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FOR IMMEDIATE RELEASE: March 5, 2013
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SCIENCE MUSEUM STUDY CONCLUDES THAT AGRICULTURAL DRAINAGE, NOT EXCESS RAIN, HAS INCREASED RIVER FLOWS AND STREAM‐BANK EROSION, A LEADING CAUSE OF EXCESS SEDIMENT IN LAKES AND RIVERS
St. Paul, Minn. – When you drive through the flat, fertile lands of Minnesota and marvel at the endless rows of corn and soybean fields, you likely don't notice the vast man‐made plumbing network, broadly referred to as tiling, which makes these fields productive. It's that unseen network that is also causing our stream banks to erode.
A new study by a team of researchers at the Science Museum of Minnesota and several major universities, published online March 1 in the journal Hydrological Processes, has shown that the extensive network of ditches and underground tiling used to enhance agricultural yields has had the unintended consequence of increasing river‐channel erosion and sediment loads.
Many rivers in Minnesota, especially those in watersheds dominated by row‐crop agriculture, are impaired by high concentrations of suspended sediment. These sediment loads degrade aquatic habitat and recreational value, both in the rivers themselves and in downstream surface waters.
One of the most visible examples of this is the vast sediment footprint that has been made on Lake Pepin, which is fed by the Mississippi River. Sediment cores from Lake Pepin show us that contribution from sources such as stream banks has increased as much as five fold over natural rates.
"Erosion of stream channels is a natural phenomenon," explains Shawn Schottler, senior scientist at the Science Museum of Minnesota's St. Croix Watershed Research Station, "but our rivers are eroding at an unnatural and increased rate so we needed to verify why this is happening. Our research concludes that agricultural drainage, more so than excess rainfall, caused the erosion of our river banks."
The purpose of the three year study was to determine why stream flow has increased in some agricultural watersheds and not in others and to determine if these hydrologic changes triggered an increase in erosion of stream channels.
Because precipitation, conversion to row‐crops, and drainage have all increased over the same time period and because each has the potential to increase streamflow, it has been a challenge to separate the relative role of each one as a driver. Other studies have related increased river flows to increases in precipitation and changes in runoff due to conversion to soybeans. Much less has been done to quantify the role of artificial drainage in increasing river flow.
The recent study by Schottler and others compared changes in flow for 21 southern Minnesota rivers from 1940 to 2009. In about half of the rivers, flows increased significantly over the past 70 years, with flows in some rivers nearly doubling since 1940. In other watersheds, river flows showed no change. These changes in flow were strongly correlated with changes in land use.
After constructing a detailed water budget, the researchers found that artificial drainage could be identified as the major driver of increased river flow, exceeding the effects of precipitation and crop conversion. Surprisingly, the primary way that drainage alters river flow is through its effects on the invisible but important process of evaporation and transpiration (ET). ET is the loss of water to the atmosphere from water bodies or plants. In Minnesota about three‐fourths of all rainfall is returned to the atmosphere through this process. Drainage reduces ET and instead routs the extra water to the rivers, increasing streamflow.
The scientists caution that while the amount of river flow attributed to drainage needs further evaluation, the process by which drainage increases flow is undeniable: a) a principal purpose of artificial drainage is to facilitate agricultural production by reducing the amount of time water is ponded in fields; b) quickly routing ponded water to rivers reduces the amount of time available for ET; c) thus, the proportion of precipitation lost to ET is reduced and instead ends up as river flow.
The observations from the Minnesota River basin offer a cautionary reminder: cropping patterns that require artificial drainage can have unintended consequences for water quality.For more detailed information on the study, go to http://onlinelibrary.wiley.com/doi/10.1002/hyp.9738/abstract.
This study was led by Schottler, and was co‐authored by Jason Ulrich of the University of Minnesota (Dept. of Biosystems and Bioproducts Engineering), Patrick Belmont of the University of Utah (Dept. of Watershed Science), Richard Moore from Minnesota State University‐ Mankato (Water Resources Center), J. Wesley Lauer at Seattle University (Dept. of Civil and Environmental Engineering), and Daniel Engstrom and James Almendinger, also from the St. Croix Watershed Research Station at the Science Museum of Minnesota. The research was funded by Environment and Natural Resources Trust Fund as recommended by the Legislative‐Citizen Commission on Minnesota Resources and the Minnesota Pollution Control Agency, Section 319 Grants.
The St. Croix Watershed Research Station is the field research station of the Science Museum of Minnesota. Founded in 1989, the SCWRS is located on the St. Croix River just south of Marine on St. Croix, MN. The station's scientific staff conduct ongoing ecological research at the watershed scale and welcome visiting researchers conducting field investigations.
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