Within the general frame of the Aquastress project a regional water balance model (WBM) was developed based on a material flow analysis software application. The aim was to provide a tool to model the dynamics of environment-society interactions in the use of water, as a way to support integrative water management. The WBM appeals to stakeholders in a decision making process and not to researchers or hydrologists. Thus it meets the challenge to simplify the hydrological system of a test site without loosing its main characteristics. The main objective of the WBM is to help identify causes of water stress, taking explicitly into account the interdependencies of water use subsystems. The WBM is primarily designed to be applied at a regional scale and on a monthly basis. Water stress is calculated by computing the water supply and demand in each sub-system or sector. Water cycle components (precipitation, evaporation, ground water storage, etc.) are considered in each subsystem as necessary, according to their specific relevance in the water balance calculation. The model divides a given site into four functional subsystems, generally referred to as sectors. These sectors are: industrial, domestic, agricultural and (semi-)natural. Water flow and use in subsystems are represented by flow arrows and functional units or boxes. Boxes represent places where water is allocated or managed, where water is processed, or where water demand is calculated. Water stress is assessed by comparing water demand and water supply.