## 4 Discussion
Another disadvantage of the water stress scalar defined by VPD is that photosynthesis is considered to be completely stopped during periods of high VPD [63]. However, soil moisture and other environmental conditions might still be favorable for maintaining photosynthetic activity at a certain level even if the atmosphere is very dry; that is, when the VPD is high, the limiting effect of water stress on photosynthesis may not reach the expected degree [61]. This is the main reason that some simulated values of the MOD17 and MODTEM models were smaller than the measured values when the stress was severe (GPP value is low) on sunny days, and it is the reason for scattered points in the figure Figures 6a,c and 8a,b). In contrast, the VPD was low on cloudy days, and the above phenomena were not obvious. [PERSON] et al. [24] pointed out that photosynthesis is co-regulated by the soil moisture and atmospheric evaporation demand. [PERSON] et al. [64] reported that VPD may decouple soil water dynamics owing to its oversensitivity to temperature, especially in summer monsoon regions. [PERSON] et al. [7] pointed out that plant moisture indicators are preferred in the LUE models. [PERSON] et al. [11] found that plant moisture indicators were more sensitive to changes in LUE than soil and atmospheric drought indicators. The principle of EF, which indicates water status, is that the decreasing amounts of energy partitioned to evaporate water suggest a stronger moisture limitation. However, our study found that the LUE model constructed based on EF had a poor performance on sunny days but a good performance on cloudy days (Figure 6e,g,f,h and Figure 8c,d). This may be related to the inaccurate measurements of ecosystem energy. The degree of energy closure of the ecosystem was not fixed. For example, [PERSON] et al. [65] found that the degree of energy closure of apple orchards varies between 0.42 and 0.75 in a year. This means that there are biases between the turbulent heat flux and the available energy of the ecosystem. In this study, the energy closure was 0.69 on sunny days, which was similar to the results of [PERSON] et al. [66]. In contrast, on cloudy days, the energy closure was 0.82. In addition, the R\({}^{2}\) between the available energy and turbulent heat flux on sunny days was lower than that on cloudy days (Figure A4). This means that the energy error was large on sunny days, resulting in poor performance of EF in indicating water status and low accuracy of the EC-LUE and ECM-LUE models. Moreover, as the available energy increases, the difference between turbulent heat flux and the available energy increases, which increases the EF. This may be the main reason for the higher simulation values of the EC-LUE and ECM-LUE models when environmental stress on sunny days was severe. [PERSON] et al. [67] also found that the EC-LUE model overestimated the GPP of moso bamboo forests under drought conditions and that EF could not characterize the effect of drought on GPP well. Because we could not obtain ground-observed short-wave infrared (SWIR) data, we did not verify the performance of the LUE model constructed based on LSWI. However, previous studies have shown that LSWI is sensitive to leaf water content rather than canopy water content [68], and the regional GPP is limited more by the water condition at the canopy level [69]. Moreover, the LSWI cannot reflect atmospheric water stress because of the weak effects of atmospheric water vapor on the near infrared (NIR) and SWIR bands [70]. Stomatal conductance has long been considered the most sensitive response of plants to water deficit [71], as a result of co-regulation by soil and atmospheric drought, and it controls the exchange of water and air between plants and the external environment. However, it is difficult to directly observe plant canopy conductance on a regional scale. An important consequence of stomatal closure under water stress is a decrease in heat dissipation, which leads to an increase in canopy temperature [72].