SIMULATING NITROGEN DYNAMICS UNDER WATER TABLE MANAGEMENT SYSTEMS WITH DRAINMOD-N

There is increasing interest in water table management to control nitrate leaching in eastern Canada. However, little is known about the impacts of this practice in the region. DRAINMOD-N, a recently developed drainage water quality model, offers the potential to evaluate nitrate leaching. The model was validated by comparing simulated results with measurements of water table depth, drain flow, cumulative nitrate leaching, and cumulative denitrification, from conventional drainage and subirrigation field plots planted to corn. Replicated plots of 15 m × 75 m were either under conventional free drainage or subirrigation at a weir setting of 0.5 m below the soil surface. DRAINMOD-N predicted water table depth to within a range of ± 160 to 210 mm, drain flow to within ± 2 mm/d and nitrate leaching to within ± 8 kg N/ha. DRAINMOD-N models denitrification using first order kinetics. This did not accurately describe field measurements of cumulative denitrification, as by day of year 270 cumulative denitrification was underestimated by 64 to 83%. Therefore, the model was modified by replacing the original denitrification function with the Michaelis-Menten relationship, which simulates denitrification as a first order process when nitrate is limiting and as a zero order process for non-limiting nitrate. This modification had little effect (< 2%) on the modified model's prediction of nitrate leaching. For cumulative denitrification, however, the prediction error with the modified model was of 23 to 60% less than with DRAINMOD-N.