2.3. Vegetation

Vegetation has two effects on the radar signal: (1) attenuate the backscattering from the underlying soils and (2) produce the volume scattering adding to the radar signal. These two effects increase the complexity of soil moisture retrieval from microwave signal. The vegetation attenuation and scattering effects were parameterized by the vegetation scattering albedo and optical depth, which are related to the vegetation water content or leaf area index.

Α. Vegetation optical depth τ is linked to the vegetation water content through b parameter:

$$
\boldsymbol{\pi} = \boldsymbol{b} \mathbf{VWC} \tag{13}
$$

The b parameter depends on the crop type, structure, and growth stage and microwave polarization. The vegetation water content is often obtained from the NDVI. Alternatively, τ can be obtained from the LAI through a linear relationship:

$$
\pi = b\_1 \text{LAI} + b\_2 \tag{14}
$$

The b1 and b2 are assumed to be dependent on the vegetation type.

B. Vegetation scattering albedo ω is set to be zero or a low value in the passive radiometer analysis.
