Application of eTank for rainwater tank optimisation for Sydney metropolitan

Abstract

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. eTank was developed based on daily water balance analysis, incorporating daily rainfall, contributing roof area and runoff generated from roof after losses (leakage and/or evaporation), daily water demand, tank storage capacity and overflow from the tank. Earlier eTank has been used for several reliability analyses within Melbourne metropolitan area to show ranges of rainwater tank outcomes depending on spatial and climatic variability. In order to assess reliability of domestic rainwater tanks in augmenting partial household water demand in Sydney area, the developed tool was used for three different climatic conditions (i.e. dry, average and wet years). In the earlier studies, a single representative year was selected for each of the dry, average and wet years. Dry, average and wet years were defined for the years having an annual rainfall of 10 percentile, 50 percentile and 90 percentile values respectively. However, as a particular year may have an unusual rainfall pattern, this study considered five respective years for each of the dry, average and wet years. eTank was used for the selected five years and average outcomes of five years were calculated. It is found that in some cases selection of a single year as dry/average/wet year lead to erroneous outcomes, as in some cases a particular dry year turned out to be better than a particular average year due to the fact that the selected average year was having some sporadic burst of rainfalls for which the storage capacity was insufficient. To assess the spatial variability, the model was used for the performance analysis at two different regions of Sydney (Australia); Central and Western. These two regions of Sydney are characterised by notable different topography and rainfall characteristics. Rainwater tank outcomes (cumulative water savings, cumulative townwater supply and cumulative overflow) were presented and compared for these two regions for different climatic conditions.