Multi-Site P-Loss Assessment from Privately Owned Cropland in Northeast Missouri.

Phosphorus exported from watersheds can accelerate eutrophication and is identified as one of the main causes of water impairment.  Accurate phosphorus (P) loss assessment has become vital for effective implementation of conservation measures.  The study objective was to assess the validity of a claypan soil parameterization of the Agricultural Policy Environmental eXtender (APEX) model for predicting runoff, sediment, and P-losses in four privately owned, monitored farm watersheds located in the claypan region of Northeast Missouri. The watersheds had one or more conservation practices such as terraces, grass waterways, and conservation crop rotation. The model’s coefficient of determination (r²) for event runoff ranged from 0.19 to 0.88, the Nash-Sutcliffe Coefficient (NSC) values ranged from -0.12 to 0.79 and the Percent bias (Pbias) values ranged from 8 to 28%. The watersheds with terraces showed poor performances for runoff requiring us to investigate model’s simulation of terraces.  The performance indices for sediment and TP are expected to improve after the model is calibrated for runoff. The target for model calibration for event outputs will be: r² > 0.6, NSC > 0.5 and Pbias < 25%. We anticipate that with calibration, the APEX model could be used as an efficient tool to evaluate conservation measure for effective control of P-losses in this region.