Domestic Roof Rainwater Harvesting Tank Sizing Calculator and Nomograph

Abstract

The importance of roof rainwater harvesting as strategy to meet domestic water demand and to reduce run-off in built-up areas is growing worldwide. Indicators that measure the performance of rainwater harvesting systems have been developed. One such indicator is reliability, which is dependent on the rainfall and water consumption patterns, tank size and effective roof area. A good design should aim at highest reliability at the lowest cost. The aim of this study was to develop a tool that can aid decision making with regard to a question such as ‘What size of tank should be installed in a specific location to provide V litres of water per day and with a reliability of R%?’

A computer-based calculator that accounts for tank inflow and outflow and computes system reliability based on monthly rainfall data, effective roof area, daily water consumption and tank size was developed. Based on the results of the calculator, a nomograph for a reliability of 67% was developed. The nomograph can be used to optimally size rainwater harvesting systems in situations where water users do not have access to computers. This tool can be employed by water users to decide on the configuration of tank size and effective roof area that meets the required daily water consumption rate at 67% reliability for a specific location. Nomographs of different reliabilities can be developed based on the rainfall data in the calculator. The higher the reliability, the greater the investment costs in water storage and roof area. However these nomographs can only be used for areas for which the rainfall data used in the calculator is representative. Hardware dealers selling rainwater tanks or contractors in building industry can use it to advice customers on the size of tank to buy or construct.