Reducing Siltation and Increasing Hydropower Generation from the Rantambe Reservoir,Sri Lanka

Working Paper no. 113-16

Hydropower in Sri Lanka is affected by unanticipated siltation from changes in upstream watersheds. In this study, we examine the costs of soil erosion associated with the Rantambe reservoir. We assess the impact of changing upstream land use patterns on soil erosion, reservoir sedimentation, electricity availability and dredging costs by using the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Sediment Retention model. We find that the current human-induced average rate of soil erosion in the watershed is 10.7 tons/ ha/year, which is approximately double the permissible rate of soil erosion. Consequently,the reservoir bears an annual hydropower loss of some half a million rupees (over three thousand US$) per year. Our model estimates that these hydel losses could be avoided by incurring an annual cost of dredging of Rs. 259,605 (1,782 US$). Three alternative watershed management strategies would also help lower sedimentation. These include: (I) adoption of a Soil and Water Conservation (SWC) by farmers, (II) re-foresting farmlands with slopes
greater than 60%, and (III) reforesting 10% of current farmlands. Strategy I would reduce the soil erosion rate relative to business as usual by 23%, strategy II would reduce the rate by 16% and strategy III would lower the soil erosion rate by 11%. Thus, SWC strategy (I) provides the best outcome. Undertaking plot level soil and water conservation (strategy I) would result in electricity revenue savings of 15% (460 US$) per year relative to the current scenario. While we focus on the hydel benefits, it is likely that there are also farm level benefits to soil and water conservation.