Water management decisions often have far reaching consequences for the every day life of the inhabitants of a river basin. To take these consequences into account during decision making processes, we aim to develop a software tool that can indicate the expected effects of different management strategies on a number of river functions. This requires knowledge domain representation that fits the perception of stakeholders, and at the same time provides a starting point for modelling. We found that UML provides a language that on the one hand facilitates the communication and is intuitively understood, and on the other hand provides a high level of consistency as well as decomposition into elements that can easily be modelled. Moreover, UML provides the space required to deal with both quantitative and qualitative information, which is desirable to incorporate knowledge of different scientific fields / disciplines.