Tensions over water resources in upland areas of northern Thailand are often attributed to reductions in water supply caused by forest clearing. This article argues that the hydrological evidence for such reductions in supply is very weak and that, rather, the key hydrological issue in upland catchments is a significant increase in water demand, especially during the dry season. The arguments are illustrated with a detailed examination of the Mae Uam catchment, located in Chiang Mai province, where the development of dry-season soybean cultivation appears to have tested the hydrological limit of the catchment, and even exceeded this limit in drier years. The author argues that a shift in focus from water supply to water demand has fundamentally important political implications. As long as the focus of public debate is on water supply, the regulatory focus will be on those resident in the forested upland areas that are seen as being crucial in securing downstream flows. But if the water management focus is shifted to water demand, then regulatory attention must shift to the diverse sources of demand that exist throughout the hydrological system.