# Quantifying Soil Moisture Impacts on Water Use Efficiency in Terrestrial Ecosystems of China
## 3 Results
### Elasticity of WUE to SM  
The variation in _SM_ can, simultaneously, directly affect the _NPP_ and _ET_, which, ultimately, leads to the changes in _WUE_. On the _ET_ pathway, the sensitivity coefficient of the _WUE_ to \(SM\) (\(\frac{\partial WUE}{\partial ET}\frac{\partial ET}{\partial\theta}\)) at the growing season scale ranged from \(-18.49\) to \(-0.04\), with an average of \(-3.02\) (Figure 4a). Generally, the negative impact of _SM_ on _WUE_ via the _ET_ pathway in the arid region of northwestern China was the most significant, followed by that in the southeast monsoon region. The impact in the central China region was relatively  
Figure 3: Temporal and spatial variation characteristics of water use efficiency (_WUE_) in the terrestrial ecosystems of China during the growing season from 1985 to 2014. (**a**) Multi-year average _WUE_ in the growing season; (**b**) Mann–Kendall test of the _WUE_ change trend (“DEC” is the decrease trend and “INC” is the increase trend) in the growing season; (**c**) multi-year average _WUE_ in the growing season in different climatic zones, whiskers indicate standard deviation; (**d**) scatter plot between _WUE_ and _SM_ in the growing season, and the blue solid line is the change trend of fitted by quadratic curve. The abbreviations of MT, SST, MST, NST, WTMP, MTMP, CTMP, PTMP, and PSF represent the marginal tropical zone, south subtropics zone, middle subtropics zone, north subtropics zone, warm temperate zone, middle temperate zone, cold temperate zone, temperate zone in Qinghai–Tibet Plateau, and the sub-frigid zone in Qinghai–Tibet Plateau (PSF), respectively. insignificant. This negative effect (Figure 4b) was mainly due to the negative feedback relationship between the _WUE_ and _ET_ (_EF_). In different climatic zones, the negative impact of _SM_ on _WUE_ via the _ET_ pathway was the lowest in the PSF, and the average value of the elasticity coefficient was \(-1.87\). The impact was the most significant in the CTMP, with an average elasticity coefficient being\(-3.80\). In other zones, the adverse effects of _SM_ on _WUE_ via the _ET_ pathway were in the order of MTMP, SST, MST, MT, WTMP, NST, and PTMP, with average elasticity coefficients of \(-3.72\), \(-3.49\), \(-3.24\), \(-3.12\), \(-2.98\), \(-2.96\), and \(-2.29\), respectively (Figure 4b). Figure 4: Spatial characteristics of the elasticity coefficient of water use efficiency (_WUE_) to soil moisture (_SM_) change in the growing season of the terrestrial ecosystem in China during 1985–2014. Panels (**a**,_c**,_e_) show the raster map of the sensitivity of _WUE_ to _SM_ variation via the evapotranspiration (_ET_) and net primary production (_NPP_) pathways and the final effects of both pathways, respectively. Panels (**b**,**d**,_f_) show the sensitivity of _WUE_ to _SM_ variation via the _ET_ pathway and _NPP_ pathway and the final effects of both pathways in different climatic zones, respectively. The abbreviations of MT, SST, MST, NST, WTMP, MTMP, CTMP, PTMP, and PSF represent the marginal tropical zone, south subtropics zone, middle subtropics zone, north subtropics zone, warm temperate zone, middle temperate zone, cold temperate zone, temperate zone in Qinghai–Tibet Plateau and the sub-rigid zone in Qinghai–Tibet Plateau (PSF), respectively. In the _NPP_ pathway, the sensitivity coefficient of _WUE_ to _SM_ (\(\frac{\partial\textit{WUE}}{\partial\textit{NPP}}\frac{\partial\textit{NPP}}{ \partial\theta}\)) had a more apparent spatial variation, and the value ranged from \(-68.66\) to \(43.19\) with an average of \(0.13\) (Figure 4c). Thus, _SM_ via the _NPP_ pathway positively affected _WUE_ in \(55.71\%\) of the area, whereas it showed a negative influence in the remaining \(44.29\%\) of the area. In addition, _SM_ via the _NPP_ pathway often had a more significant impact (larger absolute value) on _WUE_ in the southeast monsoon region and northwestern arid region of China.