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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #250984

Title: Do current models correctly predict the L-band terrestrial brightness temperature when crops begin to senesce?

item HORNBUCKLE, BRIAN - Iowa State University
item ROWLANDSON, TRACY - Iowa State University
item RUSSELL, TRACY - Iowa State University
item KRUGER, ANTON - University Of Iowa
item Sauer, Thomas

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2010
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: The microwave terrestrial brightness temperature is sensitive to soil moisture, the water content of the first few centimeters of Earth's surface. The European Space Agency will soon launch the Soil Moisture and Ocean Salinity (SMOS) mission, the world's first soil moisture satellite that will measure the terrestrial brightness temperature at L-band, the optimum radio frequency for soil moisture remote sensing. This mission could pave the way for a new era of Earth system science by providing global maps of soil moisture and useful spatial and temporal frequencies. The tau-omega model of terrestrial brightness temperature will be used by SMOS to retrieve soil moisture from satellite measurements. This model takes into account microwave emission from the soil, the attenuation of this emission by a vegetation canopy that covers the soil surface, and emission from the vegetation itself. The effect of vegetation on the brightness temperature is proportional to the mass of water contained within vegetation tissue per ground surface area. This empirical proportionality factor is called the "b" parameter. The value of this parameter is a function of radio frequency and plant type. Annual crops such as maize (corn) and soybean account for as much as 90% of the land surface cover in the US Midwest. Annual crops accumulate biomass rapidly during the summer, then begin to senesce as they reach maturity, a process during which the water content of the vegetation tissue slowly but significantly decreases. This process of sensescence takes approximately 1.5 months, or about 30% of the growing season. Are current estimates of the "b" parameter appropriate for both growing crops and crops that are in the process of senescence? Our hypothesis is that senescence not only reduces the amount of water in vegetation tissue but also significantly changes the distribution of water within the vegetation such that the characteristics of emission and scattering of microwave radiation by a canopy of vegetation will change and a different "b" parameter must be used. We will test this hypothesis with data collected during SMAPVEX2008 by the airborne National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) Passive and Active L-band System (PALS) radiometer over the Iowa Validation Site near Ames. PALS observed the L-band brightness temperature of maize (corn) undergoing the process of sensescence. The timing of the experiment will also allow us to determine if other forms of canopy water (intercepted precipitation and dew) have an effect on the L-band brightness temperature.