Assessment of outdoor water usage for performance of rooftop rainwater harvesting systems using modelled and sub-metered data

The growing popularity of rooftop rainwater harvesting systems (RRWHS) requires a realistic assessment of their performance. Based on 11 years of monitoring of 40 properties, this article analyses the effectiveness of these systems using key hydrological indicators and compares the model water mass balance with actual outdoor water consumption. It was shown that local conditions (roof area, proportion and size of green areas) and user behaviour have a key impact on the efficiency of the systems, and that theoretical models systematically overestimate water consumption for garden irrigation. The results prove that the use of fixed, hypothetical irrigation schemes in modelling does not reflect actual and behaviourally determined water consumption patterns. Consequently, the need to implement a data-driven approach based on actual sub-meter data and user preferences into calculation algorithms has been emphasised in order to optimise the selection of tank capacity and make the assessment of the hydrological performance of RRWHS more realistic.