# Precipitation-Use Efficiency and Its Conversion with Climate Types in Mainland China
## 3 Results
### NPP Estimation and Change Characteristics  
Figure 6 shows the estimated annual NPP before and after ET\({}_{\text{a}}\) improvement. As depicted, the NPP in China is small in the northwest and large in the southeast, which is consistent with previous study [72]. The NPP of the southern humid region obtained before the improvement of the ET\({}_{\text{a}}\) model was between 1400 and 1700 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\). Precipitation is proportional to the NPP. In some regions of the southeast coast, the NPP is above 1700 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\). The improved NPP range was 22-1510 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\). The region of maximum NPP shifts northward and is located south of the Yangtze River. The annual average NPP was above 1400 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\). In arid regions, water has a positive effect on vegetation productivity, which means that productivity increases with improved water conditions. However, in humid regions with sufficient water supply, NPP tends to be saturated and is no longer controlled by water [73]. In addition, this region is the cloudiest region in China [74], which is generally proportional to the light stress on vegetation growth. In coastal regions with abundant rainfall, vegetation growth is more likely to be regulated by radiation. Before the improvement, the ET\({}_{\text{a}}\) model did not consider the energy constraint in the south, the estimated NPP was overly dependent on precipitation, and the estimated values were too large. The improved NPP reflects the response of vegetation growth to the regional climate. A comparison between the estimated NPP values (average values from 2000 to 2017) and the MOD17A3 multiyear average data shows that the estimated NPP after improvement is significantly correlated with the MOD17A3 NPP (R = 0.67, \(p\) < 0.001) (Figure 7). In the arid region, the MOD17A3 NPP was larger than the estimated NPP. However, in the humid region, the MOD17A3 NPP was relatively small. Due to estimation errors regarding reflectance, maximum light-use efficiency, and radiation, MODIS NPP products are overestimated in low-productivity regions and underestimated in high-productivity regions [75]. In addition, different methods and scale conversions can also lead to different comparisons. Therefore, the estimated NPP in this study has a certain rationality and superiority. Figure 5: ET\({}_{\text{a}}\) trend (1960–2017) and (**a**) distribution of average values and trends in sub regions and (**b**) +indicates that the trend was significant at the 0.05 level. Figure 6: NPP distribution before (**a**) and after(**b**) improvement. The distribution of the average values and trends of the NPP in the sub regions in China from 1960 to 2017 are shown in Figure 8. Over the past 58 years, the NPP increased to the west of 103\({}^{\circ}\) E, while it decreased to the east. NPP in arid regions typically increases, with a rate of 4.3 g\(\cdot\)m\({}^{-2}\)\(\cdot\)10 a\({}^{-1}\). NPP in the transition zone generally exhibits a slight decrease. However, the regional differences are large. NPP in the humid region typically displays a decreasing trend of \(-\)3.3 g\(\cdot\)m\({}^{-2}\)\(\cdot\)10 a\({}^{-1}\). The distribution of the average NPP in different climatic regions shows that the NPP gradually increases from arid to humid regions. The main vegetation types in arid regions are desert grassland and lowland meadows. The soil is severely desertified and salinized in regions with low vegetation coverage. The annual average NPP is 140.4 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\). The NPP fluctuation range in the humid region is the smallest, with an average of 1287.9 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\). The average NPP in the transition zone is 807.2 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\), ranging from 226.6 to 1084.8 g\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\), thereby exhibiting the largest regional difference, which is related to the complex and diverse climate types and vegetation types in the area.