A Direct Measure of Basin-Wide Evaporation and Transpiration from the Amazon Rainforest

A water budget approach shows complex seasonal cycle and long-term changes in tropical forest function.

The Science

The Next-Generation Ecosystem Experiments (NGEE)–Tropics research team combined satellite measurements of rainfall and gravity anomalies with Amazon river flow data to derive a seasonally resolved estimate of evapotranspiration for the entire Amazon basin. The team then analyzed the seasonal cycles and long-term variation of this measurement and compared it to process-based land surface model predictions.

The Impact

The study’s results show a more complex and different seasonal cycle than current land surface models predict. The study suggests a long-term decline in evapotranspiration from the forest, due to ecosystem functional change at the scale of the entire basin.

Summary

Evapotranspiration, which comprises the sum of all moisture fluxes from an ecosystem directly to the atmosphere, is a crucial quantity at the center of the terrestrial energy, water, and carbon cycles. Because measurements of evapotranspiration are typically made at local scales, and are sparse over remote locations such as the Amazon, the larger-scale fluxes are not well known. This study combined observations of rainfall, river discharge, and time-varying gravity anomalies to construct a water budget for the Amazon basin, which allows NGEE-Tropics researchers to solve for evapotranspiration as the missing term in the budget. This water budget–based measurement shows a complex seasonal cycle, with a deeper minimum during the wet season than is estimated by other upscaling estimates or by process-based models, and also shows that models tend to increase their seasonal evapotranspiration fluxes later in the dry season than is observed. Furthermore, a long-term analysis of evapotranspiration suggests a decline in the rate over the period of observation, which could be evidence of a large-scale change in ecosystem function.

Principal Investigator

Charles Koven
Lawrence Berkeley National Laboratory
cdkoven@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

ALSS was supported by National Science Foundation grants AGS-1321745 and AGS-1553715. CDK received support from the Regional and Global Climate Modeling program through the BGC-Feedbacks SFA and the Terrestrial Ecosystem Sciences and Earth System Modeling programs through the Next-Generation Ecosystem Experiments (NGEE)–Tropics project of the Office of Biological and Environmental Research (BER) in the U.S. Department of Energy Office of Science.

References

Swann, A. L. S., and Koven, C. D. "A direct estimate of the seasonal cycle of evapotranspiration over the Amazon". Journal of Hydrometeorology 18 (8), 2173–2185  (2017). https://doi.org/10.1175/JHM-D-17-0004.1.