Drought Dynamics And Vegetation Response On The Qinghai-tibet Plateau Based On Integrated Drought Index

In recent years,research on the spatial and temporal characteristics of drought and vegetation response on the Tibetan Plateau in the context of global climate change has received extensive attention.Exploring the characteristics of drought dynamics and the response of vegetation to drought on the Qinghai-Tibet Plateau helps to deepen the understanding of the response of vegetation to drought in alpine regions.It can also provide theoretical basis and scientific support for enhancing adaptation and mitigation of climate change.This study takes the Qinghai-Tibet Plateau as the study area,and uses the daily observed precipitation,temperature and wind speed data from meteorological stations as factors to construct a comprehensive drought index(DI).After that,the applicability of DI in the study area of Qinghai-Tibet Plateau was evaluated comprehensively by using data such as drought-forming area in previous years.This lays the foundation for the analysis of the dynamic change pattern and characteristics of drought.A combination of empirical orthogonal function decomposition(EOF)and rotated empirical orthogonal function decomposition(REOF)is used to study the drought partitioning of the Tibetan Plateau.Using the methods commonly used in climate change analysis,such as Theil-Sen median trend analysis,Mann-Kendall test combined with t-test for mutation,and Morlet wavelet analysis,the spatial and temporal patterns and characteristics of drought and vegetation in different drought sub-areas of the Tibetan Plateau were analyzed from different spatial and temporal scales.Finally,the response of vegetation within different drought sub-areas of the Tibetan Plateau to different time scales and gradients of drought,the lag of response,and the threshold of response were investigated using correlation analysis and generalized additive model(GAM).The main conclusions of the study were drawn as follows.(1)The new composite drought index(DI)is based on the daily observations of meteorological stations for the past 60 years,and is improved from the composite drought index(MCI),which retains the near-30-day relative wetness index(MI30)that better reflects the balance of precipitation evapotranspiration on short time scales,and the 60-day standardized weighted precipitation index(SPIw60)that reflects the sensitivity to heavy precipitation in real-time drought monitoring.SPIw60),while the standardized precipitation evapotranspiration index(SPEI)is used instead of the original standardized precipitation index(SPI).Overall,the advantage of DI is that it retains the effect of precipitation on drought at different time scales and increases the effect of evapotranspiration caused by high temperature on drought.Subsequently,the applicability of DI on the Tibetan Plateau was analyzed using the correlation between DI and drought frequency(DF),drought-forming area(DA),and sensitivity to stronger precipitation.The results show that the correlation of DI with both DF and DA is better than that of single drought index and MCI,while DI can monitor different types of precipitation more sensitively.Besides,the correlation between DI and DF and DA increases with the increase of drought intensity.Therefore,the newly constructed integrated drought index DI has a better integrated performance in the Tibetan Plateau.(2)The regional divergence characteristics of drought on the Tibetan Plateau are not clear,if the evolutionary characteristics of drought dynamics are discussed with the Tibetan Plateau as a whole.Therefore,this paper takes DI as the basis and performs orthogonal decomposition of DI data by using the combination of EOF and REOF.The results show that REOF makes the variance contribution of each main component more uniformly distributed while ensuring the total variance contribution remains unchanged.Therefore,the partitioning results using REOF have clear partitioning results and can reflect the characteristics of the drought spatial pattern of the Tibetan Plateau in the context of climate change more accurately.The absolute value of the load vector value of 0.08 is taken as the partitioning limit when partitioning,and combined with the characteristics of the plateau geospatial distribution,the drought spatial type of the Tibetan Plateau will be divided into five types: northern plateau type(sub-areas A),western plateau type(sub-areas B),central plateau type(sub-areas C),eastern plateau type(sub-areas D),and southern plateau type(sub-areas E).(3)In the past 60 years,the frequency of droughts in sub-areas A and sub-areas C showed a significant decrease,while the frequency of droughts in sub-areas B showed an increasing trend.weak droughts in sub-areas D and sub-areas E showed an increasing trend and strong droughts showed a weakening trend.abnormal droughts in sub-areas A,sub-areas C and sub-areas E showed a significant weakening trend,while sub-areas B and sub-areas D showed a weak strengthening trend.On the whole,the northern and central areas of the Tibetan Plateau are characterized by wetting,while the western areas are more severely arid,and the eastern and southern areas show a weak aridity trend.Combined with the results of the mutation test,the aridity situation in the northern part of the Qinghai-Tibet Plateau is relatively stable in recent years,while the aridity situation in the southern part has changed significantly in recent years.(4)Spring droughts occur more frequently on the Qinghai-Tibet Plateau as a whole.All four seasons of drought show a parabolic distribution with extreme values,i.e.,there is a trend of increasing and then decreasing.The parabolic extreme value appeared between 1989 and 1992.Spring and summer droughts have shown a more obvious trend of decrease in recent years;.The frequency of weak drought increases and the frequency of strong drought decreases in autumn,while winter shows an opposite trend to autumn.Among the different subzones,summer droughts occur more often in sub-areas A and sub-areas B,and spring droughts occur more often in the remaining three subzones.In recent years,weak droughts weakened and strong droughts strengthened in sub-areas A;weak droughts strengthened and strong droughts weakened in sub-areas B;droughts in sub-areas C showed a consistent trend of weakening;weak droughts strengthened and strong droughts weakened in sub-areas D,with more obvious changes than those in sub-areas B;and droughts in sub-areas E showed a consistent trend of strengthening.(5)The periodic characteristics of drought frequency and drought intensity in different drought gradients were analyzed by using wavelet analysis.The results of the periodicity analysis of drought frequency showed that the periodicity of sub-areas A was unstable and its period was short,while the periodicity of drought frequency in sub-areas B,sub-areas D and sub-areas E was more significant and longer.The results of the periodicity analysis of drought intensity showed that the periodicity of sub-areas A and sub-areas B was unstable and short,while the periodicity of sub-areas D and subareas E was more significant and its periodicity was longer.Overall,the cycles of weak drought in sub-areas B and strong drought in sub-areas D showed a shortening trend in the last 60 years;the cycle of weak drought in sub-areas E showed an increasing trend.(6)The smoothed normalized vegetation index(SMN)was used as the basis to analyze the vegetation and its change characteristics on the Tibetan Plateau.The results show that the vegetation on the Tibetan Plateau has strong spatial differentiation,and the SMN increases gradually from northwest to southeast,showing the characteristics of low in the north and high in the south,low in the west and high in the east,and low in the mountainous and valley areas.During the past 40 years,the vegetation of the Tibetan Plateau showed a trend of "partial browning and overall greening".The vegetation in the central part of the plateau shows a distinct "browning" state,while the surrounding area shows a greening state.The browning vegetation is mainly located in the central area of sub-areas C and the central area of sub-areas D.The green vegetation is mainly located in the central area of sub-areas E.The results of Hurst index analysis also indicate that the vegetation improvement of the Tibetan Plateau has become the main theme,and the vegetation in some areas will continue to improve in the future.(7)The vegetation response to long time scale drought is becoming less and less significant on the Tibetan Plateau from north to south.The response of vegetation to drought at short time scales is becoming more and more significant.The time lag of vegetation response to drought on the Qinghai-Tibet Plateau generally showed a trend of gradually getting shorter from northwest to southeast.The time lag of vegetation response to drought in the rest of the sub-region is longer than that in Central Asia.The time lag of vegetation response to drought is more than twice as long in sub-areas B than in Central Asia.(8)The generalized additive model was used to fit the SMN and DI,and then to determine the thresholds of vegetation response to drought in each partition.The results showed that the vegetation decline stage of vegetation response to drought was very clear,and the DI thresholds that made the vegetation begin to decline were slightly different in each partition.vegetation in partition A was in the decline stage when DI>0,while partition B was slightly later than partition A,and its decline stage began when DI=-0.4.the DI of the decline stage in partition C,partition D,and partition E were1.1～-0.65,1.05～-0.76,and 1.1～-0.65,respectively.-With the gradual decrease of DI(increase of drought level),a trend of decreasing and then increasing vegetation was observed in several subzones except zone B.Among them,zone A had the fastest vegetation recovery.Among them,zone A showed the fastest recovery of vegetation,with a weak upward trend of SMN from DI=0,followed by zones C and E,with an upward trend of SMN when DI<-0.65,and zone D was the latest,with an upward trend of vegetation SMN until DI<-0.76.Overall,the sensitivity of vegetation response to drought is getting lower from north to south on the Tibetan Plateau,but the resilience of vegetation to drought is getting higher.