PASSIVE MICROWAVE OBSERVATIONS OF DIURNAL SOIL MOISTURE AT 1.4 AND 2.65 GHZ

Authors: Jackson, Thomas; O'NEILL, PEGGY - NASA; Kustas, William - Bill; BENNETT, ERIC - UNIVERSITY OF MASS; SWIFT, CALVIN - UNIVERSITY OF MASS

Submitted to: Geoscience and Remote Sensing Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/5/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Passive microwave sensors operating at low frequencies are sensitive to surface soil moisture changes. Theory describes the microwave measurement as a function of the properties of the soil profile, and the depth of the soil that significantly contributes to the measurement increases as the microwave frequency decreases. Few studies have been conducted that have involved multifrequency observations or that have considered the diurnal variations of the soil moisture and temperature and the impact of these on the microwave measurement. In this investigation, observations were made using a dual frequency microwave radiometer system over a variety of field conditions for extended periods. Results reported here clearly demonstrate that each sensor responds to a different soil depth. This result supports further efforts aimed at utilizing soil moisture observations at several depths to estimate the entire root zone soil moisture as well as monitoring moisture and energy balance. These results will ultimately impact critical design features (particularly the timing of observations) for future satellite observing systems.

Technical Abstract: Microwave radiometers operating at low frequencies are sensitive to surface soil moisture changes. Few studies have been conducted that have involved multifrequency observations at frequencies low enough to measure a significant soil depth and not be attenuated by the vegetation cover. Another previously ignored aspect of microwave observations at low frequencies has been the diurnal variations of the soil moisture and temperature and the impact of these on the brightness temperature. In this investigation, observations were made using a dual frequency radiometer (1.4 and 2.65 GHz) over bare soil, corn and grass for extended periods in 1994. Comparisons of emissivity and volumetric soil moisture at four depths for bare soils showed that there was a clear correspondence between the 2 cm soil moisture and the 2.65 GHz emissivity and between the 5 cm soil moisture and the 1.4 GHz emissivity. Observations during drying and rainfall demonstrate that new and unique information for hydrologic and energy balance studies can be extracted from these data.