随着全球气候持续变暖，极端事件的发生频率与强度总体持续攀升，对人类社会和生态系统造成了巨大的威胁。由于大气环流与陆-气耦合等复杂机制的作用，干旱与高温时常伴随发生，从而引发复合高温干旱事件，对水粮安全和社会经济安全构成更为严重的威胁。随着未来全球气温进一步升高，人类社会与自然生态等遭受复合型极端事件的威胁可能逐渐增加。作为中国最大的两个流域，长江和黄河流域具有典型的两类气候特点（湿润半湿润气候及干旱半干旱气候），是重要的人口和社会经济发展的承载体，在气候变化背景下受复合高温干旱影响的风险较高。因此本文以长江与黄河流域作为研究区域，分析了流域内复合高温干旱事件的时空变化特征，评估了复合高温干旱条件对植被生态系统及土壤水分的影响。主要研究内容及结果如下：

（1） 分析了长江与黄河流域复合高温干旱事件的时空演变特征。基于夏季干旱与高温指标，定义了热浪持续超过3天和5天的两种不同严重程度的复合高温干旱事件（即3day/90th型和5day/90th型），在此基础上探讨了复合事件频次、覆盖面积的分布及变化规律，同时，对复合事件与单一高温事件在温度上的差异进行了对比分析，研究发现1961-2014年间长江与黄河流域的复合高温干旱事件整体上变得更频繁、影响范围更广，且相较于单一高温事件，复合高温干旱事件显著放大了高温的威胁。

（2） 评估了长江与黄河流域不同植被类型对流域复合高温干旱事件的响应特征与差异。基于月尺度降水、气温及植被覆盖数据，划分植被类型，分析了长江、黄河流域夏季季节尺度与月尺度植被与降水、气温相关性，在此基础上通过构建多元高斯模型，计算归一化植被指数在单独和复合高温干旱条件下减小的条件概率，从概率的角度量化长江与黄河流域各植被类型对干旱、高温与复合高温干旱事件的响应特征。研究发现三类极端事件条件（干旱、高温、复合高温干旱条件）对黄河流域植被整体威胁更大，尤其是流域内的温带草原植被类型。

（3） 探究了长江与黄河流域复合高温干旱事件对土壤水分含量的影响机制。基于夏季月尺度降水量、平均气温、相对湿度、蒸散发量、感热通量、潜热通量及表层（0-7cm）土壤水分含量，评估了干旱、高温及复合高温干旱条件下土壤水分含量的变化规律，从相关性的角度量化了各气象因子与土壤水分含量的关系，进一步探究了土壤水分含量对夏季高温干旱的响应机制。研究表明土壤水分含量对复合高温干旱条件的响应比对单一高温、干旱事件更为强烈，且黄河流域在三类极端气候条件下的土壤水分含量异常减少程度均大于长江流域。由于气候差异，夏季极端气候条件下黄河流域的蒸散发主要受土壤水分含量限制，长江流域蒸散发主要受到太阳辐射的能量限制，导致两流域土壤水分含量对高温、干旱及复合高温干旱条件的响应过程不同，同时长江流域与黄河流域的植被类型差异也会影响夏季高温干旱条件下土壤水分含量的变化。

本研究揭示了长江与黄河流域两类典型气候区的复合高温干旱事件变化规律，探究了复合高温干旱事件对两流域的植被生态系统及土壤水循环系统的影响特征。研究结果可以服务于湿润半湿润气候区以及干旱半干旱气候区的复合高温干旱事件的监测预测工作，为水资源、农业等部门制定应对措施提供科学依据与参考。

As the global climate continues to warm, the frequency and intensity of extreme events such as droughts, heat waves continue to rise, posing a huge threat to human society and ecosystems. Due to the complex mechanism of atmospheric circulation and land-air coupling, drought and heat wave often occur together, leading to compound hot and drought events, which pose a more serious threat to water and food security and social and economic security. With the further rise of global temperature in the future, the threat of compound extreme events to human society and natural ecology may gradually increase. As the two largest river basins in China, the Yangtze River basin and the Yellow River basin have two typical climatic characteristics ( humid and semi-humid climate; arid and semi-arid climate ), which are important carriers of population and economic development, and are at high risk of being affected by compound hot and drought in the context of climate change. Therefore, the Yangtze River basin and Yellow River basin were selected as research areas to analyze the temporal and spatial evolution characteristics of the compound hot and drought events in the basins, and to evaluate the effects of the compound hot and drought conditions on vegetation ecosystems and soil moisture. The main research contents and results are as follows:

(1) The temporal and spatial evolution characteristics of compound hot and drought events in the Yangtze River basin and Yellow River basin were analyzed. Based on the indexes of summer drought and hot, two kinds of complex hot and drought events of different severity ( i.e. 3day/90th and 5day/90th ) with heat wave lasting more than 3 days and 5 days were defined. On this basis, the distribution and variation of the frequency and coverage area of complex events were discussed. A comparative analysis of the differences in temperature between compound hot and drought events and single hot events was conducted. It was found that during 1961-2014, the compound hot and drought events in the Yangtze River basin and Yellow River basin became more frequent and affected more widely, and compared with single hot events, the compound hot and drought events significantly amplified the threat of high temperature.

(2) To evaluate the response characteristics and differences of different vegetation types to combined high-temperature and drought events in the Yangtze River basin and Yellow River basin. Based on monthly precipitation, air temperature and vegetation cover data, vegetation types were classified, and the correlation between summer seasonal scale and monthly scale vegetation and precipitation and air temperature in the Yangtze River basin and Yellow River basin was analyzed. On this basis, a multivariate Gaussian model was constructed to calculate the conditional probability of reduced normalized vegetation index under single and compound hot and drought conditions. The response characteristics of vegetation types to drought, hot and compound hot and drought events in the Yangtze River basin and Yellow River basin were quantified from the perspective of probability. It is found that three extreme event conditions (drought, hot and compound hot and drought conditions) are more threatening to the whole vegetation in the Yellow River Basin, especially the temperate grassland vegetation types in the basin.

(3) To explore the influence mechanism of compound hot and drought events on soil moisture content in the Yangtze River basin and Yellow River basin. Based on monthly precipitation, average temperature, relative humidity, evapotranspiration, sensible heat flux, latent heat flux and surface (0-7cm) soil moisture content in summer, the variation law of soil moisture content under drought, hot and compound hot and drought conditions was evaluated, and the relationship between meteorological factors and soil moisture content was quantified from the perspective of correlation. The response mechanism of soil moisture content to hot and drought in summer was further explored. The results show that the response of soil moisture content to compound hot and drought conditions is more intense than that of single hot and drought events, and the abnormal reduction degree of soil moisture content in the Yellow River basin under the three extreme climate conditions is greater than that in the Yangtze River basin. Due to climate differences, evapotranspiration in the Yellow River Basin is mainly limited by soil water content under extreme climate conditions in summer, while evapotranspiration in the Yangtze River Basin is mainly limited by solar radiation energy, resulting in different response processes of soil water content to hot, drought and compound hot and drought conditions in the two basins. At the same time, the difference of vegetation types between the Yangtze River basin and the Yellow River Basin will also affect the change of soil moisture content under hot and drought conditions in summer.

This study revealed the variation regularity of the compound hot and drought events in two typical climatic regions of the Yangtze River basin and Yellow River basin, and explored the characteristics of the effects of the compound hot and drought events on the vegetation ecosystem and soil water cycle system of the two basins. The research results can serve for the monitoring and prediction of combined high-temperature drought events in humid and semi-humid climate areas and arid and semi-arid climate areas, and provide scientific basis and reference for water resources, agriculture and other departments to formulate countermeasures.