Evaluation on the Agricultural Policy Environmental Extender (APEX) Water Flux Simulations for the Chesapeake Bay Watershed.

The Agricultural Policy/Environmental Extender (APEX) is a complex mechanistic simulation model designed to evaluate agricultural management strategies and effects on yields, water and soil quality, and resource use.  Even though huge model is useful tool for investigating water related natural farm land mechanisms, it may perform differently depending on study areas or data availability. For use in the Chesapeake Bay Watershed, the model parameters have been generally calibrated using data from several test sites where appropriate information is available. The objective of this study is to evaluate APEX model performance using current parameters with observations from the OPE-3 site (Optimizing Production Inputs for Economic and Environmental Enhancement) which located at the US Department of Agriculture, Agricultural Research Center, Beltsville, Maryland, USA. Data from the OPE-3 site consists of water content at various layers and locations, runoff, corn yields, and ET covering the period from 1998 to 2014. The APEX model was run using OPE-3 characteristics for soil texture and field operation schedules from 1998 to 2014. The initial results for total water in the soil profile (to 100 cm)  showed that simulated water amounts were about 4.4 cm higher than observed but the trends were very similar. Measured yields were higher and were more variable than simulated yields, likely due to high spatial variability in the measured yields.