SUBSURFACE FLOW COMPONENT FOR ANNAGNPS

Abstract

The Annualized Agricultural Non-Point Source Pollutant Loading (AnnANPSPL) model is a watershed scale,continuous simulation, daily time step model that is currently utilized in many locations of the United States by theEnvironmental Protection Agency, Natural Resources Conservation Service, and others to estimate the impact of bestmanagement practices on non-point source pollution. The model has many unique and powerful capabilities, but prior toAnnAGNPS version 2.2, subsurface lateral flow and subsurface drainage features were not available. Subsequently,subsurface lateral flow, including a subsurface drainage feature, was incorporated into AnnAGNPS and is described in thisarticle. Subsurface lateral flow was defined based on Darcys equation and subsurface drainage was determined usingHooghoudts equation. Users have several options available within AnnAGNPS to determine the impact of subsurfacedrainage based on the availability of information on the drainage system. Subsurface lateral flow and subsurface drainagewere assumed to occur only when the soil becomes saturated. As part of an inter-agency sedimentation project, the modelwith the subsurface enhancements was applied to the Ohio Upper Auglaize watershed non-point source modeling project toevaluate alternative agricultural management scenarios in reducing soil erosion and sediment loading within the watershed.The application illustrated the importance of including subsurface capabilities in watershed models by indicating thatsubsurface drainage systems within the watershed increased total runoff, but reduced direct surface runoff that, in turn,reduced soil erosion and sediment delivery from the watershed. Sediment loadings for drained conditions were less thanloadings for un-drained conditions in all simulated scenarios; and the sediment loadings for drained conditions were reducedby 7% to 16% compared with un-drained conditions. Furthermore, the model indicated that application of various areas ofno-till or grassland to the watershed could reduce the sediment loading transported from the watershed to a range of 39%to 82% of the existing condition.