Impact of Climate Change on Reliability of Rainwater Harvesting System: A case Study in Mongla, Bangladesh

As Bangladesh has tropical monsoon with large seasonal cycle in rainfall, Rainwater Harvesting (RWH) system and its utilization could be a good and environmentally sound solution for providing safe drinking water. Although rainwater harvesting has already been promoted in different coastal areas in Bangladesh to a limited scale for drinking purpose, improper storage reservoirs prohibit people getting full benefit from it. Moreover, temporal variability of precipitation , which is the main governing factor in the design of a storage tank in Rainwater Harvesting Systems (RWHS), is expected to be altered in future under the effects of climate change. This can introduce an element of uncertainty in the design of storage reservoirs if this variability is not taken into consideration. The objective of this paper is to report on reliability problems of existing rainwater harvesting system in coastal rural areas of Mongla, Bangladesh. This study aims at developing a simulation model as tank sizing tool incorporating all the design variables and also climate change variability and assessing reliability in terms of failure time units and demand satisfaction level with the help of this model.This simulation model is developed based on mass balance equation in which historical as well as predicted future climate data are used. Historical precipitation data are collected from Bangladesh Meteorological Department (BMD) whereas predicted future data are from a climate model named PRECIS. The physical data input includes roof area, run off coefficient and reservoir size whereas social status data includes water demand and household size. The study has shown that, for a water demand of 5 lpcd, volumetric reliability lies in the range of 35-37% in case of individual household based rainwater harvesting system for historical rainfall pattern of the study area whereas the range becomes 48-50% for predicted future precipitation scenario depending on existing roof areas and corresponding storage tank sizes. The volumetric reliability of community rainwater harvesting systems for historical data (26%) is almost half of the value obtained from future precipitation scenario (56%) for a water demand of 5 lpcd. Reliability becomes lower for higher water demand as expected. Time reliability of both individual and community rainwater harvesting system is considerably lower compared to volumetric reliability for both observed and predicted precipitation data. It implies that although existing systems manage to deliver a portion of water demand, the number of months in which demand is fully met is very small. Finally, tank sizing curves for the study area have been developed by which one could obtain the reliable size of the water storage tank for given data on household size, per capita water demand and roof area. The research findings presented in this paper could be applied in areas with similar socioeconomic status and climatic condition.