Journal
BIOGEOSCIENCES
Volume 14, Issue 12, Pages 3015-3026Publisher
COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/bg-14-3015-2017
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Funding
- AmeriFlux (US Department of Energy, Biological and Environmental Research, Terrestrial Carbon Program) [DE-FG02-04ER63917, DE-FG02-04ER63911]
- AfriFlux
- AsiaFlux
- CarboAfrica
- CarboEuropeIP
- CarboItaly
- CarboMont
- ChinaFlux
- Fluxnet-Canada
- CFCAS
- NSERC
- BIOCAP
- Environment Canada
- NRCan
- GreenGrass
- KoFlux
- LBA
- NECC
- OzFlux
- TCOS-Siberia
- USCCC
- FAO-GTOS-TCO
- iLEAPS
- Max Planck Institute for Biogeochemistry
- National Science Foundation
- University of Tuscia
- Universite Laval and Environment Canada
- US Department of Energy
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Water-use efficiency (WUE) is a fundamental property for the coupling of carbon and water cycles in plants and ecosystems. Existing model formulations predicting this variable differ in the type of response of WUE to the atmospheric vapor pressure deficit of water (VPD). We tested a representative WUE model on the ecosystem scale at 110 eddy covariance sites of the FLUXNET initiative by predicting evapotranspiration (ET) based on gross primary productivity (GPP) and VPD. We found that introducing an intercept term in the formulation increases model performance considerably, indicating that an additional factor needs to be considered. We demonstrate that this intercept term varies seasonally and we subsequently associate it with radiation. Replacing the constant intercept term with a linear function of global radiation was found to further improve model predictions of ET. Our new semiempirical ecosystem WUE formulation indicates that, averaged over all sites, this radiation term accounts for up to half (39-47 %) of transpiration. These empirical findings challenge the current understanding of water-use efficiency on the ecosystem scale.
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