Concept,Modeling,and Application Of The Underlying Water Use Efficiency For Terrestrial Ecosystems

Water use efficiency?WUE?,defined as the ratio of gross primary productivity?GPP?and evapotranspiration?ET?at the ecosystem scale,is a critical variable linking the carbon and water cycles.By incoporating the nonlinear effect of vapor pressure deficit?VPD?on WUE,an underlying WUE?uWUE?was proposed and validated at the ecosystem scale.The concept of uWUE was used to partition ecoystem ET into plant transpiration?T?and soil/canopy evaporation?E?.The new method for ET partitioning was applied to three typical ecosystems in the Heihe River Basin.Responses of uWUE to global environmental changes during the 20^th century were investigated based on outputs from four terrestrial ecosystem models.The uWUE(=GPP·VPD^0.5/ET)model was theoretically derived by combining the carbon and water exchanges through plant stomata with the optimal stomatal conductance model.uWUE was found to be better than IWUE?=GPP·VPD/ET?and WUE?=GPP/ET?.A hysteresis model aiming to minimize time lags among GPP,ET,and VPD further supported the strong relationship between GPP·VPD^0.5 and ET.Daily mean VPD was shown to be a good substitute for the effective VPD that was required to preserve uWUE from half-hourly to daily time scales.Correlation coefficient between GPP·VPD^0.5 and ET?r=0.85?was higher than that between GPP·VPD^0.5 and ET?r=0.81?at the daily scale.To predict daily GPP from daily ET and mean daily VPD,uWUE?r=0.81?worked considerabilty better than IWUE?r=0.59?.A potential uWUE(uWUEp=GPP·VPD^0.5/T)and an apparent uWUE(uWUEa=GPP·VPD^0.5/ET)were proposed.The ratio of uWUEa and uWUEp was used to estimate ecosystem T/ET.uWUEp was demonstrated to be relatively constant within a uniform ecosystem,and was broadly consistent with the uWUE at the leaf scale.T/ET was strongly correlated with vegetation cover for deciduous vegetation.The uWUE method provides a novel and simple way for ET partitioning,and would enhance the role of global flux tower sites in estimating ecosystem T/ET variations.The uWUE method was applied to three typical ecosystems in the Heihe River Basin,and was compared with three traditional ET partitioning methods.Daily T/ET derived using the uWUE method agreed with that using the isotope and lysimeter/eddy covariance methods during the peak growth season.The estimated T using the uWUE method showed consistent seasonal and diurnal patterns and magnitudes with that using the sap flow method.In addition,the uWUE method is scale-independent,and can effectively capture T/ET variations in relation to vegetation changes and irrigation events.Finally,a new variance decomposition method was proposed to attribute the variation in uWUEa to the trend and inter-annual variation of environmental drivers,using outputs of four terrestrial ecosystem models during the 1901-2010period.The trend in uWUEa was dominated by atmospheric CO₂ increase,while the inter-annual variation of uWUEa was mostly driven by inter-annual climate variability.