# Impact of Climate Change on the Major Eco-Hydrological Parameters in the Dongting Lake Basin
## 3 Results and Discussion
### Contributions of Long-Term Changes to GPP and ET
#### 3.3.3 Analysis of the Time Lag in Climatic Factors on Major Eco-Hydrological Parameters  
It is widely recognized that climate change affects vegetation growth and dynamics [36], and that vegetation responses to climate are often complex and have temporal effects [37]. In particular, the time-lag effect refers to the significant influence of vegetation growth by the climatic conditions of the previous month [38]. Different climate factors showed different lag effects on the same vegetation type [38]. There is a time-lag effect in the response of key eco-hydrological parameters to climate change in the Dongting Lake Basin. Therefore, the study of the time-lag response of GPP and ET to climate factors can deeply reflect the mechanism of climate change and key eco-hydrological parameters. Based on the monthly scale data, a study on the time lag effects of GPP, ET and climate factors in the Dongting Lake Basin was accomplished. The whole correlation of GPP to the radiation of the same month shows a high level of correlation. Negative correlations were exhibited in the summer. The Dongting Lake Basin showed no lag effect of GPP with temperature and radiation, and a 2-month lag effect with precipitation. GPP responded more rapidly to temperature and radiation than to  
Figure 15: Stepwise regression model of extreme climate indices and ET in Dongting Lake Basin. (** indicates significant correlation). precipitation. Findings show that the global average optimum temperature for vegetation photosynthesis is 23 \(\pm\) 6 \({}^{\circ}\)C [39]. The vegetation cover in the Dongting Lake Basin is mostly trees and crops, and the general temperature of the basin is suitable for vegetation growth. Therefore, photosynthesis of vegetation would be more sensitive to the change in this temperature. Vegetation is generally supplemented by precipitation in the previous monthly or bimonthly period, so there is a certain lag effect of precipitation. ET in the Dongting Lake Basin showed no lag effect with temperature, precipitation, or radiation. The whole correlation of GPP to the radiation of the same month shows a high level of correlation. Negative correlations were exhibited in the summer. The Dongting Lake Basin showed no lag effect of GPP with temperature and radiation, and a 2-month lag effect with precipitation. GPP responded more rapidly to temperature and radiation than to precipitation (Figure 16a). The vegetation cover in the Dongting Lake Basin is mostly trees and crops, and the general temperature of the basin is suitable for vegetation growth. Therefore, photosynthesis of vegetation would be more sensitive to the change in this temperature. Vegetation is generally supplemented by precipitation in the previous monthly or bimonthly period, so there is a certain lag effect of precipitation. ET in the Dongting Lake Basin showed no lag effect with temperature, precipitation, or radiation (Figure 16b). The time-lag analysis on GPP and extreme climate events is shown in Figure 16c. In the Dongting Lake Basin GPP responded to the extreme precipitation events in the same month. The extreme precipitation indices used at this time were all related to the extreme precipitation strength, which was extreme precipitation events related to the maximum amount of precipitation. It indicates that there is no time lag in the occurrence of heavy rainfall events that affect GPP. When the GPP of the basin was relatively high, the occurred of extreme temperature events negatively affected the GPP in the summer and fall. The Dongting Lake Basin has a high level of crop and tree vegetation cover. With the average temperature of the basin at 17.798 \({}^{\circ}\)C to 26.132 \({}^{\circ}\)C in summer and fall, the temperature is suitable for the growth of plants and beneficial for photosynthesis. Extreme weather events would break the suitable state of plant growth and have a negative impact on GPP with no time lag effect. The time-lag analysis of ET and extreme climate events is shown in Figure 16d.