In Iowa, all open beef feedlot operations over 1,000 head are required to have runoff control systems. Iowa regulations allow the use of vegetative treatment systems (VTS) on open beef feedlots that meet regulatory siting requirements. For a National Pollutant Discharge Elimination System (NPDES) permit, the runoff control performance of VTSs must meet or exceed the performance of traditional runoff containment basins as predicted by the Iowa State University-Effluent Limitations Guideline (ISU-ELG) model. The ISU-ELG model is based on a model developed by Koelliker in 1975 to predict the performance of a holding basin at controlling feedlot runoff. In this paper, the criterion used to determine if a particular day is a “dewatering day” is investigated to determine its effect on basin performance, for wetter areas in Iowa the number of drying days has a large effect on basin performance, where as for the drier northwest region of Iowa this effect is limited. This paper compares results from the ISU-ELG model to results obtained using the Soil-Plant-Air-Water (SPAW) model to simulate traditional feedlot runoff containment basin performance. The SPAW model uses a soil moisture criterion to determine if conditions are acceptable for land application of basin effluent. The results show that the ISU-ELG model over-predicts performance of traditional containment systems in comparison to the SPAW model at all five locations investigated.