Drain Water Discharge and Nitrogen Losses in Response to Tile Drain Spacing and Depth in Southern Ontario.

Drain water discharge and nutrient losses from agricultural lands may differ depending on tile drain depth and spacing. Studies were conducted using large field plots equipped with automatic flow volume measurement and sampling systems over a 4-year period to compare two different tile drain depths (0.65 m vs. 0.85 m) and two tile drain spacing (4.2 m vs. 7.5 m) on drain water discharge and nitrate-N losses under regular free drainage (RFD) and controlled drainage and sub-irrigation (CDS) systems in a corn and soybean rotation.

Under RFD system, an increase in tile spacing from 4.6 m to 7.5 m increased total combined surface and sub-surface drain water discharges by 10 % but reduced total nitrate loss by 28 %. However, an increase in tile drain depth from 0.65 m to 0.85 m resulted in increasing total combined surface and sub-surface drain water discharges and nitrate loss by 58% and 39 %, respectively. Therefore, under RFD system, tile drain depth affected tile drain water discharge and nitrate loss more than title drain spacing. Under CDS system, an increase in tile spacing from 4.6 m to 7.5 m increased total combined surface and sub-surface drain water discharge by 20 % but reduced total nitrate loss by 109 %. However, an increase in tile drain depth from 0.65 m to 0.85 m resulted in increasing total combined surface and sub-surface drain water discharges by 59 %, but a similar total nitrate losses.

It was evident that under RFD system decreasing tile drain depth and increasing tile drain spacing reduced total nitrate losses. Further potential total nitrate reduction was also evident by CDS system. Therefore, under both RFD and CDS systems, increasing tile drain spacing or decrease tile drain depth could be highly effective for reducing excess nitrate loading from agricultural fields entering the lakes, if drainage needs for agricultural production are satisfied.