|Reponse of drought in China to global warming and their impacts on ecosystems
|刘玉芝 ; 张强
|Place of Conferral
|干旱事件 Drought events 过程特征 process characteristics 主导因素 dominant factors 气候变暖 climate warming 降水非均匀性 precipitation heterogeneity
干旱是中国最主要的自然灾害之一，在全球变暖的背景下，干旱的响应更是异常复杂，干旱研究面临许多新的挑战，深入开展干旱演变规律、主导因素及其影响方面的研究关系经济和社会的可持续发展，对于指导区域气候影响评价、风险管理及防灾减灾具有重要的科学意义。本论文利用气象台站观测资料、再分析资料以及遥感资料等，在评估多种干旱监测指标在中国适用性的基础上，利用最优指数分析了中国干旱的时空特征，重点从过程角度研究了干旱事件次数、强度、持续时间、易发区域等的变化特征，之后，为厘清影响干旱的主导因子，分析了蒸散和降水变化对区域干旱趋势的定量贡献，同时，探究了降水非均匀性和增温对干旱事件的发生、发展的影响，最后，从生态系统降水利用效率角度，探讨了生态系统降水利用效率随气候类型和干旱强度的转换特征及其机制，主要结论归纳为以下几个方面： （1） 确定了中国不同区域、不同季节的最优干旱监测指标。在我国干旱监测中，MCI指数和K指数的监测效果要优于 SPI、Pa和 SPEI指数。MCI指数对春旱的监测尤具有优越性，K指数对偏东和偏南区域夏、秋、冬季旱情的监测能力略优于MCI指数，Pa和SPI指数对夏、秋季旱情的监测准确率较高，而SPEI指数对夏旱有较强的监测能力。各指数的干旱监测能力与其考虑的主要致旱因子和因子的时间尺度密切相关，Pa、SPI以及SPEI指数监测准确率低主要是因为这些指数考虑的因子单一或时间尺度较短。 （2） 系统揭示了中国干旱的时空分布和演变规律。我国华北、东北地区中南部、西北地区东部以及西南区域是干旱的多发区域，平均每十年有14~20次干旱事件，年干旱日数在120~140d之间，局地在140d以上；干旱高发的北方大部干旱持续时间在2个月以上，而南方多为月尺度干旱事件。春旱和夏旱是我国干旱事件的主导类型，两季连旱过程以春夏连旱和夏秋连旱居多，偏南区域两季连旱以秋冬连旱和冬春连旱偏多；近60a来，变湿的西北地区中西部各季干旱日数均在减少，而变干的华北地区主要是秋、冬季趋于干旱，东北区域秋季趋于干旱，西南地区夏、秋、冬季趋于干旱，而西北地区东部和长江中下游地区主要是春、秋季趋于干旱。 （3） 量化了蒸散和降水变化对中国不同区域干旱变化趋势的贡献程度。1960-2017年，我国趋于干旱的时段和区域，蒸散的贡献均大于降水，区域变干的进程中蒸散的变化起了主导作用。干旱、半干旱区干旱变化趋势对蒸散变化高度敏感，而湿润和半湿润区干旱变化趋势对降水的变化高度敏感。蒸散对干旱区干旱趋势的影响略大于降水，蒸散减小加之降水增加使区域变湿，湿润区除春季外，其余时段降水的贡献略大，降水增加和蒸散减小使区域变湿，而过渡区蒸散的影响明显大于降水，蒸散的显著增加和降水的减少使区域变干。风速显著减小是干旱区蒸散量减少的主要原因，温度上升使过渡区蒸散增加，而湿润区日照时数和风速的减小使蒸散减小。 （4） 阐明了气候变暖背景下降水非均匀性对区域干旱的重要影响。降水越集中，降水集中期越偏晚，弱降水日数越少，干旱强度越强。干湿过渡区30%~50%的季节内干旱事件发生在季降水量偏多但降水时间分布极不均匀的条件下，此时，春、秋季降水集中度一般在0.5以上，夏季集中度在0.4以上，春、夏季降水集中期分别在5月和8月之后，秋季集中期在9月上中旬。降水非均匀性对干旱的影响在春季尤为突出，春季降水非均匀性变化是区域干旱强度增强的主要原因，而夏、秋季区域内干旱强度增强主要是因为降水量和弱降水日数的显著减少。 （5） 发现了气温突变后我国干旱事件发展速度加快的重点区域。我国不同区域干旱事件发展期随区域湿润程度的增加而减小。干旱区干旱发展期大多在35d以上，局地在60~90d，干湿过渡区在25~40d之间，湿润区大部在20~30d之间；显著变暖后，东北地区南部、华北、西北地区东部、西南东部、西藏中部、长江中下游以及华南沿海等地的部分区域干旱事件发展期明显缩短，其中，半湿润区缩短最明显；升温幅度越大、区域越湿润、干旱发展速度越快，南方湿润区干旱发展速度大于北方，东北南部、华北北部、西北地区东部干旱发展速度大于北方其余区域，显著变暖后，大部区域干旱发展速率加快，尤其是在300~600mm降水量范围内的过渡区，蒸散增加和降水减少共同作用使该区域成为我国显著变暖后干旱发展速度显著加快的重点区域。 （6） 揭示了降水利用效率随干旱强度的转换特征及其机制。中国大陆生态系统实际蒸散量和净初级生产力自西北向东南增加，以103ºE为界，以西区域1960-2017年实际蒸散量和净初级生产力增加，以东区域减小。降水利用效率自西北向东南呈“低——高——低”的空间分布格局，过渡区降水利用效率最高。干旱区土壤湿度增大是降水利用效率增加的最主要驱动力，湿润区净辐射的变化是降水利用效率变化的主要驱动力，过渡区是植被生长限制因素由水分向能量过渡的转换区域，降水利用效率变化受能量、水分和动力因子共同作用。从大气干旱与土壤干旱对降水利用效率的影响来看，土壤水分胁迫对年降水利用效率的影响大于大气水分胁迫，土壤水分是影响生态系统碳水循环的主导因子。
Drought, as a prevalent natural disaster in China, exhibits an exceptionally intricate response to global warming, thereby presenting numerous challenges for drought research. In-depth exploration of drought evolution, dominant influence factors, and assessment of the impacts is closely related to the sustainable economic and social development. Moreover, it is of great scientific significance in guiding regional climate impact assessment, risk management, and disaster prevention and mitigation. Therefore, by using meteorological observation data, reanalysis data and remote sensing data, we evaluate the suitability of different indexes for drought monitoring in this study. Then, we analyze the characteristics of drought in China using the most suitable index. We investigate the spatio-temporal characteristics of drought events, including frequency, intensity, duration and susceptible regions. To identify the primary influence factors of drought, a quantitative analysis is conducted to determine the contributions of evaporation and precipitation to regional dryness or wetness. Furthermore, this study explores the effects of precipitation heterogeneity and warming on the occurrence and evolution of drought events. Finally, we discuss the variation characteristics and related mechanisms of ecosystem precipitation use efficiency with drought intensity. The main conclusions are summarized as follows.
(1) The optimal drought-monitoring index across various regions and different seasons in China is determined. MCI (meteorological drought composite index) and K index are more effective than SPI (standardized precipitation index), Pa (precipitation anomaly in percentage) and SPEI (standardized precipitation evapotranspiration index) for drought monitoring in China. Notably, the MCI index is superior for monitoring spring droughts, while the K index is slightly better than MCI for monitoring summer, autumn and winter droughts in the eastern and southern regions. Pa and SPI are more accurate for monitoring summer and autumn droughts, and SPEI index has a stronger monitoring ability for summer droughts. Furthermore, the monitoring ability of each index closely correlates with the primary drought-causing factors and timescales. The low monitoring accuracy of Pa, SPI and SPEI indexes primarily stems from their consideration of the single factor or short timescales.
(2) The spatio-temporal characteristics of droughts in China are analyzed. North China, the southern-central Northeast China, the eastern Northwest China, and Southwest China are susceptible regions of drought. These regions experience an average of 14–20 drought events per decade and an annual average of 120–140 drought days, with more than 140 drought days in some local areas. Most droughts in the northern regions last for more than two months, whereas most of the droughts in the south last for about one month. Spring and summer droughts are the dominant types of drought events in China. Notably, spring-summer droughts and summer- autumn droughts are the most common consecutive drought events. In the south, consecutive droughts tend to occur in autumn-winter and winter-spring. In the central-western Northwest China where there is a wetting trend, the number of drought days decreases in each season, while in North China where there is a drying trend the drought days increase in autumn and winter. The Northeast China tends to be dry in autumn; the Southwest China tends to be dry in summer, autumn and winter; the eastern Northwest China and the middle and lower reaches of the Yangtze River tend to be dry in spring and autumn.
(3) The contribution of evapotranspiration and precipitation to drought trends in different regions of China was quantified. From 1960 to 2017, the contribution of ET0 is greater than that of precipitation in the regions that have drying trend in China. The sensitivity of dry/wet trends to the variability of ET0 is particularly notable in arid and semi-arid areas, while precipitation plays a crucial role in humid and semi-humid areas. In arid areas, the contribution of ET0 is larger than that of precipitation, and a decrease in ET0 combined with an increase in precipitation leads to a wetting trend. In humid areas (except for spring), precipitation makes a slightly greater contribution than ET0, and an increase in precipitation accompanied by a decrease in ET0 results in a wetting trend. Similarly, in semi-arid and sub-humid regions the effect of ET0 is significantly greater than that of precipitation. The substantial increase in ET0 and decrease in precipitation lead to a drying trend. The obvious decrease in wind speed is the main reason for the decrease in ET0 in arid areas, while the increase in temperature contributes to higher ET0 in semi-arid and sub-humid areas. Conversely, the obvious decrease in sunshine hours and wind speed in humid areas reduces ET0.
(4) The influence of precipitation heterogeneity on regional droughts is revealed. The seasonal drought intensity will be stronger when the precipitation is more concentrated, the precipitation concentration period is late and the number of weak precipitation days is few. In semi-arid and sub-humid areas, approximately 30%–50% of the intra-seasonal drought events occur when there is high seasonal precipitation but with extremely uneven distribution. During this time, the precipitation concentration generally surpasses 0.5 in spring and autumn, and exceeds 0.4 in summer. The precipitation concentration periods occur after May in spring, after August in summer, and during the first half of September in autumn. The influence of precipitation heterogeneity on drought is particularly prominent in spring, and the change in precipitation heterogeneity during this season is the primary reason for the increased drought intensity. In contrast, the increase in drought intensity in summer and autumn is mainly attributed to the decreased precipitation and a significant reduction in the number of days with weak precipitation.
(5) The hotspot areas where the rate of drought onset in China has accelerated after abrupt temperature changes have been identified. The onset time of drought events in different regions of China decreased with an increase in regional wetness.The onset time was mostly >35 days in arid areas and 60–90 days in local areas, between 25–40 days in semi-arid and sub-humid areas, and between 20–30 days in most humid areas; After 1994, in southern Northeast, North China, eastern Northwest, eastern Southwest, central Tibet, middle and lower Yangtze River, south China coast, and some other regions, the onset time have significantly shortened, among which the sub-humid zone was shortened most obviously; the greater the warming, the wetter the region, and the faster the drought onset rate. Drought onset rate in the southern humid zone was greater than that in the north, and in the southern Northeast, northern North China, eastern Northwest China, onset rate was greater than the rest of the northern regions. After 1994, onset rate was accelerated in most regions, particularly in semi-arid and sub-humid areas within the precipitation range of 300–600 mm, the combined effect of increased evapotranspiration and reduced precipitation makes this region a hotspot area in China.
(6) The conversion characteristics and mechanisms of the ecosystem precipitation use efficiency with respect to drought intensity in different climatic zones were determined. The actual evapotranspiration and net primary productivity of ecosystems in mainland China increase from northwest to southeast, with the boundary at 103º E. From 1960 to 2017, the actual evapotranspiration and net primary productivity exhibit an increasing trend in the western region but a decreasing trend in the eastern region. The precipitation use efficiency shows a "low-high-low" distribution from northwest to southeast, with the highest value in the transition areas. The increase in soil moisture in the arid regions primarily drives the increase of precipitation use efficiency, while the changes in net radiation in the humid regions contribute to the variations of precipitation use efficiency. The transition zone acts as a critical region where vegetation growth shifts from being primarily limited by water to energy, the precipitation use efficiency changes in this zone are jointly influenced by energy, water and dynamical factors. Moreover, the impact of soil moisture stress on annual precipitation use efficiency is greater than that of atmospheric moisture stress, highlighting the dominance of soil moisture in shaping the carbon–water cycle of ecosystems.
|MOST Discipline Catalogue
|理学 - 大气科学 - 气象学
|王素萍. 中国干旱对全球变暖的响应及对生态系统的影响[D]. 兰州. 兰州大学,2023.
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