By Keith E. Schilling
Drainage tiles about 4 feet below the ground surface are used throughout the Midwest to drain soils that are seasonally to perennially wet. They enable earlier planting, improve soil aeration and increase crop yields.
Questions have been raised recently regarding the role of tile drainage in flooding. On one side of the issue, tile drainage is viewed as a major cause of flooding because the tiles accelerate subsurface water delivery to rivers. But at the same time, tile drainage is considered by some in the agricultural community as a conservation practice to reduce flooding because tiles dry out the soils and make them more receptive to rainfall infiltration.
Which is it?
First consider how water leaves the landscape. Rainfall runoff occurs when when rainfall rates exceed the infiltration rate of the soil, or when soil pores are saturated and rainfall can no longer soak into the ground.
During heavy rain, the infiltration capacity of the soil is exceeded and tile drainage becomes irrelevant. A 5-inch rainfall delivered in a few hours will result in a flood regardless of whether the land has been tiled or not. While drier soils store more infiltrating water than saturated soils, floods typically occur after a sustained period of rainfall. Several days of steady rain will saturate tiled or untiled soils alike.
Large floods typically happen after a large single rainfall (rainfall exceeding infiltration) or after a sustained rainy period saturates the landscape. In either case, tile drainage is not a major factor in flood prevention or causation.
The effect of tile drainage on peak flows decreases as the scale of a watershed increases. Varying amounts of precipitation across the landscape and the variable timing of runoff from watershed to watershed makes quantifying the contributions of tile to flooding practically impossible.
When flooding does occur, its magnitude is controlled by the amount of water running off the landscape, and not the amount coming out of a drainage pipe.
My own research demonstrates that flow from drainage tiles increases the amount of water delivered to rivers after the storm event has ended. Water that might have been stored on the landscape in depressions or shallow soils is drained to the river by tiles and constructed ditches. The goal of tiling is to remove water as quickly as possible from fields, usually within 48 hours to avoid significant crop damage.
Unfortunately, discussions of tile drainage and flooding are often focused on a false choice. So, what are the relevant questions?
Tile drainage is focused on lands devoted to corn and soybeans. Rainfall runoff from croplands far exceeds runoff from land with perennial vegetation, so by making the choice to crop, we contribute to enhanced runoff and flooding, with or without tile drainage. The choice is not whether to tile or not, the choice should be whether to crop or not.
A meaningful conversation would focus on whether some crop lands should be converted to perennial systems where floodwater storage can occur (e.g., wetlands, depressions, floodplains).
Tile drainage should not be considered as part of any flood-reduction strategy. Farmers install drainage tiles for perceived economic benefits, expecting sales from increased yields to exceed the costs of installation.
Environmental by-products of tile drainage systems should be evaluated and quantified, but we should be careful not to assign motives where they do not exist. Furthermore, we should not reward farmers for installing or improving tile drainage systems as part of a flood-mitigation strategy and incentivize a practice that has little to do with flooding but everything to do with expanding crop production and increasing farm profits.
There are activities that farmers can do to reduce flooding potential from agricultural lands, but tile drainage is not one of them.Keith E. Schilling is a research geologist with the Iowa Department of Natural Resources Geological and Water Survey and an adjunct assistant professor in the Department of Geoscience at the University of Iowa. Comments: firstname.lastname@example.org