Abstract:

Under the background of global warming,there have been more and more extreme weather events which have also become frequent and intense.Research on extreme events and their impacts have been more focused than before,especially on the drought.The middle and lower reaches of the Yangtze River is an important agricultural production base of China,and one of regions with relatively abundant water resources,but droughts and floods are frequent.In recent years,the extreme precipitation events such as drought sharp reversal to flooding occurred occasionally in this region.In this research,drought was defined as qualitative extreme event,new approach of assessing the vulnerability of ecosystem to drought was attempted in a regional scale for a case study in the middle and lower reaches of the Yangtze River.The processesbased model (CEVSA) was used to simulate the dynamic responses of ecosystem to the drought and to assess the impacts of drought.According to the definition of vulnerability from IPCC,the key ecosystem function which correlated with the rainfall was selected as the indicator,the variability of indicator and its tendency of change were defined as the sensitivity and adaptive capacity of ecosystem to the drought,respectively.NPP in summer was regarded as the receptor of the drought events in this study,to assess the vulnerability of the ecosystem to drought.Specifically,the fluctuation degree of NPP in summer was used to indicate the sensitivity of the ecosystem to drought,while the trend of NPP fluctuation was used to reflect the adaptability of the ecosystem to drought.The vulnerability of ecosystems to drought in ecosystem scales was calculated and assessed by comparing with the average status in the reference period.The results of this study suggested that the spatial distribution of ecosystem vulnerability to drought had more obvious regional differences in the middle and lower reaches of the Yangtze River.Most of the ecosystems were low vulnerable and took about 65% of middle and lower reaches of Yangtze River over the period of 1961-1990, and the higher vulnerable ecosystems took about 20%,mainly distributed in the northwest of research area.Drought can increase the vulnerability of ecosystem distinctly.The main change of vulnerability was the shift from very low vulnerable ecosystems to low vulnerable ecosystems induced by drought.The medium vulnerable ecosystems and above were mainly located in the north of middle and lower reaches of the Yangtze River,and its percent was little changed due to drought.The response of different ecosystems to drought had a little difference,the vulnerability of agricultural ecosystem and forest ecosystem had increased according to the drought,but the vulnerability of agricultural ecosystem to drought was higher than the forest ecosystem.
The vulnerability of the agricultural ecosystem and the forest ecosystem would continue to rise further in subsequent years after the drought.But there was no significant difference by comparing with the baseline.It indicated that the vulnerability of the two ecosystem types could go back to the mean level of multiyears in the next year of drought generally