Effects of Climate Change on Urban Rainwater Harvesting in Colombo City, Sri Lanka

Cities are becoming increasingly vulnerable to water-related issues due to rapid urbanization, installation of complex infrastructure and changes in rainfall patterns. This study aims at assessing the impacts of climate change on rainwater harvesting systems (RWH) in the tropical urban city, Colombo, Sri Lanka. The future climate change projections are downscaled from global circulation models to the urban catchment scale using the Long Ashton Research Station Weather Generator (LARS-WG), described in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), coupled with Inter Comparison Project (CMIP3) model results. Historical rainfall data from 1981-2010 is used to simulate long-term future rainfall data from 2011-2099. The percentage change of the rainfall is calculated. The rainfall patterns are analyzed based on the daily, monthly, seasonal and annual time scales. Water requirements are calculated based on the selected scenario types. Rainfall and water demand data are incorporated into a water balance model. Climate change impacts for the selected RWH scenarios are calculated based on the water security analysis for each scenario. Analysis of the future rainfall data of Colombo reveals that several extreme weather events with very heavy rainfall may occur in the future. However, the frequency of these big events may not occur too often. Most of the selected global circulation models (GCMs) in this study predict that there will be more rainfall towards the end of this century (2080-2099). Residential RWH systems will be more affected than non-residential systems. RWH systems in Colombo should include potential future climate changes in their future design and planning and be prepared for excess runoff and additional measures against potential overflow and urban floods.