Assessing nitrogen fertilizer rates and split applications using the DSSAT model for rice irrigated with urban wastewater

When reclaimed wastewater containing more nutrients than conventional irrigation water is used for irrigation, alternative fertilization practices are required for agricultural productivity and environmental sustainability. The objective of this study was to assess the effect of the nitrogen (N) fertilizer rates and split N fertilizer application on rice yields grown in paddy fields irrigated with reclaimed wastewater using the Decision Support System for Agrotechnology Transfer (DSSAT) v4.5 model. The experimental plots employed a randomized complete block design with three treatments and four replications. The three treatments included ground water irrigation plots (GW), untreated wastewater irrigation plots (WW), and reclaimed wastewater irrigation plots (RWW). The input data for the DSSAT model, including weather data, irrigation amount, irrigation water quality, soil data, cropping practice data, and rice yields, were monitored and collected between 2006 and 2009. The DSSAT model was calibrated and validated with observed yield data using the root mean square error (RMSE), normalized RMSE (nRMSE), and the index of agreement (d) for the statistical indices. The values of RMSE, nRMSE, and d were 269-645kgha^-1, 4.1-11.7%, and 0.94-0.95, respectively, for the calibration period. The calibrated model showed good agreement with the observed rice yields, and the values of RMSE, nRMSE, and d were 155-538kgha^-1, 2.6-10.4%, and 0.95-0.98, respectively, for the validation period. The simulation results showed that the optimal N fertilizer rate for paddy field rice irrigated with reclaimed wastewater was considered to be 20-50% less than the standard fertilizer rate (SFR). Adjusting the split N fertilizer application rate was demonstrated as being enough to satisfy the target rice yield and resulted in an additional 10-20kgha^-1 reduction in the amount of N fertilizer used.