Rainwater harvesting (RWH) systems are increasingly being implemented in buildings. It is common in the UK for simple RWH tank sizing methods to be utilised, and these do not consider future climate change. This paper describes the development of a tool, which integrates elements of basic and detailed sizing approaches from the British Standard for RWH, with the latest probabilistic UK Climate Projections data. The method was initially applied to the design of a university building in Cornwall, UK. The methodology utilises 3,000 equi-probable rainfall patterns for tank sizing for each time period. Results indicate that, to ensure that it is ‘likely’ that the same non-potable demand could be met in 2080 as in the present, a tank 112% larger would be required. This increases to a 225% over-sizing for a ‘very likely’ probability of meeting the same level of non-potable demand. The same RWH system design was then assessed for three further UK locations with different rainfall characteristics. From these assessments, a simplified method was developed to enable practitioners to size RWH system tanks for current and future climates. The method provides a new approach to meet present and future non-potable demands, while preventing excessive over-sizing of tanks.

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