Abstract:
One of the most promising noninvasive techniques for monitoring surface (0-5 cm) soil moisture content over large areas is microwave remote sensing. However, as the spatial scale, satellite footprint and temporal frequency of sensor coverage increase in future missions, the problem of validating soil moisture imagery derived from microwave sensors is compounded. This is due in large part to the difficulties associated with collecting representative ground truth data sets given the increasing size of remote sensing footprints and the high degree of spatial-temporal moisture content variability within these footprints.

The purpose of this presentation is to describe a recent study of subgrid-scale soil moisture variability within remote sensing footprints in support of the Southern Great Plains 1997 (SGP97) Hydrology Experiment (a surface soil moisture mapping mission conducted between June 18 and July 18, 1997 in central Oklahoma). This study was part of a larger ground-truthing effort at SGP97 in which moisture content was measured on most days both gravimetrically (9 - 14 samples at nearly 50 sites) and using portable impedance probes (a minimum of 49 samples at 6 sites). The more dense impedance probe measurements will be compared to gravimetric data and remotely-sensed soil moisture estimates at two sites located within the Little Washita watershed, both of which are the same resolution as the microwave sensor footprint (approximately one square km). Scales and process controls of subgrid soil moisture variability will be described, as will a comprehensive impedance probe calibration effort. Implications for the accuracy and uncertainty of the remotely-sensed and impedance probe soil moisture measurements, and for the design of future ground-truthing efforts will be discussed.