There are growing evidences of climate change and it is one of the top issues facing the world today. Extreme climatic events such as typhoons, heavy storms and droughts are becoming more frequent and destructive. It is important to understand how these climate changes systems will affect the future rainfall patterns as these variables are directly related to the long standing water balance, hence leading to water related issues, consequently threatening local livelihoods. Rainwater harvesting (RWH) has been practiced for centuries and with modern system designs and technical advancements, it has become a worldwide popularity in recent decades. The historical and projected future rainfall data for Colombo, Sri Lanka and Taipei, Taiwan, downscaled from the global climate models using the LARS-WG stochastic weather generator was analysed, possible changes were identified and compared in this study. The impacts from the identified rainfall changes to the RWH systems were quantitatively analysed. Results show that there is more pronounced rainfall variability in the future date for Taipei and Colombo. However predicted rainfall results suggested that there are more extreme rainfall events in the future and the contribution to the daily rainfall from the extreme events are higher. Therefore it is evident that the rainfall patterns of these two cities will be altered, hence affecting to their established and planned RWH systems. Water security calculations for selected RWH systems under different scenarios (with and without climate change) indicate that the residential household level systems are being most affected by the potential climate change. However, impacts can still be disproportionate across the practitioners, due to differences in their adaptive capacities as well as with RWH systems go further away from the reference weather station. The study noted the absence of more specific and dynamic early warning system for the climate change and recommended such system be established in both cities to improve climate forecasting and increased awareness to cope with adverse water stresses for RWH systems and other hydrologic system practitioners.
