High Frequency Ka-band SWOT and C-band Sentinel-1 SAR Backscatter Retrieve Vegetation Optical Depth over Harvard Forest

Vegetation Optical Depth (VOD), a microwave-based parameter used to model vegetation water content and plant structure, can monitor ecosystems for water stress, drought resilience, and responses to climate change. VOD products have been made available globally from the Soil Moisture Active Passive (SMAP) L-band radiometer, but studies of VOD derived from active SAR systems at higher frequencies including Cand Ka-band have been limited. Because different frequencies penetrate vegetation canopies to varying degrees, VOD from multiple frequencies can be used to access moisture information at multiple layers of complex canopies such as forests, where current VOD measurements are not well-validated. This study develops a simplified Water Cloud Model to derive VOD from C-and Ka-band backscatter from Sentinel-1 and SWOT, respectively, over Harvard Forest in Massachusetts. The derived VOD products are compared with one another to characterize the effects of frequency on VOD, and compared with passive L-band VOD from the SMAP Dual Channel Algorithm (SMAP-DCA) to evaluate differences with an established daily product. We find a seasonal signal in both the Sentinel-1-based C-VOD and the SWOT-based Ka-VOD, demonstrating that these wavelengths-particularly Ka-band-are underutilized for studying vegetation dynamics. We also find differing temporal patterns across the C-and Ka-VOD products, suggesting that utilizing a range of frequencies can capture vegetation responses at different layers in the forest canopy. We argue that deriving VOD from multiple frequencies can enhance the understanding of minute changes in vegetation dynamics, especially in forests.