Characterization of drought in the Kerio Valley Basin,Kenya using the Standardized Precipitation Evapotranspiration Index: 1960−2016

The Kerio Valley basin in Kenya has undergone several periods of drought, yet drought patterns in the region are not well understood due to limited climatic data. Drought events in the region have resulted in crop failure and livestock deaths, exacerbating food shortages. In this study, the Standardized Precipitation Evapotranspiration Index (SPEI), a multi‐scalar drought index was used to examine the onset, duration, severity, intensity, and frequency of agricultural and hydrological drought in the region. The gridded 0.5° × 0.5° climatic datasets from Climatic Research Unit for the period 1960?2016 was used for analysis. Temporal evolutions of SPEI at 6‐ and 12‐month lags were subsequently used to evaluate agricultural and hydrological drought, respectively. Additionally, the Mann‐Kendall trend test was used to test for trends in the time series. Results from the analysis show that: 1) droughts are becoming more frequent in the region, 2) drought intensities in the arid and semi‐arid lands have weakened, 3) regions west of the Kerio River have recently recorded a wetting trend, and 4) the southern and central regions of the basin are drought‐prone. Understanding the spatial and temporal patterns of drought in the basin can assist in drought preparation and mitigation planning.     

基于SPI的黑龙江省干旱时空特征分析

利用黑龙江省10个气象站点1953—2015年逐日降水数据，计算不同尺度下的标准化降水指数(SPI)，并结合游程理论分析黑龙江省干旱的时空演变特征。结果表明：降水仍是黑龙江省干旱的主要影响因素；黑龙江省干旱夏季和秋季呈增强趋势，SPI变化速率分别为每10 a下降0.029、0.135,春旱和冬旱呈缓解趋势，速率为每10 a上升0.061、0.074，年SPI变化速率为每10 a下降0.044；黑龙江省20世纪60年代中后期~70年代末，干旱较为频繁，且干旱的持续时间长、干旱程度大、干旱强度大，出现了1974—1980年的连续干旱期，80年代~90年代中期较为正常，90年代末及之后，干旱频率及其持续时间、程度、强度呈增加趋势；干旱事件的持续时间从西北向东南呈增加—减少趋势，黑龙江省中部的干旱事件持续时间最长。     

基于日SPEI的近55 a西南地区极端干旱事件时空演变特征

利用1960~2014年中国西南地区141个气象台站的逐日气象资料,引入一个新的干旱指数——逐日标准化降水蒸散指数（日SPEI）,对极端干旱事件的年代际、年际、季节内变化及持续性特征进行了分析,结果表明：空间上,近55 a西南春季和年极端干旱程度呈一致的减弱趋势,重庆、四川与贵州的交界处及四川西北部极端干旱程度明显缓解,而夏、秋两季极端干旱表现出增强的趋势并有一定的区域性特征。时间上,春季和全年极端干旱频率、强度和持续天数逐渐减少,春季极端干旱的减弱程度较全年明显;夏、秋两季极端干旱频率、强度和持续天数呈增加趋势,夏季极端干旱的加重趋势比秋季明显。从极端干旱事件的持续性来看,20世纪60年代和21世纪初（2000~2014年）西南遭受的极端干旱最严重,持续期达60 d以上的站点分别占到站点总数的60%和73%。     

Hydrological impacts of flow regulation associated with the Upper Nepean water supply scheme,NSW

The upper Nepean River has been progressively regulated for water supply to Sydney and Wollongong since 1886 by the Upper Nepean Water Supply Scheme which consists of four large dams, two small dams and two diversion weirs. Secular rainfall changes produced periods of high rainfall and large floods (flood‐dominated regimes) between 1857 and 1900 and 1947 and the present, and an intervening period (1901–46) of low rainfall and small floods (drought‐dominated regime). Upstream impoundment and flow regulation significantly reduced flood magnitudes for most return periods during both types of flood regimes. The probability distribution of mean daily flows was also changed significantly by flow regulation such that during the drought‐dominated regime, the high and low frequency flows were reduced substantially but the moderate frequency flows were increased due to dam releases; the change from a regulated drought‐dominated regime to a regulated flood‐ dominated regime resulted in a substantial increase in discharge for most durations; and increased water diversions to Wollongong during the current flood‐dominated regime produced a marked downward shift in the whole flow duration curve. Nepean Dam reduced downstream suspended sediment yields by two orders of magnitude because it traps in excess of 99 per cent of the inflowing suspended sediment load. Streamflow releases are urgently required from the two diversion weirs to improve downstream water quality and to ensure the viability of the resident ‘potentially threatened’ eastern Macquarie perch (Macquaria nov. sp.).     

基于标准化流量指数(SDI)的石羊河流域水文干旱特征

利用石羊河流域8个水文站1961－2017年逐月实测流量数据和4个气象站点的气象观测资料，采用标准化流量指数（SDI）、游程理论来获得干旱事件的特征指标，分析了石羊河流域水文干旱演变特征，并采用与降水量、最高气温、最低气温、平均气温、蒸发量的相关性来分析石羊河流域SDI的影响因素。结果表明：1961—2017年石羊河流域干旱历时、干旱烈度和干旱强度的变化不是很一致，流域水文干旱历时20世纪70年代最长，80年代最短；干旱烈度全流域90年代最强，烈度最弱出现在80年代；干旱强度全流域60年代最强，80年代强度最弱；石羊河流域及流域中、西部年SDI变化总体上表现为在波动中呈增加趋势，西部增加幅度最大，中部增加幅度很小，流域东部年SDI在波动中呈减小趋势；石羊河全流域及流域东、中和西部年及四季干旱发生频次总体都是中旱和重旱频次最多，四季水文干旱发生频率最高的是春季，冬季干旱发生频率最低；从流域不同地段看，水文干旱发生的频率流域东部最高；构建的SDI 临界值识别出的主要水文干旱事件和实际干旱事件基本一致,干旱等级也较一致，说明使用SDI能够较好地监测到石羊河流域干旱年份及干旱等级；石羊河流域年SDI值与最高气温、平均气温和蒸发量呈负相关，与降水量和平均气温呈正相关。     

基于SPEI法的陇东地区近50 a干旱化时空特征分析

选取陇东地区近50 a平均逐月降水和气温数据,采用Mann-Kendall方法、反距离加权插值（IDW）、功率谱分析、标准化降水蒸散发指数（SPEI）等方法分析了陇东地区近50 a来干旱变化的时空特征。研究显示：近50 a来陇东地区干旱化趋势非常明显,特别是20世纪90年代以来干旱趋势显著。持续干旱事件次数增多,持续干旱累积时间增长,以春夏连旱、伏秋连旱的次数增多为显著特征。发生干旱的周期在不同的时间尺度上表现不一致,随着时间尺度的增长,干旱出现的周期也在变长。干旱发生频率不断加快,尤其是在20世纪90年代以来,极端干旱事件的频率显著上升。近50 a来干旱频率较高的区域在环县西北部和六盘山以西静宁等地,干旱高频区逐步向中南部和东部转移。通过与其他方法对比分析和历史资料比对,证明SPEI在陇东地区有较好的适用性。     

近百年中国华北农牧交错带与非洲Sahel 地区环境变化的比较分析

非洲Sahel 地区与中国华北农牧交错带在环境特征及演变过程中体现出许多相似之处, 存在明显内在联系。对两个地带近百年降水变化、植被覆盖、干旱事件和沙尘事件对比分析, 可以 对我国干旱/半干旱地区环境变化的相关科学问题有更好认识。两个地区年降水量序列的年代际 变化趋势有很大相似性。非洲Sahel 地区年降水量在1950s 以前存在明显的年际变化特征, 从 1960s 中期起主要以年代际变化为主; 其大部分地区植被覆盖表现为增长趋势; 降水量年代际变 化可能是影响该地区干旱的重要因素, 温度变化也可能存在作用; 该区沙尘暴频次年际、年代际 变化与降水量相应尺度变化有显著关系, 年均温变化和天气变率可能具有一定影响。华北农牧交 错带年降水量以年际变化为主要特征, 有明显低频波动; 该带偏北和偏东地区植被覆盖增长比较 显著, 南部部分地区植被覆盖呈下降趋势; 降水量年代际变化是影响该地区干旱的重要因素, 但 相同降水量可能引发不同程度的干旱; 该带春季沙尘暴频次与前冬温度变化及天气变率有密切 联系, 与春季降水量也有一定关系。     

近百年中国华北农牧交错带与非洲Sahel 地区环境变化的比较分析

非洲Sahel 地区与中国华北农牧交错带在环境特征及演变过程中体现出许多相似之处， 存在明显内在联系。对两个地带近百年降水变化、植被覆盖、干旱事件和沙尘事件对比分析， 可以 对我国干旱/半干旱地区环境变化的相关科学问题有更好认识。两个地区年降水量序列的年代际 变化趋势有很大相似性。非洲Sahel 地区年降水量在1950s 以前存在明显的年际变化特征， 从 1960s 中期起主要以年代际变化为主； 其大部分地区植被覆盖表现为增长趋势； 降水量年代际变 化可能是影响该地区干旱的重要因素， 温度变化也可能存在作用； 该区沙尘暴频次年际、年代际 变化与降水量相应尺度变化有显著关系， 年均温变化和天气变率可能具有一定影响。华北农牧交 错带年降水量以年际变化为主要特征， 有明显低频波动； 该带偏北和偏东地区植被覆盖增长比较 显著， 南部部分地区植被覆盖呈下降趋势； 降水量年代际变化是影响该地区干旱的重要因素， 但 相同降水量可能引发不同程度的干旱； 该带春季沙尘暴频次与前冬温度变化及天气变率有密切 联系， 与春季降水量也有一定关系。     

Geomorphic Effects of the Extreme Rainfall Event of 20–21 July, 2004 in the Latnjavagge Catchment, Northern Swedish Lapland

Mass transfers triggered by a rare rainfall event on 20–21 July, 2004, with 58.4 mm of rain within 24 h and 71.7 mm of rain within 48 h in the Latnjavagge catchment (9 km² , 950–1440 m a.s.l.; 68°20'N, 18°30'E) in the higher Abisko mountain region (Swedish Lapland), are quantified and analysed in direct comparison with mean annual mass transfers in this drainage basin. In years without rare rainfall events the Latnjavagge catchment is characterized by restricted sediment availability resulting in low mechanical denudation and mass transfers. During the rare rainfall event of 20–21 July, 2004, major stability thresholds on the slope systems (triggering debris flows and slides) and in the channel systems (break‐up of channel debris pavements and step–pool systems) in the Latnjavagge catchment were passed and mass transfers by debris flows, slides and fluvial debris transport in creeks and channels were several times higher than the mean annual mass transfers in Latnjavagge. In the calculation of longer‐term mass transfers and sediment budgets, rare events like the 20–21 July, 2004 rainfall event have to be considered as essential components. A reliable estimation of the recurrence intervals of such rare events is especially problematic. The general problem of defining an adequate length of process monitoring programmes is pointed out.     

Geomorphological characteristics of slope failures in northeast Viti Levu island,Fiji, triggered by Tropical Cyclone Winston in February 2016

The highways circumnavigating Viti Levu play a pivotal role in Fiji's socioeconomic development, and are a crucial link to transport aid during disasters. Slope failures triggered by heavy rainfall and tropical cyclones have led to considerable damage and fatalities along Viti Levu's main roads in the past. Knowledge of slope instability mechanisms is key for the successful identification and management of slope failure hazards, however, there is a paucity of such information for Fiji's main roads. Severe Tropical Cyclone (STC) Winston was the strongest cyclone on record to make landfall in Fiji and affected the Fiji Islands from the early hours of 20 February until 3 am, 21 February, 2016. We present a field reconnaissance examining the characteristics and mechanisms of slope failures induced by STC Winston along a 35 km stretch of the northern part of the Viti Levu's ring road, “Kings Road”. Approximately 61 distinct shallow, small‐scale slope failures were identified most of which were of a complex type and included earth and debris slides (planar and rotational) with a minor flow component. The trigger for the slope failures was the ~258 mm of rainfall in 24 hours after 30‐days of antecedent rainfall totalling 482 mm. This caused rapid increases in porewater pressures, especially at the contact of residual soils and the underlying weathered basement rock. Evidence of recurring slips along Kings Road indicates that contemporary slope instability is likely caused by elevated porewater pressure during high intensity rainfall events in clay‐dominated soils.     

基于日尺度SPEI的黄淮海平原冬小麦生育阶段干旱特征分析

黄淮海平原地处燕山以南、淮河以北，包含黄河、淮河和海河冲积平原及部分丘陵山区，属于半干旱、半湿润地区，年降水量500～900 mm，是我国主要的冬小麦种植区。利用黄淮海平原49个农气站点1961—2017年日值气象数据，采用Penman-Monteith模型计算的日尺度SPEI指数，从气象干旱的角度分析了黄淮海平原近56 a冬小麦主要生育阶段干旱持续时间和干旱强度的变化，并采用R/S方法对干旱未来变化趋势进行预测。结果表明：（1） 从空间分布上来看，冬小麦的营养生长期和并进期，北部干旱持续时间较短，干旱强度较小，南部干旱持续时间较长，干旱强度较大，而冬小麦生殖生长期与前两个生育期干旱分布相反。（2） 从各站点变化趋势来看，在冬小麦整个生育期内干旱持续时间与干旱强度呈相反趋势。（3） 从时间变化趋势上来看，1961—2017年干旱有明显减缓趋势，除个别站点外，在冬小麦的营养生长期和并进期，干旱持续时间和干旱强度自2003年起干旱有逐渐减缓趋势，在生殖生长期则从2007年起干旱有逐渐减缓趋势。（4） 从未来变化趋势来看，营养生长期干旱持续时间将缩短，干旱强度将减小，并进期的干旱持续时间和干旱强度的变化将和过去56 a基本保持一致，生殖生长期干旱持续时间将有所增长，干旱强度将有所加重。本研究分析了黄淮海平原冬小麦生育阶段干旱特征，旨在认识分析和掌握冬小麦生育期干旱演变特征以及干旱发生规律，对保障粮食安全和防旱减灾提供了科学依据。     

中国短历时和长历时暴雨对总暴雨贡献的空间差异性研究（1961-2015）

采用1961-2015年659站日值降水数据,以持续1d和持续2d及以上暴雨作为短和长历时暴雨标准,分析不同历时暴雨变化趋势,结果表明,中国总降雨、总暴雨和短历时暴雨从东南沿海向西北内陆依次呈“增-减-增”的分布特征,且整体以增加趋势的站点占主导,而长历时暴雨则呈现出“增-减”的分布特征,且整体以减少趋势的站点占主导,并且检测出中国自东北向西南存在一条变干带。同时中国总暴雨对总降雨、长历时暴雨对总暴雨的贡献呈现出“东南高-西北低”的分布特征,而短历时暴雨对总暴雨的贡献呈现出“东南低-西北高”的分布特征。中国总暴雨对总降雨、短历时暴雨对总暴雨贡献的变化趋势呈现出“增-增减镶嵌-增”的分布特征,且以增加趋势的站点占主导,而长历时暴雨在东部沿海地区呈现出增减镶嵌的趋势,而西北内陆地区呈略微减少趋势,且以减少趋势的站点占主导,也检测出自东北向西南存在一条气候过渡带并与上述变干带基本重合。     

近50 年云南区域气候变化特征分析

利用云南气温和降水资料, 分析了云南气候变化特征及强降水极端天气和高温干旱事件对全球气候变暖的响应。以云南香格里拉、西双版纳、昆明地区为代表, 分析了区域气象要素变化趋势。结果表明: 云南近50 年气温变化与全球、北半球、中国变化趋势基本一致, 气温变化幅度略大于全球, 弱于北半球和全国变化。云南20 世纪80 年代中后期以后出现增暖现象, 以90 年代后期增温最明显, 1986 年以来出现13 年暖冬, 大部分地区冬春季降霜日数减少。随气候变暖, 香格里拉地区降雪日数呈下降趋势, 西双版纳地区雾日明显减少, 全省降雨日数逐渐减少, 大雨频率变化不大, 暴雨、大暴雨频率上升, 高温干旱事件频率增加。进入21 世纪以后, 云南降水减少, 高温干旱事件有增强增多趋势, 由2～3 年一遇变为1～2 年一遇。2005 年春夏连旱和2006 年春旱是云南近50 年和20 年来最严重的旱灾。     

Revisit of event-based rainfall characteristics at Shapotou area in northern China

Distribution of rainfall event sizes and interval lengths between events are important characteristics of arid and semi-arid climates. Understanding their importance will contribute to our ability to understand ecosystem dynamics in these regions. Rainfall event timing and magnitude are important drivers of ecosystem processes and are instrumental in creating landscape heterogeneity in arid and semi-arid regions. Rainfall event characteristics were analyzed using an automatic tipping-bucket rain-gauge record across the entire summer monsoon season from 2008 to 2015 at the arid desert area of Shapotou in the Tengger Desert, China. Changing the minimum inter-event time (MIT) from 30 min to 24 h alters the number of rainfall events from 64 to 25 for the event depth larger than 0.1 mm. The mean rainfall intensity declined from 0.95 mm/h to 0.53 mm/h, and the geometric mean event duration rose from 0.55 h to 4.4 h. The number of rainfall events, mean rainfall intensity, and geometric mean event duration differed under different criteria of individual rainfall depths, except that for an individual rainfall depth of 0.5, 1.0, and 5.0 mm. The aforementioned features differed only at the lowest range of the mean rainfall intensity and depth for MIT=3 and 6 h. These findings suggest that identification of event-based rainfall in this specific arid region can be better achieved by setting the MIT at six hours. The wide variation in rainfall event properties indicate the need for paying more attention to the proper selection and reporting of event criteria in studies that adopt event-based data analysis. This is especially true in quantifying effective rainfall for soil water replenishment in terms of rainfall depth and intensity with infrequent rainfall events.     

A Note on Tree Deaths during the Current (2001–?) Drought in South-eastern Australia

Tree deaths over large areas during droughts are uncommon. In the semi-arid zone of New South Wales (NSW) such events were last reported during the prolonged droughts of 1895–1903 and 1939–45, though whether caused solely by drought is open to question. Anecdotal and quantitative evidence during the current prolonged drought (2001–?) in NSW's sub-humid zone (where deaths are localised) and semi-arid zone (where deaths are more extensive) suggest that climatic conditions are comparable to, and perhaps more severe than, those of the earlier prolonged droughts. It is suggested that lack of access to deep soil moisture due to inherent shallow rooting, or due to site conditions, is the most likely cause; and that this may be associated with higher than optimal tree densities in cohorts of trees that arose from previous regeneration events during periods of well above average rainfall. A more thorough examination of the extent and severity of this event is warranted.     

近半个世纪中国西北干湿演变及持续性特征分析

 利用1960-2007年中国西北地区248个气象台站的逐日降水、温度资料,通过降水和潜在蒸发均一化干湿指数,从年际演变、季节内变化以及持续性特征等方面较系统地揭示了近半个世纪来西北地区的区域性干湿演变事实,结果表明:在温度与降水双重因子驱动下西北地区的干湿演变特征同降水单一要素驱动下的干旱特征存在明显的差异,说明气候变暖对西北干旱化趋势的贡献程度比较显著;自20世纪90年代后期以来,西北各分区每年异常干旱月份发生概率呈增多趋势,而异常湿润月份发生概率除了北疆地区外,其它分区近10 a异常偏少,这种现象在西北东南部表现得尤为突出。从四季干湿变化来看,冬季表现为湿润化趋势,而春、夏、秋三季呈现出了干旱化趋势。另外西北东南部、青海高原东侧以及南疆地区自20世纪90年代以来,很少出现过持续性湿润事件,而持续性干旱事件却明显多于90年代以前,而且持续性干旱的最长持续时间长,强度大,北疆地区和青海高原48 a来持续性干湿事件发生频次、强度以及持续时间没有明显的趋势变化,而青海高原地区相比其它区域持续性干湿事件偏少,强度偏弱,最长持续时间也偏短。     

松花江区气象水文干旱演变特征

采用标准化降水蒸散指数和径流干旱指数分析了1961~2010年研究区水文干旱和气象干旱时空演变特征,并探讨了水文干旱与气象干旱的关系。结果表明：① 1961~2010年松花江区呈总体干旱化且又有明显时段性的特征,其中1967~1983年和1996~2010年气象干旱频发、覆盖范围广、持续时间长且强度大;其它时段气象干旱少有发生。其次,气象干旱空间分布差异明显,东部的平均干旱频次和强度都大于西部地区,中部（嫩江流域中下游）平均干旱持续时间最长;但在嫩江流域和黑龙江上游地区干旱略有减弱趋势。② 松花江流域和挠力河流域水文干旱呈加剧的趋势,尤其是近15 a干旱化趋势明显;挠力河流域干旱频发、强度大且持续时间很长。松花江流域水文干旱程度弱于挠力河流域,但极端水文干旱事件频发。 ③ 松花江区气象干旱与水文干旱密切相关,嫩江流域水文干旱滞后于气象干旱2个月,而第二松花江流域和松花江流域水文干旱滞后于气象干旱3个月;挠力河流域水文干旱与气象干旱无明显的时滞相关性。     

华北不同地表覆盖类型区干旱演变特征

利用华北地区气象站点1960～2008年共49年的降水旬数据集逐旬计算1个月、3个月、12个月尺度的标准化降水指数(SPI)序列。采用SPI历史序列的多年滑动平均和滑动标准差方法,解析研究区及3种主要地表覆盖类型区(草原区、农田区、阔叶林带)干旱在整个时间序列上的演变周期和幅度。采用线性回归分析干旱的演变趋势和变化频度,并对趋势线进行预测。对比了3个子区域全年12个月份干旱演变特征的差异。结果表明:研究区干旱发生的频率和强度呈逐渐加重的趋势,且各月份干旱发展趋势有较大差异,夏季的6月、7月降水较历史时期增多,春旱(4月)、秋旱(9月和10月)加重趋势显著。草原区在整个历史时期的干旱变化趋势不如南部农作物区和中部阔叶林带明显。农作物区9月、10月两月份干旱加重的趋势最显著,说明黄淮海平原区的夏玉米将越来越受到秋旱的威胁,防旱抗旱形势重于冬小麦面临的春旱,冬小麦的秋种受干旱的影响将加重。     

A Synoptic Evaluation of Frequencies and Intensities of Extreme Three-and 24-Hour Rainfall in Louisiana*

This research classifies extreme three-hour and 24-hour rainfall events in Louisiana by synoptic weather types (Frontal, Gulf Tropical Disturbance, and Air Mass). These classes were used to analyze differences in frequencies and intensities of extreme rainfall. Significant differences were found in the storm frequencies by weather type. However, no significant differences were detected in rainfall intensities between weather types during either duration. These findings have a bearing on statistical procedures associated with estimating storm probabilities in the region.     

基于SPI指数的甘肃省河东地区干旱时空特征分析

干旱是西北地区主要的自然灾害之一。本文以甘肃省河东地区作为研究区,基于该地区38个气象站1971-2010年逐月降水数据计算标准化降水指数（SPI指数）,分析了不同时间尺度SPI指数的时间序列特征,探讨了SPI₃和SPI₁₂的年代际距平和倾向率的时空变化,以及特旱、重旱和中旱的发生频次。结果表明：（1）随着时间尺度的增加,SPI指数随机性在减弱,持续性在加强。干旱出现较多的时期为1991-2005年,出现较少的时期为1974-1982年。（2）20世纪80年代以前河东地区偏湿,90年代以后河东地区有偏干的趋势。河东地区行政区划分为7个部分,70-80年代,除甘南地区外,其余地区的SPI₃倾向率均显著增加;80-90年代,河东地区SPI₃倾向率持续减少,减少最明显的为定西地区;90年代到21世纪初,河东地区SPI₃倾向率又有增加趋势。对于SPI₁₂来说,自20世纪70年代到2010年,除甘南、陇南和天水地区的SPI₁₂倾向率为先减少后增加以外,其余地区均呈现先增加,后减小,然后再增加的趋势。（3）3个月时间尺度干旱的分布范围广,发生频次高,主要发生于河东地区的北部。12个月时间尺度的特旱主要发生在河东地区的北部,重旱和中旱主要发生在河东地区的东部。