Study On Spatiotemporal Evolution Characteristics And Correlation Of Different Types Of Drought In China

Drought is one of the most serious natural disasters in the world,which is widely distributed and lasts for a long time.It will cause varying degrees of harm to the ecological environment,agricultural production,water supply and other aspects,and profoundly affect the social economy and people’s life,which has attracted the attention of various countries around the world.The causes of drought are complex,and the causes and mechanisms of different types of drought are not completely the same,but they influence each other.China has a vast territory and drought events are frequent.Therefore,it is of great scientific significance to explore the spatial-temporal evolution and correlation of different types of drought events at the watershed scale,so as to monitor and prevent drought in the watershed.Based on SPEI data and TVDI data from2000 to 2019 and from the perspective of different types of droughts,this dissertation studies the spatial-temporal evolution characteristics of meteorological drought and ecological drought in different watersheds of China in detail,and explores the impact of meteorological factors on ecological drought and the correlation between meteorological drought and ecological drought after clarifying the basic laws of drought.The main research contents and achievements are as follows:(1)Based on the grid data of precipitation and temperature from 2000 to 2019,the standardized precipitation evapotranspiration index of five time scales(1,3,6,12,24)was calculated.The Kendall rank test,R/S analysis and empirical mode decomposition were used to analyze the trend,persistence and periodicity of SPEI in different watersheds.The results show that China shows a trend of humidification.The drought trends of the nine major river basins are quite different.The Yellow River basin and Songliao basin have a significant trend of humidification,while the southwest river basins have a significant trend of drought;In the future,the areas of the southwest river basins that will maintain the trend of wetting will account for the largest proportion,while the Haihe River basin that will maintain the trend of drought will account for the largest proportion;The SPEI of each basin has a short period of 3 months,a medium period of 6 months and a long period of 12 months.(2)In terms of space,about 66% of China’s regions showed a wetting trend from2000 to 2019.At the watershed scale,Songliao Basin had the largest proportion of areas with a wetting trend,while Huaihe River Basin had the least.Through the analysis of drought frequency,duration and intensity of SPEI in China,the results show that there are large spatial differences in drought characteristics.Specifically,the southern basin of China has less drought frequency,long span and large intensity,while the northern basin has more drought frequency,short span and small intensity.(3)Based on the temperature vegetation drought index from 2000 to 2019,the trend,sustainability and periodicity of different watersheds are analyzed.The results show that the trend of TVDI in China is wetting,which is consistent with the conclusion of meteorological drought.On the basin scale,the trend of wetting in Inland River basin and Songliao basin is significant;In the periodic analysis,the southwest river basins and the Pearl River basin are mainly seasonal,while the other basins are mainly annual.There are obvious differences in TVDI in four seasons among different watersheds.The TVDI values of Haihe River Basin,Yellow River Basin,Inland River Basin and Songliao River Basin are summer>spring>autumn>winter.The TVDI values of southeast river basins and Pearl River basins are summer>autumn>spring>winter.The TVDI values of southwest river basins in spring are higher than the other three seasons,and the TVDI values of the other three seasons are staggered.(4)The Mann Kendall mutation test,Pettitte method,cumulative anomaly method and BFAST method were used to analyze the mutation of TVDI in different watersheds in China.By comparing different test methods,it can be seen that the Songliao basin had a mutation in 2010,the southwest and southeast river basins had a mutation in 2016,the inland river basin had a mutation in 2012,and the Huaihe River basin had a mutation in 2013 and 2018.The TVDI of Songliao Basin and the southwest river basins is a decreasing mutation,while the rest is a growing mutation.Under different land use types,the forest land and grassland in the southeast river basins had a sudden change in2016,the farmland and forest land in the southwest river basins had a sudden change in2016,and the grassland had a sudden change in 2009.(5)The correlation between TVDI and precipitation and temperature were analyzed by partial correlation analysis and multiple correlation analysis.The results show that 52.20% of regions in China are significantly correlated with precipitation and50.98% with temperature.On the seasonal scale,summer and precipitation pass the most significant test area,52.81%;In the partial correlation analysis with temperature,78.35% of the pixels in spring passed the significance test,with the highest number in the four seasons.In the multiple correlation analysis of precipitation and temperature,78.21% of China’s regions passed the correlation test.On the watershed scale,Songliao watershed passed the most significant test,94.27%.In the analysis of driving types,34.97% of China’s regions are strongly driven by meteorological factors,followed by precipitation,accounting for 23.34% of the total area of the country;In terms of seasons,the types of meteorological drivers in each season are quite different.In spring and winter,they are mainly driven by temperature and strong meteorological factors.In summer,they are mainly driven by non meteorological factors.In autumn,they are mainly driven by weak meteorological factors,temperature and strong meteorological factors.(6)The cross transform wavelet is used to study the correlation and hysteresis between TVDI and SPEI in various watersheds in China.The results show that there is a significant negative correlation between TVDI and SPEI in China as a whole.In terms of basin scale,Haihe River Basin,Huaihe River Basin,Yellow River Basin,Inland River Basin and Songliao River Basin are all negatively correlated.The Yangtze River Basin,Southeast River Basin and Pearl River Basin show that SPEI changes ahead of TVDI by three months.In terms of space,the correlation coefficient between TVDI and SPEI in China is-0.64,in which the Songliao basin has the most negative correlation regions and the strongest negative correlation,and the Pearl River basin has the most positive correlation regions and the weakest negative correlation.On the seasonal scale,TVDI in spring,summer and autumn is mainly negatively correlated with SPEI in each scale,while the positive correlation area in winter increases.