Sunday, 31 August 2014

Contour Plowing in the Palouse

The next time you fly over southeastern Washington, look down. The expanse of undulating hills covering the landscape is one of the more beautiful—and geologically distinctive—in the United States. In comparison to the long, linear ridges in many areas, the hills in the Palouse are arranged in a complex web of interlacing humps and hollows that look almost randomly placed.

Tectonic forces did not push up these hills, nor did rivers carve away valleys to create them. Instead, it was the wind. During the last Ice Age, winds blew in fine-grained silt, or loess, from the southwest. Much of it was rock flour, which had formed as glaciers crushed bits of rock along the land surface.

As winds transported and sculpted loess into distinctive dunes, a layer of silty soil—best known for its rich mineral content and ability to retain moisture—formed on their surfaces. Since the Palouse lies in the rain shadow of the Cascade Mountains and precipitation is often scarce, the capacity of the land to retain moisture has proven critical for farmers, which have made the area a leading wheat-producing region. Other major crops grown in the Palouse include barley, peas, and lentils.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite captured this image of farmland near LaCrosse, Washington, on July 24, 2008. The false-color image was assembled using infrared, red, and green wavelengths of light. This combination is useful for observing vegetation and monitoring its health. Areas with the most vigorous crops, grasses, and trees are bright red. Areas where vegetation is drying or dormant are darker shades of brown and gray. Areas with no red are likely fallow or being prepared for seeding.

The same geological conditions responsible for the fertile soil of the Palouse’s also created a problem for the region. The combination of hills, fine-grained soil, and farming means the soil is extremely susceptible to erosion. In 1976, the U.S. Department of Agriculture highlighted the problem in a report that noted croplands in the Palouse were losing about 20 to 30 tons of soil per acre each year, with some of the most susceptible areas losing 100 to 200 tons per acre. Forest and rangeland, in contrast, was losing less than one acre each year.

Many farmers have since made changes in order to address the problem. For instance, some have transitioned to no-till farming. Some plant different crops in adjacent strips, a practice that helps block eroding soil. The practice is even more effective when farmers till in patterns that match the natural shape of the land. This strategy, known as contour strip cropping, explains the complex, curving patterns in this image.
While erosion remains a serious issue, a report published in 1998 by the U.S. Geological Survey found that soil conservation practices had reduced erosion in the Palouse River Basin by at least 10 percent.
NASA image created by Jesse Allen, using data provided courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. Caption by Adam Voiland.
Instrument(s): 
Terra - ASTER - NASA
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