Despite increasingly scarce water resources and sometimes unreliable municipal supplies, there is limited guidance on alternative water sources such as rainwater. Some of the guidance that is available is inaccurate or misleading. Therefore to understand rainfall and how it can be integrated into building water systems I have been working on a new methodology and tool called the Rainwater Use Model (RUM). The development of the RUM aims to address some of the questions below, which are not always addressed by current guidance:

• How large should a rainwater tank be?
• Is the catchment area big enough?
• Will harvested water be able to meet needs, even over a long dry period?
• How do you take into account variability in rainfall between years?
• How do calculate water needs?
• Given the choice, should you invest in larger rainwater tanks or larger catchment areas?
• What is the payback of rainwater harvesting systems?
• How can you estimate the contingencies and reserves that should be incorporated in a rainwater harvesting system of water neutral building?

Early versions of the RUM have been applied it to a range of dry climates in South Africa and appears to work well for residential buildings. Further research is needed to test the tool on other climates and building types.