Regional Frequency Analysis of Droughts in Portugal

This study investigated the frequency of droughts for the period September 1910 to October 2004 in mainland Portugal, based on monthly precipitation data from 144 rain gauges distributed across the country. The drought events were characterized using the standardized precipitation index (SPI) applied to time scales of 1, 3, 6 and 12 consecutive months. Based on the SPI time scale series a regional frequency analysis of drought magnitudes was undertaken using two approaches: annual maximum series (AMS) and partial duration series (PDS). Three spatially defined regions (north, central and south) were identified by cluster analysis and analyzed for homogeneity. Maps of drought magnitude were developed using a kriging technique for several return periods. Similar uniform spatial patterns were found throughout the country using the AMS and PDS approaches. For several SPI time scales a comparison of the observed and estimated maximum magnitude (269-year empirical return period) showed that the AMS combined with the selected probability distribution models (Pearson type III, general Pareto and Kappa) provided better results than the PDS approach combined with the same models. A general and simplified characterization of drought duration revealed a relatively uniform pattern of droughts events across the country.     

基于SPEI的贵州省气象干旱时空演变特征及双变量区域频率分析

为深入探究贵州省干旱事件发展规律及区域重现期特征,采用贵州省1960—2017年逐月气象资料计算标准化降水蒸散指数（SPEI）,采用三阈值法游程理论、小波理论、Copula函数等方法对干旱时空演变特征、周期特征以及区域重现期特征进行分析。结果表明：1960—2017年贵州省干旱整体上呈严重化趋势,各分区存在差异,其中黔西南地区的干旱化趋势最为明显,SPEI线性倾向率为-0.003/10a;干旱强度由高到底的分区排序依次为黔东、黔北、黔西南、黔中、黔西北、黔南,且黔中地区的干旱发生频率最高;贵州省四季干旱的主周期分别为18、15、30和8 a,空间上南北分布差异显著,周期较长的区域范围呈现先增加后减少的趋势;贵州省黔北地区的干旱重现期最小,表明此区域发生极端干旱事件的概率较大,遭受干旱灾害的可能性较大。研究结果可为贵州省水资源管理及干旱风险防治提供科学依据。     

水文干旱与气象干旱临界转变条件的判定及响应关系

水文干旱与气象干旱的响应关系对于建立健全干旱监测预报系统具有重要意义。基于区域水文干旱指数(SHI)与标准化降水指数(SPI),结合游程理论和非线性关系模型分析喀什河流域水文干旱与气象干旱的特征、响应关系及驱动因素。结果表明:水文干旱的年平均干旱历时和干旱烈度大于气象干旱,且随着SPI和SHI时间尺度的增加,识别出的干旱历时和干旱烈度也有所增加。基于三参数(log 3 P1)对数函数(Logarithm)模型可以更好地表征两者的响应关系。在3个月尺度下,气象干旱历时至少为1. 10个月且干旱的烈度至少为0. 83时,将诱发水文干旱;在6个月尺度下,气象干旱历时至少为1. 60个月且干旱的烈度至少为0. 91时,易发生水文干旱。     

GIS多源数据图层叠置法研究山东省干旱分区

为揭示山东省干旱孕灾环境的自然背景和干旱空间分布格局,通过基于GIS的图层叠置方法对山东省干旱分布进行区域划分。首先根据山东省干旱成因选取降水量、土壤含水量、植被指数NDVI和气温4个干旱因子进行GIS栅格数据处理,得到相应的干旱因子图层;然后利用层次分析法确定各干旱因子的权重,把上述归一化后的4个干旱因子图层进行加权叠置,得到综合干旱因子图;最后,将综合干旱因子图与山东省高程分布图层进行叠加,得到山东省5个干旱分区,即鲁西北平原北部的农业重旱区、鲁西北平原的南部和胶莱平原农业轻旱区、胶东半岛较湿润区、鲁中南山地丘陵湿润区、山东南部沿海区域的平原湿润区,并提出了各分区的适宜干旱指标。该研究可为山东省干旱监测预警提供孕灾环境方面的理论支撑。     

基于Z指数的黄河流域旱涝等级游程分析

根据黄河流域10个监测点1951~2001年的月降水资料,依据两种不同划分标准利用Z指数进行了月尺度旱涝等级划分;即:通过事实比较,选择较合适的划分标准,并对Z指数趋势图、重大等级旱涝灾害变化趋势、空间分布、游程理论数字特征、游程数目及重现期进行分析。结果表明:10个监测点近51年均有增旱趋势,特别安阳、天水、榆林、西安、太原地区增旱趋势更明显;平凉以东,太原以南,郑州以西,西安以北这一区域发生重、大干旱等级次数比其他区域多,其次济南地区较其他监测点更易发生连续2个月的大旱和大涝。     

1962-2017年典型半干旱草原区不同时间尺度干旱特征分析

干旱是我国北方地区主要的自然灾害之一,对干旱指标的监测分析可为地区旱情评估、预警以及为制订抗旱减灾对策提供科学依据。以内蒙古高原半干旱草原区1962-2017年0.5°×0.5°降水格点数据为基础,分别计算不同时间尺度(年、季)的标准化降水指数(SPI),通过分析其干旱频次、频率以及站次比的变化,对研究区1962-2017年的干旱时空特征进行了分析。结果表明:近年来,研究区干旱程度有所加剧,且中南部地区为各类干旱事件的频发区。此外,轻旱多发生于秋季,且主要分布于研究区中部及南部,而特旱事件常在夏季发生,其高频区位于研究区西部。20世纪90年代各季均出现了大范围轻旱,夏季分别于2001和2010年出现全域性中旱及全域性特旱。     

拉尼娜事件对长江中下游旱涝的影响

为研究不同强度拉尼娜事件下长江中下游旱涝差异,利用旋转经验正交函数方法,将长江中下游划分为4个区(西北部、中部、南部和东部);基于MEI指数,提取了1961—2014年的拉尼娜事件,并采用标准化降水蒸散指数来评估旱涝等级。结果表明:不同强度的拉尼娜事件可对各分区的旱涝频率、强度和时空分布产生不同的影响。在强事件持续期,一区易发严重干旱,二到四区易发生严重洪涝;在强事件结束后,一区可出现轻度洪涝,二到四区可出现中等以上干旱。在中等事件持续期,一区洪涝的频率和强度略大,二区到四区干旱的频率和强度略大;在中等事件结束后,一区洪涝的频率和强度非常大,二区到四区干旱的频率和强度非常大。弱事件对旱涝影响较小,且在南部和北部有反向变化特征,南旱北涝。旱涝频率、强度和分布差异与不同等级的拉尼娜事件影响水汽输送和垂直运动的强度有关。     

Clustering Quantile Regression-Based Drought Trends in Taiwan

Drought is a normal, recurring climatic feature and occurs in all climatic zones. Imbalanced water availability induced by droughts has far-reaching and adverse impacts both on human lives and natural environments. This study aims to summarize temporal and spatial drought variations in Taiwan by combining quantile regression and cluster analysis. Three-monthly rainfall series covering the 1947–2012 period for 12 rainfall stations are used in this study. Quantile regression is applied to 3-month SPI, drought duration, drought severity, and drought frequency series for exploring temporal drought trends at different quantiles. Various quantile slopes for these 12 stations are then analyzed by hierarchical agglomerative clustering algorithm to detect regional variation patterns. The results show considerable spatial diversity over Taiwan. Stations along east coast are prone to more severity due to declined SPI trends associated with increasing drought duration and severity. Positive SPI slope associated with decreasing drought duration and severity are noted at stations located in the west and lead to lessened droughts. However, temporal variations in drought-duration and drought-severity series are insignificant at most quantiles and stations. In addition, a distinct behavior is found in drought frequency since severe droughts may not accompany frequent droughts.     

Frequency and Spatial Characteristics of Droughts in the Conchos River Basin,Mexico

Abstract

The temporal and spatial characteristics of droughts are investigated to provide a framework for sustainable water resources management in a semi-arid region. Using the Palmer Drought Severity Index (PDSI) as an indicator of drought severity, the characteristics of droughts are examined in the Conchos River Basin in Mexico. This basin is important to both the United States and Mexico, because the Conchos River supplies approximately 80 percent of the flows of the Lower Bravo/Grande River above the binational reservoirs of Amistad and Falcon. The temporal and spatial characteristics of the PDSI are used to develop a drought intensity—areal extent—frequency curve that can assess the severity of a regional drought in the basin. The analysis of the PDSI suggests that the Conchos River Basin had a severe drought in the 1990s, which the basin has not experienced before. Based on this analysis, the recent drought that occurred in the 1990s has an associated return period of about 80 to 100 years over the basin.     

Changing Pattern of Droughts during Cropping Seasons of Bangladesh

There has been a growing concern on temporal variations on drought characteristics due to climate change. This study compares meteorological drought characteristics for two different periods to quantify the temporal changes in seasonal droughts of 18 weather stations of the country. Fifty-five years rainfall and temperature data are divided into two different thirty-year periods, 1961–1990 and 1985–2014 and standardized precipitation evapotranspiration index (SPEI) for those periods are calculated to assess the changes. Four seasons in this study are selected as two major crop growing seasons namely, Rabi (November to April) and Kharif (May to October) and two critical periods for crop growth in term of water supply namely critical Rabi (March–April) and critical Kharif (May). Results show that moderate, extreme, and severe Rabi droughts has increased in 11, 9, and 4 stations out of 18 stations, respectively, and Kharif severe and extreme droughts has increased in 8 and 9 stations, respectively, In addition, the frequency analysis shows that the return periods have decreased during 1985–2014 at the stations where it was high during 1961–1990 and vice versa. This has made the spatial distribution of return periods of droughts more uniform over the country for most of the seasons. Increased return period of droughts in highly drought prone north and northwest Bangladesh has caused decrease in average frequency of droughts. Consequently, this result corresponds that Bangladesh experiences fewer droughts in recent years. Trend analysis of rainfall and temperature data reveals that significant increase of mean temperature and no significant change in rainfall in almost all months have increased the frequency of droughts in the regions where droughts were less frequent.     

Large Scale Probabilistic Drought Characterization Over Europe

A reliable assessment of drought return periods is essential to help decision makers in setting effective drought preparedness and mitigation measures. However, often an inferential approach is unsuitable to model the marginal or joint probability distributions of drought characteristics, such as drought duration and accumulated deficit, due to the relatively limited number of drought events that can be observed in the historical records of the hydrological variables of interest. As an alternative, the marginal and multivariate probability cdf’s of drought characteristics can be derived as functions of the parameters of the cdf of the underlying variable (e.g. precipitation), whose sample series is usually long enough to obtain trustworthy estimates in a statistical sense. In this study, the latter methodology is applied to investigate space-time variability of drought occurrences over Europe by using the CRU TS3.10.01 precipitation dataset for the period 1901–2009. In particular, a methodology able to take into account autocorrelation in the underlying precipitation series is adopted. First, a spatial analysis of historical droughts at European level is carried out. Then, the joint probability distributions of drought duration and accumulated deficit are derived for each cell, with reference to both historical and design drought events. Finally, the corresponding bivariate drought return periods are computed, as the expected values of the interarrival time between consecutive critical droughts.Results show that several heavy drought episodes have widely affected the continent. Among the most recent events, drought occurred during the period 1985–1995 was the worst in terms of extent of the regions characterized by return periods greater than 250 years. Besides Euro-Mediterranean regions, North Western and Central Eastern regions appear more drought prone than the rest of Europe, in terms of low values of return periods.     

西南地区干旱时空变化特征研究

干旱是一种严重的自然灾害,西南地区是我国受干旱影响比较严重的地区。分析干旱的时空变化特征,可以为 应对气候变化和抗旱减灾提供科学的决策依据。本文利用西南地区 31 个气象站 1951—2010 年的月降水量数据,计算尺度分别为 1 个月、3 个月和 1 年的标准化降水指数（SPI）,分析了 60 年西南地区月、季和年干旱的时空变化特征。西南地 区干旱发生频率具有空间差异性,四川大部、云南南部发生干旱的频率高,而四川中部和广西东南部发生重旱的频率高。 60 年中西南地区降水量总体没有发生显著性变化,但是阶段性变化显著。年干旱、秋季的站次比具有明显增加的趋势,给西南地区抗旱带来更大的挑战。     

Evaluation and Assessment of Meteorological Drought by Different Methods in Trarza Region,Mauritania

Drought Indexes (DIs) are commonly used for assessing the effect of drought such as the duration and severity. In this study, long term precipitation records (monthly recorded for 44 years) in three stations (Boutilimit (station 1), Nouakchott (station 2), and Rosso (station 3)) are employed to investigate the drought characteristics in Trarza region in Mauritania. Six DI methods, namely normal Standardized Precipitation Index (normal-SPI), log normal Standardized Precipitation Index (log-SPI), Standardized Precipitation Index using Gamma distribution (Gamma-SPI), Percent of Normal (PN), the China-Z index (CZI), and Deciles are used for this purpose. The DI methods are based on 1-, 3-, 6-, and 12 month time periods. The results showed that DIs produce almost the same results for the Trarza region. The droughts are detected in the seventies and eighties more than the 1990s. Twelve drought years might be experienced in station 2 and six in stations 1 and 3 in every 44 years, according to reoccurrence probability of the gamma-SPI and log-SPI results. Stations 1 and 3 might experience fewer drought years than station 2, which is located right on the coast. In station 1, which is located inland, when the annual rainfall is less than 123 mm, it is likely that severe drought would occur. This is 63 mm/year for station 2 and 205 mm/year for station 3 which is located in the south west on the Senegal River. DI results indicate that the CZI and the gamma-SPI methods make similar predictions and the log-SPI makes extreme drought predictions for the monthly period for all the stations. For longer periods (3-, 6-, and 12 month period), for all the stations, the log-SPI and the gamma-SPI produce similar results, making severe drought predictions while the normal-SPI and the CZI methods predict more wet and fewer drought cases. The log-SPI, the gamma-SPI, PN and Deciles were able to capture the historical extreme and severe droughts observed in early 1970s and early 1980s.     

21st Century Drought Scenarios for the UK

A 6-month drought severity index (DSI6) is applied to each of the 11-member perturbed-physics ensemble (HadRM3-PPE-UK) monthly precipitation dataset from 1950 to 2100 to investigate projected 21st century droughts in the UK. Four main drought characteristics are investigated: intensity, drought covariance, frequency of drought months and frequency of drought events at a given duration. Changes in these characteristics are analysed for 30-year periods: 1970–1999 (1980s), 2010–2039 (2020s), 2040–2069 (2050s) and 2070–2099 (2080s) and described in terms of their seasonal behaviour, for both moderate and extreme droughts. Projections of drought characteristics are expressed in the forms of ensemble-mean change relative to the 1980s and model consensus, and analysed over 23 water resource regions. In general, drought characteristics show profound increases (and widespread) for the 2050s and 2080s with larger change occurring during the wet season and under moderate drought conditions. Drought covariance sees greater increase during the dry season with greater change magnitude (but less widespread) under extreme drought conditions. Results also show that droughts can persist over long durations. However, the projected frequency of droughts at longer durations is low compared with droughts with shorter duration of persistence. Water resource regions (WWRs) mostly show negative change in drought characteristics, except for drought covariance. However, intensity and duration of droughts also generally increase over most of the WRRs in England, which are already highly exploited. Of particular relevance to water management, results from this ensemble have a strong influence on dry season water availability, especially in parts of England.     

An analysis of multiyear droughts in Greece

A regional analysis was conducted regarding quantiles of the fundamental features of multiyear droughts, severity, magnitude and duration. Basin geomorphic and climatic characteristics were used as explanatory variables in multiple linear regression equations of the logarithmically transformed variables. Results of satisfactory accuracy were obtained, permitting their utilization in ungauged basins of the study area, which encompasses a significant part of the hydrologically important central and northern Greece. Finally, comparisons are made with a similar study in California and some observations advanced regarding the sensitivity of drought characteristics to climatic warming.     

从黄河流域2009年初的旱情看建设南水北调西线工程的必要性和紧迫性

黄河流域大部分位于大陆干旱、半干旱气候地带,严重缺水,旱灾频繁.2009年初黄河流域5省发生的特大干旱,以及历史上重大旱灾事件表明,更要警惕黄河出现严重水资源危机,抓紧推进南水北调西线工程,以抗御可能出现的特大和连年严重旱灾.     

黄河流域气象干旱与水文干旱时空特征及其传递关系EI北大核心CSCD

基于标准化降水蒸散指数(SPEI)和标准化径流指数(SRI),比较了黄河流域气象干旱和水文干旱的时空分布差异,分析了二者时间尺度上的关联性,并选取典型干旱事件进一步探讨了两种干旱类型的传递关系。结果表明:两种干旱类型空间上有相似的趋势和频次,但在黄河源区和黄河中南部(渭河流域)差异显著,其干旱历时均有随年代延长的趋势,水文干旱历时增长尤为明显;在时间尺度上,SPEI与SRI在大部分区域基本一致,但在黄河源区和渭河流域差异较大,尤其是短时间尺度上差异更显著;气象干旱与水文干旱并非一一对应,多场短历时间断气象干旱受时滞效应、异常气象波动等影响,可能引发一场长历时连续水文干旱或多场短历时间断水文干旱,一场长历时连续气象干旱强度衰减可能引发多场短历时间断水文干旱。     

Regional Dry Spells Frequency Analysis by L-Moment and Multivariate Analysis

The spatial variation of the statistical characteristics of the extreme dry events, such as the annual maximum dry spell length (AMDSL), is a key practice for regional drought analysis and mitigation management. For arid and semi arid regions, where the data set is short and insufficient, the regionalization methods are applied to transfer at-site data to a region. The spatial variation and regional frequency distribution of annual maximum dry spell length for Isfahan Province, located in the semi arid region of Iran, was investigated using a daily database compiled from 31 rain gauges and both L-moment and multivariate analysis. The use of L-moment method showed a homogeneous region over entire province with generalized logistic distribution (GLOG) as the regional frequency distribution. However, the cluster analysis performed two regions in west and east of the province where L-moment method demonstrated the homogeneity of the regions and GLOG and Pearson Type III (PIII) distributions as regional frequency distributions for each region, respectively. The principal component analysis was applied on at-site statistics of AMDSL and found the L-coefficient of skewness (LCs) and maximum AMDSL the main variables describing the spatial variation of AMDSL over the Isfahan Province. The comparison of two homogeneous regions also proved the difference between two regions. Therefore, this study indicates the advantage of the use of multivariate methods with L-moment method for hydrologic regionalization and spatial variation of drought statistical characteristics.     

ENSO影响下安徽省旱涝灾害及农业生产损失时空变化特征

基于1961—2014年25个气象站资料,采用线性趋势法、标准化降水蒸散指数(SPEI)、M-K趋势检验以及皮尔逊相关分析法,分析了ENSO影响下安徽省1961—2014年近54年旱涝时空特征及对农业生产影响。研究表明:(1)与ENSO事件有关的中度以上干旱(洪涝)发生次数占中度以上干旱(洪涝)总次数的68%(83%),且干旱事件受厄尔尼诺次年及拉尼娜年影响大,洪涝受厄尔尼诺当年影响更大;(2)春、秋季SPEI波动幅度大于夏季和冬季,洪旱灾害风险增大。春季呈干旱化趋势,冬季在20世纪90年代湿润化达到最大,随后趋向于干旱化。与皖北地区相比较,皖南地区各月份趋势变化大;(3)安徽各地区的旱涝指数与SSTA的相关性在ENSO冷暖事件中不同,ENSO暖事件对皖南地区相关系数最大达0.32,超过99%置信度检验,影响更为显著,而ENSO冷事件对皖北地区相关系数最大达0.28,超过99%置信度检验,影响更为显著。随着滞后性月份的增加,安徽各区域的旱涝指数与SSTA的相关系数逐渐增大,皖南地区与SSTA相关性大于皖北地区,SSTA对未来3个月皖南旱涝有明显的影响;(4)近20年皖北、皖南地区稻、麦减产主要发生在ENSO事件年或者前一年,且减产率高。皖北地区和江淮地区的小麦减产发生次数较少,小麦减产率较大,而皖南地区小麦减产发生年份较多,小麦减产率较小。ENSO对农业生产的影响与旱涝分布状况有关,江淮地区良好的灌溉条件会降低ENSO年农业旱涝受灾风险。     

近50年来浙江省干旱规律时空分析

利用浙江省近50年降水资料,基于最优区域旱涝指标采用克里金空间插值方法对浙江省旱涝进行时空规律分析和等级评定。结果表明,浙江省发生干旱频繁,两年一旱,发生大旱以上的频率较高,连旱严重,近年干旱有加重的趋势;同时干旱呈现明显的区域性,发生重旱时波及范围广,各分区发生干旱频率在16%~26%之间,仅有4个分区不易波及。