# Remote Sensing of Ecosystem Water Use Efficiency in Different Ecozones of the North China Plain
## 3 Results
### WUE Temporal Variation  
The spatial distribution of change trends in GPP, ET and WUE in the North China Plain are shown in Figure 5. It can be observed that the significantly increased GPP areas in North China are concentrated in the northwestern and eastern parts of the North China Plain, and are distributed in the MGMG, CCLPMF and HHPMF ecological function areas. The area with increased GPP accounted for 66.05% of the area of the North China Plain, and the area with a significant increase (\(p<0.05\)) in GPP accounted for 21.24% of the area of the North China Plain. The significantly increased ET was concentrated in the western, southern and eastern parts of the North China Plain, and distributed in the CCLPMF, CJPEF and HHPMF ecological function zones. The area with increased ET accounted for 66.23% of the area of the North China Plain, and the area with a significant increase (\(p<0.05\)) in ET accounted for 28.30% of the area of the North China Plain. The trend of WUE mainly showed a mild decrease and a mild increase, accounting for 73.22% of the area of the NorthChina Plain. In approximately the past decade, the change trend of WUE in the North China Plain has had a clear dividing line. The north of the central part has mainly shown an upward trend, and the south of the central part has mainly shown a downward trend. The area where WUE increased accounted for 58.65% of the area of the North China Plain, among which the area where WUE increased significantly (\(p<0.05\)) accounted for 20.41% of the area of the North China Plain. The different trends in WUE in different regions of China indicate that the factors leading to changes in WUE are spatially complex. Figure 4: Influence of GPP and ET on the spatial distribution of WUE in different ecological functional areas (in Figure 6d, the symbol “+” indicates the increasing trend of annual WUE (GPP or ET), the symbol “-” indicates the decreasing trend of annual WUE (GPP or ET), and the blank indicates the uncertainty of annual WUE (GPP or ET)). Figure 3: The spatial distribution of multiyear averages of GPP, ET and WUE for seven ecoregions in the North China Plain. The spatial distribution of the stability of GPP, ET and WUE in the North China Plain from 2001 to 2017 is shown in Figure 6. The GPP, ET and WUE in the North China Plain are relatively stable. Within the study area, the areas with medium-low fluctuations in GPP and WUE accounted for the largest proportion at 54.22% and 59.90% of the area of the North China Plain, respectively, and they were concentrated in the CCLPMF and HHPMF ecosystem functional areas. The areas with high fluctuations in GPP and WUE accounted for the smallest proportion, at 0.83% and 0.66% of the total study area, respectively, and were sporadically distributed in the study area. The spatial distribution and statistical results of WUE stability were similar to GPP. The area with small ET fluctuation accounts for 63.87% of the area of the North China Plain, and is mainly distributed in the QLMDF, CCLPMF, CJPEF and HHPMF ecosystem functional areas. Furthermore, there were no image elements with large fluctuations. Different ecosystem types produce units of organic matter by consuming different amounts of water, which affects the spatiotemporal variation in WUE in different ecosystems. Therefore, it is important to study the characteristics of the WUE of different ecosystem types, as shown in Figure 7. The multiyear average values of WUE in the ecological function areas in North China were QLMDF > CCLPMF > MGMG > OPS > CJPEF > HHPMF > BHSSM in descending order, among which the multiyear mean values of WUE in the MGMG, OPS and CJPEF ecological function areas were not significantly different from each other. From 2001 to 2017, the WUE of the MGMG, CCLPMF and OPS ecosystems showed a growing trend, among which the WUE of the MGMG ecological function area grew the fastest, which might be related to the construction of the \"Three Norths\" protection forest project.