The importance of radiation for semi-empirical water-use efficiency models
Sven Boese, Martin Jung, Nuno Carvalhais, and Markus Reichstein
Max Planck Institute for Biogeochemistry, Dept. for Biogeochemical Integration, Hans-Knoell-Strasse 10, 07745 Jena
Received: 02 Dec 2016 – Accepted for review: 26 Dec 2016 – Discussion started: 05 Jan 2017
Abstract. 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 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 semi-empirical ecosystem WUE formulation indicates that, averaged over all sites, this radiation term accounts for up to half (40–49 %) of transpiration. These empirical findings challenge the current understanding of water-use efficiency on ecosystem-scale.
Boese, S., Jung, M., Carvalhais, N., and Reichstein, M.: The importance of radiation for semi-empirical water-use efficiency models, Biogeosciences Discuss., doi:10.5194/bg-2016-524, in review, 2017.