RZWQM simulation of long-term crop production, water and nitrogen balances in Northeast Iowa
Introduction
Agricultural system models have been improved tremendously in the last two decades because of advancements in computer technology, integration with field research, and real-time data collection. The Root Zone Water Quality Model (RZWQM) has evolved steadily since its debut in 1992 (Ahuja et al., 2000). By partnering with scientists at MSEA (Management Systems Evaluation Areas) sites, team members developing RZWQM have been able to integrate field research results (Watts et al., 1999) and test the model for: 1) water and pesticide movement in Minnesota (Wu et al. 1999); 2) surface runoff, nitrate and pesticide losses to seepage and runoff, and crop yield in Missouri (Ghidey et al., 1999); 3) crop yield, and water, nitrate and pesticide movement in Iowa (Jaynes and Miller, 1999); 4) crop yield, N uptake, plant biomass, leaf area index, soil water content, and soil N in Nebraska (Martin and Watts, 1999); and 5) leaf, stem, and seed biomass of corn in Ohio (Landa et al., 1999).
At another MSEA site near Nashua, IA the RZWQM has been evaluated extensively under tile-drained conditions to estimate pesticide leaching, N loss, and crop growth (Bakhsh et al., 2004a, Bakhsh et al., 2004b, Singh et al., 1996, Kumar et al., 1998a, Kumar et al., 1998b). This site is composed of 36 0.4-ha plots and has gone through three phases of study (1978–1992, 1993–1998, 1999–2003). In previous model applications, RZWQM was used only for selected plots (management systems) and years. Kumar et al. (1998a) tested RZWQM on three plots (Plot #13, #22, and #35) and for 1993 and 1995 growing seasons to study swine manure application on nitrate leaching in tile drains. In a subsequent study, they evaluated RZWQM on six plots (#14, #25, #31 for no till and #13, #22, and #35 for moldboard plow) from 1990 to 1992 to investigate macroporosity effects on pesticide transport to tile drains (Kumar et al., 1998b) and pesticide distributions in soils (Azevedo et al., 1997). Singh and Kanwar, 1995a, Singh and Kanwar, 1995b) and Singh et al. (1996) simulated tile drainage and nitrate-N losses to tile drains using RZWQM for 12 of the plots (continuous corn) from 1990 to 1992. The same set of data was later used by Kumar et al. (1999) to study tillage effects on water and nitrate movement with RZWQM. Bakhsh et al. (1999) applied RZWQM to three manure plots (#11, #23, and #27) for 1993–1996 data to study swine manure application on nitrate-N transport to tile drains. In order to fully integrate RZWQM with the long-term field study near Nashua, we need to simulate all the management practices (crop rotation, tillage, and N and manure management) for all the years on all the plots. Doing so will: 1) quantify weather variability; 2) identify possible experimental errors due to equipment failure or mismanagement; and 3) fill in data gaps that will allow analysis of water and N balances in the soil–plant–atmosphere continuum.
Since most RZWQM applications at other locations used only 2 to 4 years of field data as shown above (Ma et al., 2000, Ma et al., 2003, Bakhsh et al., 2004a, Bakhsh et al., 2004b, Jaynes and Miller, 1999, Kumar et al., 1998a, Kumar et al., 1998b), in this study, we recalibrated an improved version of RZWQM for one plot using the 26 years of data collected in Nashua, Iowa of the USA and then applied the calibrated soil and plant parameters to other plots with various management practices. Therefore, our objectives were to (1) demonstrate the calibration of RZWQM for plant and soil parameters using long-term field experiments and (2) analyze the overall goodness-of-simulation of RZWQM for crop yields as well as long-term water and N balances under different management practices. However, detailed analyses of simulation results for a specific management effect (rotation, tillage, and N management) were discussed by Ma et al., 2007a-this issue, Malone et al., 2007-this issue.
Section snippets
Materials and methods
The experiment was conducted at Iowa State University's Northeast Research Center near Nashua, IA. The three dominant soils at this site are Floyd loam (fine-loamy, mixed, superactive, mesic Aquic Hapludolls), Kenyon silty-clay loam (fine-loamy, mixed, superactive, mesic Aquic Hapludolls), and Readlyn loam (fine-loamy, mixed, superactive, mesic Aquic Hapludolls). These soils are moderately well to poorly drained, lie over loamy glacial till, and belong to the Kenyon–Clyde–Floyd soil
Model calibration
Model calibration was conducted following the procedure outlined by Ma et al. (2003). Measured soil hydraulic properties were used for the 10 soil layers (Table 2). Since the lateral saturated soil hydraulic conductivity (Lksat) was not sensitive except for the layers immediately above (#6) and between (#7) the tile drains, Lksat was therefore assumed to be the same as the vertical saturated soil hydraulic conductivity (Ksat) for all the layers except the two (#6 and #7) layers. Lksat for these
Summary and conclusions
We evaluated RZWQM using long-term (1978 to 2003) field data for 30 out of 36 plots. RZWQM was calibrated using data for Plot No. 25 and then used with the same soil and plant parameters, except for lateral hydraulic gradient, to simulate responses for the other 29 plots. Simulated water and N balances and crop production were compared to experimental measurements whenever possible. In general, the model correctly simulated year to year variations in tile flow and N loading in the tile flow.
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