| Under the background of global warming,the impact of drought on China is becoming more and more serious.Drought problems are complex,drought types are diverse,and different drought types are closely related.Understanding the propagation process among different types of droughts can not only deepen the understanding of the spatial and temporal evolution characteristics of drought,but also further elucidate the formation mechanism of drought,which has important theoretical significance and application value for improving the drought resistance level and resisting drought risk in the basin.Based on the surface observation data,remote sensing,model,assimilation,reanalysis and climate model,this paper explores the temporal and spatial evolution characteristics of meteorological,hydrological,agricultural and groundwater droughts in China from 1990 to 2019,and the propagation process between various types of droughts.The impacts of different types of droughts on human social systems and natural ecosystems were further assessed,and the evolution patterns of each type of drought in China under future climate change and the changes in population size exposed to drought were elucidated in depth.Main findings are presented as follows:(1)The drought indexes of meteorology,hydrology,agriculture and groundwater were constructed to reveal the spatiotemporal evolution characteristics of various types of droughts.The constructed drought indices have good applicability in China and can characterize the drought characteristics of various types.Meteorological droughts are characterized by high intensity and frequency in the southern region,and there is a non-significant trend of enhancement in the severity of meteorological droughts nationwide.Hydrological drought has the characteristics of low frequency,long duration and high intensity in southern China.The area affected by extreme drought accounts for a large proportion in the same period,which is obviously different from other drought types.The agricultural drought in China has undergone a change from "wet to dry",and the area affected by agricultural drought and the degree of"drying" of soil water are both on the increase.Groundwater drought is characterized by high frequency,short duration and low intensity in the south,while it is dominated by long duration and high intensity in the north.(2)The propagation characteristics from meteorological drought to hydrological,agricultural and groundwater drought are expounded.The hydrological drought in the southern region of China is most closely related to the meteorological drought,and the propagation time is basically dominated by one month in the whole country.The propagation time at the basin scale is faster in summer and autumn.The propagation time of meteorological to agricultural drought is faster in most of the southern regions and slows down with increasing latitude.The seasonal propagation time characteristics of the basins further indicate that soil moisture changes are mainly influenced by precipitation and temperature.From the perspective of spatial scale,the propagation path of drought in the process of water cycle was identified as meteorological-hydrological-agricultural-groundwater drought.(3)A drought-triggered threshold model was created and the drivers affecting the dynamics of the threshold were analyzed.In the long chain propagation process of meteorological-hydrological-agriculturalgroundwater drought,the risk probability triggering the next stage is gradually decreasing.Under meteorological drought stress,the probability of hydrological drought in the Yangtze River,Huaihe River,Southeast and Pearl River basins is much higher than that in other basins.The probability of agricultural drought in eastern China is significantly higher,and the trigger threshold gradually decreases with the direction of coastal to inland.In addition to agricultural drought induced groundwater drought,groundwater drought is also affected by other factors.North China,the middle reaches of the Songliao River Basin,the upper reaches of the Yangtze River and the Yellow River and northern Xinjiang are high-risk areas prone to groundwater drought.The trigger thresholds in the eastern part of the country showed a significant(p<0.05)increasing trend,indicating that their resistance to drought is weakening.Temperature,dryness and soil evaporation were the driving factors affecting the dynamics of trigger thresholds.(4)Quantifying the exposure of human social systems and the vulnerability of natural ecosystems under the impact of various types of drought.In 2007,2004,2011 and 2019,China ’ s population was exposed to the most drought years of meteorology,hydrology,agriculture and groundwater,reaching 272 million,266 million,475 million and 392 million,respectively.North China is the high exposure area of all types of drought population,and agricultural drought is the main type of drought affecting population exposure in China.Vegetation responses to water changes are usually longer in arid and semiarid and semi-humid areas than in humid southern areas.From the perspective of population exposure and vegetation vulnerability,the impact of drought on China’s human social system is mainly concentrated in the east of Hu Huanyong line,and the impact on natural ecological system is mainly concentrated in the west of Hu Huanyong line.(5)Predicting the evolution of multiple types of drought in China under future climate change and its impact on population size.With the intensification of emission scenarios,the interannual variability and regionality of precipitation are stronger at the national scale.Under the RCP8.5 scenario,the temperature rise in China increased from southeast to northwest.At the end of the 21st century under RCP6.0 and RCP8.5 scenarios,higher frequency,longer duration and higher intensity meteorological droughts are expected to occur in most of the southern part of China.The change of hydrological and meteorological drought frequency has a similar spatial pattern,and the duration of hydrological drought in China is significantly shortened.Under the three emission scenarios,it is expected that the agricultural drought in Northwest China has a significant weakening trend,but the southern region still faces the risk of high outbreak frequency and long duration.In the future,North China and South China will have the largest population exposed to various types of drought as high-density areas of population.Population exposure to meteorological and agricultural droughts is projected to be highest by the middle and late 21st century. |
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