Impact of residential irrigation area and roof size on the economics of rainwater harvesting systems

![CDATA[Rainwater harvesting (RWH) has grown in popularity over the last 15 years and has attracted a significant amount of research. The economic viability of RWH systems has been reported with various outcomes. The water demand profile is complex and of all domestic demands, outdoor irrigation use is potentially the largest and most variable. The quantity of water available for harvest is influenced not only by the rainfall pattern and tank size, but also by the area of the roof used to harvest the water (the RWH system's catchment). Roof area can vary considerably with the size of the house, or because parts of the roof are unsuitable for harvesting (e.g. due to overhanging trees, practicality and/or cost of the guttering arrangement). A versatile economic evaluation tool named ERain has been developed to analyse the economics of various RWH system arrangements. ERain combines performance analysis using daily rainfall data with life cycle cost analysis. Outputs from the model include economic indicators such as benefit cost ratio (BCR) and net present value (NPV) reported against performance indicators such as reliability (% of days the demand is met) and efficiency (% of available water used - i.e. that not lost to overflow). Here ERain has been used to assess the effects of varying roof size or irrigation area on the economic viability of RWH systems for tank sizes ranging from l-7kL. Results show that excluding outdoor use, the BCR increases with roof size along with reliability while efficiency decreases. Interestingly, the larger roof area has the most significant effect in terms of reliability on the smaller tanks. Including outdoor use reduced reliability overall and increases efficiency and BCR indicating that it is better financially to use the RWH system for outdoor use when reliability is not a concern. However, the pattern of BCR and NPV is different from when no outdoor use is attached. When outdoor uses are not connected reliability is high even with a smaller tank, and so a larger tank offers little advantage. However, once outdoor use is connected small tanks have a low reliability which can be increased with a larger tank. When analysing various irrigation areas tank sizes up to l 5kL were considered. The larger NPVs and BCRs occur with the larger irrigation areas as this increases water use and hence monetary water savings. The highest BCR occurs with a l 5kL tank; while the highest (least negative) NPV occurs with a 10kL tank as it did without outdoor use connected. Within the l-7kL tank range, the 7kL tank is the most favourable when outdoor irrigation use is connected.]]