Historically, in the Netherlands more than 70% of the public sewer systems are combined systems. These combined systems thus treat relatively clean storm water as sewage in sewage treatment plants. Newly installed combined systems now separate storm and sewage water. Several techniques were developed to prohibit storm water draining to the existing combined systems. For coping with the different regulations and technical solutions, the decision model IBOS Rainwater was developed. The basic idea of IBOS was to create a web-based interface that provides an individual user with information on the local hydrological and drainage situation anywhere in the Netherlands, with which water harvesting decisions can be made.