Jimmy R. Williams (B.S. Texas A&M University)


Model category CSM
Plant part Shoot
Scale Field
Licence open_source
Operating system Windows, Linux
Programming language Fortran
Format of model inputs and outputs .DAT files and other files
Species studied Generic-crops
Execution environment Console, Stand-alone application

Scientific article

The Agricultural Policy/Environmental Extender (Apex) Model: An Emerging Tool for Landscape and Watershed Environmental Analyses
Gassman,Philip W.; Williams,Jimmy R.; Wang,Xiuying; Saleh,Ali; Osei,Edward; Hauck,Larry; Izaurralde,Roberto C.; Flowers,Joan
Transactions of the ASABE, 2010 View paper

Model description

The Agricultural Policy / Environmental eXtender (APEX) model was developed to extend EPIC’s capabilities of simulating land management impacts for small-medium watersheds and heterogeneous farms. It can be configured for land management strategies such as irrigation, drainage, furrow diking, buffer strips, terraces, waterways, fertilization, manure management, lagoons, reservoirs, crop rotation and selection, pesticide application, grazing, and tillage. The routing of water, sediment, nutrient, and pesticide capabilities are some of the most comprehensive available in current landscape-scale models.

Some case studies

APEX can perform long-term continuous simulations for modeling the impacts of different nutrient management practices, tillage operations, conservation practices, alternative cropping systems, and other management practices on surface runoff and losses of sediment, nutrients, and other pollutant indicators. Example applications include:

- evaluate effects of global climate and carbon dioxide changes on crop yields,
- assess alternative manure applications and other management scenarios,
- design biomass production for energy,
- demonstrate hydrologic balance components and pollutant transport for different cropping and forestry production systems,
- simulate intensive rotational grazing scenarios depicting movement of livestock between paddocks,
- evaluate the effects of different tree harvesting treatments on forested watersheds,
- analyze the potential benefits of “woody draws” (relatively small, natural drainage areas covered by trees or shrubs in agricultural landscapes),
- determine cropping and conservation practice effects on lake systems,
- simulate landscape management decisions such as harvest unit size, total area harvested, and rotation length,
- assess environmental impact of pollutants into lakes,
- evaluate effects of buffer strips,
- simulate nonpoint-source pollution impacts from cultivated cropland,
- design economical and environmentally safe landfill sites