Ground-based dual-frequency radiometry of bare soil at high temporal resolution

We report on an experiment linking microwave brightness temperatures and emissivities to the dynamics of in situ measured water content. The bare plot of 14 x 20 in was instrumented with two ground-based radiometers (1.4 and 11.4 GHz, respectively) and in situ time domain reflectometer (TDR) and temperature probes installed at five depths in three soil profiles. All data were recorded every 30 min from May to July 2002. The measured brightness temperature dynamics is a superposition of three patterns. The largest variations were caused by the changes in water content due to precipitation and the subsequent drying process. During dry periods, we observed daily oscillations of brightness temperature. Calculating effective soil temperatures based on the in situ measured temperature profiles revealed that these oscillations were partially due to changes in water content in the very topsoil. This effect was especially pronounced for the 11.4-GHz measurements, indicating that the very surface experienced the strongest changes in water content. At 1.4 GHz and horizontal polarization, the brightness temperature measurements turned out to be very sensitive to sun reflection at the ground. This reflection was visible through exceptionally high brightness temperature values at distinct times of the day. We observed pronounced hysteresis loops when comparing the emissivities of the two radiometers or the radiometric signals with the water content measured in situ at 2-cm depth. Such loops could be seen for the daily wetting and drying cycles as well as in the longer term drying and wetting behavior. This demonstrates that the gradient of the water content in the topsoil is a dynamic property depending on various time scales. These gradients affected the measurements of radiometers and TDR probes to different degrees and at different times. Despite an almost smooth surface, slight changes in the surface structure during the experiment caused a substantial variability with respect to the relationships between water content and emissivity for both radiometers. This is an indication that surface structures smaller than the wavelength may exert important effects on the emissivity of a bare soil.