|Yuan, Yongping - UNIVERSITY OF MISSISSIPPI|
|Theurer, Fred - NRCS|
Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 17, 2005
Publication Date: February 26, 2006
Citation: Yuan, Y., Bingner, R.L., Theurer, F.D. 2006. Subsurface flow component for AnnAGNPS. Transactions of the ASAE. 22(2): 231-241. Interpretive Summary: Effective tools are needed to evaluate the impact of agricultural production practices on how water drains from fields. Drainage can be a significant issue in soils having high permeabilities in surface layers and an impermeable or semi-impermeable layer at a shallow depth. Enhancements to the USDA AnnAGNPS watershed conservation planning model are described in this study needed to assess the impact of subsurface drainage systems. An Ohio agricultural watershed was studied containing tile drainage for most every field. The model was able to quantify the value of tile drainage in reducing the sediment load from the watershed because of reduced surface runoff that causes erosion. Sediment loadings under tile-drained conditions were always less than loadings under undrained conditions for otherwise identical land uses. The average sediment load of all alternative agricultural practices for drained loadings was 89.2 percent of the load for the corresponding undrained loadings. Through the application of the model it was established that while many conservation incentive programs treat tile drainage as a production practice, there are significant erosion and sediment control benefits provided by the practice.
Technical Abstract: The Annualized Agricultural Non-Point Source Pollutant Loading model (AnnAGNPS) is a watershed scale, continuous simulation, daily time step model. It is currently utilized in many locations of the U.S. by EPA, NRCS and others to estimate the impact of BMPs on non-point source pollution. The model has many unique and powerful features, but until recently, did not include the simulation of subsurface flow processes such as subsurface lateral flow, subsurface drainage, and groundwater flow. However, subsurface flow can be significant in soils having high hydraulic conductivities in surface layers and an impermeable or semipermeable layer at a shallow depth. Subsurface lateral flow, including a subsurface drainage feature, was incorporated into AnnAGNPS and is discussed in this paper. Subsurface lateral flow was determined using Darcy’s equation. Users have several options available within AnnAGNPS to determine the impact of subsurface drainage based on the availability of information on the drainage system. The subsurface lateral flow and subsurface drainage were assumed to only occur when the soil becomes saturated. The model was applied in the Ohio Upper Auglaize Watershed non-point source modeling project to evaluate alternative agricultural management scenarios in reducing soil erosion and sediment loading within the watershed. The application shows that adding subsurface lateral flow and subsurface drainage increases total runoff, but reduces direct surface runoff which in turn reduces soil erosion and sediment delivery from the watershed. Sediment loadings under drained conditions were less than loadings under undrained conditions in all simulated scenarios; and the average of all alternatives for drained loadings was reduced by 10 percent comparing with undrained conditions. Furthermore, the model indicates that application of alternative land management scenarios could realistically reduce the sediment loading transported from the watershed to a range of 75-82 percent of the existing condition.