eTank: A Decision Support Tool for optimizing rainwater tank size

One of several common water conserving techniques is on-site stormwater harvesting for non-drinking purposes. However there is a lack of knowledge on the actual cost-effectiveness and performance optimisation of any stormwater harvesting system. At present stormwater harvesting systems are proposed and installed without any in-depth analysis of its effectiveness in various climate conditions. In particular the proposed design storage volume could be overestimated or underestimated. The biggest limitation of stormwater harvesting schemes is the rainfall variability, which will control the size of the storage needed and can't be based on long-term average annual rainfall data. A stormwater harvesting system designed considering average annual rainfall will not provide much benefit for a critical dry period. Similarly, a stormwater harvesting design for a particular region will not be similar for stormwater harvesting design in other regions. With all these uncertainties, even with several awareness campaigns and financial incentives, there is a general reluctance to adopt any potential stormwater harvesting measure. The main reasons behind this are that people are not aware of the payback period for their initial investment and the optimum size of the storage required satisfying their performance requirements. It is necessary to quantify the expected amount of water that can be saved and used through any particular harvesting technique based on contributing catchment size, tank volume, geographic location, weather conditions and water demand. Without proper analysis and quantification, any adopted tank size may not be cost-effective. This paper presents development of a comprehensive decision support tool to analyse and optimize a potential rainwater tank. The tool was developed based on daily water balance analysis, incorporating daily rainfall, runoff generated from roof after losses, daily water demand, tank size and overflow from the tank. For insufficient/no rainwater, the analysis assumes augmented supply from townwater supply. The developed tool enables a simple quantitative analysis of the expected water that can be saved based on the relevant constraints. The input data require are: daily rainfall, roof area, expected loss from roof to tank, tank volume and daily rainwater demand. The tool produces graphs showing cumulative yearly rainwater used, overflow and augmented townwater supply. To account for climate variability, provision has been made in the tool to analyse for a particular option in three different years (climate conditions), for which often a dry year, an average year and a wet year are considered. Also, the tool enables a life cycle costing analysis and payback period of any particular proposed tank size through the simulated outputs related to expected water savings per year, initial construction costs and operational costs related to tank. Expected water savings are calculated by tanking average of three (dry, average and wet) separate year's cumulative annual water savings. For the cost analysis, additional input data require are: water price, water price increment rate and maintenance cost increment rate. Also, the tool calculates the reliability of a particular size tank, connected with a particular roof size to fulfill the expected rainwater demand. Reliability is a measure of percentage of days in a year, when the tank was able to supply the expected demand. The paper illustrates different scenario results produced by the tool for daily rainfall data near Melbourne City, under different climatic conditions. The simulated results were compared with an earlier published spreadsheet based model results. The developed tool and the earlier spreadsheet based model produce exactly same results. The developed tool is a user-friendly tool which will make end-users decision making process easy, effective and knowledgeable.