Advanced Water Budget Analysis for Rainwater and Related Harvesting Applications

This session describes an advanced water budget analysis (WBA) model and approach for rainwater harvesting applications developed by Rainwater Recovery Inc. A case study is presented which illustrate the features of the model and the approach in general. The WBA model is a spreadsheet-based application that uses 20 or more years of the most recent climatic data local to the subject site to predict performance of rainwater harvesting systems for particular project geometries and demand patterns. Important project geometry parameters include cistern usable volume, catchment area type and areal extent. Performance is measured in terms of water savings (Utilization), runoff reduction (Efficiency), and Reliability (probability of a system meeting the total imposed water demands applied to the system on a given day). Performance for various scenarios can be analyzed next to each other in tabular and/or figure format to obtain optimum cistern size, collection area, and demands. The model is flexible enough to accommodate simple residential irrigation projects and more complex commercial or institutional projects with various collection areas, alternate sources of harvested water, and multiple types of seasonal and year-round demands. The most typical and useful application of the model is to compare performance of various cistern sizes with a fixed collection area and demand scenario. Conversely, the model can predict performance of a fixed cistern size over several collection area and demand scenarios, providing the ability to evaluate the option of connecting additional collection areas to the system in order to maximize its performance and cost-effectiveness. Many other useful statistics and graphics can be compiled from the model data such as statistics for LEED® credits or for educational purposes such as showing tank levels over a typical season, as well as quantifying water savings or reliance on a domestic back-up water supply across varying seasonal or annual weather patterns. Model output can be used to calculate system return on investment (ROI) once a harvesting system has been designed and priced by using data on water cost and use reduction and, if relevant, data on runoff volume reduction. A properly sized harvesting system can substantially reduce runoff to receiving waters or combined sewer systems, resulting in a reduction in volume and/or frequency of combined sewer overflow (CSO) events and potential savings in fees.