Ammonia Volatilization from Flooded Soil Systems: A Computer Model. III. Validation of the Model
- G. R. Jayaweera *,
- K. T. Paw U. and
- D. S. Mikkelsen
An NH3-volatilization model predicting NH3 loss as a function of five input variables was validated using a wind tunnel to simulate rice paddy conditions and direct field experiments. A total of five variables in a central composite statistical design were compared to study the interactive effects of NH4-N concentration, pH, temperature, wind speed, and water depth. Experiments were also conducted in a flooded rice field with polypropylene basins placed at water level. Samples were collected every hour for determination of NH4-N concentration. Temperature, pH, and wind speed were recorded continuously, and water depth was constant. Wind-tunnel data showed that the model predicted observed values with excellent accuracy in the range of conditions found in flooded rice systems. The regression of predicted NH3 loss on observed losses resulted in an r2 of 0.98 and a regression slope of 0.99. Field experiments also showed very close agreement between predicted and experimental values with 6-, 12-, and 24-h averages of pH, temperature, and wind speed. The model validation confirmed the theory that NH3 volatilization is a function of NH3(aq) concentration and the volatilization rate constant for NH3, which are dependent on five variables: floodwater NH4 concentration, pH, temperature, water depth, and wind speed. The model is theoretically sound and predicts NH3 loss with a high level of accuracy using a menu-driven computer program with easily measurable variables, and can be used in comparison studies of NH3 loss at the same site.
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