What can cosmic rays from deep outer space tell us about soil moisture levels in Colorado and Wyoming?

Contact for reporters:
Josh Rhoten
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What can tiny, highly charged particles arriving on Earth from outer space tell us about soil moisture content within wildfire burn scars in Colorado and Wyoming? 

Quite a lot, as it turns out. 

Colorado State University civil engineers are testing the capabilities of a cosmic ray neutron rover to evaluate soil moisture across a variety of situations. Their ongoing work to further develop this mobile sensing technology may be useful in assessing long-term hydrological changes from wildfires such as increased erosion or changes to soil composition. The tool may also be particularly useful to measure changes to snowpack levels that occur due to increased sun exposure after fires and are key factors in seasonal runoff levels. 

Cosmic rays are a type of radiation that travels through space at the speed of light. Scientists believe these particles originate from powerful events, such as supernovas and solar flares. They often travel long distances before arriving on Earth, where they then mostly glance harmlessly off our atmosphere. However, when the cosmic rays do make it through that protective layer, they scatter and create a shower of secondary particles – thousands of which pass through us every day with no problem. 

A key byproduct of this scattering process is the creation of high-energy neutrons. These newly formed subatomic particles then interact with the hydrogen commonly found in our soils, Jeffrey Niemann, a professor in CSU’s Department of Civil and Environmental Engineering, said. 

“Hydrogen atoms are very effective at slowing down and absorbing the energy of neutrons,” he said. “Because hydrogen in the environment is often contained in soil moisture, wet soils reduce the number of neutrons that occur in a certain energy range, while dry soils allow more of those neutrons to be detected. Measuring those differences allows us to estimate soil moisture across entire fields instead of at just a single point.” 

Stationary versions of these types of cosmic ray sensors have been used in agricultural research for more than a decade. They can passively monitor an area about once an hour. 

The rover CSU is testing, however, is much larger and can be housed in the back of a truck or other vehicle. That allows it to provide measurements over faster, minute-long timescales, and it can be moved across a landscape to better map the moisture. Niemann said the rover can measure water content in about a 200- to 300-meter radius as it travels service roads through difficult terrain. 

The idea is to link the new, detailed readings it can gather to data from other measurement methods to better understand the overall hydrology of an area. 

“Traditional sensors offer a very close-up picture of a few points on the ground, while remote sensing by satellites in space can provide useful but coarse or un-detailed data,” Niemann said. “Our approach with the rover offers a unique solution to find that missing middle-scale data set, which will help improve many important hydrological modeling activities.” 

A key example of the potential of the rover comes through a project funded by the U.S. Army Corps of Engineers to study the effects of the 2020 Mullen Fire near Laramie, Wyoming. To the CSU team’s knowledge, these types of sensors have not been used in post-wildfire environments, said graduate student Dawson Carney, who will lead field work for the project beginning this fall. 

“We are picking sites and working on permits for the project right now. I am looking forward to hopefully establishing a basic understanding of how this technology can work in these environments,” he said. “A key question will be how topography, vegetation type and density, and burn severity all impact the rover’s effectiveness for measuring changes in snowpack or soil moisture storage, all of which are important for evaluating watershed recovery after these events.” 

Niemann added that the team hopes to compare variables such as snowpack levels between pre- and post-fire events to see if there were noticeable changes in volume, for example. 

“There has also been some post-fire management in that area where dead trees were removed or mulched. And because we are doing this on U.S. Forest Service land, there are a lot of access roads,” he said. “So, the overall situation there really lends itself to gathering useful information for a variety of post-wildfire management scenarios.” 

Graduate student Luke Eddington is leading a related project at the Central Plains Experimental Range in Weld County. His work seeks to better estimate the moisture content of rural gravel roads.  

Vehicles traveling these paths during dry spells can kick up plumes of what is known as fugitive dust. That impacts air quality in the area and is increasingly a topic under consideration for regulation at the state and federal levels. Applying water to roads can limit the effect, and the team is hopeful their work will help managers monitor and proactively determine when dust suppression is needed. The work also considers how to best account for things like the surrounding landscape moisture and ambient moisture in the air, which can influence readings. 

Eddington said it has been a challenging project to undertake. 

“As a civil engineer, there was a lot to learn about how the rover works with cosmic rays before even considering its applications and possible use in hydrology,” he said. “But it has been a great experience working in the field with it and then analyzing the data. It has really pushed my learning.” 

Niemann said that project is funded by the Center for Transformative Infrastructure Preservation and Sustainability and the Agricultural Research Service, with extensive cooperation from Weld County. He added that it could also be relevant to understanding the effects of drought in similar environments. 

He said he is pleased with how these projects have demonstrated the potential of taking the sensor on the road. 

“We have other technologies that can accurately measure snowpack in the mountains – LIDAR that can measure snow depth or stationary observatories that have been active for years – but the rover can be used across a watershed and relays good information about the water content of the snow that those approaches can’t,” he said. “I look forward to continuing our work to hone the data collection processes with this tool and expand the possible uses for it.”