Experience in northern Thailand suggests that rapid commercialisation of upland catchments can have significant, and often unpredictable, social, economic and biophysical impacts. The purpose of this paper is to describe a multi-disciplinary methodology that is being used to assess water resource management in northern Thailand. The methodology identifies and integrates contributions from biophysical, economic and socio-cultural disciplines in the development of a scenario-modeling Decision Support System (DSS). The key modeling 'unit' in the DSS is the 'Resources Management Unit' (RMU). The RMU situates the socially and economically constituted agricultural household (the main decision maker) in its biophysical environment. This paper describes the application of the methodology to a case study of water allocation between upstream and downstream villages in the small subcatchment of Mae Uam in northern Thailand. It concludes with an indication of the potential for the methodology to be applied in other catchments.