A revised hydrological model for the Central Amazon: The importance of emergent canopy trees in the forest water budget

The Amazon forest is integral to the global climate system in part because of the high rate of rainfall recycling through tree transpiration and biodiversity (size and species composition). However, the partitioning of precipitation into evaporation, transpiration and runoff, has been quantified at only a few sites. At our study site in the central Amazon, annual rainfall in 2013 was 2302 mm and latent heat flux measurements made using eddy covariance revealed that 1360 mm (59%) was returned to the atmosphere through evaporation and transpiration. Runoff accounted for 41% of the net ecosystem water loss. Combining annual xylem sap flux estimates with total stand sap wood area, we estimated annual stand transpiration rate to be 851 mm (36% of annual rainfall). Emergent canopy trees (diameter >30 cm; average height of 28 m) were responsible for the majority (71%) of the transpired water flux, recycling potentially 26% of the rainfall back to the atmosphere. By difference, we estimate that 510 mm of intercepted rainwater (22% of rainfall) was evaporated directly back to atmosphere from the canopy. Highest stand transpiration rates occurred during the dryer months due to both increased water vapor pressure deficit and the onset of new leaf flush. This study provides further evidence for convergent water use characteristics of tropical trees and highlights the importance of large trees in tropical moist forests. Large trees have been demonstrated to be vulnerable to drought-related mortality, and thus potentially will make up a critical component of the response of tropical forests to climate change.