**4. Discussion**

#### *4.1. Effect of Energy and Water on ETa in Different Climatic Regions of China*

The Budyko curve shows the relationship between ETa-Pre-ET0 and can reveal the limiting relationship between energy and water on ETa [79–81]. Figure 14 shows the Long-term mean values of annual ETa, Pre, and ET0 together with Fu's curves with the regional average values of parameter ω, where ω is the plant-available water coefficient, representing the relative difference in the way plants use soil water for transpiration, and larger values of ω tend to promote evapotranspiration. It can be seen that the relationship in most areas conforms to the Budyko curve, and the data are within the boundary conditions of the hydrothermal coupling assumption, with ω in the humid region greater than in the arid region. In humid regions, ETa is limited by available energy. As it asymptotically approaches ET0, in arid regions, ET0 exceeds P, and where ETa is mainly controlled by water, evapotranspiration ratio (ETa/Pre) tends to 1.

**Figure 14.** Relationship among ETa-Pre-ET0 in different climatic regions of China. The ratio of mean annual ETa to Pre as a function of the index of dryness (ET0/Pre) for different values of plant-available water coefficient ω.

Evapotranspiration ratio at some stations in the arid and semi-arid zones (in the lower right dashed box) is less than 1; as such, this may be attributed to the way different kinds of vegetation use soil water and the particularity of precipitation conversion to evaporation in arid areas. In the arid region, small precipitation events are just able to wet the soil surface and are quickly evaporated, whereas large precipitation events increase potential water losses from the ecosystem through runoff or deep soil water percolation [13]. Therefore, evaporation can be lower than precipitation in arid regions, this is consistent with the study of Yang et al. [82] in northern China. In addition, evapotranspiration ratio in arid regions also shows a large fluctuation, which may be related to the large variation in precipitation in arid regions [83,84]. The amount of precipitation in arid and semi-arid regions of China has a large variation [85]; the sparser the precipitation in a region, the greater the variation, and interannual fluctuations in precipitation and regional differences cause large fluctuations in evapotranspiration.

### *4.2. Complexity of PUE Change in Transition Zone*

Figure 15 shows a schematic diagram of the controlling factors of PUE in different climatic regions in China, which shows the opposite change of PUE in arid and humid regions and the complexity of PUE change in transition zones.

**Figure 15.** Schematic diagram that illustrates the control factors of the PUE in different climatic regions of China.

Warming and humidification in Northwest China [86] have a grea<sup>t</sup> impact on the increased ETa, NPP, and PUE in arid areas. Warming and humidification increase the amount of water that can evaporate, thereby increasing ETa. Temperature increase results in earlier onset of the greening period while delaying the yellowing period, enhancing vegetation activities, then increasing NPP. Multiple research based on empirical observations and process-based models have also confirmed that aboveground production in arid ecosystems has exhibited an increasing trend [87]. Decreased ETa in the humid region is related to the decrease in ET0, which is consistent with the evaporation paradox [88]. In the southern humid region with sufficient water supply, changes in ETa and ET0 are consistent [89]. Thus, when the ET0 decreases, ETa also decreases. Rainfall is abundant in humid areas. Vegetation productivity is positively correlated with air temperature and negatively correlated with precipitation. In recent decades, the increase in precipitation in humid regions has reduced photosynthetically active radiation. Enhanced radiation restrictions reduce vegetation NPP.

Factors influencing ETa, NPP, and PUE in the transition zone are more complex because this region is a competitive zone for water and energy [90]. In the highland region in the western part of the transition zone, NPP increases along with warming and humidification, and a significant decrease in precipitation in the eastern region of the transition zone and the warming and drying caused by temperature increases are the main reasons for the NPP decrease [91]. At the same time, the region is affected by the interannual fluctuation of the intensity of the East Asian summer monsoon, so the interannual and interdecadal precipitation fluctuations in the region are large [92].

### *4.3. Transformation Characteristics of PUE*

The transition zone exhibited the highest PUE. PUE reached its highest value of 2.2 g· <sup>m</sup>*<sup>−</sup>*2·mm*<sup>−</sup>*<sup>1</sup> in the area where the annual precipitation was 414 mm. The unimodal PUE distribution, which first increases and then decreases with increasing precipitation, has been confirmed by other studies. Paruelo et al. [8] indicated that in American temperate grasslands with 200–1200 mm precipitation, PUE first increased and then decreased with increasing precipitation, peaking at 475 mm. PUE in extremely arid and extremely humid regions is low. Hu et al. [5] reported that the PUE initially exhibited a rising trend but subsequently decreased as precipitation increased from 200 to 1200 mm. PUE peaked at 400–500 mm. Huxman et al. [6], Lauenroth and Paruelo [7], and Yu et al. [11] also showed that in regions with annual precipitation less than 600 mm, PUE increased with increasing precipitation. In humid regions with annual precipitation above 650 mm, PUE decreased with increasing precipitation, and when the annual precipitation was above 1500 mm, PUE was approximately constant. Zhang et al. [9] reported that the spatiotemporal PUE pattern in alpine grasslands in northern Tibet initially increased in the arid region and subsequently decreased along the precipitation gradient toward the humid region, reaching a peak at approximately 500 mm precipitation.

The PUE distribution pattern with precipitation in this study is consistent with the above studies, and different precipitation thresholds of the maximum PUE conversion point may be partially attributed to the different research methods and study regions. In addition, the distribution patterns of annual PUE with ΔT, ΔSM, and ΔU changes in different precipitation climate types also indicate that the PUE transition interval coincides with the northern edge of the monsoon. Therefore, the results of this paper are reasonable.

Our research shows that transition regions with limited rainfall have the strongest NPP and PUE processes, and the response of the carbon fluxes in these fragile regions deserves more attention.

### **5. Conclusions**

Based on the improvement of the ETa model, this study characterized the responses of ETa, NPP, and PUE to climate change in different climatic regions of China, revealed the PUE conversion characteristics with the precipitation distribution, and clarified the driving force of PUE changes in different climate regions. The main conclusions are as follows:

The improved ETa model fully reflects the energy limitation on ETa in humid regions, and the estimated ETa is more reasonable and reliable. The distribution of ETa and NPP in China shows a gradually increasing trend from northwest to southeast, and the trends of ETa and NPP both change from an increase to a decrease from the arid to the humid region. ETa and NPP fluctuations in arid regions are mainly controlled by water, and the increase in precipitation and soil moisture is the main reason. ETa and NPP in humid regions are mainly controlled by energy. ETa in the transition zone is affected by both water and energy, and regional differences in ETa and NPP changes in the transition zone are large.

There was a conversion zone of PUE in mainland China. Arid and humid regions had the lowest PUE, and the transition zone with annual precipitation of 200–600 mm had the highest PUE. In the past 58 years, PUE in arid regions has exhibited an increasing trend, whereas PUE in the transition zone generally exhibited a slightly decreasing trend. PUE displayed a decreasing trend in most of the humid regions.

PUE changes in arid regions are dominated by water conditions, whereas changes in energy in humid regions largely determine PUE changes. The transition zone is the conversion zone where the prevailing factor transitions from water to energy. PUE changes are caused by the interaction of energy, water, and dynamic factors. Among them, soil moisture plays the most prominent role, followed by temperature and wind velocity.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/rs14102467/s1, Table S1: Regression equation between environmental factors and PUE in each sub regions (annual averaged air temperature (Tmean), net radiation (Rn), wind velocity (U), relative humidity (RH), and soil moisture (SM), respectively).

**Author Contributions:** Conceptualization, P.Y. and Q.Z.; methodology, P.Y., S.W., J.Y. and H.Z.; software, S.W., W.W. and J.W.; formal analysis, S.W. and P.Y.; writing—original draft preparation, S.W.; writing—review and editing, S.W., P.Y. and Q.Z.; visualization, S.W., X.R. and J.W.; funding acquisition, P.Y. and S.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No. U2142208, 41975016), the Basic Research Innovation Group Project of Gansu Province (Grant No. 20JR5RA121), the Natural Science Foundation of Gansu Province (Grant No. 21JR7RA698), and the Foundation of drought Meteorological Science Research (Grant No. IAM202001).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The GLDAS\_Noah025\_M.2.0 and 2.1 datasets can be accessed online (https://disc.gsfc.nasa.gov/datasets?page=1&project=GLDAS (accessed on 25 February 2021)); The MOD17A3 surface vegetation NPP data provided by the EOS/MODIS (TERRA satellite) of NASA can be accessed at https://earthdata.nasa.gov/ (accessed on 2 November 2020); and China FLUX ETa data are available at http://rs.cern.ac.cn/data/initDRsearch?classcode=SYC\_A02 (accessed on 19 December 2019).

**Conflicts of Interest:** The authors declare no conflict of interest.
