# Precipitation-Use Efficiency and Its Conversion with Climate Types in Mainland China
## 3 Results
### Driving Force of PUE Changes and Its Corresponding Conversion Characteristics
The contribution of the factors is consistent or opposite depending on the factor and region. Rn exhibits a negative effect in southern humid regions and a positive effect in northern arid regions. Rh exhibits a positive effect in northern arid regions and a negative effect in southern humid regions. Furthermore, the effects of Tmean, SM, and U follow apparent \"V\" shapes, with positive and negative directions or turns from large to small and subsequently to large in the transition zone. To further reveal the turning characteristics of the PUE responses to water, energy, and dynamics and to clarify the precipitation climate zone where turning occurs, the multiyear average precipitation at each station was taken as a spatial climate type at every 200 mm. For example, the P100 climate type represents a spatial climate type with average annual precipitation between 0 and 200 mm. The NPP and PUE responses to changes in Tmean, SM, and U in different precipitation climate types were further analyzed. Figure 13 shows the distribution patterns of annual NPP and PUE with \(\Delta\)T, \(\Delta\)SM, and \(\Delta\)U changes in different precipitation climate types. Therefore, \(\Delta\)T, \(\Delta\)SM, and \(\Delta\)U are the increments of Tmean, U, and SM, expressed as \(\Delta\)X = X - X\({}_{\text{min}}\), where X is T, SM, or U, and Xmin is the minimum value of each factor. In the arid region and transition zone (i.e., P100, P300, and P500), NPP and PUE were the most sensitive to various factors, especially \(\Delta\)SM (Figure 13b,e). NPP and PUE increased significantly with positive \(\Delta\)SM. In the humid region, NPP and PUE changed slightly with \(\Delta\)SM. However, in the extremely humid area (i.e., P \(\geq\) 1400), NPP and PUE decreased with positive \(\Delta\)SM. Soil moisture in the arid region and transition zone is close to the withering humidity. Vegetation growth is affected mainly by water factors. Soil moisture in the humid region always maintains a relatively high value. However, vegetation is less sensitive to soil moisture. Hence, extremely humid soil can restrict the oxygen supply to vegetation roots and soil microorganisms due to excessive  
Figure 12: Contribution of environmental factors to precipitation-use efficiency (PUE). Tmean is air temperature, Rn is the net radiation, U is the wind speed at 10 m, Rh is the relative humidity, and SM is soil moisture. moisture. The NPP and PUE responses to a positive \(\Delta\)SM range from significantly increased to decreased between the arid and extremely humid regions, respectively. The NPP and PUE responses to \(\Delta\)T and \(\Delta\)U also have conversion characteristics (Figure 13a,c,d,f). Water available for evapotranspiration in arid regions is limited. Changes in NPP and PUE with \(\Delta\)T were not obvious. However, in the transition zone, more obvious air temperature increases resulted in stronger water restriction for vegetation growth and smaller PUE. In the humid region, NPP and PUE were less sensitive to \(\Delta\)T, but they increased slightly with positive \(\Delta\)T in the extremely humid region, which reflects the promotion of vegetation growth in extremely humid regions by improved energy conditions. Wind velocity had the greatest impact on vegetation growth in the arid region and transition zone. Increased wind velocity was more conducive to evapotranspiration, thereby causing faster water loss and decreased NPP and PUE. In the humid region, a positive \(\Delta\)U resulted in more favorable evapotranspiration of super humid water vapor, which indirectly promoted NPP and PUE.