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

Abstract:

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·VPD^0.5/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·VPD^0.5 and ET relationship (r = 0.844) is better than that between GPP·VPD and ET (r = 0.802). The hysteresis model supports the GPP·VPD^0.5 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.