Spatiotemporal Variations Of Aridity In China During 1961-2015

Water is one of the most important factors affected by climate change and changes in global climate led to intensified hydrological cycle,directly influence the precipitation,evaporation,runoff and soil humidity and cause the re-distribution of water resources in time and space;therefore,using an aridity index(AI)that is defined by the ratio of annual precipitation(P)to annual potential evapotranspiration(PET)to classify climate zones and analyses inter-annual and inter-decadal variations,meanwhile,the change characteristics and main influencing factors of AI in China and different regions were also analyzed.Northwest China is one of the most sensitive areas to global climate change,so analyses dry and wet change using the Empirical Orthogonal Function(EOF),Rotated Empirical Orthogonal Function(REOF)and Piecewise linear fitting model(PLFIM)method in the last chapter.Conclusions are as follows:(1)Based on AI the climate can be divided into 5 types,Hyper-arid,Arid,Semi-arid,Mid,Dry and humid,respectively.The analysis of year-to-year variations in every climate type from 1961 to 2015 show that hyper-arid climate areas significantly decrease and semi-arid significantly increase,which means that a part of hyper-arid area convert into semi-arid areas.The analysis of every climate area using average 10 years AI and its variation show that the climate is drier-wetter-drier – wetter between 1961 to 2015.The changed area in climate type is mainly concentrated in the Xinjiang province,the Greater Hinggan and the Lesser Hinggan regions,the North China Plain,the north China plain,Qinling Mountains surroundings and the eastern Tibetan Plateau.(2)The national average AI shows a breakpoint in 1993,increasing(wetter)before 1993 and decreasing(drier)after 1993.From 1961 to 1993,temperature increase in most parts of the country,wind speed and available energy decrease,while the trend of relative humidity and precipitation is not obvious;From 1993 to 2015,the increase trend of temperature strengthen and the decrease trend of wind speed and available energy weaken,some areas even turn into increasing,relative humidity decrease.The change of climate factor's trend causes the change national average AI's trend and main influencing factor which from available energy to relative humidity.(3)AI has obvious regional characteristics;therefore,China can be divided into 8 regions,Northeast,North China,The middle and lower reaches of the Yangtze river and jianghuai region,South China,Southwest,Tibetan plateau,The western part of Northwest China and The eastern part of Northwest China.As a turning point in 1993,wet first dry later in the middle and lower reaches of the Yangtze river and jianghuai region,South China and Southwest region,the west part of Northwest China first wet and become stable later,Northeast and east part of Northwest China continuous wet,Tibetan plateau continuous dry,and dry first wet later in North China.The most important factors also change during two phases.In Northeast and North China It is continuous precipitation;In The middle and lower reaches of the Yangtze river and jianghuai region,it changes from precipitation and available energy to relative humidity;In South China,it changes from precipitation and available energy to precipitation.;In Southwest,it changes from precipitation and available energy to precipitation and relative humidity;In Tibetan plateau,it changes from precipitation,temperature and available energy to precipitation and relative humidity;In the western part of Northwest China,it is continuous precipitation;In the east part of Northwestern China,it changes from precipitation and wind speed to precipitation.(4)The climate in northwest China is mainly arid or semiarid and its dry-wet degree has obvious spatial distribution characteristics.According to the result of REOF,Northwest China can be divided into five regions: 1,northern Xinjiang;2,southern Xinjiang;3,western Inner Mongolia region;4,plateau region and 5,eastern of northwest region.there were two breakpoints,1978 and 1993.The “dry-wet-dry” and “dry-wet-slightly wet” pattern were shown in the northern Xinjiang and southern Xinjiang,respectively.Breakpoint was not found in western Inner Mongolia region,it got wetter during 1961-2015 in this region.There are two breakpoints in Plateau region,it was wet during 1961-1985,and then dry during 1985-2000,after 2000 it got wet again.In eastern of northwest region,it was dry before 2000 and then wet after 2000.The precipitation was the most important factor in affecting the change of AI,however other factors also play the important and indispensable roles in their changes.