The Effect of Vapor Pressure Deficit on Water Use Efficiency at the Sub-Daily Time Scale

Friday, 19 December 2014
Sha Zhou1, Bofu Yu2, Yuefei Huang1 and Guangqian Wang1, (1)Tsinghua University, Beijing, China, (2)Griffith University, Nathan, Australia
Water use efficiency is a critical index for describing carbon-water coupling in terrestrial ecosystems. The nonlinear effect of vapor pressure deficit (VPD) on carbon-water coupling has not been fully considered. To improve the relationship between gross primary production (GPP) and evapotranspiration (ET) at the sub-daily time scale, we propose a new underlying water use efficiency (uWUE = GPP·VPD0.5/ET) and a hysteresis model to minimize time lags among GPP, ET, and VPD. Half-hourly data were used to validate uWUE for 7 vegetation types from 42 AmeriFlux sites. Correlation analysis shows that the GPP·VPD0.5 and ET relationship (r = 0.844) is better than that between GPP·VPD and ET (r = 0.802). The hysteresis model supports the GPP·VPD0.5 and ET relationship. As uWUE is related to CO2 concentrations, its use can improve estimates of GPP and ET, and the effect of CO2 fertilization on carbon reservation and water loss.