Published in partnership with the St. Croix Watershed Research Station:

There are lots of things farmers can do to clean up the water flowing into Lake St. Croix on the Minnesota-Wisconsin border. Popular practices include buffer strips, smart tillage, and cover crops.

The more complicated question is which projects are most worth the effort.

What to do where – to have the biggest impact for the lowest investment – has important implications for improving the water quality of the federally-protected river. Research Station scientist Jim Almendinger has been wrestling with that question, thanks to funding included in the St. Croix Crossing in Stillwater.

“In the face of commodity prices and government policies that encourage planting more corn and soybeans, and development resulting from the new bridge at Stillwater, Lake St. Croix will likely degrade further without efforts at curbing phosphorus loads from all sources,” Almendinger says.

Further degradation is unacceptable. Minnesota and Wisconsin actually want to cut phosphorus in Lake St. Croix by 27 percent – to 360 tons/year, the same level as existed in the 1940s – by 2020. Efforts to curb phosphorus are underway across the watershed. Ensuring those efforts are effective is where computer models comes in.

Simulating the St. Croix

Almendinger has developed a model of the watershed meant to serve as a tool for people and organizations working across two states, 19 counties, and other jurisdictions. It will help paint the big picture, and put local efforts in context.

The project essentially provides a way to answer “what if we do X upstream? How will it help or hurt Lake St. Croix?”

Because agriculture is the biggest source of sediment and phosphorus that is flowing into Lake St. Croix, Almendinger has been studying how changes in farm practices have the potential to improve the situation. Covering soil with another crop between harvesting and planting corn or soybeans is the clear winner.

Winter Crops Work Best

Almendinger plugged winter wheat as a cover crop into the model (although he says any crop that can become well established by December will work), and found it would provide the biggest bang for the buck – reducing phosphorus in Lake St. Croix by 23 percent if implemented on every acre planted with corn and soybeans in the watershed. Buffer strips and grassed waterways are also promising, with the capability of reducing by 15 percent the amount of phosphorus running off a field.

“When large portions of the landscape that were formerly tilled and protected only by scattered crop residue are replaced with living cover crops during the fall and through the spring runoff season, sediment and nutrients in runoff will decline substantially,” Almendinger says.

The algorithms are part of a computer program called SWAT – the Soil and Water Assessment Tool. By entering data like about whether an area is residential, agricultural, wetlands, or other land types, the steepness of slopes, and the types of soil, the interconnected watershed can be translated into a reasonably reliable pile of computer code.

Nothing can match the real-world, though, Almendinger hastens to point out. Despite imperfections, models have an essential job.

“Models are not totally wrong or right, we have to live in the uncertain reality in between,” he says.

An Achievable Challenge

The model took years to fine-tune, matching computer predictions with real-world measurements. The result was the news that hitting cleanup goals will be a big challenge. “If we do everything possible on the landscape, we should get close to the goal,” he says.

The reality is that an “all-of-the-above approach” is needed to reduce phosphorus in Lake St. Croix, decreasing the danger of toxic algae blooms and the overall fouling of the water. But the software also pointed to some of the most effective methods.

Making such major changes to the landscape and farming practices may seem intimidating, but the alternative is not an option either: worsening water quality in Lake St. Croix, algae and other impacts having highly negative effects on people, fish, and other wildlife.

This article is cross-posted from Field Notes, a digital newsletter from the St. Croix Watershed Research Station, part of the Science Museum of Minnesota. Sign up for the email list here.