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United States Department of Agriculture

Agricultural Research Service

Research Project: DEVELOPMENT OF ALTERNATIVE PRACTICES FOR IMPROVED WATERSHED MANAGEMENT

Location: Cropping Systems and Water Quality Research

Title: Epic and Apex: Model Use, Calibration, and Validation

Authors
item Wang, Xiuying -
item Williams, Jimmy -
item Gassman, Philip -
item Baffaut, Claire
item Izaurralde, Cesar -
item Jeong, Jaehak -
item Kiniry, James

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 27, 2012
Publication Date: August 27, 2012
Citation: Wang, X., Williams, J.R., Gassman, P.W., Baffaut, C., Izaurralde, C., Jeong, J., Kiniry, J.R. 2012. EPIC and APEX: Model use, calibration, and validation. Transactions of the ASABE. 55(4):1447-1462.

Interpretive Summary: The Environmental Policy Integrated Climate (EPIC) and Agricultural Policy/Environmental eXtender (APEX) models have been developed to assess a wide variety of agricultural water resource, water quality, and other environmental problems. The EPIC model is designed to be applied at the field level. APEX is a direct extension of EPIC that can be used to simulate more complex areas: landscape, whole farm, and watersheds. Key model components are described for EPIC and APEX, including different options for simulating the main processes. The procedure by which input parameters are adjusted to obtain a good match between model results and observed data is described for both models, with an emphasis on important parameters and guidance regarding logical sequences of steps in the process. Two case studies are presented: one for a 35-ha field in north central Missouri and one for the 5720-ha Clear Creek watershed in central Texas. Together they provide examples of APEX applications to simulate streamflow, crop yields, and sediment and atrazine yields. Examples of EPIC field-scale testing for crop yields and soil organic carbon are also presented. Future research and development needs for both models are also discussed. This paper will benefit scientists and engineers who are using the model as well as resource managers who have an interest in the model results and need to understand how it needs to be calibrated and validated.

Technical Abstract: The Environmental Policy Integrated Climate (EPIC) and Agricultural Policy/Environmental eXtender (APEX) models have been developed to assess a wide variety of agricultural water resource, water quality, and other environmental problems. The EPIC model is designed to be applied at a field-scale level. APEX is a direct extension of EPIC which can also be applied at the field-scale, as well as for more complex multi-subarea landscape, whole farm, and watersheds. Key model components are described for EPIC and APEX, including different options for simulating surface runoff, evapotranspiration, soil erosion and other processes. Field-scale calibration and validation procedures are then described for both models, with an emphasis on important calibration parameters and guidance regarding logical sequences of calibration steps. Additional calibration and validation guidance is further provided for applications of APEX at the landscape and watershed scales. Two calibration and validation case studies are presented: one for a 35-ha field in north central Missouri and one for the 5720-ha Clear Creek watershed in central Texas. Together they provide examples of APEX calibration and validation for streamflow, crop yields, and sediment and atrazine yields. Examples of EPIC field-scale testing for crop yields and soil organic carbon are also presented. Future research and development needs for both models are also discussed.

Last Modified: 8/1/2014
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