Multifrequency Microwave Radiometer Measurements of Soil Moisture

Ground-based microwave radiometer experiments were performed to investigate the effects of moisture, temperature, and roughness on microwave emission from bare soils. Measurements were made at frequencies of 0.6-0.9, 1.4, and 10.7 GHz using van-mounted radiometers to observe prepared soil sites in Kern County, CA. The sites were instrumented for monitoring soil characteristics and surface meteorological conditions. Brightness temperature variations of approximately 15 K at 1.4 GHz and 25 K at 10.7 GHz were observed as a result of diurnal changes in the soil temperature. Increasing the soil moisture content from 2 to 15 percent by volume resulted in brightness temperature decreases of approximately 70 K at 0.775 and 1.4 GHz, and 40 K at 10.7 GHz, depending, to a lesser extent, on polarization and viewing angle. The results show the significance of soil temperature in deriving soil moisture from microwave radiometer measurements. Comparisons of the microwave measurements with theoretical predictions using a smooth surface model show reasonable agreement and support previous results of this nature obtained with other soil types. Approximately equal sensitivity to soil moisture was observed at 0.775 and 1.4 GHz, although the sampling depth is greater at the lower frequency.