This paper presents the underlying assumptions and results for the Agricultural Policy/Environmental eXtender (APEX) model, which was applied within an integrated modeling system for the National Polio Program (NPP) environmental baseline simulations for the Upper North Bosque River Watershed area in Texas. The system consists of an economic model linked to an environmental component comprising the field-scale APEX and watershed-scale Soil Water Assessment Tool (SWAT) models. The results are edge-of-field environmental indicators obtained from the APEX model.