Cascade Effect And Risk Assessment Of Drought In Xilin Gol Grasslan

In the context of global change,the suddenness,concurrency,unpredictability,and associated risks of various natural hazards are becoming increasingly prominent.Recent studies indicate that greenhouse gas emissions will contribute to the simultaneous or sequential occurrence of multiple meteorological hazards,and extreme climate events will increase.However,a large number of studies have tended to focus on the risks posed by single-hazard,single-bearer meteorological hazards,which may overlook the superimposed or cascading effects of these hazards with other hazards,resulting in incomplete assessment of the impacts of climate change.Although the current meteorological hazard monitoring system has developed considerably,most of the hazards are monitored only for a single hazard-bearing body or a single hazard type,rather than the whole process of hazard development,which leads to inconsistent monitoring conclusions often given by different sectors or different hazard monitoring products.In this context,this study selects the Inner Mongolia temperate Xilin Gol grassland drought chain as the research object to carry out basic research on chain disasters under climate change and intensification of human activities,in order to provide scientific basis for curbing the evolution and spread of disaster systems and mitigating disaster losses at the source.The standardized precipitation evapotranspiration index SPEI（precipitation P-potential evapotranspiration PET）,standardized evapotranspiration drought index SEDI（actual evapotranspiration ET/potential evapotranspiration PET）,and soil moisture index SSI were selected as indicators of meteorological drought,ecohydrological drought,and soil drought,respectively.The modified Granger-Gray（GG）model was used to estimate the actual evapotranspiration,and combined with the maximum Pearson correlation coefficient method（MPCC）and structural equation modeling（PLS-SEM）and Mann-Kendall trend method to detect the characteristics of spatial and temporal pattern changes of the three types of droughts in the study area during 1990-2019and their propagation characteristics in different seasons.The cascading effects among the three drought types were also quantified and analyzed by considering the effects of vegetation,snowpack and human activity factors.The main conclusions are as follows:（1）The spatial and temporal characteristics of meteorological drought,soil drought and ecohydrological drought in the Xilin Gol grassland were detected.It was found that the warm-drying trend of Xilin Gol grassland was obvious in the last 30 years,especially during 1990-2005,when it experienced a severe warm-drying climate.Although the drought was alleviated after 2005,the drought has persisted.Spatially,SPEI and SEDI at the 12-month scale showed high consistency,and both showed a significant decreasing trend in the east and northwest,indicating that meteorological drought and ecohydrological drought may have aggregated and appeared to be aggravated to some extent.It is noteworthy that the SEDI has a wider range of significant decreasing trends（i.e.,drought trend）than the SPEI.The temporal and spatial trends and pattern differences of the three drought indices illustrate the complexity of the drought phenomenon.（2）We explored the use of partial least squares structural equation modeling（PLS-SEM）for integrated risk assessment of grassland drought hazard cascade effects,quantitatively described the empirical and mechanistic causal mechanisms of multiple latent variables including natural and social factors in the evolution of drought hazards.On this basis,the interrelationships between meteorological drought hazard risk evolution process and other drought hazard events are clarified.We found that meteorological drought had a significant effect（p<0.05）on both soil drought(f²_east=0.50,f²_south=0.20,f²_northwest=0.34,f²_centre=0.27)and ecohydrological drought(f²_east=1.17,f²_south=0.64,f²_northwest=0.77,f²_centre=1.21)throughout the Xilin Gol grassland region.The strong cause-effect relationship indicates that warm-dry meteorological conditions have a significant cascade effect on both soil drought and ecohydrological drought,i.e.,persistent meteorological drought is more likely to trigger the other two types of droughts,and there is a tendency for the propagation of meteorological drought to ecohydrological drought to increase in intensity and scope.（3）The drought cascade effect differs for different time scales（seasonal drought,annual drought and multi-year persistent drought）.The effect of multi-year persistent drought on soil drought and ecohydrological drought cascade effects is decisive.Persistent meteorological drought and reduced intense precipitation are the main causes of deep soil drying in grasslands in the region over the past 30 years.For seasonal or annual scale droughts,vegetation amplifies or mitigates the drought cascade effect,where soil moisture,surface evapotranspiration,and the relationship between the two play important roles.The propagation of different drought types has a distinct seasonality.In spring ecohydrological drought was not sensitive to soil drought.In the cold winter,the propagation of different drought types was longer.In the summer with more precipitation and higher temperature,the propagation relationship of different drought types was stronger.If insufficient precipitation triggers meteorological drought,coupled with rapid temperature increase will accelerate evaporation of surface soil moisture,which may lead to rapid decrease of soil moisture and trigger soil drought,which in turn affects ecohydrological drought.（4）The cascade effect of drought varies across regions.Land use and cover changes as well as winter snow conditions affect the cascade effects between meteorological drought,soil drought to ecohydrological drought.Land cover change plays an important role in ecohydrological drought conditions by changing evapotranspiration.In the east,where the vegetation cover is better,the deterioration of the previous vegetation aggravates the cascade effect of meteorological drought to reduce evapotranspiration and aggravates ecohydrological drought.However,in the northwestern sparsely vegetated areas,evapotranspiration was mainly influenced by meteorological factors,and the cascade effect between soil drought and ecohydrological drought was not obvious.Population growth,urbanization,overgrazing and other human activities can also amplify the impact of meteorological drought on grassland ecosystems.In recent years,the implementation of local"fencing"and"returning grazing to grass"policies have improved the vegetation cover,which has a certain impact on the cascade effect of meteorological drought and to some extent alleviated the local ecohydrological drought.