Soil and Water Assessment Tool (SWAT) is a free and open source hydrology model and GIS computer simulation sponsored by the USDA.
SWAT is a well known geographic hydrological model in use by many universities and government agencies around the world, and integrates with commercial products like ArcGIS.
Some users of SWAT
*The River Systems Research Group of the Department of Oceanography at the University of Washington
*The Hydro and Agro Informatics Institute under the Ministry of Science and Technology of the Royal Thai Government
*The Departments of Agricultural & Biological Engineering and Civil & Environmental Engineering at Purdue University