Non-dimensional design parameters and performance assessment of rainwater harvesting systems

The use of non-dimensional parameters is proposed in this study to investigate optimum performance of rainwater harvesting systems. A suitable behavioral model is implemented to assess the inflow, outflow and change in storage volume of a rainwater harvesting system according to a daily mass balance equation under historical precipitation observations. The model determines the water-saving efficiency, overflow ratio and detention time. These are interpreted as a measure of the system performance over the long-term simulation period. Performance is examined under various scenarios in terms of environmental conditions that are typical of the Italian territory (3 precipitation regimes and 3 levels of water demand) and system characteristics (10 storage capacity levels). Optimum sizing of the rainwater harvesting system is investigated as a function of two non-dimensional parameters: the demand fraction and the storage fraction. The demand fraction is shown to affect significantly the water-saving efficiency and overflow ratio, while the storage fraction controls the detention time thus influencing the water quality degradation within the system. Sensitivity analysis is carried out to investigate the influence of the length of the time series climate records on the reliability of the selected performance indices. Results demonstrate that 30 years of daily rainfall records are sufficient to allow suitable assessment of the system performance. Optimum system design based on the medium demand fraction and the low storage fraction determines system performance that appears almost independent on the three precipitation regimes investigated.