Climate Change Adaptation for Municipal Water Supply in Colombo, Sri Lanka
Colombo city, with a population of 2.2 million is important for Sri Lanka’s economy. The Kelani River is the primary source of water to the city. Impacts of climate change on the Kelani River Basin have put the water supply system at risk.
Context
Colombo, covering 37.29 km? with a population of 2.2 million, is vital for Sri Lanka’s economy, hosting 80% of industries and contributing 50% of the GDP. The Kelani River is the city’s primary water source, with the Ambatale Water Treatment Plant purifying over 500,000 m?/day. The Kelani River Basin, spanning 2,292 km?, experiences a hot, humid climate with rainfall mainly from the South-West Monsoon between April and September. It receives an average annual rainfall of 3,450 mm, with runoff of 5,500 million m?. Climate change impacts on the basin put the water supply system at risk.
The city of Colombo is located in the “wet zone” of Sri Lanka with a tropical rainforest climate.
Actions
To establish the decision context for CRIDA Step 1, stakeholders were involved to define the climate change risk. The main concern of the water supply system is the inability to meet demand. The critical thresholds of the system were identified through interviews with personnel from the National Water Supply and Drainage Board (NWSDB). The flow requirement at the intake to guarantee the per capita demand of 180 liters/day were identified as main thresholds.
Subsequently, CRIDA Step 2 entailed a series of stress tests to identify the vulnerability of the water supply system to climate change. Future climate and hydrological data for the period 2030-2059 were obtained using Regional Climate Models (RCMs) and a hydrological model. The system was then stress tested based on these future hydrological projections to assess potential failures. Future hydrology suggests that flow will likely increase, but the demand will likely increase as well due to population growth and/or increasing per capita water demand.
During CRIDA Step 3, the search for adaptation measures focused on demand management. This resulted in three scenarios with varying per capita demands (both increasing and decreasing demand), and the related minimum flow. These scenarios were used in Step 4 to determine the annual number days of failure with the future hydrology under the different scenarios.
Lastly, as part of CRIDA Step 5, strategies to decrease the per capita water demand were listed as recommendations to (future) policymakers. These include programs for raising awareness, limitation of supply through usage thresholds, water pricing, reduction of non-revenue water and relying on alternative sources, and water reuse to reduce stress on surface water.
Outcomes
The case study demonstrated that CRIDA offers an effective methodology for identifying climate risks that could affect the proper functioning of the hydrological system amidst climatic and demographic uncertainties. The stress test revealed that, although future flow is expected to rise, failures are still likely due to the combined effects of hydrological changes and population growth, as determined through the decision-making process involving all stakeholders. The evaluation of the adaptation actions showed that for no failure, the per capita water demand should be reduced to 69 liters/day. This is a big reduction and can only be achieved through drastic measures.