Evaluation of Root Zone Water Quality Model for Simulation of Nonpoint Source Pollution from Agricultural Lands

Abstract

Calibration and evaluation of the Root Zone Water Quality Model (RZWQM98), using measured datafrom different geographical locations, is an important component of model improvement strategy. This study wasdesigned to evaluate the latest version of RZWQM98 using 6 yr (1992-97) of field-measured data from a field atWalnut Creek Watershed located in central Iowa. Measured data included subsurface drainage or tile flows,nitrate-nitrogen (NO3 -N) concentrations and loads with subsurface drainage water, and corn (Zea mays L.) andsoybean (Glycine max (L.) Merr.) yields. The dominant soil within this field was Webster Soil (fine-loamy) in theClarion-Nicollet-Webster Soil Association. The cropping system was a corn-soybean rotation with chisel plowingafter corn harvest. Simulations of subsurface drainage flow (mm) closely matched observed data showing modelefficiency of 99% (EF = 0.99), and difference of 0.6% (D = 0.6), between measured and predicted values. Themodel 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-applicationrate exceeded 90 kg ha -1 . Fraction of applied nitrogen lost with subsurface drainage water varied from7 to 16% when N-application rate varied from 0 to 180 kg ha -1 after accounting for the nitrate loss with no-fertilizerapplication. The RZWQM98 has the potential to simulate the impact of nitrogen application rates on cornyields and NO3 -N losses with subsurface drainage flows for different agricultural fields.