Equation for rainwater tank efficiency: Considering demand, roof area, tank size and pluvial regime

A rainwater harvesting system (RWHS) is essential to meet the water domestic needs for many semi-urban, remote communities, where there is no centralized potable water supply system. This research evaluated the behavior of rainwater tanks sized by the modified Rippl method applied in regions of different rainfall regimes in Rio Grande do Norte (Brazil) and developed two empirical equations: one to determine the rainwater tank volume and the other to determine its efficiency. The independent variables studied were the roof area, demand, annual precipitation, critical precipitation, and precipitation concentration degree. The general empirical equation developed to determine the volume considering the entire study area proved to be adequate, with no need to divide the area into homogeneous sub-regions. In the case of efficiency, obtaining a general equation proved to be feasible, but it was necessary to exclude the eastern region during the acquisition, due to the high discrepancy in the rainfall regime in that region in relation to the others. Using a general tank efficiency equation, it is possible to evaluate the behavior of tanks of different sizes, not only measured by the Rippl method. This study also examined the behavior of rainwater tanks sized by the Rippl method, but the emphasis was on the study of reservoirs efficiencies. It argues the necessity to study the rainwater tanks by efficiencies indexes, as this approach provides essential information so that the users can choose the best size considering their RWHS degree of dependence in meeting their demands