# Impact of Climate Change on the Major Eco-Hydrological Parameters in the Dongting Lake Basin
## 3 Results and Discussion
### Spatial-Temporal Changes in Climate Variables, GPP and ET
#### 3.2.3 Changes in GPP and ET  
The annual GPP and ET from the Dongting Lake Basin are shown in Figure 9. The average GPP in the Dongting Lake Basin from 2000 to 2017 exhibited high fluctuations. The mean value of GPP in the Dongting Lake Basin from 2000 to 2017 was 6.375 g C m\({}^{-2}\) d\({}^{-1}\), with the maximum value occurring in 2000, being 6.779 g C m\({}^{-2}\) d\({}^{-1}\). The minimum value appeared in 2005, being 5.842 g C m\({}^{-2}\) d\({}^{-1}\). Research has shown that GPP is generally increasing in most regions of the world as a result of climate change [23; 24; 25]. An observation can be made that the GPP in the study area had the same trend.  
The average ET in the Dongting Lake Basin from 2000 to 2017 showed a fluctuating decreasing trend with a significant decreasing rate of \(-3.309\) mm/year. The mean ET was 472.339 mm, with the maximum value occurring in 2000, being 549.568 mm. Meanwhile, the minimum value occurred in 2011, being 423.903 mm.  
Figure 9: Interannual trends in GPP and ET in the Dongting Lake Basin from 2000 to 2017.  
According to Figure 9, GPP and ET had similar trends. Essentially, ET and GPP have similar response characteristics under climate change scenarios [11]. ET and GPP are interrelated and significant environmental factors.  
Figure 10 shows the seasonal variations of GPP and ET in the Dongting Lake Basin.  
During the spring season (March-May), GPP exhibited an upward trend with a rate of increase of 0.0052 g C m\({}^{-2}\) d\({}^{-1}\)/year. At the same time, ET had a downward trend in the study area with a rate of decrease of \(-\)1.2961 mm/year.  
For GPP in the summer season (June-August), an upward trend was identified with a rate of 0.0168 g C m\({}^{-2}\) d\({}^{-1}\)/year. ET exhibited a significant downward trend in the study area with a rate of decrease of \(-\)1.6004 mm/year, being the most prominent among the four seasons.  
In the autumn season (September-January), GPP in the Dongting Lake Basin exhibited an upward trend with a rate of increase of 0.0011 g C m\({}^{-2}\) d\({}^{-1}\)/year. On the other hand, ET displayed a downward trend with a rate of decrease of \(-\)0.6933 mm/year.  
In the winter season (December-February of the following year), GPP in the Dongting Lake Basin exhibited an upward trend with a rate of increase of 0.0028 g C m\({}^{-2}\) d\({}^{-1}\)/year. Meanwhile, ET showed a fluctuating downward trend with a rate of decrease of \(-\)0.332 mm/year, being the least evident among the four seasons as shown in Figure 9.  
The spatial distribution characteristics of GPP and ET in the Dongting Lake Basin are shown in Figure 11. The GPP of the Dongting Lake Basin between 2000 and 2017 ranged from 0.88 g C m\({}^{-2}\) d\({}^{-1}\) to 7.17 g C m\({}^{-2}\) d\({}^{-1}\). Meanwhile, the ET ranged from 336.60 mm to 546.48 mm. The spatial distribution characteristics of GPP and ET in the Dongting Lake Basin were found to be similar, with both demonstrating a pattern of high values in the west and low values in the north. GPP and ET in the Dongting Lake Basin showed higher levels in the Yuan River Basin and the Li River Basin, which have high tree, grass, and crop coverage.  
Figure 10: Trends of GPP and ET in different seasons in the Dongting Lake Basin from 2000 to 2017.