SWAT is a hydrologic model, developed by the Agricultural Research Service (ARS) of the United States Department of Agriculture (USDA) and Texas A&M AgriLife Research, that operates in order to assess the impacts of land management on water quality and quantity in watersheds over long periods. It has the capacity to simulate important nutrient and pesticide processes in the land phase and in the in-stream phase. Besides, it allows to apply a number of climate customization options for simulations that are looking at the impacts of climate change.

More information and the executables of the model can be found at the official SWAT website:

http://swat.tamu.edu/