Validity Regions of Soil Moisture Retrieval on the $\mbox{LAI}$– $\theta$ Plane for Agricultural Fields at L-, C-, and X-Bands

To quantitatively examine the sensitivity of the soil moisture retrieval over agricultural fields, first, we generated a database of radar backscattering coefficients using the first-order vector radiative transfer model for various leaf area indices (LAIs), incidence angles, frequencies, and polarizations. Then, soil moisture contents were retrieved from the backscattering coefficients of the database using the water-cloud model (WCM) and Oh et al.'s scattering model. Based on the sensitivity examination for LAI and θ (incidence angle) on soil moisture retrieval, we propose validity regions of soil moisture retrieval over agricultural fields on the LAI-θ plane at L-, C-, and X-bands for vvand hh-polarizations in the form of O
_Fpp
<; -A
_Fpp
LAI + θ
₀
,
_Fpp
, where the constants A
_Fpp
and θ
₀
,
_Fpp
are determined in this letter for F-frequency band and pp-polarization. The validity regions were verified with inversion results from synthetic aperture radar (SAR) data of bean and wheat fields.