Realistic simulation of rainwater tank systems: revealing the detail

The benefits derived from allotment-scale rainwater tank systems are usually modelled based on input data including rainfall, the attributes of the dwelling and the attributes of the rainwater tank system. However, reliable determination of mains water savings requires a realistic, continuous approach in simulating the numerous flows between the rooftop, the rainwater tank and wastewater system in order to fully realise the "hidden" benefits of integrated water systems. Also, the timing and availability of water demand and the annual rainfall regime for a range of climatic zones shows greater influence on evaluating system benefits than previously acknowledged. This study uses three commonly used modelling tools, namely PURRS (v7.2), MUSIC (v3) and a spreadsheet approach, to highlight the differences between outcomes and to reveal the hidden detail of using realistic, continuous simulation techniques. Input data to each model was selected as advised by user guidelines, including climate files, suggested water demands and time-steps. Models were run with climate data of unequal duration and time-step, which highlighted significant differences between modelled outcomes. Using climate data of equal duration still resulted in major differences. The reasons for these differences are explained as a function of the duration and time-step of climate data, the time-step and diurnal patterns of indoor/outdoor water demand and tank configuration. The differences and similarities between models produce variable results for different climatic regimes and water demands. Results imply that the length and time step of climate inputs and simulating the distribution and time-step of daily water demand and rainwater tank configuration are significant factors in robustly evaluating mains water savings for a range of Australian climates, particularly for smaller tank sizes.