The Use of the Soil-Plant-Air-Water Model to Predict the Hydraulic Performance of Vegetative Treatment Areas for Controlling Open Feedlot Runoff

Several Iowa beef feedlots have interim, National Pollution Discharge Elimination System (NPDES) permits for vegetative treatment systems (VTS) to control and treat feedlot runoff. In Iowa, performance of these systems is predicted for permitting purposes using either the Iowa State University-Vegetated Treatment Area (ISU-VTA) Model or the Iowa State University -Vegetated Infiltration Basin/Vegetated Treatment Area (ISU-VIB/VTA) Model. For an Iowa NPDES permit, these systems must be shown through modeling to have equal or better performance than a conventional runoff containment basin on the basis of median nutrient mass released over 25 years. Modeling is also a useful design tool for both Concentrated Animal Feeding Operations (CAFOs) and non-CAFO sized operations wishing to utilize VTS systems. Field-scale VTS performance monitoring conducted over the past two years by ISU has shown that the current ISU models do not accurately predict actual hydraulic performance at the monitored VTSs. The ISU models are being revised to improve their performance. Along with improving the ISU-VTS model performance, other modeling alternatives are being investigated. The Soil-Plant-Air-Water (SPAW) model is one possible alternative for modeling the hydraulic performance of a VTA. For this paper, the SPAW predicted performance was compared to monitoring results at four VTAs located in Iowa. Two different methods were used to model the VTA performance, the first method utilized the field module of SPAW; this method was found to have Nash-Sutcliffe modeling efficiencies ranging from 0.16 to 0.57. At all locations, the SPAW model underestimated the amount of release that occurred from the VTAs. The second modeling method utilized the pond module of SPAW, for this method the Nash-Sutcliffe modeling efficiencies ranged from 0.26 to 0.83. Again, the SPAW model underestimated the cumulative volume of effluent released from the VTAs