Abstract
Monitoring global vegetation dynamics is of great importance for many environmental applications. The vegetation optical depth (VOD), derived from passive microwave observation, is sensitive to the water content in all aboveground vegetation and could serve as complementary information to optical observations for global vegetation monitoring. The microwave vegetation index (MVI), which is originally derived from the zero-order model, is a potential approach to derive VOD and vegetation water content (VWC), however, it has limited application at dense vegetation in the global scale. In this study, we preferred to use a more complex vegetation model, the Tor Vergata model, which takes into account multi-scattering effects inside the vegetation and between the vegetation and soil layer. Validation with ground-based measurements proved this model is an efficient tool to describe the microwave emissions of corn and wheat. The MVI has been derived through two methods: (i) polarization independent ( MVI B P ) and (ii) time invariant ( MVI B T ), based on model simulations at the L band. Results show that the MVI B T has a stronger sensitivity to vegetation properties compared with MVI B P . MVI B T is used to retrieve VOD and VWC, and the results were compared to physical VOD and measured VWC. Comparisons indicated that MVI B T has a great potential to retrieve VOD and VWC. By using L band time-series information, the performance of MVIs could be enhanced and its application in a global scale could be improved while paying attention to vegetation structure and saturation effects.
Highlights
Vegetation plays an important role in our ecosystems and its interactions with the Earth system
The results provide a satisfactory level of accuracy for the Tor Vergata model to be accepted as the theoretical basis for this study
We found the linear relationships between TB and bare soil emissivity were valid in all angles
Summary
Vegetation plays an important role in our ecosystems and its interactions with the Earth system. MPDI is used to minimize the effect of physical temperature and reflects the dielectric properties of vegetation canopy and soil Both MPDT and MPDI have an inverse correlation with vegetation water content [10]. This issue has restrained the application of global vegetation monitoring and further soil moisture retrieval [5] To overcome this drawback, Shi et al [5] introduced a new multi-frequency passive microwave vegetation index (MVI) by using the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) data, that can significantly minimize the surface emissivity signals [5,12]. The τ-ω emission model is a well-known zero-order solution of the radiative transfer equations that simulate brightness temperature (TB) as a function of soil emissivity, single scattering albedo, and optical depth This model ignores multi-scattering effects inside the vegetation and between the vegetation and soil layer.
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