A Simple and Objective Method to Partition Evapotranspiration into Transpiration and Evaporation at Eddy-Covariance Sites (Invited Presentation)

Separating evapotranspiration (ET) into evaporation (E) and transpiration (T) is challenging but key for a better understanding and prediction of the hydrological cycle and plant water use. In this study, flux data at routine eddy-covariance sites were used to develop a new and simple method for ET partitioning based on the separation of soil and canopy conductances, with the main assumption that the latter is proportional to gross primary productivity (GPP). The result of T:ET across different plant functional types (PFTs) was consistent with recent modeling or empirical results. The mean annual T:ET was highest for evergreen needleleaf forests (0.75 ± 0.17), followed by croplands (0.62 ± 0.16) and grasslands (0.56 ± 0.15). The leaf area index (LAI) was shown to explain only small (20%) variations of mean annual T:ET across sites. However, at each site, the correlation of T:ET with LAI was strong at the seasonal scale, during which T:ET increased nonlinearly with LAI. The results didn’t show significant relationship of T:ET versus precipitation across sites. However, the partitioned soil evaporation after each precipitation pulse is consistent with three-stage soil evaporation theory. This ET partitioning method is an objective assessment as it is totally data-driven. It is also simple so it can be readily applied to global flux tower networks at different temporal and spatial scales, enabling continuous estimation of T:ET to monitor ecosystem dynamics and hydrological responses to environmental change.