## 3 Results  
### Temporal and Spatial Variation Characteristics of ET in the YRB  
#### 3.1.1 Interannual Variation Characteristics of ET  
The annual distribution of ET and the interannual variation of average ET in the YRB from 2001 to 2020 are shown in Figure 2. The varied range of the annual average value of ET in the YRB from 2001 to 2020 was 549.23\(-\)622.34 mm/yr, and the average value of multi-year average annual ET was 592.58 mm/yr (dotted line in Figure 2). The years in which the annual average ET values exceeded the multi-year average annual ET value were 2005 (596.53 mm/yr), 2006 (619.59 mm/yr), 2011 (622.07 mm/yr), 2012 (600.26 mm/yr), 2013 (617.87 mm/yr), 2015 (622.34 mm/yr), 2016 (606.09 mm/yr), 2018 (619.10 mm/yr), 2019 (606.19 mm/yr), and 2020 (595.64 mm/yr). The minimum ET value occurred in 2001 with 549.23 mm, and the maximum value was 622.34 mm in 2015, and the difference between the highest and lowest interannual ET values was 73.11 mm. The relative change rate of the interannual average ET fluctuated significantly over the study time period, with a maximum of 8.20% (2011) and a minimum of \(-\)7.06% (2007). The average relative change rate was 0.48%, showing an overall upward trend.  
As shown in Figure 3a, the average value of ET in the YRB in the last 20 years ranged from 0 to 1599 mm, with significant geographical differences, generally showing high values in the east and southwest and low values in the central and northwest. Among them, the five sub-basins with the largest annual ET were K (831.8 mm), H (688.88 mm), L (684.86 mm), J (684.54 mm), and M (671.64 mm) in order from large to small. From the perspective of spatial distribution, these five sub-basins were all distributed in the eastern part of the YRB. The sub-basins with the smallest annual ET were C (531.19 mm), E (532.55 mm), and D (550.13 mm) from small to large, all of which were located in the middle of the YRB. It is worth noting that the plateau peaks in the northern high\(-\)altitude regions of sub-basin A were covered with snow all the year round, which made the average annual ET in this area generally lower than 500 mm, while the average annual ET was generally higher in the southern area of this sub-basin.  
As shown in Figure 3b, the variation trend of ET in most areas of the YRB was not significant, accounting for 58.10% of the total area. The ET of 34.99% of the areas showed a significant or extremely significant increase trend, of which the significant increase area accounted for 12.50%, and the extremely significant increase area accounted for 22.49%, which were mainly distributed in most areas of sub-basin A, B, and J. The ET of 6.90% of the  
Figure 3: (**a**) Spatial distribution of mean ET on a per pixel basis and (**b**) spatial distribution of ET change trend in the YRB from 2001 to 2020. A – M represent 13 sub-basins of the YRB, which are as follows: the Jinsha River Basin; the mountainous areas in the upper reaches of Dadu River and Jialing River; the confluence area of Sichuan Basin; the main stream area in the upper – middle reaches of the Yangtze River; the Wujiang River Basin; the middle reaches of the Yangtze River; the confluence area of Nanyang Basin; the Jianghan – Dongting Lake Plain; the Dongting Lake Basin; the mountainous area of Ganjiang River Basin; the Poyang Lake Plain; the confluence area of Chao Lake and the plain along Wanjiang River; and the confluence area of the Yangtze River Delta.  
Figure 2: Interannual variation of average ET in the YRB from 2001 to 2020.  
areas showed a significant or extremely significant decrease trend, of which the significant reduction area accounted for 3.83%, and the extremely significant reduction area accounted for 3.07%, which were mainly distributed in parts of sub-basin C, H, and G. In general, the average value of the Z statistic was 0.97, which showed a certain increasing trend.