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Among the alternative water sources, rainwater harvesting is the most popular and readily useable. However, studies related to the knowledge of future sustainability of this resource and its usage are limited. In this paper, projected future rainfall data for two distinct future set of periods (2020–39 and 2080–99) are used to calculate future water savings potentials using an earlier developed daily water balance model for five different regions in Sydney, the largest city in Australia. The rainfall data is collected from four different projection tools provided by the Australian government's climate data portal. With the assumptions of average household water demand as 400 L/day, a tank size of 5000 L and a roof catchment area of 200 m2, water savings and reliabilities of water availability are calculated for future scenarios for five different locations within a broader region of Sydney. Water savings and reliabilities are then compared with the same for current average rainfall condition based on historical data for the same locations. It is found that for most of the studied cases, both in the near (2020–39) and distant (2080–99) future, water savings as well as reliabilities are expected to be decreased compared to the current average scenario. However, in the distant future, both water savings and reliabilities are expected to be increased compared to the near future scenario. Also, it is predicted that spatial variability of water savings within the selected region will increase compared to the current scenario. [ABSTRACT FROM AUTHOR]

Rainwater harvesting (RWH) has become a popular water saving and conservation means across the globe. In many cities, a pre-determined rainwater tank size is generally selected without a detail water balance modelling at the location on interest. However, to achieve maximum benefit from a RWH system at a given location, it is desirable to design the system based on the site specific conditions (e.g. local rainfall and loss characteristics) and other relevant design parameters. There have been limited studies on regionalization of RWH system, which can account for the spatial variability in rainfall, loss and water demand characteristics over a given region to provide site-specific design of RWH system. This paper presents a RWH regionalization approach to develop a set of design curves, which assist in the selection of an adequate RWH system at a given location. We use rainfall data from 159 rain gauges across the Greater Sydney region in Australia to develop and test the proposed regionalization approach. It has been found that there is a significant variation in rainfall characteristics across Greater Sydney region and that a common tank size across this region does not present an adequate solution. It has been found that a 5 kL tank can achieve a reliability (i.e. percentage of days the RWH can meet the given demand) of 31–93% for four people depending on the location in the Greater Sydney region and the types of water use. The developed regional design curves could result in increased water savings across Greater Sydney. The proposed regionalization approach can be adapted to other cities across Australia and the world which present a high rainfall gradient. • Water savings by rainwater harvesting at 159 locations in Sydney are evaluated. • Reliability of a rainwater tank is moderately correlated with mean annual rainfall. • A set of design curves is developed for Sydney to enhance rainwater harvesting. [ABSTRACT FROM AUTHOR]