The Evolution Of Dry-wet Compound And Their Effects On Vegetation In Typical Areas Of The Yellow River Basin

Compared with a single drought or flood disaster,dry-wet compound events may have more adverse impact on crop production and food security.However,existing studies have paid seldom attention on the evolution characteristics of dry-wet compound events,and their driving factors and losses have not yet been revealed.To end this,firstly,the dry and wet events in adjacent seasons(spring-summer,summer-autumn,autumn-winter,winter-spring)in typical areas of the Yellow River Basin were evaluated based on the standardized precipitation index(SPI),and the joint return periods of various combinations of dry-wet,wet-dry,continuous dry,and continuous wet events under two scenarios(moderate and severe)were calculated using a copula function.The Mann-Kendall test was used to explore the joint return period dynamics with a 31-year sliding window.The Copula-based likelihood-ratio method and cross-wavelet transform were adopted to explore the driving forces of dry and wet combination dynamics.Secondly,taking the Wei River Basin(WRB),which is the largest tributary of the Yellow River Basin,as an example,the drought-flood abrupt transition index was defined to identify the drought-flood abrupt alternation(DFAA)events during the flood season of the WRB,and spatiotemporal evolution characteristics and future trend variability of DFAA events were explored.The driving factors of DFAA events were comprehensively evaluated using qualitative and quantitative combination framework.Finally,based on the annual scale and monthly scale,the vegetation cover dynamics in the WRB were analyzed using the GIMMS NDVI 3G dataset from 1982 to 2013.Pearson correlation coefficient method was used to determine the optimal response time of vegetation to spring-summer and summer-autumn DFAA events.A probabilistic different degrees under DFAA event stress.The main research results of this paper are as follows:(1)spring-summer and summer-autumn are prone to continuous dry(wet)events,while the probabilities of the combinations of dry and wet conditions occurring in autumn-winter and winter-spring are almost the same.Spatially,continuous wet events frequently occur above Longyang gorge and below Huayuankou,while the Inner flow area is prone to continuous dry events.(2)the risk of dry and wet combinations with high frequency decreases,whereas the risk of compound events with low frequency increases.The dependency structure dynamics of the SPI series between adjacent seasons show the strongest correlation with the El Nino-Southern Oscillation(ENSO),followed by the Arctic Oscillation and sunspot activities,implying that the ENSO has a dominant control on the dry and wet combination dynamics in the YRB.(3)the flood season is dominated by flood to drought(FTD)events in the WRB,and the upstream of the WRB and Jing River basin(JRB)were dominated by the drought to flood(DTF)events before 1976.Spatially,the frequency of DTF events in the WRB presents a "less-more-less" variation pattern from southwest to northeast and shows a significant spatial difference.However,the FDT events are vice versa.DTF events dominated the WRB after 2010,and the events more likely occur the BLR and east of the JRB.Average water vapor pressure is the dominant factor of DFAA events in the WRB compared with other meteorological factors,whereas Arctic Oscillation among multiple teleconnection factors exerts strong impacts on DFAA dynamics.(4)Vegetation coverage in the WRB improved from 1982 to 2013,and the NDVI obviously increased after 2000.The monthly average NDVI shows a weak unimodal trend,and the vegetation activity is the best from April to October.The distribution of NDVI gradually increased from northwest to southeast,and the vegetation coverage is higher in the middle and lower reaches of the WRB and the south of Yan'an.(5)Compared with DTF events,FTD events have more inhibition effect on vegetation in the basin.Lag response time of vegetation coverage to DFAA events in the WRB is more than 4 months.Under the same vegetation loss scenario,the vegetation loss probability under spring-summer DFAA events stress in the WRB is higher than that in the summer-autumn season,indicating that the vegetation activity in the basin is more sensitive to the spring-summer DFAA events.Spatially,the vegetation activity in the JRB is more sensitive to FTD events,whereas the vegetation in the lower reaches of the WRB has stronger resistance to FTD events in summer-autumn.Under the stress of spring-summer and summer-autumn DTF events,the vegetation vulnerability in the upper and middle and lower reaches of the WRB is relatively high,which should be paid more attention to.