Critics charge that tiling increases stream flows, causing fragile riverbanks to drop more sediment, Baloun says.

However, the effects of tile drainage on stream hydrology are far from clear, Sands says. “There are so many things going on that it’s difficult to understand and tease apart all the individual effects. It’s very hard to say what annual row cropping or drainage do to water balance, hydrology and stream flow.”

Matt Helmers, Extension agricultural engineer at Iowa State University (ISU), underscores that uncertainty. “We have seen changes in annual stream flow, but there are many reasons for that. It’s hard to point to one thing.”

It also depends on what else is going on in the field, Greg McIsaac adds. “Are the tiles draining a pond or wetland? Are you tiling an area that would otherwise produce a lot of surface runoff? Are you draining terraces, which hold soil in the field? The impact of tile on stream flow is not straightforward.”

That’s why farmers and society “need more research on how tile systems of different types affect hydrology,” says Warren Formo, director of the Minnesota Agricultural Water Resources Coalition.