What happens to water that is held back in the field through drainage water management?

Good question, says Matt Helmers, an Iowa State University agricultural engineer. Researchers don't yet have a clear picture.

Besides subsurface drains, water leaves the field along several pathways. It can return to the air through evaporation and transpiration by plants. It can run off the surface, percolate down into the aquifer or flow laterally below the tile depth. But these movements are very hard to measure, Helmers says.

Still, “It is generally thought that these pathways tend to have water lower in nitrate concentration than tile flow water,” says Don Pitts, a water quality and drainage expert at the Illinois Natural Resource Conservation Service. That helps explain why curbing tile flow also reduces nitrate losses.