## 3 Results
### WUE Analysis of the Five Natural Terrain Areas  
#### 3.2.1 WUE Changes in Five Natural Terrain Regions  
Anhui Province is divided into five different natural topographic regions: the Huaibei Plain, the Jianghuai hilly region, the hilly mountainous area of southern Anhui, the Dabie Mountain area in western Anhui, and the plain alongside the river. The temporal changes in ET, the GPP, and WUE in these five regions are illustrated in Figure 10. The WUE in the five regions differs, with the highest observed in the hilly and mountainous region of southern Anhui, followed by the Dabie Mountain area, the Huaibei Plain, the Jianghuai hilly region, and the plain alongside the river; the GPP was highest in the hills and mountains in southern Anhui > the Dabie Mountains > the Jianghuai hilly region > the Huaibei Plain > the plain alongside the river, in order from high to low; and ET was highest in the hills and mountains of southern Anhui > the Dabie Mountains > the Jianghuai hilly region > the plain alongside the river > the Huaibei Plain, in order from high to low.  
The hilly and mountainous region in southern Anhui and the Dabie Mountains are both blessed with rich forest vegetation, abundant rainfall, and good light conditions, meaning that the vegetation in these two areas contributes a large amount to the GPP. At the same time, under conditions of sufficient light and heat, the ET of these two areas is also the highest in Anhui Province; thus, the WUE is generally high. The Huaibei Plain is the main maize- and wheat-growing region in Anhui Province, and crops are important GPP contributors in this region. However, high rainfall and low ET means that the GPP is the main factor affecting the WUE in the Huaibei Plain [61]. The Jianghuai hilly region and the plain alongside the river are associated with the Huaibe River and Yangtze River running from west to east, respectively; thus, both regions suffer from higher ET in general, with weak photosynthesis and low transpiration, lowering the WUE.  
The hills and mountains in southern Anhui showed the largest growth rate, and the plains alongside the river, which was the only region showing a decreasing trend in WUE, the lowest. A growth trend was maintained in the GPP in all regions, with the hilly and mountainous area in southern Anhui showing the highest growth rate, and the Dabie Mountains the lowest growth rate. ET also increased in all regions, with the fastest growth rate in the Dabie Mountains and the slowest in the hilly and mountainous region of southern Anhui. Although the lowest GPP growth was observed in the Dabie Mountains,  
Figure 9: Trend of average WUE in each month: (**a**) January, (**b**) February, (**c**) March, (**d**) April, (**e**) May, (**f**) June, (**g**) July, (**h**) August, (**i**) September, (**j**) October, (**k**) November, and (**l**) December.  
the highest GPP of the five regions was observed in this region; thus, the highest WUE and the most stable change trend were observed in this region. In contrast, the slow growth trend in the GPP and steady and insignificant increase in the ET in the plain alongside the river indicates that the WUE is mainly influenced by the GPP of this region, with significant interannual changes.