Satellite Imagery Helps USDA Tackle Problem of Soil Salinity

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SAN FRANCISCO — Scientists with the U.S. Department of Agriculture (USDA) have developed a new way to use satellite imagery to assess soil salinity, a problem that plagues farmers around the world and leads to significant reductions in agricultural productivity, according to scientists who participated in the research.

The technique relies on imagery from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) to create maps that depict varying salinity levels for entire regions. MODIS imagery was used as part of a study that also relied on soil samples from local fields to create detailed maps of salinity levels for approximately 300,000 hectares of the Red River Valley, a fertile agricultural region in the Northern Great Plains of North America.

“The approach is quite viable,” said Dennis Corwin, soil scientist at USDA’s Salinity Laboratory in Riverside, Calif., who led the Red River Valley project. “We have shown how to map roughly 300,000 hectares, but it certainly can be applied at scales of a million or more hectares.”

Farmers around the world wrestle with problems of soil salinity. When the soil’s salt content is too high as a result of human activity such as irrigation or natural events including the breakdown of underground minerals, plants suffer. “Worldwide, 350 million hectares are salt-affected each year, and the problem is getting worse and worse,” Corwin said.

Soil salinity in the Red River Valley has become an increasingly serious problem during the last 17 years due to a combination of increased precipitation and changing agricultural practices, according to Michael Ulmer, senior regional soil scientist for USDA’s Natural Resources Conservation Service in Bismarck, N.D. Higher levels of rainfall have caused water tables to rise, bringing more salt toward the surface.

That salt is decreasing crop yields, and as the land becomes less productive, property values decline, Corwin said.

“We tried to put a dollar figure on the problem of salinity,” Ulmer said. “It is costing well over $75 million a year in lost production.”

To gain a better understanding of salinity levels in the region, scientists at the center asked USDA’s Salinity Laboratory to explore cost-effective techniques, including the use of remote sensing, to map salinity levels in the Red River Valley, an ancient lake bed that extends from Minnesota and North Dakota to Manitoba, Canada. “The basic idea was to be able to assess salinity for a large area — 100,000 hectares or larger — be able to map the salinity and do it at low cost,” Corwin said. Previous measurements, which were made by researchers taking soil samples throughout the region, were simply ineffective on such a large scale, Ulmer said.

Salinity Laboratory researchers began exploring how remote sensing could be used to assess salinity levels in the region in 2007. That work, which was described in a June 29 news release issued by USDA’s Agricultural Research Service, used MODIS imagery captured from 2000 through 2006 to assess changes in the health of plants in the region. MODIS instruments are orbiting onboard NASA’s Terra and Aqua environmental monitoring satellites.

While Corwin said there is no way to identify the exact salt content of soil through satellite imagery, scientists can gain insight into salinity levels by looking at the Normalized Difference Vegetation Index and the Enhanced Vegetation Index, two MODIS data products designed to assess the overall health and robustness of plants based on their ability to reflect light in different wavelengths.

By evaluating seven years of data, scientists were able to factor out the impact of intermittent problems hurting the crops, such as disease or hail. “The notion was that over a long period of time, you would weed out these yearly effects and see the overall effect of salinity,” Corwin said.

That technique did, in fact, help scientists create regional maps of varying salinity levels, Corwin said. The results of the imagery analysis were confirmed by soil samples taken from 60 fields in three counties of the Red River Valley, he added.

In addition, the remote sensing data gathered by Corwin’s team offered clues to the variable nature of soil salinity levels in the region. “We learned a tremendous amount about how salinity affects the soil and how salinity levels change with time, weather and how people manage the soil,” Ulmer said.

Based on the results of the Red River Valley salinity assessment, North Dakota state legislators are considering a plan to conduct a detailed assessment of the cost of increased soil salinity in the region. There is also growing interest among state officials in hiring salinity specialists to help farmers understand salinity levels and learn how to reduce their impact on crops, Ulmer said. When soil salinity is at low to moderate levels, farmers can modify their agricultural methods to reduce the impact of the salt and improve overall yield, Corwin said.

The Red River Valley salinity study also may lead to similar projects around the world. Corwin is working with scientists from Iraq who have expressed interest in mapping soil salinity levels in that country.