Ecosystem transpiration and evaporation: insights from three water flux partitioning methods across FLUXNET sites

We apply and compare three widely applicable methods for estimating ecosystem tran spiration (T) from eddy covariance (EC) data across 251 FLUXNET sites globally. All three methods are based on the coupled water and carbon relationship, but they differ in assumptions and parameterization. Intercomparison of the three daily T estimates shows high correlation among methods (R² between 0.80 and 0.87), but a spread in magnitudes of T/ET (evapotranspiration) from 45% to 77%. When compared at six sites with concurrent EC and sap flow measurements, all three EC based T estimates show higher correlation to sap flow based T than EC based ET. The partitioning methods recover expected tendencies of T/ET increasing with dryness (vapor pressure deficit and days since rain) and with leaf area index. Analysis of 140 complete site-years shows that T/ET variability was 1.6 times higher across sites than across years. Spatial variability of T/ET was primarily driven by vegetation and soil characteristics (e.g. crop or grass designa tion, minimum annual leaf area index, soil coarse fragment volume) rather than climatic variables. Overall, T and T/ET patterns are plausible and qualitatively consistent among the different water flux partitioning methods implying a significant advance made for estimating and understanding transpiration globally, while the magnitudes remain uncertain. Our results represent the first extensive estimates of ecosystem T from EC data permitting a data driven perspective on the role of plants' water use for global water and carbon cycling in a changing climate.