Characteristics Of Extreme Precipitation In Qinghai-Tibet Plateau And Estimation Of Critical Rainfall For Flood Disasters

With the background of climate change characterized by warming,the global and regional water cycle speeds up,which makes the spatial distribution of precipitation different,and the precipitation in some areas shows an increasing trend.The results of the second comprehensive scientific investigation on the Qinghai-Tibet Plateau indicate that the imbalance of the solid-liquid structure of the water body in the Qinghai-Tibet Plateau and the transformation of more and more glaciers,snow and other solid water into liquid water have resulted in an increase in the amount of water entering the atmospheric circulation,which is bound to increase the occurrence of extreme precipitation events and thus increase the risk of flood disasters in the Qinghai-Tibet Plateau.Based on the daily precipitation data of meteorological stations on the Qinghai-Tibet Plateau from 1961 to 2017 and historical flood disaster records,this paper analyses the characteristics and relationship between extreme precipitation and flood disaster on the Qinghai-Tibet Plateau,and estimates the critical rainfall of flood disaster on the Qinghai-Tibet Plateau.Finally,the following conclusions are drawn:(1)The average annual precipitation of Qinghai-Tibet Plateau is 472.36 mm from1961 to 2017.The annual precipitation shows an upward trend at the rate of 8.06mm/10 a.The annual precipitation changed abruptly in 1987,and since then the precipitation has changed from less to more.The annual precipitation of the plateau is unevenly distributed in space,showing a decreasing trend from southeast to northwest.The precipitation is mainly concentrated in the Hengduan Mountains in the south of the plateau,while the precipitation in the Qaidam Basin in the north and west of the plateau and the Qiangtang Plateau is relatively low.The range of precipitation tendency rate of meteorological stations in Qinghai-Tibet Plateau ranges from-25.46mm/10 a to 43.02mm/10 a.The annual precipitation of 66 stations in 78 stations shows an upward trend,while the precipitation of only 12 stations shows a downward trend.The increasing trend of annual precipitation is more significant in the southern part of the plateau than in the northern part of the plateau.(2)The threshold of extreme precipitation over the Qinghai-Tibet Plateau ranges from 7.84 mm to 51.90 mm,with an average of 23.11 mm.The annual average extreme precipitation is 37.59 mm,showing an upward trend at the rate of2.00mm/10a;The average annual extreme precipitation days are 1.22 days,and the rate of 0.06 days/10 a is rising in the whole region;The mean value of extreme precipitation intensity is 29.31 mm/d,rising at a weak rate of(0.02 mm/d)/10a;The average contribution rate of extreme precipitation is 7.94%,which is rising at the rate of 0.30%/10 a.From the spatial distribution of extreme precipitation index,the spatial distribution of extreme precipitation is consistent with that of annual precipitation,showing a decreasing trend from southeast to northwest;the spatial distribution of extreme precipitation days is mainly concentrated in Hengduan Mountains and Zoige Plateau,southern Qinghai-South Plateau and local areas of southern Tibet valley;the stations with high contribution rate of extreme precipitation mainly distribute in the southwest and north of the plateau in the Himalayas,Ali Plateau,Qaidam Basin,northern Kunlun Mountains and Zoige Plateau.(3)A total of 1506 flood events were recorded in the Qinghai-Tibet Plateau from1961 to 2010.The vast majority of flood disasters are caused by precipitation.Only a small number of flood disasters are caused by dam breaks of reservoirs,rivers,glaciers and snow melting water.The average annual occurrence of flood disasters in the Qinghai-Tibet Plateau is 30.12 times,and the frequency of flood disasters shows an upward trend,with a tendency rate of 10.10 times per 10 a.According to the time distribution of flood disasters,81.20% of the flood disasters occurred in June to August,mainly in summer,and relatively few in spring and autumn.In terms of spatial distribution,the high-incidence areas of flood disasters in the Qinghai-Tibet Plateau are mainly in the Hehuang Valley and Hengduan Mountains in the eastern part of the plateau;The sub-high incidence area is located in the southern Tibetan Valley and the periphery of the Hehuang valley;the middle area is located in the Qaidam Basin,the northern wing of Kunlun Mountains,the southern valley of Tibet and the Hengduan Mountains;the northern flank of Qilian Mountains and the periphery of the southern Tibetan valley are the sub-low-incidence areas of flood disasters.In addition,the whole Qiangtang Plateau,the southern Qinghai Plateau and most of the Hengduan Mountains are low-incidence areas of floods.(4)The critical rainfall value of flood disasters in the Qinghai-Tibet Plateau is the highest in the southern part of the plateau,followed by the eastern and southeastern parts of the plateau,and the lowest in the central,western and northern parts of the plateau.The high value area of critical rainfall value is Nujiang River Basin,and the critical rainfall value of flood disaster is 38.52 mm.The sub-high critical rainfall areas are Yarlung Zangbo-Ganges River Basin River Basin and Lancang River Basin.The critical rainfall values of these two zones are 31.74 mm and 29.87 mm,respectively.The median critical rainfall area is the Hexi Corridor-Alxa Inland Flow Area,the Yellow River Basin and the Yangtze River Basin.The critical rainfall values of these basins are 27.60 mm,28.28 mm and 28.12 mm,respectively.The sub-low critical rainfall area is the inland flow area of Qaidam-Tarim basin,and the critical rainfall is 22.42 mm.The inward flow area of Qiangtang Plateau is the low value area of critical rainfall for flood disaster,and the critical rainfall is 15.56 mm.