Identifying Soil Hydraulic Heterogeneity by Detection of Relative Change in Passive Microwave Remote Sensing Observations

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Abstract
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 characteristics from PMRS. We show that relative change in surface soil moisture can be determined accurately from PMRS measurements of brightness temperature, given the commonly observed linear relationship between 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 caused by spatial variability in initial wetness rather than soil heterogeneity. 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 the 1992 Hydrologic Atmospheric Pilot Experiment (HAPEX‐Sahel) experiment can be proven. Areas of relatively fast or slow drying can be mapped. These areas coincide with geomorphological features and soil‐type patterns 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 homogeneity hypothesis. Furthermore, the variogram indicates the existence of an effective correlation length much larger than that commonly observed 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.