Spatiotemporal Evolution Of Ecological Drought In The Future Over Northwestern China

Global warming has led to more serious ecological problems and ecological drought is becoming a hot topic of research in the field of ecohydrology.Ecological drought refers to the phenomenon of"intermittent shortage of water available to an ecosystem caused by natural and human activities,resulting in the deviation of the ecosystem from its optimal performance".In recent years,research on ecological drought theory has made some progress in the definition,vulnerability and monitoring of ecological drought,but little research has been done on ecological drought prediction.Elucidating the evolution of ecological drought in the context of climate change can provide theoretical support for mitigating ecological drought risk and conducting ecological drought early warning.In this study,meteorological elements of five climate models（GCMs）from the 6th International Coupled Model Comparison Program（CMIP6）were downscaled based on observations and the data accuracy was assessed based on the bias-corrected spatial decomposition（BCSD）method.The monthly-scale ecological water demand,effective precipitation,and ecological water scarcity in the Northwest from 1982 to 2100 were then calculated using the Penman-Monteith[CO₂]method considering CO₂ concentration,the single-crop coefficient method,and the Saxton equation based on LUH2 land-use data,using the meteorological elements of the GCMs averaged over the multi-model ensemble as input.A standardized ecological drought index（SEWSI）based on ecological water shortage was constructed,and the characteristic variables of future ecological drought were extracted by tour theory,and the spatial and temporal evolution patterns and risk characteristics of future ecological drought were analyzed by using the rotating empirical orthogonal decomposition method（REOF）and Copula function.The main conclusions reached are as follows:（1）The application of the BCSD downscaling method in Northwest China is systematically evaluated.The BCSD method can better reproduce the spatial distribution characteristics of the annual and monthly mean values of meteorological elements,but there are some shortcomings in the reproduction of the trends of each element in the study area,which are mainly affected by the simulation accuracy of the GCMs themselves.The trend reproduction of temperature and radiation is good,that of relative humidity is poor,that of precipitation and 10m wind speed as a whole is good,and that of month-by-month is slightly inadequate.After downscaling by BCSD,the simulation accuracy can be significantly improved,and the NSE of all meteorological elements can reach above 0.5,which can ensure the quality of the data.（2）A method for calculating ecological water demand based on climate model data was proposed,and ecological water demand,effective precipitation and ecological water deficit were estimated for the SSP2-4.5 and SSP5-8.5 climate scenarios in Northwest China from1982 to 2100.The average monthly water deficit in Northwest China during the base period（1982～2014）was 11.3 mm,with 78.9%of the water deficit area,and the water deficit-free areas were the Yili Basin,southern and eastern Qinghai,southern Shaanxi,and near the Kunlun Mountains.From 2015 to 2100,ecological water demand,effective precipitation and ecological water deficit showed a significant upward trend;compared with the base period,ecological water demand increased by 8.3%and 15.5%,effective precipitation increased by7.9%and 12.4%,and ecological water deficit increased by 8.7%and 18.7%.（3）The standardized ecological drought index SEWSI was constructed to reveal the spatial and temporal evolution patterns of ecological drought in different divisions and drought risks under the future SSP2-4.5 and SSP5-8.5 climate scenarios in Northwest China.The constructed SEWSI has a high correlation with the commonly used drought index SPEI and vegetation coefficient,and can better monitor the ecological drought in Northwest China.Both the SSP2-4.5 and SSP5-8.5 scenarios show an increasing trend of ecological drought in the future,occurring once every 2-3 years on average.Compared with the SSP2-4.5 scenario,the SSP5-8.5 scenario has a higher probability of ecological drought,with 2060-2065,2070-2075 and 2092-2095 being the more serious periods of ecological drought in Northwest China.The ecological drought in Northwest China has obvious spatial and temporal heterogeneity,and is divided into six sub-regions based on REOF.In the future,Shaanxi-Southern Gansu is the region where ecological drought occurs most frequently,Northern Xinjiang and Tarim basin are the regions where ecological drought occurs most severely,while the ecological drought in Northwest Qinghai,East-Central Gansu and the Yili Basin will be alleviated...