Megacities development is putting an increasing pressure on water resources in their peri-urban areas. It particularly raises the challenge of the competition between potable water supply and irrigated agriculture, in a context of rapid land use changes, which results in increased pressures on water availability and quality. This is the case of the Tietê-Cabeceiras catchment in the Metropolitan Region of São Paulo (Brazil), the only metropolitan catchment where agriculture is being represented in the water committees - the negotiation and discussion body for water management at catchment level. While agriculture directly competes with domestic water uses, it also provides some environmental and social services: it contributes to the preservation of forest fragments, prevents illegal urban land occupation and probably contributes to the preservation of social network in its rural district which could counterbalance the negative social effect of urbanization. But urbanization is putting an high pressure on agriculture and its evolution is difficult to anticipate. An approach based on the development of simulation tools as mediating tools for concertation on this issue is presented.