From official site:
SWAT is a public domain model actively supported by the USDA Agricultural Research Service at the Grassland, Soil and Water Research Laboratory in Temple, Texas, USA.
This is not an official repository for SWAT. It exists mostly because of the lack of official public one.
This repository is an attempt to bring various revisions under one umbrella to make it easier to track code changes and keep up with newer revisions. CMake build system is used to make compiler and platform independent build process. Testing is mostly done with gfortran compiler on MS Windows and Ubuntu GNU/Linux.
More information can be found in wiki.