Amazonian Terrestrial Water Balance Inferred from Satellite-Observed Water Vapor Isotopes

The Science

Atmospheric humidity and soil moisture in the Amazon forest are tightly coupled to the region’s water balance, or the difference between two moisture fluxes, evapotranspiration minus precipitation (ET-P). Changes in the drivers of evapotranspiration (ET), such as aboveground biomass, could have a larger impact on soil moisture and humidity in the dry Amazon relative to the wet Amazon. The Atmospheric Infrared Sounder (AIRS) observations are sensitive to spatiotemporal variations of ET-P, enabling investigation of the spatial, seasonal, and interannual variability of ET-P over the Amazon.

The Impact

This research reaffirms an increasing contribution of ET to atmospheric moisture for forest regions farther from the Atlantic, with the largest contributions happening during the dry season of the Amazon. The deuterium-based estimates of ET-P have the potential to further investigate the hydrological dynamics that control changes in the carbon and water exchanges within the Amazon.

Summary

Atmospheric humidity and soil moisture in the Amazon are closely linked to the region’s water balance, defined as evapotranspiration minus precipitation (ET-P). However, significant uncertainties in both fluxes complicate the assessment of water balance variations and their dependence on ET or P. By using satellite observations of deuterium content in water vapor, this research finds that the HDO (semi-heavy water)/H₂O ratio is sensitive to changes in ET-P across the Amazon. When calibrated with basin-scale estimates from terrestrial water storage and river discharge, the water vapor deuterium data reveal that rainfall primarily drives water balance variability in the wet Amazon, while ET plays a more crucial role in the dry Amazon. Consequently, changes in factors influencing ET, such as aboveground biomass, could significantly affect soil moisture and humidity in the southern and eastern regions of the Amazon compared to the wet areas.

Principal Investigator

Mingjie Shi
Pacific Northwest National Laboratory
[email protected]

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This research was partly conducted at Pacific Northwest National Laboratory, which is operated for the U.S. Department of Energy by Battelle Memorial Institute under contract DE-AC05-76RL01830. This study was partly supported by the Department of Energy’s Biological and Environmental Research program as part of the Environmental System Science program through the Next-Generation Ecosystem Experiments Tropics project.

References