# Precipitation-Use Efficiency and Its Conversion with Climate Types in Mainland China
## 3 Results  
### \(ET_{a}\) Estimation and Its Change Characteristics  
ET\({}_{a}\) was calculated using Equation (3). Compared with the observations from the flux stations (Figure 2), both the observed and estimated ET\({}_{a}\) were found to increase significantly with increased precipitation in the region where P \(<\) 600 mm. Furthermore, when P \(\geq\) 600 mm, the observed ET\({}_{a}\) first increased and then decreased with increasing P, although the estimated ET\({}_{a}\) still increased significantly with increased P. Many factors affect ET\({}_{a}\), the most important of which are water and energy conditions. In arid and semiarid regions, the amount of precipitation is relatively small, but the energy is sufficient. The main factor affecting ET\({}_{a}\) is water, and a change in P largely determines the change in ET\({}_{a}\). In the semi humid region, increased P makes energy the controlling factor of ET\({}_{a}\) instead of water, and the rate at which ET\({}_{a}\) increases with P slows down. In humid areas with abundant rainfall, ET\({}_{a}\) no longer increased with increasing P. Due to more precipitation, energy conditions limit ET\({}_{a}\), and higher P results in smaller ET\({}_{a}\). Earlier studies confirmed the switch of the ET\({}_{a}\) controlling factor from water to energy with increased precipitation [70]. Equation (3) does not consider this switch when estimating the ET\({}_{a}\) in the humid region and does not reflect its energy constraint. Estimates are largely dependent on precipitation, which exaggerates the results. We further used P = 600 and P = 1400 mm as the boundaries and conducted regression corrections on the estimated ET\({}_{a}\). The specific regression equation is given by Equation (8). \[ET_{a\_new}= \begin{cases}\text{ET${}_{a}$ if $\text{Pre}\leq 600\text{mm}$}\\ 0.815*\text{ET${}_{a}$}+26.2\text{ if 600 mm $<$\text{Pre}\leq 1400\text{mm }($\text{r}=0.82$,$\text{a}<0.01$)}\\ -0.615*\text{ET${}_{a}$}+1249.8\text{ if $\text{Pre}>1400\text{mm }($\text{r}=0.53$,$\text{a}<0.01$)} \end{cases}  \tag{8}\]  
The improved ET\({}_{a}\) in the southern humid region is significantly smaller than the ET\({}_{a}\) before improvement (Figure 3). Regions with the highest ET\({}_{a}\) move north from the southeastern coastal area to the middle and lower reaches of the Yangtze River. To further verify the accuracy of the improved ET\({}_{a}\) model, we compared the estimated values with  
Figure 2: Distribution of observed and estimated ET\({}_{a}\) with P.  
the ET\({}_{\text{a}}\) observations from the flux stations during the validation period and the ET\({}_{\text{a}}\) from GLDAS during the same period. This comparison showed that the improved ET\({}_{\text{a}}\) is highly consistent with the observed and GLDAS ET\({}_{\text{a}}\) (Figure 4). The correlation coefficients were 0.95 and 0.85, respectively, and the standard deviation between the estimated and observed values was much smaller than that between the GLDAS assimilation and observed values. GLDAS data are sparser in the high evapotranspiration regions, and the values are too large. [PERSON] et al. [71] also identified an overestimation in the high ET\({}_{\text{a}}\) regions of southern China in the GLDAS data. Hence, our improved ET\({}_{\text{a}}\) model can simulate the ET\({}_{\text{a}}\) in the study region more accurately. Over the past 58 years (1960-2017), the ET\({}_{\text{a}}\) trend was roughly bounded by 103\({}^{\circ}\) E, with ET\({}_{\text{a}}\) increasing to the west and decreasing to the east (Figure 5a). ET\({}_{\text{a}}\) in the arid region is the smallest, with an average of 68.8 mm and a fluctuation range of 21.3-142.7 mm. ET\({}_{\text{a}}\) in the transition zone is 350.2 mm with a 79.2-485.0 mm fluctuation range. The ET\({}_{\text{a}}\) fluctuation range in the humid region is between 439.8 and 745.6 mm, with an average of 602.5 mm. Over the past 58 years, ET\({}_{\text{a}}\) in the arid region increased at an average rate of 2.9 mm\(\cdot\)10 a\({}^{-1}\).