Rainwater harvesting potential for southwest Nigeria using daily water balance model

Abstract

For the performance analysis and design of rainwater tanks, a simple spreadsheet based daily water balance model was developed using daily rainfall data, contributing roof area, rainfall loss factor, available storage volume, tank overflow and rainwater demand. This water balance model was then used to design an optimum size of domestic rainwater tank to be used for southwest Nigeria. The optimisation criterion was set to provide uninterrupted intended demand from the selected rainwater tank during the critical (dry) months. For the tank water, two demand scenarios were assessed: (i) toilet flushing only; and (ii) toilet flushing and laundry use. Analysis was performed for a typical dry year (1998) in southwest Nigeria. Current analysis outcomes were compared with an earlier analysis using monthly average rainfall data. It is found that analysis using monthly average rainfall data overestimates the required rainwater tank size. In addition, the newly developed model was used to assess the reliability of domestic rainwater tanks in augmenting partial household water demand. This analysis showed that a reliability of 100% is possible to achieve with a tank size of 7000L under low demand. However, with higher demand a bigger tank size (∼10,000L) is required to achieve 100% reliability even though very high reliability could also be attained with a tank size of 7000L. From overflow analysis, the results of this study showed that a large quantity of water is lost as overflow, even in a dry year with a tank size of 10,000L. Thus, harvested rainwater could be used for other purposes if larger tanks are used as these would capture more of the excess rainwater which could then be tasked to other purposes without compromising the reliability of water availability for primary uses.