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Title:   Author  item
                                            Baffaut, Claireitem
                                            Bolster, Carlitem
                                            NELSON, NATHAN - Kansas State Universityitem
                                            VAN LIEW, MIKE - University Of Nebraskaitem
                                            Arnold, Jeffreyitem
                                            WILLIAMS, JIMMY - Texas A&M Agrilife
  Submitted to: Annual International SWAT Conference
                                    Publication Type: Proceedings
                                    Publication Acceptance Date: 12/2/2013
                                    Publication  Date: 12/11/2013
                                    Citation: Baffaut, C., Bolster, C.H., Nelson, N., Van Liew, M., Arnold, J.G., Williams, J. 2013. SWATing your APEX model: a how-to from the trenches. In: Proceedings of 2013 International SWAT Conference, July 17-19, 2003, Toulouse, France. p. 86-97.Interpretive Summary: Modelers and scientists need methods to link edge-of-field observations to stream loadings, and integrate what we know of soil and water quality at the field scale to watersheds and river basins. Computer simulation models are useful tools that can be used for this purpose. In particular, the Soil and Water Assessment Tool (SWAT) was designed for the watershed scale and the Agricultural Policy Environmental eXtender (APEX) was designed for the field scale. APEX and SWAT are particularly well adapted to this integration because they belong to the same family of models, have many common inputs, and use similar equations. However, they are not identical and the differences need to be recognized before both models can be used jointly. This paper reviews significant differences identified in how APEX and SWAT calculate major components of the water and nutrient cycle.  Since APEX is more flexible, we define how it can be set up so that it becomes equivalent to SWAT, whenever possible. When model setup does not take these differences into account, the models do result in different output values. This analysis will help model users separate the model effects from true scale effects obtained with these models, and interpreting their results.
                                Technical Abstract: Methodologies to link edge-of-field observations to stream loadings need to be developed to integrate what we know of soil and water quality at the field scale to watersheds and river basins. The Soil and Water Assessment Tool (SWAT) can be used to scale up results obtained at the field scale with the Agricultural Policy Environmental eXtender (APEX). The APEX and SWAT models are particularly well adapted to this integration because they belong to the same family of models and have many common input parameters. However, they are not identical and the differences need to be recognized before both models can be used jointly. For example, while runoff can be calculated with the Curve Number method in both models, calculation of the soil water retention parameter is different and leads to different curve numbers, and thus runoff. Similarly, both models can use the MUSLE equation to calculate sediment loss but the equations used to calculate peak runoff rate differ. This paper reviews some differences in algorithm and parameterization between APEX and SWAT. Since APEX parameterization is more flexible, we define how it can be parameterized so that it becomes equivalent to SWAT, whenever possible. Differences were identified in how APEX and SWAT calculate runoff, potential evapotranspiration, peak runoff rate, harvest index, and transfer between the phosphorus pools. When parameterization does not take these differences into account, the models do result in different values of output variables. This analysis will help model users separate the model effects from true scale effects when parameterizing these models, and interpreting their results.


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