Generalized equations, climatic and spatial variabilities of potential rainwater savings: A case study for Sydney

Optimised harvesting of rainwater has been a challenge as the reliability of this water supply depends on many factors i.e. rainfall amount, tank size, roof area and rainwater demand. Various studies have been done in this context considering water usages, rainfall & roof area; and different methods were proposed to maximise the water supply reliability for an optimum rainwater tank. Among the analysis methods, a daily water balance method is the most acceptable and reasonably accurate, although, general end-users hardly can interpret outcomes of these analyses. This paper presents relationships of expected water savings under different climatic conditions for specific tank sizes with two major contributing factors; demand and roof area for an Australian city, Sydney. Expected annual water savings were calculated for different combinations of tank size, roof area and demand using an earlier developed daily water balance model, eTank. For a given tank size expected water savings for different roof areas and demands are presented in the form of charts for a user-friendly presentation to the end-users. Also, produced charts for a particular climatic condition were converted to a generalized equation having independent variables of roof area, tank size and demand to facilitate quick calculations through apps or computer. For the validity check of the developed equations, results from the equations were compared with the eTank simulated results and it is found that the results from the generalized equations are very close to the eTank produced results. To present spatial variability, similar charts and equations were produced for four different regions of Sydney.