The efficiency of nature based solutions (NBS) for improved water quality and flood mitigation is often evaluated ex-ante by means of simulation models. Realistic estimates of this efficiency are important, because NBS compete with other land use for space and other resources. Informed societal decisions depend on reliable evaluation of mitigation strategies. The changes that make up the implementation of such strategies are represented in hydrological models by changing the values of existing parameters and/or the inclusion of new parameters and processes. The goal of this session is to compare approaches to numerical NBS representation in models, how parameter values are estimated, and how the latter can be validated.