Technological advances in recent years have fundamentally changed how water resources systems can be modeled. These advances include low-cost, easy access to cloud computing, open source application development frameworks, data transfer protocols, advances in online collaboration tools, and a wide range of specific web-based user interaction tools. The dominant water resources planning software today do not currently utilize most of these advances. This presentation describes a new, web-based application for collaborative water system modeling called OpenAgua, a collaboration between the University of California, Tec de Monterrey, and the University of Massachusetts. Key features of OpenAgua include a water system simulator; intuitive interfaces for entering and viewing water system data for multiple scenarios; utilization of cloud computing; and advanced visualization tools. Collectively, these features enable exploring a wide range of water management options under future uncertainty. In addition to demonstrating these core capabilities, this talk will also highlight several exciting future possibilities for advanced uses of cloud computing, including using emerging large global datasets for local water system planning.