AMSR-E satellite observed rainforest responses during the 2005 and 2010 Amazon droughts.

The Amazon rainforest plays an important role in the global hydrological cycle and effects the climate at both regional and global scales. Continuous large-scale deforestation and associated fire activities have been an issue of international significance over the last two decades, while in more recent times two historic droughts impacted vast areas of the Amazon in 2005 and 2010. These interrelated issues have considerable ecological response, and underline the need for a consistent observation system to comprehensively characterize the rainforest dynamics. Satellite-based remote sensing offer the only viable approach at monitoring the vegetation state at large space and time scales.
Most studies examining these issues have used the Enhanced Vegetation Index (EVI), a vegetation greenness index with characteristics similar to NDVI derived from remote optical sensors. Like any optical observation, EVI is affected by water vapor, clouds and aerosols in the atmosphere, and there has been some debate about their influence on inferred vegetation changes during the recent droughts over the Amazon. Unlike optical remote sensing, passive microwave observations are not affected by such atmospheric phenomena and can measure both leaf and woody components of aboveground living biomass through a retrieved property termed the vegetation optical depth (VOD), by virtue of the sensitivity of passive microwave emissions to water in the environment. The microwave-based vegetation record therefore provides a unique opportunity to examine forest response during the 2005 and 2010 drought that is complementary to the vegetation index analysis.
In this study, we apply the VOD data retrieved from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), rainfall data from Tropical Rainfall Measuring Mission (TRMM), water level data obtained from in situ measurements and fire observations based on the MODIS instrument to investigate large scale forest dynamics. Results indicate that the VOD clearly delineates regions suffering from severe droughts in 2005 and 2010 and in addition can capture the less intense drought affecting the southeast Amazon in 2007.