|Bakhsh, A - IOWA STATE UNIVERSITY|
|Kanwar, R - IOWA STATE UNIVERSITY|
Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 22, 1999
Publication Date: January 5, 2000
Citation: Bakhsh, A., Kanwar, R.S., Jaynes, D.B., Colvin, T.S., Ahuja, L.R. 2000. Prediction of NO3-N losses with subsurface drainage from manured and uan-fertilized plots using gleams. Transactions of the ASAE. 43(1):69-77. Interpretive Summary: There is great concern over the appropriate use of swine manure as a nitrogen fertilizer source and the possible contamination of tile drainage water in fields that are subsurface drained. It would be extremely expensive and time consuming to test the effects of manure application on water quality for the full extent of soils, possible manure application rates, and methods and timing of application. For this reason, scientists are developing computer modeling that can simulate the impact of manure application on crop growth and water quality. If accurate and reliable, these models would allow exhaustive testing of the impacts of manure use for a wide range of soil, weather, and management conditions. We evaluated the commonly used research computer model, GLEAMS, for its ability to accurately predict the cycling of N from manure applications to soil and the leaching of nitrate into subsurface tile drainage systems. Our results clearly demonstrated that the model is not sufficiently sensitive to interactions between different forms of soil nitrogen, weather, and plant requirements to accurately predict the impact of manure management on either crop growth and productivity or water quality. This information is helpful to soil scientists and agronomists trying to improve computer models for simulating agronomic systems and to regulators and decision makers wishing to use these models for management system evaluations.
Technical Abstract: Predictive tools are needed to help reduce the probability that using swine manure for crop production will contaminate shallow groundwater resources. The objective of this study was to use the GLEAMS (V.2.1) model to compare measured versus simulated effects of swine manure application with urea-ammonium-nitrate (UAN) on subsurface drain water quality from beneath long-term corn (Zea Mays L.) and soybean plots. Four years (1993-96) of field data from an Iowa site were used for model calibration and validation. The SCS curve number (CN2) and effective rooting depth (RD) were adjusted to minimize the difference between simulated percolation below root zone and measured subsurface drain flows. Model predictions of percolation water below the root zone followed the pattern of measured drain flow data, giving an average difference of +9.9%, and -5% between predicted and measured values for manured and UAN fertilized plots, respectively. Model simulations for overall NO3-N losses with percolation water were comparable to measured NO3-N losses with subsurface drain water giving an average difference of +20% for manured plots. The model over-predicted NO3-N losses, particularly for soybean on plots which received manure the previous year. Predicted NO3-N losses with subsurface drainage from fertilized plots were much lower than measured values with an average difference of -32%. The results of the study indicate that the rate factors based on soil temperature and soil water levels used in N-transformation processes of the model must be refined for consistent simulations of NO3-N losses with subsurface drainage water for corn-soybean rotation plots fertilized with either swine manure or UAN for corn phase of production.