -1), moderate (10 - 20 t ha-1 yr-1), high (20 - 50 t ha-1 yr-1) and very high ( >50 t ha-1 yr -1). A critical 1.5% area was identified as a considerable soil erosion hazard area, necessitating urgent measures for erosion control. Findings indicate significant LULC changes, driven primarily by deforestation, agricultural expansion, and urbanization, which have intensified mean soil erosion rate from 2.39 t ha-1 yr-1to 11.22 t ha-1 yr-1, highest mean sediment export rate as 2.29 t ha-1 yr-1 and highest mean sediment deposition rate as 5.36 t ha-1 yr-1 in different watersheds. Importantly, the identified erosion hotspots and sediment export zones indicate a substantial risk of accelerated sedimentation in downstream reservoirs, potentially reducing storage capacity and affecting hydropower efficiency in the Upper Mahaweli system. These findings support targeted watershed management interventions to mitigate long-term sedimentation impacts.