| Drought is a kind of long-term natural disaster with high frequency and universal existence all over the world.Due to the comprehensive influence of hydrometeorological factors in the occurrence and development of drought,a single type of drought index is not enough to represent the characteristics of multiple droughts and the drought condition of vegetation.How to construct comprehensive drought index which can capture multiple types of droughts more comprehensively has become one of the hot issues in the current study.Therefore,the construction of a comprehensive drought index that can not only represent vegetation status but also comprehensively monitor different types of drought,and clarify the temporal and spatial evolution of drought and its risk distribution characteristics can provide scientific basis for making emergency measures against drought in advance,effectively planning water resources,improving drought resistance and disaster reduction ability,monitoring and warning vegetation health status,etc.Standardized meteorological,agricultural,groundwater and ecological drought indices(SPI,SSI,SGI and SEWDI)were constructed by parametric or non-parametric methods to represent meteorological,agricultural,groundwater and ecological drought in Northwest China based on precipitation,soil water,land water reserves data and ECWMF ERA5 reanalysis data from 1982 to 2020.Five comprehensive drought indexes,i.e.,CDI＿E,CDI＿C,CDI＿P,CDI＿K and CDI＿Kn,were constructed by coupling with four single type of drought indexes based on entropy weight method,correlation coefficient weighting method,principal component analysis method,kernel entropy component analysis method and Kendall-Copula function method.The correlation between five comprehensive drought indexes and standardized chlorophyll fluorescence(SSIF),Self-calibrating Palmer drought index(sc PDSI)and four univariate drought indexes was compared and analyzed.The ability of various comprehensive drought indexes to monitor drought was evaluated and verified according to typical drought events recorded in Northwest China.The temporal and spatial evolution characteristics of comprehensive drought were analyzed based on linear regression and run distance theory.The spatial distribution,dynamic changes and transmission characteristics of drought risk were evaluated by using equal-weight method of risk factors,10-year sliding window and Markov chain.The main conclusions are as follows:(1)The variation trend of drought in northwest China was analyzed and compared.In most regions of Northwest China,meteorological,agricultural and ecological drought showed a trend of remission,while groundwater drought showed a trend of aggravation.The proportion of extremely significant drying area was ranked as groundwater drought >agricultural drought > ecological drought > meteorological drought,while the proportion of alleviating area of each type of drought was ranked as ecological drought > meteorological drought > agricultural drought > groundwater drought,and with the increase of time scale,the proportion of wetting area increased,the correlation between single type drought index and SSIF in each climate region was stronger,which can better reflect the long-term vegetation change.(2)The correlation between the CDI＿C and SSIF at 1-,3-and 12-month scales is strong in different climate zones,but decreases with the increase of time lag.Short-term and longterm vegetation status changes can be predicted according to the CDI＿C of different time scales.The correlation between CDI＿C and sc PDSI,SPI,SSI,SGI,SEWDI is strong,and the significant correlation area accounts for about 90%,which can better represent the vegetation status and synthesize the signals of various types of droughts,and can best reflect the comprehensive drought status in Northwest China.The drought events recorded in Northwest China were gradually shortened in duration and the regional distribution area was gradually reduced.Severe droughts occurred in 1986,1991,1995 and 1997,the drought conditions reflected by CDI＿C were consistent with them.(3)A comprehensive drought index suitable for northwest China was constructed and optimized.The change trend of comprehensive drought represented by CDI＿C showed that the proportion of arid area increased first and then decreased,and most areas showed a significant trend of wetting.The frequency of drought in Northwest China is light drought >medium drought > extreme drought > severe drought,and the spatial distribution is higher in arid and semi-arid regions than in humid and sub-humid regions.Compared with 1-and 3-month time scale CDI＿C,the frequency of drought reflected by 12-month time scale CDI＿C decreased,the proportion of drought area,the frequency of drought,the proportion of significantly wetted area,the duration and intensity of average drought were larger.(4)The change and transmission characteristics of comprehensive drought risk in northwest China were clarified.In arid area,the drought risk,exposure and vulnerability decreased first and then increased,and the resilience increased first and then decreased.The long-term change of drought risk and risk factors was small.While the drought risk characteristics in humid,subhumid and semi-arid areas were opposite to those in arid area.The regions with high drought risk are mainly distributed in the arid and semi-arid areas of Xinjiang and Qinghai,which are characterized by long duration of drought,high intensity,high frequency,high exposure,strong vulnerability and weak resilience.The probability of drought risk shifting in the same level for two consecutive years in each region is greater.In arid and humid regions,low and medium-low drought risk levels were easy to maintain or transfer to a higher level;in medium-high and high-drought risk levels were easy to maintain or transfer to a lower level;in semi-arid and sub-humid regions,low,medium-low and medium-high drought risk levels were easy to maintain or transfer to a higher level;and high drought risk levels were easy to maintain or transfer to a lower level.Therefore,alleviating the long-term drought in arid and semi-arid areas and coping with the change of drought risk in the subhumid areas are the focus of drought resistance in Northwest China. |
PDF Full Text Request