The integrated management of river basin is an important problem for social, economical and environmental issues. It requires to take into account many dynamics and interactions. In order to model the interactions between population and water resource, we propose hère an object oriented model, linking an hydrological model to 5 sub models describing the human activities: population dynamics, land-use, houslng, économie indicators, water withdrawals. Our model is centred on the "sub-bassin" spatial unit. Within the European project FIRMA, we apply this model to the Orb basin (southern France) using a participatory approach. We illustrate how such models can be used to compare différent scénarios at the river basin scale. Using simple simulations, we show the impact of an economical shift on both water resource and population migration. Final discussion addresses the limits of the models, the définition of a relevant spatial unit, and the possible links with agent techniques.