This paper reports the successful implementation of a prototype model integrating representations of both natural and social systems and developed in collaboration with stakeholders. The model integrates a hydrological model parameterised to represent the effects of precipitation and temperature on water availability in the Thames region of southern England (including Oxford, London and the Southeast) with a model of demand for water by households. It is the first of a series of models to be developed as part of a project on Freshwater Integrated Resource Management with Agents (FIRMA). Models are being developed for five regions of Europe where the consequences of climate change for water use and availability could well be significant. The model reported here differs from economic models of demand in that its empirical validity is achieved by taking interaction a`mong agents explicitly into account whereas economic models typically (for simplicity) find some means of excluding dense patterns of agent interaction.