Tank Sedimentation, Soil Erosion Simulations and Conservation Interventions of the Sub-catchments in Palugaswewa Tank Cascade System, Sri Lanka

Tank Cascade Systems (TCS) in the dry zone of Sri Lanka is threatened by soil erosion and high levels of sedimentation. Despite these challenges, the nation lacks studies exploring spatial soil loss variations within TCS contexts. Consequently, this research aimed to assess the sedimentation levels of five tanks and to analyze the spatial distribution of potential soil erosion rates across six selected sub-catchments within the Palugaswewa TCS. By utilizing sediment depth contour maps, the current sedimentation volume for each tank was computed. The study employed the revised universal soil loss equation (RUSLE) and geographic information system techniques to evaluate the potential average annual soil erosion rate, considering both existing land use scenarios and conservation interventions. The potential annual sediment yield was calculated using the sediment delivery ratio and potential average annual soil erosion rate. At present, 40 to 50 % of the tank storage capacity has been filled with sediments under existing land use. The potential average annual erosion rates of the sub-catchments of Palugaswewa TCS ranged from 19 t/ha/yr to 44 t/ha/yr. Notably, Sri Lanka's acceptable erosion rate stands below 12 t/ha/yr, rendering the erosion rates within Palugaswewa TCS unsuitable and destructive to sustained land productivity. The sediment delivery ratio varied from 0.18 to 0.9. This study suggests that adapting appropriate conservation measures such as cover cropping and soil contour bunding reduces the potential average annual erosion rate by 8.9 t/ha/yr to 14.5 t/ha/yr in the Palugaswewa sub-catchments.