Active and passive microwave measurements of soil moisture in FIFE

During the intensive field campaigns of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) in May–October of 1987, several nearly simultaneous measurements were made with low‐altitude flights of the L‐band radiometer and C‐ and X‐band scatterometers over two transects in the Konza Prairie Natural Research Area, some 8 km south of Manhattan, Kansas. These measurements showed that although the scatterometers were sensitive to soil moisture variations in most regions under the flight path, the L‐band radiometer lost most of its sensitivity in regions unburned for many years. The correlation coefficient derived from the regression between the radar backscattering coefficient and the soil moisture was found to improve with the increase in antenna incidence angle. This is attributed to a steeper falloff of the backscattering coefficient as a function of local incidence at angles near nadir than at angles greater than 30°. Applying an initial slope correction to the measured data generally improves the correlation coefficient. At C band the improvement factor is found to be about 0.08 near vertical and gradually decreases to 0.04 at 45° incidence angle.