Restoring Prairies Helps Reduce Flooding

By Chris Anderson

During the past century and a half, much of the original grassland in the central United States was converted to farmland. The once-vast sea of grass encountered by American Indians, explorers and settlers has been replaced with an ocean of corn and soybeans.

Scientists believe this nearly complete conversion of grassland to cropland may contribute to flooding, since prairies feature diverse and closely packed plants with deep roots.

At the 24,500-acre Glacial Ridge Project in northwest Minnesota, The Nature Conservancy is undertaking one of the nation’s largest prairie-wetland restoration projects. spoke with Phil Gerla, a Conservancy hydrologist at Glacial Ridge, about whether restoring prairies can help protect people and businesses from floods.  

The Conservancy has restored 173 wetlands and 11,500 acres of prairie at Glacial Ridge. Most of the land will be added to a national wildlife refuge.

“A lot of attention is focused on restoring wetlands to mitigate flooding downstream. Few people, however, have suggested that restoring areas to grassland also can reduce floods. Prairies store and use a lot of water.”

—Phil Gerla, hydrologist for the Conservancy in Minnesota

Tell us about your research.

Phil Gerla:

I started in 2002 by looking at a paired site - a cropped field within the Glacial Ridge project that was planted in a soybean-wheat rotation and a well-established prairie about 300 feet away, which as far as we know was never plowed.
The goal was to look at the difference between soil moisture, infiltration and recharge between the cropped field, which we’ve now restored to grassland, and the intact native prairie.
Over the last six years we have been monitoring soil moisture using a technique called time domain reflectrometry (or TDR) and recording changes in the water table.
Basically, TDR measures the rate at which an electromagnetic pulse travels down and back up two parallel steel rods. The rate is a function of the soil's moisture content. I’ve been recording it hourly for the last six years at different depths in the soil. The data provides a profile of soil moisture through time.

What have you learned?

Phil Gerla:

Three features stand out. First, the native prairie tends to be much more resistant to large changes in soil moisture. Rather than it being really dry or really wet, it stays moderately dry for a longer period. Compared to the prairie, the former cropland is in a sense much “flashier” - the soil tends to become wetter much faster and then rapidly dries out.
Another observation has shown us that infiltration and recharge have decreased over the last six years since we restored the cropland to grassland. The native grasses are expanding their root systems and using more water over a longer period of the growing season. It's essentially drying out the landscape. The water table in the former cropland has dropped an average of one-half foot since 2001 compared to no change in the adjacent native prairie.
There is one other finding that's surprising. Although the soil has become moderately dry under the native prairie, there tends to be a resiliency in the amount of soil moisture. Under drought conditions the soil tends to hold the moisture longer at deeper levels in the native prairie. I'm not exactly sure why, but it's something that deserves more study and observation.

What do your findings mean for grassland restoration?

Phil Gerla:

Prairie restoration might be a way of mitigating floods at their source – where runoff is actually forming.
A lot of attention is focused on restoring wetlands to mitigate flooding downstream. Few people, however, have suggested that restoring areas to grassland also can reduce floods. Prairies store and use a lot of water. Prairies often create dense surface vegetation that helps slow runoff at the source.
Another interesting outcome is that although native prairie tends to create a moderately dry soil profile, deeper soils remained moist even during dry periods.
By maintaining or restoring prairie on a larger scale, we might be able to provide a small but steady amount of water to nearby streams under drought conditions. If it’s all cropland, then that base flow is lost and the streams tend to be more intermittent or dry, which may diminish aquatic biodiversity.

So, big grassland restoration projects like Glacial Ridge can help mitigate flooding and drought beyond their boundaries?

Phil Gerla:

We have to keep in mind that under unusually wet conditions - such as excessive spring runoff or storm occurrences with a very long recurrence interval - floods will follow. Runoff will occur whenever the ground is saturated and water on the landscape fills available storage.
But prairie restoration can help alleviate the peak flows in small and intermediate floods. There are farms and communities downstream from Glacial Ridge that could benefit. We’re holding back much more water on the landscape and that mitigates peak flow lower in the watershed. Even at 24,500 acres, however, Glacial Ridge covers only a tiny fraction of the Red River basin, so the effect far downstream is small.
A lot of money and resources have gone into building dikes and levees, and they're essential to flood protection for many communities. Far fewer funds have gone toward a more conservation-oriented approach to flood mitigation. Engineering solutions should be balanced with strategically designed conservation methods. Perhaps we need to emphasize this more in the future.

Chris Anderson is a senior media relations manager with the Conservancy.

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