Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/9/2003
Publication Date: 1/5/2004
Citation: Bakhsh, A., Hatfield, J.L., Kanwar, R.S., Ma, L., Ahuja, L.R. 2004. Simulating nitrate losses from walnut creek watershed. Journal of Environmental Quality. 33(1)114-123. Interpretive Summary: Observations of crop production and the environmental impact of farming practices are often collected from small research areas. It has been difficult to extrapolate the findings from one set of experiments to a larger area without conducting a series of experiments at each of the locations. Simulation models that estimate crop production and environmental quality parameters, for example, nitrate-nitrogen losses from corn fields in relation to amount of nitrogen fertilizer applied, offer the potential of being able to extend research results to larger areas. We evaluated one of the current simulation models for the conditions in central Iowa typical of the corn-soybean production system. This model predicted the amount of water that was lost from the bottom of the root zone and the amount of nitrate-nitrogen in this water. There was good agreement between the observed data and the estimated data from the model. This gives us confidence that we can use this model to help assess different nitrogen management scenarios. When we used this model to determine the optimum nitrogen application rate we found this value agreed well with observed data from this area. As we gain confidence in these results we will be able to use this model to aid producers in understanding the potential environmental impacts of different management systems balanced against the efficiency of crop production. This will allow us to help producers determine the most optimum farming practice for their particular farm.
Technical Abstract: Calibration and evaluation of the Root Zone water Quality Model (RZWQM98), using measured data from different geographical locations, is an important component of model improvement strategy. This study was designed to evaluate the latest version of RZWQM98 using 6 yr (1992-1997) of field-measured data from a field at Walnut Creek Watershed located in central Iowa. Measured data included subsurface drainage or 'tile' flows, nitrate-nitrogen (NO3-N0 concentrations and loads with subsurface drainage water, and corn (Zea mays L.) and soybean (Glycine max L. Merr.) yields. The dominant soil within this field was Webster soil (fine- loamy) in the Clarion-Nicollet-Webster Soil Association. The cropping system was a corn-soybean rotation with chisel plowing after corn harvest. Simulations of subsurface drainage flow (mm) closely matched observed data showing model efficiency of 99% (EF = 0.99), and difference of 0.6% (D = 0.6), between measured and predicted values. The model simulated NO3-N losses (kg ha**-1) with subsurface drainage water reasonably well with EF = 0.77 and D = 11. The simulated corn and soybean yields (kg ha**-1) were in close agreement with measured data with D = 5. Nitrogen-scenario simulations demonstrated that corn yield response function reached a plateau when N-application rate exceeded 90 kg ha**-1. Fraction of applied nitrogen lost with subsurface drainage water varied form 7 to 16% when N-application rate varied from 0 to 180 kg ha**-1 after accounting for the nitrate-loss with no-fertilizer application. The RZWQM98 has the potential to simulate the impact of nitrogen application rates on corn yields and NO3-N losses with subsurface drainage flows for agricultural fields.