Background

Erosion-Productivity Impact Calculator (EPIC) Model

EPIC, a cropping systems simulation model, was developed to estimate soil productivity as affected by erosion throughout the United States during the 1980’s. It was a response to the first Resources and Conservation Act (RCA) appraisal conducted in 1980, which revealed a significant need for improved technology for evaluating the impacts of soil erosion on soil productivity.

EPIC simulates all crops with one crop growth model using unique parameter values for each crop. The processes simulated include leaf interception of solar radiation; conversion to biomass; division of biomass into roots, above ground mass, and economic yield; root growth; water use; and nutrient uptake.

EPIC is a field scale, daily time step model composed of physically based components for soil and crop processes such as erosion, nutrient balance, crop growth, and related processes. It is designed to simulate drainage areas that are characterized by homogeneous weather, soil, landscape, crop rotation, and management. Since the initial development, EPIC has been continually improving through the additions of algorithms to simulate water quality, climate change and the effect of atmospheric CO₂concentration, and nitrogen and carbon cycling.

Agricultural Policy/Environmental eXtender (APEX) Model

The APEX model was developed to extend the EPIC model capabilities to whole farms and small watersheds. In addition to the EPIC functions, APEX has components for routing water, sediment, nutrients, and pesticides across complex landscapes and channel systems to the watershed outlet. APEX also has groundwater and reservoir components. A watershed can be subdivided as much as necessary to assure that each subarea is relatively homogeneous in terms of soil, land use, management, and weather. The routing mechanisms provide for evaluation of interactions between subareas involving surface runoff, return flow, sediment deposition and degradation, nutrient transport, and groundwater flow. Water quality in terms of nitrogen (ammonium, nitrate, and organic), phosphorus (soluble and adsorbed/mineral and organic), and pesticides concentrations (GLEAMS pesticide model is used to estimated pesticide fate.) may be estimated for each subarea and at the watershed outlet.