| Drought is one of the most widespread natural disasters in the world with the most serious economic losses.Even with the rapid development of science and technology,the consequences caused by drought hazards are unavoidable.Likewise,China is also a frequently drought-impacted country,and drought has become a major obstacle to restrict China's economic and social development and agricultural production.Especially in recent years,droughts in China have become increasing serious,frequent and widespread due to the impacts of climate warming and intensive human activities(underlying surface/land use change),which has seriously threatened the ecology,food and even the safety of human life.Hence,reasearches on drought from a systematic perspective is urgent and important.In this study,the Yellow River Basin(YRB),known as the 'Mother River'of China,is taken as a case study to put forward a set of drought theoretical system integrating drought evaluation,propagation,driving and prediction systematically.The YRB is divided into six subzones based on different climate characteristics.For each subzone,the mathematical model for drought assessment and risk calculation is established,and based on the model,the meteorological,agricultural and hydrological drought evolution laws and driving factors are identified,the drought driving and propagation mechanism is revealed,the internal relationship among meteorological,agricultural and hydrological drought is analyzed,the risk probabilitis of meteorological,agricultural and hydrological drought under future climate and land use change scenarios are predicted,and finally the comprehensive drought resistance measures and suggestions are given.The research,achievements are expected to provide a complete theoretical system and technical guidance for basin's drought prevention and resistance scientifically and effectively.The main research contents and results are as follows:(1)Change trends of precipitation and temperature in the YRB are analyzed based on the observation data from 1966 to 2010 and NEX-GDDP downscaling data.The results show that the precipitation in the past 45 years showed decreasing trend except that in zone A(high-cold belt semi-arid and semi-humid region)and zone F(warm temperate belt humid region)with slight increasing trends,while the temperature showed increasing trend in the whole basin.Compared with the past 40 years(1971-2010),the precipitation during the period of 2011-2050 will decrease obviously except that in zone C(middle temperate belt arid and semi-arid region)and zone F(warm temperate belt humid region)with increasing trends,while the temperature will increase greatly except that in zone B(cold-temperate belt semi-arid and semi-humid region)and area C(middle-temperate belt arid and semi-arid region)with no obvious variations.(2)Based on CA-Markov model and the remote sensing images of land use patterns in 1980,1990,2000 and 2010 of the YRB,the future land use patterns of the YRB are predicted.And the land use change characteristics from 1980 to 2050 in each subzone of YRB are analyzed by using GIS technology and the land use transfer matrix.The results show that the main land use types in the YRB are farmland,woodland and grassland,but the conversions among them are highly frequent.Compared with 2010,the farmland and grassland areas will decrease while the woodland and build-up areas will increase in 2050.(3)An alternative approach of time-varying parameters in SWAT model is proposed to reflect the changes in underlying land surfaces,and the simulation performance is compared with that by modeled with traditional static parameters.The result show that the time-varying parameter calibration is not only effective but also necessary to ensure the validity of the model when dealing with significant changes in underlying land surfaces.While static model parameters can degrade model performance and the indexes such as Ens and |Rel|fail to reach the benchmark,further verifying the rationality and necessity of time-varying parameter in hydrological model proposed by this study when dealing with significant changes in underlying land surfaces caused by high intensity of human activities.(4)Based on the calibrated SWAT model and drought assessment methods,the temporal and spatial evolution characteristics and propagation mechanism of meteorological,agricultural and hydrological drought conditions in the YRB are analyzed and discussed.The results show that the meteorological and agricultural droughts were the most serious in the 1990s,while in the 2000s,the meteorological and agricultural droughts in autumn and winter in most areas of the YRB were significantly alleviated,but the meteorological droughts in spring and summer were intensified.Hydrological drought in the 1990s was prone to occur in autumn,but that in the 2000s was more prone to occur in spring and was easily escalated to the severe and extreme drought categories in the middle and lower reaches.The propagation time from meteorological to hydrological drought showed obviously seasonal characteristics,which was the shortest in summer with 1-2 months,but the longest in winter with 5-13 months.Compared with agricultural drought,the propagation time from meteorological to hydrological drought increased.Before 2000,the propagation time in the upstream was longer than that of the middle and lower reaches.The development and utilization of water resources has made the hydrological process in the YRB more complicated,and prolonged the propagation time from meteorological to hydrological drought(5)On the basis of identifying the main impacting factors of meteorological,agricultural and hydrological droughts,the relative driving contributions to these three drought types are quantified by setting drought driving simulation schemes.The results show that the impacts of atmospheric circulation anomaly and land use change on meteorological drought are about 95%and 5%,respectively.The impacts of climate variation and land use change on agricultural drought are about 75%and 25%,respectively.The impacts of water resources development and utilization,climate variation and land use change on hydrological drought are about 50%,30%and 20%,respectively(6)Based on future climate scenario data(NEX-GDDP)and land use prediction patterns,the drought risk assessment mode is established and the risk probabilities of meteorological,agricultural and hydrological droughts in the next 30 years(2021-2050)of the YRB are predicted.The results show the YRB will be more prone to experience short-term(1-3 months)meteorological drought in the next 30 years,and the risk in the upstream will be slightly higher than that in the middle and lower reaches.The high-risk regions for experiencing agricultural droughts will mainly distribute in zone A(alpine belt semi-arid and semi-humid region)and zone F(warm temperate zone humid region),while the high-risk regions for experiencing hydrological droughts will mainly distributed in zone C(middle temperate belt arid and semi-arid area)and zone E(warm temperate belt semi-arid and semi-humid region).And finally the coping measures and suggestions for agricultural and hydrological drought resistances are put forward based on the topographic conditions and water supply characteristics of corresponding high-risk zones of agricultural and hydrological droughts,respectively. |
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