A companion modeling process about water resources supply and demand at the scale of a catchment in partnership with the catchment management institution and with intermediary level stakeholders of the different sectors of water use, production and distribution in the territory is being held in the Thau catchment, near Montpellier in South of France. This companion modeling process has been shaped by both a scientific questioning about interactions between land and water dynamics and governance within a catchment and the history of collaborations between the research team and the case study partners. It was initiated by the catchment management institution’s request to our team for a methodological support during a prospective exercise within the catchment management plan development process. It was then agreed to keep on collaborating through the development of a computer aiming at supporting a real joint reflection on the integrated management of land and water resources of the territory. During a first phase, causal maps were used for developing and discussing individually the conceptual model with the stakeholders. The objective was to elicit and discuss which variables we should consider and how they are linked. We ended up with a conceptual model where the complexity was not arising from feedback loops between variables but from the interlinkage of differentiated decision-making centers, resources and management entities. For this reason, we’ve shifted to agent-based modeling and implemented our simulation model under Netlogo. A first stakeholders’ workshop for validating the simulation model and discussing stakes and scenarios was held in March and this companion modeling process has been institutionalized as an official action of the newly born catchment integrated management contract.