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Field-scale and watershed-scale environmental models are only capable of simulating mechanistically a limited number of practices individually. Furthermore, there is need for a mechanism to simulate environmental and economic models within a unified interface program in order to determine cost-effective practices. This study was conducted to develop an automated program, SWAPP (Soil and Water Assessment Tool (SWAT)-Agricultural Policy/Environmental eXtender (APEX) Interface Program), to convert SWAT files to-and-from APEX format and simulate SWAT and APEX simultaneously; integrate SWAPP and an economic model (Farm-level Economic Model; FEM); and evaluate the resulting program (Comprehensive Economic and Environmental Optimization Tool-SWAPP; CEEOT-SWAPP) using measured data.