Efficient equitable and sustainable water management at the river basin level plays a crucial role in solving growing water shortages. However water allocation decisions are highly complex due to the temporal and spatial variation in supply and demand. The increasing competition for water among different uses and across countries further complicates basin management. An aggregate integrated economichydrologic river basin model is developed to analyze these issues for the Mekong River Basin in Southeast Asia. Model results indicate competition for Mekong waters and significant tradeoffs between offstream and instream uses during the dry season and point to the importance of appropriate institutions for enhanced economic efficiency in water allocation across waterusing sectors and countries.