When Groundswell began stewarding the land at Westport Prairie, sheets of rain would flow off of a cornfield and pool in the tobacco shed where we store equipment. It got so bad that we thought about digging ditches, culverts, or raising the shed floor. Then something amazing happened. We planted the hillside to prairie, and the flooding stopped. Rainfall no longer sheeted down the hill, it infiltrated into the ground.
This got me thinking. Let’s run a simple experiment to see how well water soaks into our soils. This week, we set out across Westport Farm and Westport Prairie with a tool called an infiltrometer. We used it to compare how fast water soaks into prairies, conventional row crops, and vegetable fields. The results were shocking.
Yahara Lakes Watershed map showing land areas that drain directly to each lake. Yellow denotes agricultural areas that comprise most of the 385-square-mile watershed. The map was created by Mike Smale of Clean Lakes Alliance.
Why infiltration matters
When rain hits the ground, it has a few paths. It can wash away over the surface as runoff, evaporate into the air, pass through a plant, or infiltrate into the soil. Some of that infiltration may stay in the soil, and some may “recharge” the groundwater we drink. In general, the more water that soaks into the ground instead of running off, the cleaner our rivers and lakes will be.
Healthy soil can soak up a lot of water. A healthy soil is generally one teeming with roots, worms, microbes, and organic matter. Roots and worms dig channels that help water move. Microbes create tiny rooms we call “pore space,” which increases the water retention of the soil. And organic matter works as a sponge.
Degraded soil, or soil that’s been heavily tilled and eroded, doesn’t give water anywhere to go. It fills up fast, then water pools on the surface or runs off. When we talk about “soil health,” we don’t just mean healthy crops. Poor soil practices cause floods and carry pollution into our lakes and rivers.
The map above shows the significance of farm runoff to the Yahara Lakes. Westport Farm and Westport Prairie are located in the farmland north of Lake Mendota. Our ability (and responsibility) to store water where it lands dictates the health of our lakes and whether our downstream neighbors flood.
Infiltrometer in the field. Thanks to Will Fulwider of UW–Madison Division of Extension for loaning us this tool. Photo by Sam Douglass.
The Experiment
We used a double-ring infiltrometer to measure the rate at which water soaks into the soil. This tool mimics what happens when water pools in a field during a heavy rain. We tracked how much water soaked into the soil in 20 minutes and then repeated the experiment in 11 places. The question was simple: how much rain can this soil soak up in 20 minutes?
Our most surprising result came from the area pictured below, where a soybean field meets a nine-year-old prairie. We tested the prairie first.
The prairie soaked up 10.5 inches of water in 20 minutes. That’s a lot of water! Then, just twenty feet away, we ran the same test in the soybean field. We poured water into the rings and started the timer. After 20 minutes, the water level had not changed.
This soil was so compacted, so degraded, that it couldn’t absorb water. Any rain that falls on this patch of the field that day would run straight towards the pond. Days after conducting this experiment, I am still grappling with the impacts of this. We’re not just talking about one farm, but the entire watershed.
Soybean field (left) and nine-year-old prairie (right). Photo by Sam Douglass.
Next, we took the infiltrometer across the street to Westport Farm to see how farming practices like cover crops, organic inputs, subsoiling, minimizing tillage, and crop rotation affect how well water infiltrates.
Here’s what we found: compared to a neighboring conventional corn field, an organic rice field soaked up twice as much water. A field we’re cover cropping soaked up four inches of water, or six times more than the corn field next to it. A lemongrass crop soaked up eight inches, matching what we saw in the prairie.
It is not inevitable that farming harms our waters. Many of our supporters and easement holders are farmers themselves, or own and lease farmland. If this is you, we’d love to connect you with resources that help pay for soil conservation practices.
This experiment was a great reminder for me that soil health affects everybody, not just farmers. We all share a watershed.
Take care,
Sam Douglass
Farm & Land Manager
P.S. If this work inspires you, please consider making a gift to Groundswell to keep the momentum going!