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Erosion can't hide from laser scanner.

Erosion Can't Hide From Laser Scanner

Particles of topsoil blown by wind will bounce along the soil surface and finally escape a field, leaving it less able to support crops. Water will wash away valuable topsoil and nutrients. And how rough the soil surface is influences whether the soil will erode.

Until now, soil scientists have had no suitable technique to measure soil roughness - or microtopography - on the small scale.

ARS soil scientists Joe M. Bradford and Chi-hua Huang, of the National Soil Erosion Research Laboratory in West Lafayette, Indiana, have developed a portable scanner that can. It measures the tiny ridges left in the soil by tilling or clods of soil particles that clump together naturally.

What does the scanner do? It measures soil elevation by shining a low-power laser beam onto the surface and detecting the position of the laser spot reflected from the soil with a 35-mm camera. In place of film, the scanner camera uses electronic circuitry somewhat similar to that in a video camera to transmit the spot's position to a small computer about 30,000 times a minute.

The laser and camera are mounted on the frame of a motor-driven carriage. The computer controls the carriage movement.

At the end of a scan, a microtopographic map is stored in the computer. Scientists can analyze it immediately and can compare it to previous maps to see whether erosion has occurred.

Before the scanner, researchers used to lower a single pin or row of pins onto the soil surface and register the pin positions.

This procedure was laborious and yielded elevation data only at grid spacings on the order of 1 to 5 centimeters (one-half to 2 inches) - too big to measure roughness at the microtopographic level. Also, the pin would sometimes sink if the soil was soft. The laser beam, working like an optical pin but without touching the surface, gives the true surface elevation.

Why Measure So Small?

"With water and wind erosion, soil particles being detached and transported have sizes in the order of millimeters," says Huang. "So to understand these processes, we need to study the surface microtopography resulting from soil grains and aggregates (soil clumps) that move."

"Water infiltration, soil erodibility, and ease of tillage depend on soil texture," says Bradford. "And soil texture is determined by its percentages of clay, sand, and silt." Clay particles are less than 0.002 mm in diameter, silt is 0.002 to 0.05 mm, and sand is 0.05 to 2.0 mm.

The scanner has the spatial resolution required to detect roughness from millimeter-scale soil particles. It can digitize a 1-meter-long profile in 6 seconds, taking data as close as 0.5 mm (20-thousandths of an inch) apart. For a typical 1-meter-square erosion test plot, it can collect about 1 million elevation points in 50 minutes.

Bradford and Huang use the scanner to measure surface lowering or rising from two processes: erosion-deposition and wetting-drying. Minute changes such as either of these can be detected only by the scanner.

Erosion-deposition is soil eroding from one place and being deposited somewhere else. Wetting-drying is the process where wet soil swells and dry soil shrinks, which changes the properties of the soil surface.

Since it is portable and can fit into the back of a van or station wagon, the scanner can be used in both laboratory and field studies that quantify how soil roughness affects erosion.

Other ARS researchers also use the scanner. Lawrence J. Hagen, an agricultural engineer at the ARS Wind Erosion Unit at Manhattan, Kansas, uses it to study the processes of wind erosion. "Data collected by the scanner in wind tunnel tests enable us to quantify how soil clods protruding from the soil surface shelter it and prevent erosion," says Hagen. "It also calculates the capacity of the surface roughness to trap and store detached soil particles among the sheltered areas between the soil clods."

Besides studying soil erosion, a team of scientists from the newly established ARS National Soil Tilth Laboratory at Ames, Iowa, uses the scanner to measure water storage on the soil surface from tillage operations and cattle grazing. "Ponding on the surface decreases runoff - and hence erosion - by holding the water in place until it can infiltrate the soil," says soil scientist Jerry Radke.

Entomologist Edwin Berry at the Ames lab also uses the scanner to estimate earthworm activity at the surface - which can affect soil tilth. "Some earthworms feed at the surface and move organic material to their burrow openings," says Berry. "They often form a cast near the opening, and the change in size and shape of these casts reflects their activity."

Other USDA agencies also benefit from data collected by the scanner. The USDA Forest Service Intermountain Research Station at Moscow, Idaho, uses it to measure soil erosion from forest road surfaces, roadside ditches, and wheel ruts from logging trucks, as well as from recently harvested areas. "The major advantage of the laser scanner is that it can measure roughness very accurately," says Edward Burroughs, project leader. "There are other methods to measure surface contours and roughness that are faster and that analyze data more easily, but the measurements are coarser."

PHOTO : Technicians Wayne Carstenson and Brent Schroeder use a newly developed laser system to record the roughness of a soil sample.

PHOTO : Measuring soil surfaces doesn't have to be this arduous. Photographic pin meter operated by technicians Wayne Carstenson and Brent Schroeder will be replaced by the new laser recording system.
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Title Annotation:soil erosion
Author:Konstant, Dvora Aksler
Publication:Agricultural Research
Date:Sep 1, 1991
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