## 3 Results  
### Spatial Distribution Patterns of SWUE  
The multi-year average GPP, SM, and SWUE data during the growing season in northeast China from 2007 to 2015 are shown in Figure 2. The spatial distribution of GPP showed significant heterogeneity (Figure 2a). In the Songnen Plain, Liaohe Plain, and Hulun Buir Plateau, GPP values of cropland and grassland were lower. The lowest value was about 94.3047 gC/m\({}^{2}\). The GPP values of vegetation in Changbai Mountain and the Greater and Lesser Khingan Mountains were relatively high with the maximum value of 1343.26 gC/m\({}^{2}\). The multi-year average surface SM in northeast China was 3.001-9.431 m\({}^{3}\cdot\)m\({}^{-3}\). Similar characteristics of SM spatial distribution to that of GPP were found (Figure 2b). The SM values were lower in the west and southwest of northeast China. Relatively high SM was observed in the middle, east, and southeast of northeast China and Sanjiang Plain wetland. The multi-year average SWUE ranged from 0.341 gC/kg H\({}_{2}\)O to 6.173 gC/kg H\({}_{2}\)O (Figure 2c), showing consistent spatial distribution of GPP. Low SWUE values were found in the middle and southwest, whereas SWUE in the east and southeast of the study area was higher.  
Ecosystem SWUE varied in different soil types (Figure 3a). SWUE values on dark-brown forest soils were the highest (3.187 gC/kg H\({}_{2}\)O), followed by albic soils (3.084 gC/kg H\({}_{2}\)O). The lowest SWUE (1.752 gC/kg H\({}_{2}\)O) was observed in solonetz, and marine solonchak showed an average SWUE of 1.920 gC/kg H\({}_{2}\)O. During the past nine years, the order of growing season SWUE of different vegetation in northeast China was as follows: CBMF \(>\) BF \(>\) CF \(>\) HE \(>\) SH \(>\) SW \(>\) ME \(>\) CR \(>\) GR \(>\) OT. Highest SWUE was observed in coniferous and broadleaf mixed forests (3.319 gC/kg H\({}_{2}\)O), followed by broadleaf forests (3.314 gC/kg H\({}_{2}\)O). The lowest SWUE was found in area covered by other type (1.942 gC/kg H\({}_{2}\)O) (Figure 3b). SWUE values for grasslands were relatively lower (2.231 gC/kg H\({}_{2}\)O). SWUE also varied with the elevation. Highest multi-year average SWUE was found at an elevation of 800-1000 m (3.051 gC/kg H\({}_{2}\)O). The average SWUE values in the areas of below 200 m and above 2000 m were 2.301 gC/kg H\({}_{2}\)O and 2.583 gC/kg H\({}_{2}\)O, respectively. From 0 to 1000 m, SWUE increased gradually. When the elevation was over 1000 m, there was a decreasing trend in SWUE (Figure 3c).  
As shown in Figure 3d, SWUE of vegetation was generally high in the ecological function protection areas for water conservation in northeast China. Average SWUE values in the Changbai Mountains and the Great Khingan Mountains were 3.388 gC/kg H\({}_{2}\)O and 3.073 gC/kg H\({}_{2}\)O,  
Figure 3: The variations of multi-year average soil water use efficiency (SWUE) in growing season with soil types (**a**), vegetation types (**b**), elevation (**c**), and ecological function protection areas (**d**) in northeast China.  
Figure 2: The multi-year average gross primary productivity (GPP) (**a**), soil moisture (SM) (**b**), and soil water use efficiency (SWUE) (**c**) in the growing season of northeast China from 2007 to 2015.  
respectively. The Sanjiang and Songnen Plain wetland ecological function protection areas play important roles in flood regulation, with average SWUE values of 2.669 gC/kg H\({}_{2}\)O and 2.100 gC/kg H\({}_{2}\)O, respectively. As the only ecological function protection area of species resources in northeast China, average SWUE in the Liaohe River Delta Wetlands reached 2.179 gC/kg H\({}_{2}\)O. In Horqin Sandy Land, the ability and role of ecosystems to prevent land desertification and reduce the risk of sandstorm is mainly emphasized. However, vegetation SWUE in this area was the lowest (1.945 gC/kg H\({}_{2}\)O).