An Approach for Monitoring Global Vegetation Based on Aquarius L-band Scatterometer and Radiometer Observations

Vegetation monitoring is important for the study of the global carbon cycle and ecosystem. Aquarius/SAC-D mission that launched in 2011 is the first satellite with both active and passive L-band microwave instrumentation. The Aquarius instrument consists of three dual polarized L-band (1.413 GHz) radiometers each with its own feedhorn and a fully polarimetric L-band (1.26 GHz) scatterometer that makes use of the radiometer feedhorns. In this study, the impact of soil moisture and vegetation on L-band backscatter and emission is studied using Aquarius scatterometer/radiometer measurements. Considering that the passive microwave data are more sensitive to changes in surface water and active microwave data are more sensitive to vegetation cover and roughness, we attempt to use radar vegetation index (RVI) for monitoring global vegetation. The relationship between RVI and VWC is explored at regional and global and continental scales based on Aquarius observations. The VWC data are derived using satellite estimates from MODIS/NDVI. It is found a good statistical relationship between RVI and VWC, especially for agricultural areas. Although statistical relationship and seasonal variability is captured, the dynamic range of the RVI is insufficient for a meaningful signal-to-noise. Further vegetation optical depth (VOD) is estimated using the τ−ω model by reconstructing the microwave vegetation indices (MVIs) and Microwave Polarization Difference Index (MPDI) derived from Aquarius radiometer data. The results indicate that the VOD can generally reveal vegetation seasonal changes and can provide unique information for vegetation monitoring. The uniqueness of this approach is that the RVI and VOD are retrieved directly from the backscatter and brightness temperature without any field correction or auxiliary vegetation data. This method can provide a new opportunity for long-term global vegetation monitoring for Aquarius or SMAP mission.