Apex Simulation of Runoff and Total Phosphorous for Three Adjacent Row-Crop Watersheds with Claypan Soils.

The Agricultural Policy Environmental Extender (APEX) model can simulate crop yields, runoff, and the transport of sediment and nutrients in small watersheds that have combinations of farm level landscapes, cropping systems and/or management practices.  The objectives of the study were to parameterize, calibrate and validate the APEX model for three (East, Center, and West) long-term monitored adjacent corn/soybean watersheds with claypan soils at the Greenley Memorial Research Center, Knox County, Missouri. Additionally, the study aimed to evaluate the long-term benefits of existing waterways.  ArcAPEX was used to create input files. The APEX0604 model was calibrated for daily events using measured data from the Center watershed from 1993 to 1997and validated with data from the West and East watersheds. Crop yield predictions were within ±13% percent of the measured yields of all watersheds.  Daily runoff was well simulated as indicated by r² and Nash-Sutcliffe Coefficient (NSC) values ranging from 0.92-0.94 and 0.7-0.8, respectively.  Goodness of fit indicators for total phosphorus (TP) model predictions for higher storm events (average 50mm) were 0.78-0.79 and 0.47-0.59, for r² and NCS, respectively.  The r² and NCS values for sediment and total N (TN) predictions were less than 0.4 and 0.2 respectively.  Model simulations for average annual values for 40 years showed an18-20% reduction in runoff and 35-38% reduction of TP when grass waterways are in place.  Model was able to simulate the differences observed in measured runoff, sediments and nutrients among the three watersheds.  APEX provides a useful tool in evaluating best management practices for long-term benefits.