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Title: Sensitivity analysis of the agricultural policy/environmental extender (APEX) for phosphorus loads in tile-drained landscapes

item Ford Iii, William
item King, Kevin
item Williams, Mark
item WILLIAMS, JIMMY - Texas A&M University
item Fausey, Norman

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2015
Publication Date: 4/24/2015
Publication URL:
Citation: Ford III, W.I., King, K.W., Williams, M.R., Williams, J., Fausey, N.R. 2015. Sensitivity analysis of the agricultural policy/environmental extender (APEX) for phosphorus loads in tile-drained landscapes. Journal of Environmental Quality. 44:1099-1110. DOI:10.2134/jeq2014.12.0527.

Interpretive Summary: Phosphorus exported from agricultural fields in tile drained landscapes contributes to water quality problems in downstream water bodies, such as the summer algal blooms seen in Lake Erie. Phosphorus load reductions are thus a major point of emphasis and computer models can provide assistance at determining the most effective strategies. Models that simulate these processes are relatively untested in these landscapes, hence the objective of this paper was to test if a widely used modeling tool can simulate phosphorus loads at the edge of field for a suite of tile-drained midwestern fields. Results of the study point to the ability of the Agricultural Policy/Environmental eXtender (APEX) model to effectively simulate agricultural field phosphorus loadings at seasonal and annual timescales. Complexity in the interactions occurring in the field has significant implications for management when considering different pathways for P to make it from the field to the stream.

Technical Abstract: Numerical modeling is an economical and feasible approach for quantifying the effects of best management practices on phosphorus (P) loadings from agricultural fields. However, tools that simulate both surface and subsurface P pathways are limited and have not been robustly evaluated in tile-drained landscapes. The objectives of this study were to test the ability of the Agricultural Policy/Environmental eXtender (APEX), a widely used field-scale model, to simulate surface and tile P loadings over management, hydrologic, biologic, tile and soil gradients, and to better understand the behavior of P delivery at the edge-of-field in tile-drained, Midwestern landscapes. To do this, a global, Sobol’-based, sensitivity analysis was performed and model outputs were compared to measured P loads obtained from 14 surface and 15 subsurface edge-of-field sites across central and northwestern Ohio. Results of the sensitivity analysis showed that P dynamics were highly sensitive to coupled interactions between presumed important parameters, suggesting non-linearity of P delivery at the edge-of-field. Comparing model results to edge-of-field data showcased the ability of APEX to simulate water and sediment discharges, and their associated P loadings, at both seasonal and annual timescales. Results provide new insights regarding baseline tile P loadings which exceed thresholds for algal proliferation. Further, negative feedbacks between surface and subsurface P delivery suggest caution is needed when implementing P-based BMPs designed for a specific flow pathway.