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TECHNICAL REPORTS

Ground Water Quality

Simulating Nitrate Drainage Losses from a Walnut Creek Watershed Field

^a Dep. of Agricultural and Biosystems Engineering, Iowa State Univ
^b USDA-ARS, National Soil Tilth Laboratory, Ames, IA 50011
^c USDA-ARS, Great Plains System Research Unit, Fort Collins, CO 80522

^* Corresponding author (rskanwar{at}iastate.edu).

Received for publication July 5, 2002. This study was designed to evaluate the improved version of the Root Zone Water Quality Model (RZWQM) using 6 yr (1992–1997) of field-measured data from a field within Walnut Creek watershed located in central Iowa. Measured data included subsurface drainage flows, NO₃–N concentrations and loads in subsurface drainage water, and corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields. The dominant soil within this field was Webster (fine-loamy, mixed, superactive, mesic Typic Endoaquolls) and cropping system was corn–soybean rotation. The model was calibrated with 1992 data and was validated with 1993 to 1997 data. Simulations of subsurface drainage flow closely matched observed data showing model efficiency of 99% (EF = 0.99), and difference (D) of 1% between measured and predicted data. The model simulated NO₃–N losses with subsurface drainage water reasonably well with EF = 0.8 and D = 13%. The simulated corn grain yields were in close agreement with measured data with D < 10%. Nitrogen-scenario simulations demonstrated that corn yield response function reached a plateau when N-application rate exceeded 90 kg ha^–1. Fraction of applied N lost with subsurface drainage water varied from 7 to 16% when N-application rate varied from 30 to 180 kg ha^–1 after accounting for the nitrate loss with no-fertilizer application. These results indicate that the RZWQM has the potential to simulate the impact of N application rates on corn yields and NO₃–N losses with subsurface drainage flows for agricultural fields in central Iowa.

Abbreviations: A_s, age parameter • CRM, coefficient of residual mass • D, difference between predicted and observed data • DOY, day of year • DP, drainable porosity • EF, model efficiency • ET, evapotranspiration • FWANC, flow-weighted average nitrate concentrations in subsurface drainage water • MSEA, management system evaluation area • N_max, nitrogen-uptake coefficient • RMSE, root mean square error • RZWQM, Root Zone Water Quality Model with Windows interface

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