SOIL MOISTURE UNDER THE SURFACE
The radar signal penetrates into the soil for 0,5 to 5 cm. This depends mainly on moisture. However for the growth of a plant this top soil moisture is relevant, but so is the moisture in the deeper soil. In order to make estimates for this deeper soil water, different approaches are in use (e.g., 'Soil Water Index').
The DRYMON Soil Water Index provides the soil moisture over a profile of 1 meter deep. As water percolate differently among soils, the soil water index is callibrated for soil texture. The calibration is described in:
Scatterometer-Derived Soil Moisture Calibrated for Soil Texture With a One-Dimensional Water-Flow Model
de Lange, R.; Beck, R.; van de Giesen, N.; Friesen, J.; de Wit, A.; Wagner, W.;
Geoscience and Remote Sensing, IEEE Transactions on
Volume 46, Issue 12, Dec. 2008 Page(s):4041 - 4049