Vegetative Treatment Systems (VTSs) are currently being used at several open beef feedlots across Iowa as an alternative to traditional feedlot runoff containment systems. There are two types of VTS: a VTA system which is comprised of solids settling basin (SSB) followed by a vegetative treatment area (VTA), and a VIB-VTA system which is comprised of a solids settling basin followed by a vegetative infiltration basin (VIB) and a VTA. Iowa State University developed two computer models to simulate VTS performance. When model predictions were compared with data collected from four Iowa sites, the models were found to under predict the VTA outflow, VIB outflow, and nutrient concentrations in the SSB outflow. This paper focuses on the modifications made to the Iowa State University VTA model. To identify the problems with the model, the graphical and numerical outputs were examined for values that were either not reasonable or did not fit the expected behavior of the system. Three major problem areas were identified in the VTA model: an extremely high rate of water removal from the VTA, incorrect calculation of soil moisture, and incorrect tracking of the water table (especially in high water table situations). Each of these problems was isolated and the code controlling this function of the model was examined. Potential solutions were tested to see if they accurately simulated VTS behavior. If successful, the solutions were then implemented. These modifications and their impact on model performance are discussed in this paper
