Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2008
Publication Date: November 4, 2009
Citation: Thorp, K.R., Youssef, M.A., Jaynes, D.B., Malone, R.W., Ma, L. 2009. DRAINMOD-N II: Evaluated for an agricultural system in Iowa and compared to RZWQM-DSSAT. Transactions of the ASABE. 52(5):1557-1573. Interpretive Summary: Loss of nitrate (NO3) from artificially-drained agricultural fields in the Midwestern United States has negatively impacted water quality throughout the region for many years. Some cities have spent millions of dollars to install facilities to remove NO3 from drinking water. In addition, the effects of NO3 losses have had economic and ecological impacts as far south as the Gulf of Mexico where water quality issues from excessive NO3 levels have damaged the natural ecosystem and disrupted the fishing industry. To solve this problem, changes must occur in the way agricultural fields in the Midwestern United States are managed. Computer simulation is a cost effective approach to assess the efficacy of new management practices for agricultural nitrogen, and ARS and university researchers have worked diligently to develop computer simulation models capable of predicting the fate of nitrogen from agricultural fields. In this research, we have evaluated and compared two of these models, the ARS Root Zone Water Quality Model (RZWQM) and the DRAINMOD-N II model. Evaluations of the models were performed using 10 years of observed data from a production field in central Iowa. Once thoroughly tested against observed field data, the models can be applied to answer further questions about how to most effectively manage agricultural nitrogen. Results of this study will be useful for researchers and extension personnel who wish to utilize a computer simulation tool to provide growers with effective nitrogen management recommendations, particularly in the Midwestern United States.
Technical Abstract: A new simulation model for N dynamics, DRAINMOD-N II, has been previously evaluated for only a few sites. We evaluated the model using ten years (1996-2005) of measured data from a subsurface-drained, corn-soybean agricultural system near Story City, Iowa. Nitrogen fertilizer was applied to plots at low, medium, and high rates (57-67, 114-135, and 172-202 kg N ha-1) during corn years, and nitrate (NO3) losses from subsurface drains under each plot were monitored biweekly for ten years. Average annual simulated and measured NO3 losses in drainage water were 21.4 and 20.1 kg ha-1 for the low N rate, 25.9 and 26.5 kg ha-1 for the medium N rate, and 35.9 and 37.0 kg ha-1 for the high N rate, respectively. The model efficiency statistics for DRAINMOD-N II simulations of annual subsurface drain NO3 losses were 0.88, 0.94, and 0.94 for low, medium, and high N rates, respectively. A comparison of DRAINMOD-N II simulations to that of the RZWQM-DSSAT hybrid model for the same experimental dataset demonstrated that the two models were most different in their simulations of evapotranspiration, soybean N fixation, plant N uptake, and net N mineralization. Future field investigations should focus on generating better understandings of these processes. The results suggest that DRAINMOD-N II can reasonably simulate the effects of different corn-year N rates on losses of NO3 through subsurface drainage lines and that simulations of subsurface drainage NO3 losses by DRAINMOD-N II are comparable to that of RZWQM-DSSAT.