Code modernization and modularization of APEX and SWAT watershed simulation models

SWAT (Soil and Water Assessment Tool) and APEX (Agricultural Policy/Environmental eXtender) are respectively large and small watershed simulation models derived from EPIC (Environmental Policy Integrated Climate), a field-scale agroecology simulation model. All three models are coded in Fortran and have evolved over several decades. They are widely used to analyze anthropogenic influences on soil and water quality and quantity. Much of the original Fortran code has been retained even though Fortran has been through several cycles of development. Fortran now provides functionality originally restricted to languages like C, designed to communicate directly with the operating system and hardware. One can now use an object-oriented style of programming in Fortran, including inheritance, run-time polymorphism and overloading. In order to enhance their utility in research and policy-making, the models are undergoing a major revision to use some of the new Fortran features. With these new programming paradigms the developers of SWAT, APEX, and EPIC are working to make communication between the two models seamless. This paper describes the ongoing revision of these models that will make them easier to use, maintain, modify and document. It is intended that they will converge as they continue to evolution, while maintaining their distinctive features, capabilities and identities.