IFC and NASA to Study Soil Moisture

Researchers from the Iowa Flood Center have been invited to participate in a NASA field experiment this summer that will focus on global observations of soil moisture gathered by the Soil Moisture Active Passive (SMAP) satellite. IFC researchers, working with colleagues at Iowa State University and many other agencies and universities from around the country, will evaluate the satellite data by deploying instrumentation in the South Fork watershed, a tributary of the Iowa River. They hope to learn the effects of vegetation and agricultural practices on satellite-based soil moisture maps.

SMAP essentially takes pictures of Earth’s surface at a specific wavelength of microwave radiation. Humans can only see visible light, which is composed of wavelengths around hundreds of nanometers.  The wavelength SMAP uses is about 20 centimeters, which allows SMAP to “see through” vegetation and into the soil.  As the water in the soil increases, Earth’s surface looks “darker” to SMAP.

“As with many remote-sensing products, there is a continued need for evaluation,” explains IFC Director Witold Krajewski. The South Fork watershed is one of several validation sites around the world. Researchers here will answer a specific question: How does Iowa’s abundant crops, which also contain water, affect the accuracy of the satellite’s soil moisture data?

The Iowa campaign will be a two-part effort. The first will analyze satellite data from the end of May through early June, when crops are barely emerging. In August, researchers will take another look at soil moisture conditions when corn, soybeans, and other crops are fully developed.

Many members of Iowa’s research community will participate in the field experiment, and many will also deploy their own instruments to measure soil moisture or other relevant environmental data. The Iowa Flood Center will deploy two mobile X-band polarimetric radars to study rainfall at high spatial and temporal resolution. They will also operate other rainfall measuring instruments provided by NASA.

“Understanding the rainfall variability gives you an idea how much water gets into the soil and how it dries out,” Krajewski explains.

ISU researcher Brian Hornbuckle will study the characteristics of soil surface roughness. In the spring, the surface is typically rough from tillage and planting. Rain causes the soil to become smoother. “To understand that process is important,” Krajewski says. “Soil surface roughness does affect the satellite observations.”

Two other IIHR faculty members will also contribute specific expertise.   Bill Eichinger will help characterize the soil surface roughness with a Lidar instrument, and Anton Kruger will deploy a new ground-based remote sensing technique that can measure daily and seasonal changes in crop water.