Tiling is a widely accepted agricultural practice that’s been around for decades, but recently it’s been called into question.
The banks in the Minnesota River basin have been eroding, and a recent study by a St. Croix laboratory claimed that tiling is causing increased flows in rivers and streams, which in turn eats away at the riverbanks. A University of Minnesota Professor has some questions about the methodology and the findings in this study.
“Certain environmental groups have accepted that bank erosion is the primary reason for sediment buildup in the Minnesota River and Lake Pepin,” said Dr. Satish Gupta, the Raymond Allmaras Professor in the Department of Soil, Water, and Climate. “So the next question is ‘what’s causing the sediment banks to slump’”? They’re saying the river flow has gone up and it’s undercutting some of the banks.”
Gupta said the question now is why the river flow is higher than it’s been in the past.
“They’re saying it’s because we have put tile drainage in the landscape that we have so much more water coming into the river,” said Gupta. “This is based on work people did in Iowa and at a laboratory in St. Croix, Minnesota. They did some analysis and suggested it is tile drainage that’s causing the rivers to go up.”
He noticed that some of the analysis did not account for increased precipitation and mis-applied the soil hydrology principles.
“They’re saying when immigrants came to this country, they started draining the land and tiling it,” said Dr. Gupta. “Over the years, they say the practice has been increasing runoff into the river, which in turn erodes the riverbanks. That’s their argument.”
Dr. Gupta and his associates put together their own study of the matter, and the findings will soon be published as a paper in a journal called “Water Resources Research, a journal of the American Geophysical Union.
“We started analyzing river flow and annual precipitation data, using 1976 as a starting point,” said Gupta. “The reason we did that is plastic tile wasn’t manufactured in the USA until 1967. People were initially worried the plastic wouldn’t be able to withstand the pressure from freezing soil in the winter and they’d end up replacing it. They didn’t really adopt plastic tile line until after 1976.”
He added, “Even the Research and Outreach Station at Lamberton used the clay tile lines in 1975.”
So they began their study to see if annual river flows had risen significantly over the next several decades.
“What we found was riverflows were higher because we’re getting an average of 4 inches more precipitation every year,” said Gupta, “and that water has to go someplace. In fact, in Waseca we get an average of 8 more inches of precipitation per year now than the past. In the areas we studied in southern Minnesota and northern Iowa, there’s no space to hold the extra water.”
Gupta said those numbers were confirmed by a recent national climatic assessment issued by the White House.
“A recent study commissioned by President Obama said we are getting about 10-15 percent more precipitation than in the past,” said Gupta. “If you think we have 30 inches of rain in Minnesota on average, then 10-15 percent is 3.5-4 inches more rain per year.”
Dr. Shawn Schottler and associates of the St. Croix Watershed Research Station wrote the initial paper that blamed tiling as a major cause in rising river levels. Gupta said he questioned the methodology used in coming to that conclusion.
“Dr. Schottler had made some mistakes in his analysis,” said Gupta. “He also made some assumptions which are not right. He showed that quite a bit of water is coming from tile drainage, and also because we replaced previous crops with more soybeans.”
Gupta added, “His argument for soybeans was ‘we plant them later into May, whereas corn is planted in mid-April, so during that period there’s some evapotranspiration (moisture released from the plant into the atmosphere) going on, and that water also shows up in the river. That’s simply not true.”
“Evapotranspiration of corn and soybeans are about the same,” he said. “They both lose about 20-22 inches of evapotranspiration. The previous crops included a lot of small grains, which only uses 12-14 inches of evapotranspiration. As a result, we have actually increased evapotranspiration than what we had previously.”
He said the previous crops like small grains actually left more water in the soil than the corn and soybeans we grow currently. That’s just one of the errors Gupta feels the St. Croix lab made in it’s analysis.
“They made some high-school-level math mistakes too,” said Gupta. “Taking an average value of a linear relationship is okay, but the average value of an exponential relationship is wrong, because it should be like more of a geometric mean. And when you take an average value, you ignore the variability, and Schottler called this variability the tile drainage effect. That is simply wrong!”
He added, “Those kinds of mistakes aggravated the problems in their analysis.”
The other problem is their technique showed tile drainage effects where there is no tile drainage.
“We took their technique and applied it to northern Minnesota,” said Gupta. “We found a 139 percent increase in tile drainage, when there’s no tile drainage in the entire area.
Gupta said they studied a wide range of watersheds in Iowa and Minnesota.
“We studied a total of 29 different watersheds,” said Gupta. “In almost all cases, we found an increase in precipitation that’s been happening for the last 30-40 years is causing higher flows in rivers, while tile drainage and cropping systems have had a very minimal impact.”