Table 1 Various definitions and methodologies of WUE estimation and responses of WUE to climatic variation

Leaf level

\( \mathrm{WUE}=\frac{\mathrm{A}}{\mathrm{T}}=\frac{{\mathrm{C}}_{\mathrm{a}}-{\mathrm{C}}_{\mathrm{i}}}{1.6\Delta \mathrm{w}} \)

C_a and C_i, ambient and inner-leaf partial pressure of CO2, respectively; ∆w, the difference of water vapor

Gas exchange

Reflects that plants control stomata to optimally satisfy the trade-off between the amount of carbon assimilated and the amount of water transpired; easy to measure; adaptable to small-scale and several species; not appropriate for all biomes

Leaf WUE decreases with precipitation increase; effect of climate warming is uncertain for leaf WUE.

Beer et al. (2009); Niu et al. (2011)

\( {\mathrm{WUE}}_i=\frac{\mathrm{A}}{\mathrm{g}}=\frac{{\mathrm{C}}_{\mathrm{a}}-{\mathrm{C}}_{\mathrm{i}}}{\mathrm{Pa}} \)

Pa, atmospheric pressure

Gas exchange; stable isotope discrimination

More appropriate than WUE for describing the biochemical functions of vascular plants; easy to measure; not applicable for all biomes

WUEi increases during dry seasons in plants with smaller and narrower leaves.

Picotte et al. (2007); Beer et al. (2009)

Ecosystem level

\( {\mathrm{WUE}}_{\mathrm{et}}=\frac{\mathrm{GPP}}{\mathrm{ET}} \)

\( \mathrm{or}=\frac{\mathrm{GEP}}{\mathrm{ET}}, \)

\( =\frac{\mathrm{NPP}}{\mathrm{ET}}, \)

\( =\frac{\mathrm{NEE}}{\mathrm{ET}} \)

Models with RS products; EC flux data

Could be estimated by models with RS data at large scale; but the inherent carbon–water process of plants were not clear

Precipitation increases WUE, while climate warming reduces WUE; droughts increase WUE in Northeast China and central Inner Mongolia but decrease WUE in central China; WUE response to drought in arid ecosystems (WUE increases with drought) contrasts that in semi-arid/sub-humid ecosystems (WUE decreases with drought).

Niu et al. (2011); Liu et al. (2015); Yang et al. (2016)

\( {\mathrm{WUE}}_{\mathrm{t}}=\frac{\mathrm{GPP}}{\mathrm{T}} \)

Models with RS products

Other ecosystem WUE could be calculated by WUEt, but the T is not easy to measure or model at large scale

Precipitation increases WUEt, while climate warming reduces WUEt; WUEt shows lower sensitivity to precipitation than WUE.

Niu et al. (2011); Sun et al. (2015)

\( {\mathrm{IWUE}}_{\mathrm{et}}=\frac{\mathrm{GPP}\times \mathrm{VPD}}{\mathrm{ET}} \)

EC flux data; models with RS products

Indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment; easy to estimate at large scale;

IWUEet increases during short-term moderate droughts.

Beer et al. (2009); Sun et al. (2015)

\( {\mathrm{IWUE}}_{\mathrm{t}}=\frac{\mathrm{GPP}\times \mathrm{VPD}}{\mathrm{T}} \)

EC flux data; models with RS products

Indicates the biophysical process at large scale; but T is not easy to measure or model

IWUEt is conservative with precipitation.

Beer et al. (2009); Sun et al. (2015)