APEX Calibration and Validation

Simulation modeling is widely used to assess water contamination risk associated with pesticide use and for evaluating effectiveness of agricultural conservation practices. Currently the Agricultural Policy/Environmental eXtender (APEX) model is being used for this purpose in the USDA Conservation Effects Assessment Project (CEAP) Cropland National Assessment. In support of the CEAP modeling effort, APEX was calibrated and validated using a 9-year record (1999-2007) of surface runoff and tile flow, and an 8-year record (1999-2006) of soluble pendimethalin and fluometuron herbicide losses from fields in a cotton-peanut rotation located in the Atlantic Coastal Plain region of south-central Georgia (USA). Conventional and strip tillage management were directly compared. Monthly surface runoff was calibrated by adjusting three variables: the Curve Number Index Coefficient (CNIC), which is a driver of the Natural Resources Conservation Service Runoff Curve Number; the runoff curve number for the average soil moisture condition 2 (CN2); and Irrigation Runoff Ratio (IRR). Monthly statistics met satisfactory criteria between observed and simulated surface runoff for both tillage types, with R values from 0.64 to 0.79 and Nash-Sutcliffe efficiency (NSE) values from 0.55 to 0.73 during calibration and validation periods. Monitored and predicted annual runoff and fluometuron and pendimethalin runoff losses for both tillage systems and the tile drainage from the strip-till system were well aligned. Values of R ranged from 0.57 to 0.93 and NSE values from 0.51 to 0.89 based on annual comparisons between simulated and observed values. APEX correlation was less satisfactory for annual tile flow from the conventional tillage system, and relatively poor for fluometuron transport in tile flow from both tillage systems. Overall, APEX effectively replicated annual means for these three variables with simulated annual means within 10% of the observed values. The percent bias statistical parameter, PBIAS, was below 26% for all annual