Stationary L-Band Radiometry for Seasonal Measurements of Soil Moisture

The results of seasonal L-band (1.4 GHz) soil moisture (SM) measurements and their analysis are described. The measurements were performed using a radiometer mounted at the top of the 300-m tall ETL Boulder Atmospheric Observatory (BAO) tower during period of 2004-2005. In situ soil moisture measurements were made in the center of the radiometer antenna footprint using time-domain reflectometer (TDR) probes installed at 5, 10 and 15 cm depth. Radiative transfer theory predicts that L-band radiation from soil originates from deeper within the soil compared with commonly used remote sensing microwave bands at higher frequencies. Results from this experiment indicate such generally higher sensitivity to subsurface SM content. On the other hand, a significant amount of scattering variability is also observed for the same level of in situ SM measured at 5 cm depth. Our radiometric experiments support the hypothesis that different vertical gradients of SM can lead to different reflectivity values for the same SM level at a fixed depth. Further investigation is needed to quantify the role of grassy vegetation for L-band soil reflectivity during spring and summer seasons.