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Title: CALIBRATION OF THE ROOT ZONE WATER QUALITY MODEL FOR SIMULATING DRAINAGE AND LEACHED NITRATE IN THE GEORGIA PIEDMONT

Author
item Abrahamson Beese, Deborah
item RADCLIFFE, D - UGA-CRSS
item Steiner, Jean
item CABRERA, M - UGA-CRSS
item Hanson, Jonathan
item Rojas, Kenneth
item Schomberg, Harry
item Fisher, Dwight
item SCHWARTZ, L - UGA
item HOOGENBOOM, G - UGA-BAE

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2005
Publication Date: 11/1/2005
Citation: Abrahamson Beese, D.A., Radcliffe, D.E., Steiner, J.L., Cabrera, M.L., Hanson, J.D., Rojas, K.W., Schomberg, H.H., Fisher, D.S., Schwartz, L., Hoogenboom, G. 2005. Calibration of the root zone water quality model for simulating drainage and leached nitrate in the Georgia Piedmont. Agronomy Journal. 97:1584-1602.

Interpretive Summary: A model is a description of a real system or an object in a system. Scientists use models to help farmers and others understand what happens to the soil, crops, and water when they use different management practices. We used a research model to describe how cotton grows, how much water and fertilizer nitrogen it used and how much drained into ground water. Scientists from the USDA-ARS-J. Phil Campbell, Sr., Natural Research Conservation Center in Watkinsville, Geogia, and the University of Georgia in Athens, Georgia, set up a model called the Root Zone Water Quality Model to describe Piedmont soils and how cotton grows in them. The model predicted how much water and fertilizer nitrate was lost to ground water when the soil was tilled and when it was not tilled. We compared the amount of water that drained into ground water and the amount of leached nitrate predicted by the model to drainage and leached nitrate that we measured in a cotton field experiment. The model predictions did not match the measured data very well, but it was sensitive to whether the soil had macropores, large pore spaces in the soil that water can flow rapidly through and carry nitrates with it into groundwater. If the model can predict the results of different fertilizer applications in cotton fields, it will help Southeastern farmers know how much fertilizer to apply and when to apply it. It will also increase our understanding of drainage and nitrate leaching in these soils so that we can keep the water clean.

Technical Abstract: Many model parameters and processes are sensitive to small adjustments and can affect final results. This can lead to erroneous conclusions unless the model has first been calibrated to the system of interest. In addition, parameters required to run the model but that do not significantly affect simulations may require unnecessary time and effort in further applications. This study was conducted to calibrate the Root Zone Water Quality Model for simulating drainage and leached nitrate under no-tillage and conventional-tillage management practices in the Georgia Piedmont. Several key processes in the model have recently been refined based on studies where soils and climate are very different from the Piedmont. The current version has not been tested in the Piedmont region for tile drainage and nitrate leaching since these revisions. We focused on the calibration procedure recommended by the model developers but also on macroporosity and saturated hydraulic conductivity parameters, and initial soil nitrate. Adjustments to saturated hydraulic conductivity improved simulated tile drainage, and soil macroporosity was sensitive to simulated drainage. Simulations of leached nitrate, however, did not compare well to measured values. The calibration analyses used in this study will clarify aspects of the model's performance during a subsequent testing phase, and contribute to our analyses of the model's ability to accurately simulate drainage and nitrate leaching in Georgia Piedmont tillage systems.