Laser beam focus on soil moisture

Scientists at the University of Kansas have started using a new tool to help understand how water moves, or doesn’t move, in the soil.

Daniel Hirmas and his team of soil hydrologists are using a Multistripe Laser Triangulation (MLT) scanner to measure the size of pores in soil horizons.

“We hand dig seven foot deep pits during the day so they’re ready for us to crawl into with our equipment once it’s dark enough to use the laser,” Hirmas said.

“Actually, it’s my graduate students who do the digging.”

It’s important to get a grasp on pore size in soil because doubling the size of the pore allows 16 times more water to flow.

In turn, better knowledge of how water moves through soil will give farmers in arid regions a better understanding of how to manage their scarce water resources.

The laser scanner lets his team examine five main factors:

  • better understanding of how plant nutrients are transported in the soil
  • accurate prediction of aquifer recharge rates in arid regions
  • more efficient use of water for crop and livestock production
  • better prediction of how dry areas will react to climate change
  • understand water runoff and sedimentation of surface water bodies

Hirmas’s specific area of study is called water retention analysis, which is based on the fact that the size of pores in the soil determines how easily water moves through the soil.

The degree of ease with which water moves through soil is called hydraulic conductivity. Water moves more easily through large empty spaces called macropores, which prompted the research team to concentrate on macroporosity.

“A finer textured soil typically has more aggregation and larger pores,” he said. “The fine textured soils have a lot of clay and a lot of silt.”

This makes the soil sticky so that it clumps together into aggregates, he added.

Large spaces between the aggregates create large pores. Most water is held by the medium and small pores, which means there’s not much point working with them because there’s no way to affect them.

“There are more smaller pores, but they don’t contribute to the flow of water as much as the large pores,” he said.

“They become saturated, but then the water just stays there. It takes forever for water to move because of the tightness of the grains. There’s so much friction, water has to follow a much more tortuous path.”

Hirmas said “saturated hydraulic conductivity” refers to the condition of the soil when all the pores are filled with water, while “preferential water flow” means that a significant portion of the water will be transported through a small number of large pores.

“It’s interesting that a small number of large pores control most of our soil hydrology,” he said.

“It’s when water gets into these large pores that we get downward water movement.”

An apt comparison is fracking in the oil industry. Drillers concentrate on shale, which is basically clay rock, because it has so much porosity compared to sandstone. It’s easier to extract oil out of shale in the Bakken formation than it is to get it out of sandstone in Alberta.

“Coarse textured sandy soils are very permeable, and water moves through them very easily, but they don’t actually have a lot of pore space. All the pores are limited to the spaces between the large grains,” Hirmas said.

“But fine textured soils have a lot of internal porosity. There’s a lot of pore space inside the aggregates and few pores between the aggregates.

“There’s also a lot of porosity created when roots decompose or earthworms travel around. Those are new conduits that water will flow through, especially if that conduit reaches the surface.”

Hirmas said the math of pore size is interesting.

Firefighters, hydraulic engineers and geometry teachers know that a slight increase in the size of a pipe creates a major increase in the flow of liquid through the pipe.

It’s necessary to first ground truth the MLT before using it in a specific soil type.

Hirmas used water with a blue dye to saturate the soil on pit walls. Color photographs of the cutaway profile determine how much blue dye is concentrated in the large pores.

Armed with that information, he scanned the same cutaway with the MLT to see if it picked up the same large pores.

A side by side comparison of the color photograph and the MLT image showed a high correlation between the two images. This meant that the tedious and time-consuming blue dye method was no longer the only way to document pore size.

The MLT does the same job in far less time, which allows Hirmas to study more soil profiles and his graduate students to dig more pits.

“In order to get an accurate picture of the pores on the pit wall, we actually freeze the vertical wall surface and then peel off a thin layer so we’re sure the soil we scan isn’t disturbed,” he said.

“We do all this at night because we don’t want ambient light from the sun to dry the wall or to give the laser false information. The MLT stays cooler and it’s easier to work with the frozen layer of soil when we work at night.”

The MLT differs from other laser scanners because it emits a pair of parallel laser light stripes that are focused on the soil wall surface. The camera that receives the feedback is offset from the twin beams, which gives Hirmas the triangulation he needs to measure pores.

“The camera watches the apparent deformation of the laser stripes and the distance between the stripes as they’re swept across the target surface,” he said.

“The computer calculates the surface topography as the beams bounce back to the camera from the wall.

“Some portion of those straight laser lines fall into the cracks and pores. They don’t bounce back to the camera. They disappear into the vacant spaces. The camera registers that as missing data. But missing data can be as useful as data bouncing back to the camera.

“We take the digital gaps and use image analysis to quantify those cracks where the signal disappeared. Those gaps represent real pores in the soil. So with a triangulated laser scan, we document the number of pores and sizes of those pores relatively quickly compared to using dyed blue water.”

For more information contact Hirmas at 785-864-5542.


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