As the basis of food and fiber production, gross primary production (GPP) plays a critical role in the growth of vegetation. Understanding the response of GPP to climate extremes is important for ensuring food security under ongoing global warming. Plenty of evidence shows that the recent widespread dry or hot events across the globe have significant influences on GPP, yet little is known about their joint impacts. Here, we reveal a high risk of compound dry and hot events globally, in response to the strong negative dependence of precipitation and temperature, which leads to a substantial negative impact on GPP for both crop and pasture ecosystems. Using a meta-Gaussian model, we show that the probability of a reduction in global terrestrial GPP increases significantly under compound dry and hot conditions relative to their individual counterparts. Further, the risk of GPP reductions increases with the intensified severity of compound dry and hot events across the globe. These results unravel the sensitivity of GPP to compound dry and hot conditions and highlight the need to account for the influence of compound events when assessing the carbon budget.

In this study, Yulin city and Yan’an city in northern Shaanxi Province were taken as the study area. Based on the diurnal dry–wet events abrupt alternation index DWAAI, the joint probability distribution of two characteristic variables of “urgency” and “alternation” of dry–wet events abrupt alternation was established by using copula function, and the characteristics of dry–wet events abrupt alternation were analyzed. DWAAI was calculated from daily precipitation data and the applicability of the index was verified. On this basis, the two characteristic variables of “urgency” and “alternation” were separated, and the appropriate marginal distribution function was selected to fit them, and the correlation between the two variables was evaluated. Finally, the appropriate copula function was selected to fit the bivariate of each station, and the joint cumulative probability and recurrence period of the two variables were calculated. The results show that the DWAAI index is suitable for the identification of dry–wet events abrupt alternation in the study area. Light and moderate dry–wet events abrupt alternation occurs more frequently, while severe events rarely occur in the study area. The frequency of severe dry–wet events abrupt alternation in Jingbian station and its northern area is greater than that in the southern area, and the risk of dry–wet events abrupt alternation of disasters in the northern area is higher. The greater the degree of “urgency” and “alternation”, the greater the joint cumulative probability and the greater the return period. The return period of severe dry–wet events abrupt alternation was more than five years, while the return period of light and moderate dry–wet events abrupt alternation was less than five years.

根据长江源区直门达水文站1957—2021年径流资料,使用长、短期径流旱涝急转指数(LRDFAI和SRDFAI),分析长江源区旱涝急转事件发生变化情况,采用希尔伯特-黄变换方法,分析长江源区旱涝急转强度周期变化。结果表明:长江源区直门达水文站短期径流旱涝急转事件主要发生在3—9月份,“旱转涝”和“涝转旱”事件发生频次相当。发生频次在不同年代波动较大,总体上呈现“多-少-多-中-多-多”的趋势。短期旱涝急转事件中“旱转涝”事件强度呈现微弱的增长趋势,而“涝转旱”事件强度呈现微弱的递减趋势,分别存在7.1 a和12.3 a的主周期。共发生长期径流旱涝急转事件20次,“旱转涝”和“涝转旱”事件发生频次分别为8次和12次,存在8.3 a和14.2 a的周期。

基于长江中下游流域75个雨量站1960-2015年的日降水资料,通过对原有的旱涝急转指数加以改进,定义了日尺度旱涝急转指数（Dry-Wet Abrupt Alternation Index, DWAAI）,全面分析长江中下游流域夏季（5-8月）旱涝急转事件的时空演变特征,并讨论了旱涝急转事件与事件发生前太平洋海表温度的关系。结果表明：① 改进的DWAAI综合考虑了事件前后期旱涝差异与急转快慢程度,筛选事件更加全面。② 总体来说,自20世纪60年以来,流域内发生旱涝急转事件的区域范围越来越广,事件频率和强度均具有逐年增长趋势。旱涝急转事件主要发生在5月和6月,且汉江水系、中游干流区间、洞庭湖水系北部和鄱阳湖水系西北部地区为事件高发区。③ 旱涝急转事件与事件发生前Nino 3.4区域海温持续异常偏低存在一定关系。在发生时间上,La Ni?a现象具有一定的先兆作用,41.04%的事件发生在La Ni?a现象衰亡期或现象结束后8个月内;在事件强度上,流域内站点的DWAAI与事件发生前第1~6个月的Nino 3.4区域海温异常值存在显著的负相关性,尤其是在鄱阳湖水系和中游干流区间,二者负相关性最强。研究结果可以为长江中下游流域防洪抗旱工作提供一定的依据。

Based on the daily precipitation data from 75 rainfall gauging stations covering 1960-2015 in the middle and lower reaches of the Yangtze River Basin, we analyzed the temporal and spatial distribution characteristics of dry-wet abrupt alternation (DWAA) events during the summer (from May to August) and its relationship with ENSO by defining the daily scale dry-wet abrupt alternation index (DWAAI) based on the modification of original index. The results showed that: (1) Modified DWAAI, which was defined by taking into account the differences of dry-wet degree between the earlier and later periods as well as how slowly or quickly the process changes from dry to wet in the abrupt alteration period, could be used to identify DWAA events accurately and effectively. (2) On the whole, areas where DWAA events occurred had expanded gradually since 1960. Meanwhile, the frequencies and intensities of such events had gradually increased over time. DWAA events mainly occurred in May-June, and the Hanjiang River watershed sub-basin, the middle reaches of the Yangtze River, the northern Dongting Lake watershed sub-basin and the northwestern Poyang Lake watershed sub-basin were high-incidence areas of such events. (3) There were some relationships between DWAA events and phenomena of continuously low SST in Nino3.4 region before such events occurred. Specifically, La Ni?a early-warning reacted to the occurrence of DWAA events. About 41.04% of such events occurred during decline stages of La Ni?a or within the first 8 months after La Ni?a ended. In terms of intensity, there were significant negative correlations between DWAAI at all the stations and SST anomalies in Nino 3.4 region within 6 months before DWAA events occurred, especially in the Poyang Lake watershed sub-basin and the middle reaches of the Yangtze River. The conclusions indicated that these methods and results were meaningful for the fighting against drought and flood in the Yangtze River Basin.

为了识别我国旱涝交替事件以及探究其时空演变规律,引入降雨距平指数(RAI),并提出了一种在月尺度上识别旱涝交替事件的简便易操作的研究方法。结果表明:我国旱涝交替事件具有明显的年内分布规律,4—8月份旱涝交替事件发生频次明显增加,8月份以后,旱涝交替事件发生次数减少。不同月份,旱涝交替事件高频区的分布也不同,4—6月份主要在华南地区,6—9月份主要在西南、华中、华北和东北地区。总体而言,全国旱涝交替发生的频率呈现东南高西北低的特点。而旱涝交替强度的空间格局与频率完全不同,呈现出西北高东南低的特点。21世纪以来,珠江流域、黄河、海河流域片旱转涝事件发生频率呈上升趋势;全国旱涝交替事件强度无明显变化趋势。研究成果能为我国旱涝灾害管理与防控提供科学参考,具有较强的实用价值。

To identify dry-wet transition events over China and analyze their temporal and spatial distribution characteristics, we present a method to identify dry-wet transition events on monthly scale based on the rainfall anomaly index (RAI). Our findings indicate that the frequency of dry-wet transition events increased from April to August, while declined after August. The spatial patterns of dry-wet transition frequency vary among different months. During April to June, the high-frequency area mainly located in south China, while southwestern, northern, northeastern parts of China experienced more frequent dry-wet transition events during June to September. In general, dry-wet transition events occurred more frequently in southeastern part of China than the northwestern part, while intensity distribution of such events showed an opposite pattern. We further examined the variations of both the frequency and intensity in nine large river basins over different periods. Our results demonstrate that the occurrence of dry to wet events showed an increasing trend in Pearl River Basin, Yellow River Basin, Haihe River Basin, since 2000. However, there was no significant trend in the intensity of transition events. The study would provide a valuable reference for the flood and drought managements in China.

During recent decades, more frequent flood-drought alternations have been seen in China as a result of global climate change and intensive human activities, which have significant implications on water and food security. To better identify the characteristics of flood-drought alternations, we proposed a modified dry-wet abrupt alternation index (DWAAI) and applied the new method in the middle and lower reaches of the Yangtze River Basin (YRB-ML) to analyze the long-term spatio-temporal characteristics of dry-wet abrupt alternation (DWAA) events based on the daily precipitation observations at 75 rainfall stations in summer from 1960 to 2015. We found that the DWAA events have been spreading in the study area with higher frequency and intensity since 1960. In particular, the DWAA events mainly occurred in May and June in the northwest of the YRB-ML, including Hanjiang River Basin, the middle reaches of the YRB, north of Dongting Lake and northwest of Poyang Lake. In addition, we also analyzed the impact of El Niño Southern Oscillation (ENSO) on DWAA events in the YRB-ML. The results showed that around 41.04% of DWAA events occurred during the declining stages of La Niño or within the subsequent 8 months after La Niño, which implies that La Niño events could be predictive signals of DWAA events. Besides, significant negative correlations have been found between the modified DWAAI values of all the rainfall stations and the sea surface temperature anomalies in the Nino3.4 region within the 6 months prior to the DWAA events, particularly for the Poyang Lake watershed and the middle reaches of the YRB. This study has significant implications on the flood and drought control and water resources management in the YRB-ML under the challenge of future climate change.

基于 1960—2018 年的逐日降水观测数据，采用日尺度旱涝急转指数（DWAAI）计算方法，以生态地理区为框架，识别并分析了西南地区旱涝急转事件的时空变化特征。结果表明：西南地区旱涝急转事件的发生次数在 1960—2010年有增加趋势，在 2011—2018 年快速减少；旱转涝事件多发生在春夏季（4~8 月），涝转旱事件则跨越了春、夏、秋季（5~11 月）；在云南高原常绿阔叶林、松林区（VA5），西双版纳山地季雨林、雨林区（VIIA3），闽粤桂低山平原常绿阔叶林、人工植被区（VIA2），旱涝急转事件主要发生在夏季，而在湘黔高原山地常绿阔叶林区（VA3）和四川盆地常绿阔叶林、人工植被区（VA4），旱涝急转事件主要发生在春季；旱涝急转事件发生次数的空间分布呈现东北多、西南少的格局；2000 年以来旱涝急转事件在滇中南亚高山谷地常绿阔叶林、松林区（VIA3）和闽粤桂低山平原常绿阔叶林、人工植被区（VIA2）发生次数减少，但有加重趋势，呈现极端化特征。

With the warming climate and enhancing human activities, the frequency and intensity of dry-wet abrupt alternation (DWAA) events have been increasing, largely affected on natural environment and social-economy development. Using observed daily precipitation from 1960 to 2018, this study analyzed the spatio-temporal variations of DWAA events in Southwest China by calculating Dry-Wet Abrupt Alternation Index (DWAAI) based on eco-geographical regions. The DWAA events in Southwest China exhibited an increasing trend before 2010 and decreased after 2011. The dry-to-wet events mainly concentrated in spring and summer (April to August). Meanwhile, the wet-to-dry events were primarily found in spring, summer and autumn (May to November). At seasonal scale, DWAA events mainly occurred in summer, distributed in the area of Yunnan Plateau evergreen broadleaved forest and pine forest region (VA5), Xishuangbanna mountains seasonal rainforest and rainforest region (VIIA3), and Fujian, Guangdong and Guangxi low mountain and plain evergreen broadleaved forest and cultivated vegetation region (VIA2). In the area of Hunan and Guizhou mountains evergreen broadleaved forest region (VA3), and Sichuan Basin evergreen broadleaved forest and cultivated vegetation region (VA4), the DWAA events were examined in spring. Spatially, majority of DWAA events existed in the northeast of Southwest China, and the less were found in the southwest. Although frequency of DWAA events declined since 2000, the intensity of DWAA was obviously enhanced, especially in Xishuangbanna mountains seasonal rainforest and rainforest region (VIIA3), and Fujian, Guangdong and Guangxi low mountain and plain evergreen broadleaved forest and cultivated vegetation region (VIA2).

以长江源区为研究区域,采用沱沱河站、直门达站实测径流,利用北大西洋涛动指数(NAOI)分析北大西洋涛动(NAO)的强度,研究NAO对两站冬季、夏季及全年径流丰枯变化的影响,采用相关分析和交叉小波变换分析NAO与两站径流的多尺度相关特性,从海-气耦合影响大气环流角度分析NAO对长江源区径流的可能影响机制。研究结果表明,长江源区冬季径流受NAO影响较大,2000年之前NAO强年对应径流偏枯概率较大,NAO弱年对应径流偏丰概率略大;夏季径流受长江源区气温影响更为显著,对于NAO强弱的响应不如冬季明显。NAOI与长江源区沱沱河站、直门达站径流在年代际变化规律上具有很好的趋同性,而对于年际或更短时间尺度上的相关关系不甚密切。沱沱河站、直门达站月径流与NAOI两者之间在整个1960—2020年时间轴上具有8~16个月时间尺度上的共振周期。两站与NAOI分别在1970年代、1980年代之前呈现同频同位相变化态势;之后呈现反位相变化态势。分析其作用机制可知,NAO作为北大西洋地区最重要的气候模态,通过大气遥相关和Rossby波列直接影响季风和西风带的强弱,进而调整青藏高原上空水汽输送和辐合、辐散场的分布,从而影响长江源区降水和径流的时空分布。

基于昆明市1965 -2016年逐日降水资料， 计算昆明市汛期（5 -10月）的长周期旱涝急转指数、 短周期旱涝急转指数和旱涝急转强度， 采用小波分析、 M-K突变检验、 灰色关联度等方法， 分析了昆明市的旱涝急转特征及其与城市化的相关性。结果表明： 在1965 -2016年， 昆明市长周期旱涝急转指数LDFAI （Long-term drought-flood abrupt alternations index）呈-0.066·（10a）^-1的下降趋势， 反映出昆明市的旱转涝事件减少、 涝转旱事件增多的状况， 并且昆明市汛期长周期旱涝急转指数LDFAI序列出现2个突变点， 但突变并不明显； 昆明市长周期旱涝急转指数存在18年的主周期变化； 从短周期旱涝急转指数SDFAI （Short-term drought-flood abrupt alternations index）来看， 昆明市5 -7月和8 -9月旱转涝事件增多、 涝转旱事件减少， 而7 -8月和9 -10月表现出相反的趋势； 从旱涝急转与城市化的相关性来看， 在分辨率为0.5时， 城市化指标的6个因子对旱涝急转产生不同程度的影响， 所有指标关联度都在0.6以上， 表明城市化指标与旱涝急转显著关联， 与旱涝急转强度关联度最大是烟粉尘排放量， 最小的是非农业经济总产值， 关联度分别为0.91和0.63。

Based on the daily precipitation data of Kunming from 1965 to 2016， the index of the long-term drought-flood abrupt alternations index （LDFAI） in flood season （from May to October）， short-term drought-flood abrupt alternations index （SDFAI） and the intensity of drought-flood abrupt alternations are calculated in this study.The characteristics of the drought-flood abrupt alternations in Kunming， and the correlation between the characteristics of drought-flood abrupt alternations and urbanization are analyzed by using the methods which include the wavelet analysis， Mann-Kendall （M-K） mutation test， grey correlation degree and other methods.The results indicate that the LDFAI shows the declining trend （-0.066 per 10 years） from 1965 to 2016， which means the decrement of drought-to-flood events but the increasement of flood-to-drought events in Kunming.Besides， LDFAI shows two mutation points in Kunming， but they are not significant.There is an 18-year main period of LDFAI in Kunming.On the other hand， according to the results of SDFAI， there is an increasement of drought-to-flood events， but a decrement of flood-to-drought events from May to July and from August to September.However， the opposite results are appeared from July to August and from September to October in Kunming.According to the correlation between the drought-flood abrupt alternations and urbanization， the six factors of urbanization index have effect of varying degrees on the drought-flood abrupt alternations at the resolution ratio of 0.5， All the correlation degree indexes are above 0.6， the urbanization index is significantly related to the drought-flood abrupt alternation.The most significant factors are smoke and dust emission， while the least important one is the total output value of non-agricultural economy， and their correlation coefficients are 0.909 and 0.630 respectively.

Research results of the formation mechanism of drought-flood abrupt alternation and the   waterlog prevention and reduction measures in China and abroad are summarized. It is widely believed that the formation mechanism of drought-flood abrupt alternation is closely related to atmospheric circulation anomaly, magnetic index change in solar magnetic field, and monsoon and subtropical anticyclone. In the aspect of city waterlogging mechanism, research results have been achieved in urban runoff yield and concentration mechanism, mathematical modeling, and urban underlying surface change affecting on the rainfall runoff process, which laid a solid foundation for further research. Nevertheless most researches so far focused on the droughts and floods in single city instead of seasonal changes. The following problems need to be further explored: the regularity of the evolution of drought-flood abrupt alternation, the influence of drought-flood abrupt alternation on urban drainage system standards and disaster prevention and mitigation, the establishment of city waterlogging model of urban runoff yield model coupled with valley hydrological and hydrodynamics process.

为探讨旱涝急转对花生生理生化的影响，以不同旱涝耐性的3个花生品种中花8号（抗旱）、豫花15（耐涝）和湘花2008（旱涝兼耐）为材料，进行不同梯度的先旱、后涝连续处理，比较不同花生品种的叶片相对含水量，过氧化物酶（POD）、过氧化氢酶（CAT）、超氧化物歧化酶（SOD）、谷胱甘肽还原酶（GR）活性，以及丙二醛（MDA）含量的变化情况。结果表明，在旱、涝急转的胁迫过程中，干旱对叶片相对含水量的影响甚于湿涝。一定强度的旱或涝胁迫总体上均可使花生叶片的4种抗氧化酶POD、SOD、CAT、GR活性提高，随着胁迫的加深活性降低，复水后活性回落至对照水平，且旱甚于涝。MDA含量总体变化趋势是干旱胁迫下各品种花生叶片中MDA含量有所升高，并于复水解除干旱胁迫后恢复至对照水平；而湿涝处理阶段，MDA含量呈直线上升趋势，在胁迫解除后有所降低。中花8号、豫花15、湘花2008等3个花生品种具有显著不同的水分适应性，中花8号抗旱性更强而耐涝性较弱，豫花15抗旱性较弱而耐涝性强，湘花2008抗旱性中等而耐涝性更强，为水分广适性品种。

Drought-flood abrupt alternation (DFAA) is an extreme hydrological phenomenon caused by meteorological anomalies. To combat the climate change, the watershed integrated management model—Soil and Water Assessment Tool model (SWAT)—was used to simulate DFAA, total nitrogen (TN) and total phosphorus (TP) from 1961 to 2050, based on measured precipitation data in the Hetao area and the downscaled Representative Concentration Pathways (RCPs) climate scenarios. In the future, the increase in temperature and the increase in extreme precipitation will aggravate the pollution of water bodies. Results indicate that the risk of water quality exceeding the standard will increase when DFAA happens, and the risk of water quality exceeding the standard was the greatest in the case of drought-to-flood events. Results also indicate that, against the backdrop of increasing temperature and increasing precipitation in the future, the frequency of long-cycle and short-cycle drought-flood abrupt alternation index (LDFAI, SDFAI) in the Hetao area will continue to decrease, and the number of DFAA situations will decrease. However, the zone of high-frequency DFAA situations will move westward from the eastern Ulansuhai Nur Lake, continuing to pose a risk of water quality deterioration in that region. These results could provide a basis for flood control, drought resistance and pollution control in the Hetao and other areas.