# Remote Sensing of Ecosystem Water Use Efficiency in Different Ecozones of the North China Plain
## 4 Discussion  
### WUE Spatial Variation and Driving Factors  
The ecological restoration projects implemented in North China have an impact on the ecosystem carbon and water cycle [40]. The annual variation of WUE in the Beijing-Tianjin sand source region ranges from 0.03 to 4.10 g C m\({}^{-2}\) y\({}^{-1}\)[26], similar to the results of this paper. In 2001-2017, the area showing an increasing trend in WUE accounted for 58.65% of the area of the North China Plain, and the findings were consistent with those of [PERSON], et al. [21]. These findings may be related to ecological restoration (e.g., reforestation and desertification management) causing land use/land cover changes, leading to an increase in vegetation greenness and thus affecting changes in carbon and water cycles [41]. Because most species and environmental conditions are similar within an ecological zone, there is little variation in the spatiotemporal distribution of WUE for an ecological functional area. The WUE of different ecological functional areas is closely related to the distribution of vegetation in the North China Plain; for example, the highest mean value of WUE in the QLMDF ecosystem functional area may be related to the high distribution of forest in this ecosystem functional area. The trend of WUE changes in the North China Plain was mainly influenced by the trend of GPP changes, which accounted for approximately 44.38% of the total study area. This may be related to the related engineering of vegetation restoration. Compared with bare land, the increase of vegetation cover increases both GPP and ET. However, the greater increase in GPP than ET is the main reason for the increase in WUE [8]. Figure 8: Spatial correlation of WUE with precipitation and temperature. Different driving factors have different effects on the spatiotemporal variation of WUE in different study areas [31]. There are many factors that affect the variation of WUE, such as vegetation type and climatic factors. Land cover change has large effects on physico-biogeochemical processes that can alter WUE [42; 43], indicating that different land cover WUE has some differences [21; 44]. Vegetation types and environmental conditions are very similar in the same ecosystem functional area, but vegetation types and environmental conditions are very different in different ecosystem functional areas, so there are great differences in WUE in different ecosystem functional areas, i.e., ecosystem functional areas with high cover vegetation types have higher WUE values. The influence of climatic factors on the WUE of terrestrial ecosystems is extremely important, and temperature and precipitation determine the basic structure and distribution of climate and vegetation. From the analysis of the driving factors of the spatiotemporal variation of WUE, the response of precipitation to WUE in the North China Plain is greater than that of temperature, which is consistent with the research results of [PERSON] et al. [16]. In addition, WUE is positively correlated with precipitation in most areas of the Beijing-Tianjin sand source [26], and in the Loess Plateau, WUE and precipitation are also positively correlated [45; 46]. However, in some grasslands, excessive precipitation also has a negative spatial effect on WUE. In addition, WUE at flow stations in China all show a positive sensitivity to temperature [47]. In addition, in the Loess Plateau and the Beijing-Tianjin sand source region, the area where WUE is positively correlated with temperature accounts for most of the area [26; 45]. It shows that WUE in the ecosystem of northern China may decrease with warming and increase with the increase of precipitation, but the degree of change may vary according to vegetation type and water conditions [26]. Furthermore, humidity rather than temperature is the main control factor of regional WUE, which is also consistent with the above view [48].