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 Policy Program (NPP) environmental baseline simulations for the Upper North Bosque River Watershed (UNBRW), located in Earth County, Texas. The integrated modeling system consists of an economic model that is linked to an environmental component comprising the field-scale APEX and watershed-scale Soil Water Assessment Tool (SWAT) models. The environmental baseline simulations test and calibrate the complete environmental component, using monitoring data obtained during 1993-94 in the UNBRW. The economic model was not executed for the environmental baseline, although underlying assumptions built into the economic model were used. The results are edge-of-field environmental indicators obtained from the APEX model.