IDENTIFYING SOIL HYDRAULIC HETEROGENEITY BY DETECTION OF RELATIVE CHANGE IN PASSIVE MICROWAVE REMOTE-SENSING OBSERVATIONS

The ability of passive microwave remote sensing (PMRS) to assess soil moisture has been amply demonstrated. Building on the expertise, this research is a first attempt at obtaining near-surface soil hydraulic c haracteristics from PMRS. We show that relative change in surface soil moisture can de determined accurately from PMRS measurements of brigh tness temperature, given the commonly observed linear relationship bet ween the two variables. Relative, rather than absolute, change within a series of PMRS images, obtained during a period of drying, is also a very useful statistic for filtering out the drydown heterogeneity cau sed by spatial variability in initial wetness rather than soil heterog eneity. We develop a test of soil heterogeneity based on the detection of deviations of relative change from the average rate in a series of images by more than the instrument error. With the test the existence of soil heterogeneity for a pair of images showing a drydown during t he 1992 Hydrologic Atmospheric Pilot Experiment (HAPEX-Sahel) experime nt can be proven. Areas of relatively fast or slow drying can be mappe d. These areas coincide with geomorphological features and soil-type p atterns that are expected to show the observed moisture dynamics. The variogram of relative change in the HAPEX images differs from those of brightness temperature, corroborating the rejection of the soil homog eneity hypothesis. Furthermore, the variogram indicates the existence of an effective correlation length much larger than that commonly obse rved in ground-based soil surveys. These findings encourage the use of PMRS for assessing soil hydraulic characteristics that are valid at a scale appropriate for hydrometeorological models.