# Quantifying Soil Moisture Impacts on Water Use Efficiency in Terrestrial Ecosystems of China
## 4 Discussion
### Plausible Changing Trend and Adjustment of _Wile_  
Under the background of climate change, especially with the increase in temperature and drought, the trend of change in _WIE_ is uncertain. Studies have shown that a temperature rise would increase the _NPP_ of global terrestrial ecosystems [66; 67] However, the continuous temperature rise may eventually lead to a significant decrease in _NPP_[51], mainly owing to the limitation of the optimum temperature for plant growth [68; 69]. Thus, _WIE_ may show two completely different changes in future scenarios of a continuous temperature rise. First, the temperature rise in the early stage could result in a general increase in _NPP_. Meanwhile, the temperature rise in arid areas (water-limited areas) might not contribute much to _ET_, but may cause a linear increase in humid areas [65; 66]. Therefore, under the early warming scenario, _WIE_ in the arid region of northwestern China could tend to increase significantly. In the humid area, _WIE_ may only slightly change. In contrast, in the late warming period, _WIE_ in the arid northwestern region may become smaller, owing to the decrease in _NPP_. Correspondingly, the _WIE_ in the humid eastern area may decline because of the possible continued increase in _ET_. In addition, future climate changes may lead to an increase in the frequency, intensity, and duration of drought events [70; 71]. Therefore, drought will cause a decrease in _SM_, which will affect the trend of change in _WIE_.  
Although studies have shown that an ecosystem can respond to increased drought by increasing _WIE_[7; 8], this issue may not be straightforward. Because the _WUE_ of different vegetation types has various responses to drought, the _WIE_ may be positively or negatively correlated with drought and show a quadratic function relationships [59]. In the arid region, the decrease in _SM_ caused by drought could lead to a much greater decline in _ET_ compared with the change in _NPP_ (increasing or decreasing trend) (Figure 4). Finally, a drought may lead to an increase in _WIE_ in most areas of the arid region. In humid areas, the energy conditions mainly determine the evaporation process. Therefore, the decline in _SM_ in the early drought stage might not cause a significant change in _ET_. At this time, the trend of change in _WIE_ might mainly depend on the changing in _NPP_, which could show an upward or downward trend. With the aggravation of drought, _SM_ could decrease significantly, which could cause a decline in _ET_. As a result, in the humid area, the original upward trend of _WIE_ may be strengthened, and the original downward trend of _WIE_ may be stabilized or even reversed in some cases.  
To enhance the adaptability of ecosystems to future climate change or mitigate the adverse effects of climate change, it may be necessary to introduce the following suggestions:(1) In arid regions, the existing desert vegetation and grassland should be prevented from shrinking or degrading owing to intense human factors because these vegetation have a high ecosystem resilience [72]. (2) In arid areas with sparse vegetation, greening should be achieved by expanding grassland areas rather than artificial afforestation. C4 plants, which are usually dominated by herbaceous plants, have a higher internal WUE and can better adapt to the natural conditions of limited water and sufficient energy [73]. With the possible aggravation of drought in humid areas, expanding the C4 vegetation area can also be considered.