| Under the background of climate change,the flood and drought problems in the river basin are prominent,and understanding and mastering the occurrence and development status of flood and drought events is of great significance to social and economic security and prevention of hazards.The Yarlung Zangbo River is located on the Tibetan Plateau,a world water tower and fragile ecosystem.Its drought evolution not only has an important impact on the local socio-economic,but also involves the use of water resources and cross-border water security on the Tibetan Plateau.Drought in the Yarlung Zangbo River Basin is an urgent issue.This manuscript uses the daily meteorological data in the Yarlung Zangbo River Basin from1955 to 2017 to calculate the Standardized Precipitation Index(SPI),and analyzes the characteristics of drought spatial and temporal evolution.Then the Copula function is used to construct the distribution function of the drought duration and drought severity,and the distribution characteristics of drought frequency and drought recurrence period in river basin were discussed.Moreover,the drought frequency is correlated with the spatial population and GDP to calculate the drought impact in the typical years,revealing the watershed drought exposure degree.We should fully understand the law of drought occurrence and development in the Yarlung Zangbo River Basin,and take some suggestions to prevent and reduce disasters.The results show that:(1)In the past 63 years,the drought was more frequent in the 1980 s and declined in the1990 s.And increased in the types of extreme,severe,moderate,and light drought after 2010 s.Droughts in the Yarlung Zangbo River Basin mainly occurs in the central and eastern,and is distributed in Shannan,Nyingchi,Nagqu,Lhasa,and Yadong County in Xigazê.The change of SPI drought index shows that the worst drought in the 63 years was 1983,and there was less drought from the 1990 s to the early 2000 s.The drought increased in 2010 s,and the drought was more severe in 2009.The drought in the Yarlung Zangbo River Basin and cities mainly occurs in summer,and there is little drought in winter.The number of dry years in each city is as follows: Nyingchi > Lhasa > Shannan,Nagqu,Qamdo >Xigazê > Ngari.The annual mean frequency of the drought grades is generally high in the east and middle,low in the west generally.The distribution of light and moderate drought is very wide,and thedistribution range of severe drought and extreme drought is relatively concentrated.Moderate drought mainly occurs in the middle or east of the Yarlung Zangbo River basin.The extreme drought in Lhasa is more prominent,and the light and moderate drought in Yadong County of Xigazê is relatively frequent.(2)The distribution of drought frequency in the whole basin is high in the middle,and low in the west and east.The drought recurrence period in the basin shows the characteristics of long in the west and east and short in the middle,indicating that the middle of the basin is more vulnerable to long-term and high-intensity drought.The number of drought in Nyingchi station was the most,while that in Baxoi station was the least.The Clayton Copula function can better describe the correlation of the drought event with the lower-tail correlation.With the increasing of drought duration and drought severity value,the joint cumulative probability value also increases,but the variability gradually decreases.Nagqu station has the highest drought frequency and the shortest recurrence period in the weather station of the most populous county in the prefecture-level city.The drought in Nagqu Station in April 2002 lasted for one month,with an severity of 0.32 mm,a drought recurrence period of 1.11 years,and a drought frequency of 82.05%.The highest average drought frequency in the basin over the years occurred at the Lhari Station in the Nagqu and the lowest at the Baxoi Station in Qamdo.The drought recurrence period of Baxoi station in Qamdo is about 4.51 years,and the drought is more likely to occur.The highest drought recurrence period is the Bomê Station of Nyingchi,about 14.11 years.(3)Over the past 30 years,the population and GDP of the seven prefecture-level cities in the Tibet Autonomous Region have generally shown trends of development and enhancement.Through the calculation of the exposure model,the drought exposure degree at different time scales in the Yarlung Zangbo River Basin is obtained.The maximum value of drought exposure degree in the basin: 2015> 2010> 1995> 2000> 2005,and the spatial differences of the basins were large.The meteorological drought exposure degree in the Yarlung Zangbo River basin is generally increasing.The spatial difference of drought exposure level is higher in the middle of the basin than in other areas,especially in areas with better socioeconomic conditions along the main stream.Highly exposed areas are distributedin the north of Lhasa,the southeast of Xigazê,the north of Nyingchi,the north of Shannan,and the west of Qamdo.The range and intensity of drought exposure degree are generally concentrated and increasing,and disaster risk is transferred to areas with good socioeconomic conditions in the middle of the basin.Affected by global climate change,the spatial distribution of precipitation in the Yarlung Zangbo River Basin is uneven.The droughts and disasters in the river basin have a wide area,and the proportion of moderate and severe droughts has increased.At the same time,drought-prone areas coincide with socio-economic areas,exacerbating the degree of disaster.In order to effectively deal with the sudden drought events in the river basin,we need to start with the following aspects: strengthening the optimal allocation of water resources in space and time,exerting economic leverage,advocating water-saving and efficient water-use measures,and improving scientific management of early dry disasters.Through run length theory,Copula model and exposure index,the drought evolution characteristics and exposure degree analysis of the Yarlung Zangbo River Basin are analyzed.Some suggestions are put forward,which can provide support for the prevention and control of flood and drought disasters and risk avoidance. |
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