Reliability analysis of rainwater tanks : a comparison between South-East and Central Melbourne

A spreadsheet based daily water balance model is used for the performance analysis and design optimisation of rainwater tanks at two different regions of Melbourne, South-East and Central Melbourne. These two different regions of Melbourne are characterised by notable different topography and rainfall characteristics. From historical rainfall data, three representative years (dry, average and wet) are selected. Reliability is defined as percentage of days in a year when rainwater tank is able to supply the intended partial demand for a particular condition. For the three climatic conditions, a number of reliability charts are produced for domestic rainwater tanks in relations to tank volume, roof area, number of people in a house (i.e. water demand) and percentage of total water demand to be satisfied by the harvested rainwater. It is found that for a relatively small roof size (100 m2), 100% reliability cannot be achieved even with a very large tank (10,000 L). Reliability becomes independent of tank size for tank sizes larger than 5000–7000 L depending on the location. This is defined as threshold tank size, relationships with threshold tank sizes and annual rainfall amounts are then established for both the locations. It is found that for a particular annual rainfall amount, threshold tank size for South-East Melbourne is higher than that of Central Melbourne. However, for a relatively large roof size (200 m2), approximately 90% reliability can be achieved with a tank size of 10,000 L and with further increase in tank size, a 100% reliability is achievable, except in a dry year. Furthermore, it is found that reliabilities for South-East Melbourne maintain consistent higher values as compared to Central Melbourne.