Following the catastrophic floods of 2008, many fingers pointed toward agricultural drainage tile as a major contributor to the state’s worst natural disaster.
While the plumbing beneath 9 million acres of Iowa farmland certainly affects the volume and quality of water in the state’s rivers and streams, it’s not clear that it increased the severity of the 2008 floods.
‘It’s a controversial and very important issue, and we need to understand it better than we do today,’ said Dean Lemke, chief of the Iowa Department of Agriculture and Land Stewardship’s Water Quality Bureau.
Since the late 1800s, farmers have used subsurface permeable pipes, often in conjunction with excavated ditches, to remove excess water from poorly drained fields. The practice improves crop yields and in many cases permits farming of land otherwise too wet to farm.
In the 1970s, perforated polyethylene tubing began to replace the clay cylinders known as ’tile,’ but the original nomenclature remains in common usage.
Today pipes either are strategically placed in a field to remove water from isolated wet spots or installed in a pattern to drain an entire field, commonly with lines 3 to 5 feet deep and 4 rods, 66 feet, apart.
About 39 percent of Iowa’s 23 million acres of corn and soybean fields have been drained with an estimated 800,000 miles of tile.
Though many farmers continue to improve and upgrade their tile networks, Lemke said little has changed in the extent of tile drainage in Iowa during the past 25 years.
‘It’s hard to believe that tile changes during that period had much to do with flood peaks in 2008,’ he said.
Simply put: We’ve had more rain
Chuck Gipp, soil conservation director for the Iowa agriculture department, said the state needs an irrefutable scientific study to document the effect of tile drainage on stream flow.
Gipp, a former state legislator and Winneshiek County farmer, said he believes such a study would show that ag tile drainage has minimal effect on the severity of flooding.
‘The real cause is that we’re getting much more rain than we did in the past,’ said Gipp. He noted that climate scientists at Iowa State University have reported a 19 percent increase in statewide average annual precipitation since Iowa started keeping records in 1873.
Most researchers agree that large-scale, basinwide floods are largely attributable to catastrophic precipitation, not the presence of subsurface or surface drainage systems, tile drainage expert Gary Sands, an associate professor at the University of Minnesota, said.
In 2008, ‘there was so much rain in such a short time that you were going to have disastrous flooding with or without drainage tile,’ said Dan Jaynes, a soil scientist with the U.S. Department of Agriculture in Ames.
Tile capacity is restricted by the size of the pipe and often can drain no more than half an inch of rain per day, he said.
During the heavy rains of June 2008, Jaynes said, water ran off the surface of tiled fields almost to the same extent it did from untiled fields.
Jaynes said he’s not certain that even a highly unlikely return to pre-settlement prairies and wetlands could have averted the floods of 2008.
Ag tile has impact
Hydrologist Michael Burkart of Ames, who worked for two federal agencies before becoming a consultant, said he agrees with Gipp that the flood-exacerbating effects of ag tile drainage have not been conclusively proven. But most scientists think farmland drainage negatively affects the environment, he said.
‘Tiling opens up land for more production, but by draining it you increase the amount of water going into streams,’ he said.
Ag tile drainage also increases the concentration of nitrates, the principal corn fertilizer, in lakes and streams, with adverse effects on aquatic life and potentially adverse effects on human health, Burkart said.
Burkart said state drainage policies so far have favored farmers, with the general public suffering the consequences. ‘We need to debate whether increased profits for farmers justify the negative consequences downstream,’ he said.
Lemke, Gipp and other prominent state water resources officials say the big change in the state’s hydrology occurred long before the floods of 1993 or 2008.
The conversion of native prairie and wetlands to farmland beginning more than 160 years ago, followed by the mid-20th century conversion of hay, oats and pasture to corn and soybeans, did more to hasten and increase runoff following heavy rains than anything else, Lemke said.
They acknowledge, however, that the early 20th century drainage of 6 million acres of lowlying ground in the Des Moines lobe region of north-central and northwest Iowa – massive native wetlands that could store and slowly release excess rainfall – increased the annual and peak flows of central Iowa rivers like the Des Moines and the Raccoon.
The later drainage of sloughs and wetlands in other parts of Iowa had a similar if less pronounced effect, they said.
Burkart and other environmental advocates worry that a proposal by the Iowa agriculture department to upgrade and enlarge drainage systems in the Des Moines lobe, the so-called ‘Iowa Plan,’ could worsen stream flow and pollution problems associated with ag drainage.
During the excessively wet 2010 growing season, the inadequate drainage capacity of that aging infrastructure has been painfully obvious to farmers lamenting their yellowed, stunted and drowned crops.
The plan calls for constructing wetlands that in theory would reduce nutrient loads in the water drained from crop fields, but Burkart said he fears the wetlands would be mere ‘tokens,’ incapable of neutralizing the increased nutrient loads.
Before the plan proceeds, he said, proponents should submit detailed wetland specifications for review.
Farming practices called into play
Environmental activists Larry Stone of rural Elkader and Bob Watson of Decorah both say Iowans need to rethink the annual row crop agriculture model that rushes water off the land as quickly as possible.
Water quality will improve, flood damage will diminish and wildlife will flourish when deep-rooted perennial vegetation is reestablished on the landscape, they said.
Curt Zingula, 57, of rural Marion, who farms 1,460 acres in northeast Linn County, said farmers tend to roll their eyes when their urban neighbors make anti-tile comments. In addition to making their land more productive, farmers ‘really believe that ag tile benefits the environment by reducing erosion’ and consequent siltation of streams, Zingula said. It allows farmers to significantly reduce tillage, the leading cause of soil erosion, Zingula said. Before the advent of drain tile, farmers relied on spring tillage to warm and dry the soil to facilitate seed germination and early growth, but tiling serves the same purpose, he said. Tile also reduces the amount of compaction caused by heavy equipment traversing wet soil, another major reason for tillage.
When heavy rains fall on Iowa farm fields, the surplus water either can drain through tile or run off the surface of untiled fields, carrying with it soil particles, manure, phosphate fertilizer and other chemicals.
‘Which would you rather have: clear water leaving a tile outlet or brown water running over the top of farm fields?’ Zingula asked.
Acknowledging that drainage tile removes nitrates from the soil, Zingula said it also prevents soil saturation, which causes denitrification, a process that releases the greenhouse gas nitrous oxide into the atmosphere.
Zingula said he regards tiling, which costs about $500 per acre, as an investment. It increases the value and productivity of farmland, which in turn increases property tax proceeds for schools and other government services, he said.