Vegetation Optical Depth: Remote Sensing Retrievals and Applications

Vegetation optical depth (VOD) accounts for the attenuation of microwaves through vegetation and is a function of vegetation water content and its structure. VOD estimated at different frequencies provides complementary information on these vegetation properties. It has been shown that VOD can be used as a proxy of other vegetation properties, such as tree height, sap flow, leaf fall and above ground biomass.

Unlike traditional optically-based technologies, microwave-frequency sensors can observe through clouds. Furthermore, one of the strengths of particularly low frequency VOD is its sensitivity to vegetation changes in dense forests, where optical indices tend to saturate. The relationship between VOD and AGC (above ground carbon) was recently discovered. The estimation of AGC allows scientists to assess interannual variations in carbon stocks, a main actor in climate change mitigation, on Earth.

The availability of data from ASCAT, Sentinel-1, SMOS, SMAP, AMSR-E, and AMSR2 missions allow the study of vegetation seasonality and, subsequently, information about the plant status, from leaves to stems. Multi-frequency retrievals could reveal the partitioning of the extinction, in microwave radiation caused by vegetation, into scattering and absorption.

In this special issue on VOD, we invite contributions on the following topics:

• VOD retrieval algorithms, based on both physical models and data-driven methods.
• Various complementary VODs (L-, C-, X-band) and information derived from other sensors (optical).
• Downscaling VOD merging data from different sensors with different spatial resolutions.
• Approaches for the harmonized processing of data coming from different sensors to construct longer, coherent, VOD records.
• Field measurements of VOD, relationships of vegetation water content and validation of satellite VOD products.
• Applications of remotely sensed VOD data including data assimilation and disaster assessment (fires, droughts, and C-stocks monitoring).
• Theoretical modeling of vegetation for quantifying optical depth and transmissivity.

Dr. Arnaud Mialon
Dr. Nemesio Rodriguez-Fernandez
Dr. Mariette Vreugdenhil
Dr. Tianjie Zhao
Dr. Roberto Fernández Morán
Guest Editors

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