We have developed a software framework called Tethys to aid in the creation of web-based water resource modeling applications. This suite is a Python-based scripting environment that leverages open source tools for geoprocessing of spatial data, map rendering and visualization, distributed computing, and database management. The system makes it possible to deploy a calibrated, high-resolution watershed or surface water model as a web-based application for decision support. The framework provides data managements, access to computing resources, and pluggable components (e.g. plots, maps and user controls) that enable rapid development of modeling applications. We have used the system to develop prototype applications for land use change impacts such as burned area analysis and urban master plan development, impact of snowmelt on spring runoff in mountainous regions, and flood early warning.