Assessing Time-varying Risk Of Drought And Flood From A Multivariate Perspective In The Pearl River Basin,China

Droughts and floods are recurrent extreme phenomena in the hydrological cycle,which subsequently triggered about half of the natural disasters worldwide as well as one-third of economic damage and form the persistent threat to human lives and surroundings.Meanwhile,compound extreme events—concurrence or consecutive occurrence of multiple hydro-meteorological extremes in a short time—have emerged with ascending frequency recently.As compared to a single extreme phenomenon,a compound event joining multiple interactive climatic extremes tends to have a greater influential extent and result in more severe socio-economic costs,currently becoming a hot topic among the hydrological community in a global warming context.Therefore,it is of great importance to assess the time-varying risk of drought and flood from the multivariate perspective in a changing environment.In the study,risk assessment is conducted for both human communities and ecosystems in the Pearl River basin(PRB)which is subject to frequent droughts and floods.A framework is proposed to evaluate the time-varying risk of drought,flood and compound dryness and wetness events(CDWE)risk in a changing environment by closely following the risk concept(i.e.,Risk=hazard probability×exposure×vulnerability)highlighted by the IPCC.The results of the study are the premise of efficient disaster risk management,in favor of the enhanced community and ecological security.Main findings are presented as follows:(1)Droughts,floods and CDWEs as stressors for human society and ecosystems are initially identified using the threshold method and their spatial and preference is analyzed.Results indicate that the western part and eastern edge of the PRB are subject to less frequent drought with,however,long duration and high severity.In the West River basin as well as its two major tributaries,diminished flood duration and volume are noted downstream of the Yantan and Xijin Reservoir,which is attributed to the role of hydrological infrastructure in mitigating flood impacts.The recent four decades withness the occurrence of inter-seasonal compound events of dry and wet extremes with the mean frequency of 21.5 in the PRB and its spatial preference in the middle and eastern parts.(2)Potential nonstationarity in univariate variables and bivariate dependence structure are tested for identified droughts,floods and CDWEs,based on which bivariate nonstationary frequency analysis is performed mainly via the GAMLSS model.Results show that the probability of drought attributes exceeding the specified duration and severity is remarkably higher in the west of the Nanbeipan River,the western and northern parts of the Hongliu River and upper and lower streams of the East River.Floods with peak or volume exceeding design values tend to happen in the upper stream,while the concurrence of peak and volume beyond design values is more frequent in the lower stream,which represents the more adverse situation than the former.Return periods of CDWEs are estimated to be 2.8,14.2 and 83.3 years under the moderate,severe and extreme scenarios.Persistent wetness and dryness have a higher probability to occur relative to the dryness-wetness transition events.(3)Exposure and vulnerability with multi-faceted nature are expressed as indicators driven by gridded big data including the exposed population,the gross domestic product,human development index,water resource stress,water body area,percentage of human settlement and other diverse influencing factors.Eastern PRB,particularly the Pearl River Delta,maintains a higher level of human-economic exposure relative to the western proportion.Human-economic exposure increase with an annual rate of 4.49×10-3.In comparison,the vulnerability of human community exhibits the reverse spatial distribution and decrease(-9.33×10-3/yr)across the PRB When ecosystems are investigated as well,vegetation in winter has the highest vulnerability(0.601)to drought stress,which is 7.15%,11.3%and 15.6%greater than its counterparts in autumn,spring and summer.Vegetation vulnerability to drought stress maintains high levels in the middle PRB,which is followed by the Nanbeipan River.As for the exposure of ecosystems to drought stress,merely slight heterogeneity is noted with a little higher values in the southwest and northeast portions in the PRB.(4)Risk of droughts,floods and CDWEs is derived by multiplying hazard likelihood,exposure and vulnerability and spatio-temporal dynamics of estimated risk are investigated.Atlas of the above-mentioned risk is specifically developed for the PRB.More than half of the PRB experiences alleviated risk over the recent 40 years,while drought risk presents upsurge in Northwestern Nanbeipan River,Northern Hongliu River and inter-boundary of North River,East River and middle-lower streams of the West River.Over 1997-2017,decreasing trends of flood risk are noticed across the West River,which is most significant at gauge sites of Wuxuan,Dahuangjiangkou and Wuzhou with a decline of 26.9%?31.6%? 31.7%respectively.The descending vulnerability is found to dominate the decline in flood risk.Over the recent four decades,CDWE risk presents a slightly increasing trend(0.286%/yr).Risk of persistent wetness increases at the highest rate,which is followed by the abrupt transition from wetness to dryness.Analysis of time-varying risk of CDWEs in space and time domain reveals that high values of risk gradually aggregate to Nanbeipan River and risk in autumn-winter and winter-spring climbs up more rapidly within a year.As for ecosystems under drought stress,Nanbeipan River,Hongliu River,Yu River and East River have larger ecological risk and high risk is found to mainly concentrate in the western PRB and the East River(especially its upper stream).Ecological risk under droughr stress mainly escalates in the eastern PRB and the western edge.(5)To sum up,Nanbeipan River jointly has high drought,CDWE and ecological risk with intensifying trends,which is the region where disaster risk management ought to primarily be implemented.Flood risk at the lower stream of the West River,although having been alleviated,is significantly higher than other gauges.It,therefore,highlights that persistent efforts are needed to reduce the flood risk at the lower stream of the West River.The highly urbanized Pearl River Delta is densely populated and economically developed.Drought risk and ecological risk within the domain of Pearl River Delta maintain at low levels and favorably dropped over the recent decades.However,its CDWE risk ranks the highest place in the whole PRB,deserving sufficient attention of decision-makers.