Rainwater Harvesting (RWH) systems are recognized as a widely accepted solution to save potable water in buildings. In addition, RWH systems may play an important role in mitigating the impact of increasing imperviousness in urban areas by contributing to increase both at-source retention and detention of storm water runoff. The chapter provides an overview of methodologies for designing RWH systems, with specific focus on numerical models based on the long-term water balance simulation of the tank. An overview of metrics to evaluate RWH system performance is presented, with regard to the estimation of both water saving potential and storm water control benefits. The accuracy of the modelling results with reference to the length of the available rainfall series and to the selected resolution time step of the used model is discussed. Results of an application to six cities in southern Europe are also discussed in order to highlight the impact of different precipitation regimes as well as the influence of rainwater demands on the system design and performance. Finally, the German, British and Italian standards on RWH are analysed and compared to identify differences and common design approaches.
Keywords: Behavioural model, Detention time, European standards, Long-term simulation, Operational parameters, Precipitation, Rainwater harvesting, Rainwater demand, Runoff control, System performance, Tank sizing, Water saving, yield.