Tile drainage is a widely adopted water management practice in the eastern Canadian provinces of Québec and Ontario. It aims to improve the productivity of poorly drained agricultural fields. Nevertheless, studies have also shown that subsurface drainage is a significant pollution pathway to surface water. This study was undertaken to evaluate the effect of tile drain spacing on surface runoff, subsurface drainage flows, and phosphorus (P) loss from two tile-drained agricultural fields located near Bedford, Québec. Field data were used with the DRAINMOD model, and in developed regression models in order to perform the analysis. Both DRAINMOD and the regression models showed good performance. Simulation results indicated that when lateral tile drain spacing is increased, the volume of subsurface drain flow decreases, and the volume of surface runoff increases, at sites with sandy and clay loam soils. For every 5 m increase in drain spacing, total phosphorus (TP) loads in subsurface drainage decreased by 6% at a site with sandy loam soil, and increased by 20% at a site with clay loam soil. TP loads in surface runoff increased as a result of increased drain spacing.

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