THIEF RIVER FALLS, Minn. | With six propellers whirring, a drone rises from a narrow gravel road about a half-mile from the Red Lake River in Pennington County.
It slowly moves along a channel that funnels water from miles of farmland into the river.
The channel is nearly invisible under a shroud of dense tree cover. But the drone will capture a precise image of the ground contours — with an accuracy down to 3 centimeters — using LIDAR, a surveying tool that employs lasers.
"It's got a little spinning mirror in there and a laser bounces off that mirror and it measures how long it takes that laser to bounce off the ground and come back," explains pilot Zackary Nicklin, an instructor at Northland Community and Technical College in nearby Thief River Falls.
In farm country, a vast network of these ditches drains water from the fields. Eventually, that water, and whatever pollutants it carries, flows into a river.
Most ditches are in the open, easily visible. But often, the last mile or so to the river is wooded and inaccessible. Those locations are often overlooked when county water quality managers try to identify sources of nutrient pollution in their waterways.
Bryan Malone knew some of those ditch outlets to the river in Pennington County were in bad shape.
"They have steep slopes, the banks are bare, no vegetation. Looks like they could be contributing a fair amount of sediment to the river system," Malone said.
So he applied for and received a grant from the Minnesota Board of Water and Soil Resources, with money from the state's Clean Water Fund, to monitor 53 ditch outlets across the county "and measure what ditch is eroding the most, what volume (of soil) is moving from year to year, and make a priority list of what ditch outlet needs the most help," said Malone.
He manages the county's soil and water conservation district, overseeing a range of conservation programs.
Malone sees drones as tools that make it faster and easier to inspect these often inaccessible ditches. The flight over this half-mile ditch to the Red Lake River takes three minutes. With setup, the whole process takes about a half hour.
"We're looking to get somewhere between five and seven ditches a day," Nicklin said.
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The cost of these flights is significantly less than a private company might charge because as part of the approximately $400,000 project, the college was able to buy a new drone, a camera and LIDAR that students can help operate.
"They get the benefit of coming out here, using state-of-the-art sensors, state-of-the-art aircraft and getting real world experience on how we collect data," explained Nicklin.
The college also teaches imagery analysis — how to find the useful information in all the data collected by drones.
Instructor Steve Sorenson said he and his colleagues have worked with the soil and water conservation district to understand things like ditch erosion and sediment blockages.
"I'm looking at this drainage culvert and I'm like, 'OK, so what am I looking for?'" said Sorenson. "We need those experts to tell us this is what right looks like or wrong looks like. What we've been doing is developing what we call a guidebook, which helps us identify what we're looking for."
Malone will use that data to apply for grants to repair the most severely eroded sites and reduce soil and nutrients flowing into the rivers.
He also plans long-term monitoring of the sites.
This is the second year of the three-year grant to collect images and LIDAR. Comparing information over several years will allow analysts to see changes.
"If we come in 10 years from now and take the same shots, we'll know the exact volume (of soil) that's going into the river," he said.
The protocols the teams are developing as part of this project will make it easier for other agencies to adopt a similar approach using drone technology to monitor ditch erosion.
"Our goal is to make this something that every conservation organization around the state can utilize to more easily survey and design conservation practices in the field," said Malone.