Due to mounting human pressure, stakeholders in northern Thailand are facing crucial natural resources management issues. Among others, the impact of upstream irrigation management on the downstream agricultural viability is a usual source of conflict. It has often both biophysical and social origins. As different ethnic groups with tense relationships are involved, appropriate solutions should only emerge from negotiation. CATCHSCAPE has been developed through a Multi-Agent System approach that enables us to describe the whole catchment features as well with farmer's individual decisions. The biophysical modules simulate the hydrological system with its distributed water balance, irrigated schemes management, crop and vegetation dynamics. The social dynamics are described as a set of resources management processes (water, land, cash, labour force). Water management is described according to the actual different levels of control (individual, scheme and catchment. Moreover, the model's architecture is presented in a way that emphasizes the transparency of the rules and methods implemented. Finally, one simulated scenario is described with its main results as well, according to different viewpoints (economy, landscape, water management).