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.     

Computation of Drought Index SPI with Alternative Distribution Functions

The Standardized Precipitation Index (SPI) is widely used as drought meteorological index, to identify the duration and/or severity of a drought. The SPI is usually computed by fitting the gamma probability distribution to the observed precipitation data. In this work, the possibility to calculate SPI by fitting to the precipitation data the normal and the log-normal probability distributions was studied. For this purpose, 19 time series of monthly precipitation of 76?years were used, and the assumption that the gamma probability distribution would provide better representation of the precipitation data than log-normal and normal distributions, at various time scales (1, 3, 6, 12 and 24?months) was tested. It is concluded that for SPI of 12 or 24?months, the log-normal or the normal probability distribution can be used for simplicity, instead of gamma, producing almost the same results.     

Regional Drought Modes in Iran Using the SPI: The Effect of Time Scale and Spatial Resolution

In the present paper, regional drought modes in Iran are identified applying the Principal Component Analysis (PCA) and Varimax rotation to the Standardized Precipitation Index (SPI) computed on different time scales. Data used include gridded monthly precipitation covering the period 1951–2007 retrieved from the Global Precipitation Climatology Centre (GPCC) archive with different spatial resolutions (2.5, 1 and 0.5° resolution). The objective of the study is twofold: (i) Investigate the stability of drought spatial modes as a function of the SPI time scales used for monitoring the different kinds of drought, (ii) Evaluate the impact of the spatial resolution of gridded data on drought regionalization. For the coarse spatial resolution of 2.5°, results show four drought modes of distinct variability, which remain quite stable when the SPI time scale is varied from 1- to 24-month. Differently, for higher spatial resolutions drought modes appear more sensitive to the index time scale and become less spatially homogeneous as the time scale is increased. Moreover, the number of identified modes (sub-regions) may reduce to three or two, but in all cases the most well defined sub-region appears to be the southern one. This suggests that both the spatial resolution of precipitation data and the time scale may affect drought regionalization, i.e. the number of drought modes and their spatial homogeneity.     

Spatial and Time Variability of Drought Based on SPI and RDI with Various Time Scales

The spatial and temporal variability of droughts were studied for the Northeast Algeria using SPI and RDI computed with monthly precipitation data from 123 rainfall stations and CFSR reanalysis monthly temperature data covering the period 1979–80 to 2013–14. The gridded temperature data was interpolated to all the locations having precipitation data, thus providing to compute SPI and RDI with the time scales of 3-, 6- and 12-month with the same observed rainfall data. Spatial and temporal patterns of droughts were obtained using Principal Component Analysis in S-Mode with Varimax rotation applied to both SPI and RDI. For all time scales of both indices, two principal components were retained identifying two sub-regions that are similar and coherent for all SPI and RDI time scales. Both components explained more than 70% and 74% of drought spatial variability of SPI and RDI, respectively. The identified sub-regions are similar and coherent for all SPI and RDI time scales. The Modified Mann-Kendall test was used to assess trends of the RPC scores, which have shown non-significant trends for decreasing drought occurrence and severity in both identified drought sub-regions and all time scales. Both indices have shown a coherent and similar behavior, however with RDI likely showing to identify more severe and moderate droughts in the southern and more arid sub-region which may be due to its ability to consider influences of global warming. Results for RDI are quite uniform relative to time scales and show smaller differences among the various climates when compared with SPI. Further assessments covering the NW and NE of Algeria using longer time series should be performed to better understand the behavior of both indices.     

基于多尺度SPI指数的哈巴河地区近53 a的干旱分析

为了对新疆哈巴河地区开展最新的旱情研究,以期全面了解该地区干旱特征及演变规律,基于不同时间尺度下的标准化降水指数(SPI)方法,分析了哈巴河地区1962—2014年间的干旱演变特征。结果表明①近53 a来,SPI指数方法在哈巴河地区有较高的适用性,不同时间尺度SPI皆可监测旱情变化,且与记载的流域实际旱情相符。②从SPI₁,SPI₃,SPI₆指数过程线可以看出,哈巴河地区发生不同干旱等级事件的概率为20%左右;SPI₃指数较SPI₁,SPI₆指数评价的旱情程度要重;SPI₁₂呈现显著的上升趋势,长时间尺度说明地区气候在逐步变湿,有助于减轻地区干旱。③不同季节SPI的年际过程差异显著,其中年SPI最小值为1963年的-1.97,53 a间发生轻旱、中旱、重旱等不同等级干旱事件的年份分别为9,4,0 a。④四季SPI发生轻旱级别以上干旱的年份约占26%~28%,平均约11 a。⑤冬季SPI呈增加趋势,其余季节及年SPI变化趋势不明显,且夏季SPI与年SPI过程较为相似。研究结果可为区域旱情监测与预警、风险管理提供科学依据。     

基于响应单元的太湖流域干旱分析方法研究

提出一种将区域或流域划分成若干个不同的响应单元,把每一个响应单元看作一个点,借助于点干旱指标评价的方法分析面干旱,并应用到太湖流域。通过太湖流域干旱记录与三种基于降雨的常用指标的对比分析,得出Z指数法的精度最高,标准化降水指标(SPI)次之,降水距平百分率(PAI)最低,而且Z指数法显示太湖流域的旱情主要发生在夏秋季节。Z指数法考虑了降水的时空分布特性,具有多时间尺度特性,反映出了不同时间尺度的水分状况。     

南盘江流域水文干旱对气象干旱的响应特征研究

选取南盘江流域3个站点40年(1970年-2009年)的逐月径流资料和20个站点同期逐月降水资料,计算标准化径流指数(SSFI)和不同时间尺度的标准化降水指数(SPI),从中选取水文干旱样本和同期的SPI进行统计分析,研究水文干旱对气象干旱的响应特征。结果表明:南盘江流域SSFI对SPI具有良好的响应关系;SSFI对SPI的响应关系随时间尺度的不同相关性不同;总体来看,流域内水文干旱对气象干旱的响应时间约为6个月。     

标准化降水指数与有效干旱指数在新疆干旱监测中的应用

基于干旱监测理论,根据新疆测站1961—2010年逐日降水资料,对比分析了基于不同时间尺度(1、3、6、9、12、24个月)标准化降水指标(SPI)与有效干旱指数(EDI)对干旱监测的有效性与实用性。结果表明:不论针对短期干旱还是长期干旱,EDI监测能力均明显优于SPI。短时间尺度的SPI受短时间降水影响较大,能反映短时间新疆地区的干旱变化特征;随时间尺度的增加,SPI对短时间降水的响应能力较差,但仍可反映长时间序列下明显的干旱变化趋势。与SPI相比,EDI能够很好地反映出短期干旱和当时降水量的影响;同时能够随时间迁移,给每日降水量分配以不同权重,考虑前期降水量对当前干湿状况的影响。相关研究结果可为湿润区及其他气候区洪旱灾害监测提供重要理论依据。     

鄱阳湖干旱多尺度特征及其与月均水位的相关性

以鄱阳湖13个气象站1957~2013年的逐月降水量、平均气温、各站点纬度和同期水位站逐月平均水位为实验数据,分别计算1、3、6、12、24、48个月尺度下标准降水指数(SPI)和标准降水蒸散指数(SPEI)时间序列,并利用Morlet小波分析理论,分析了该序列多时间尺度变化特征。基于Mann-Kendall检验,分析了鄱阳湖气象干旱趋势特征;利用Spearman秩相关系数,研究了不同时间尺度SPI和SPEI序列与月平均水位的相关关系。研究表明,鄱阳湖流域SPI和SPEI序列存在约68个月变化的主周期,两个主要特征时间尺度变化的强分布;气象干旱与湖水位的相关关系随时间尺度的增大而减弱。     

基于SPEI的南水北调中线水源区干旱时空特征

利用多种统计方法对南水北调中线水源区降水和气温序列进行趋势诊断,识别了不同气象站点序列的时空变化趋势;以标准化降水蒸散指数(SPEI)为干旱指标,建立考虑降水和气温共同对干旱过程影响的评价模型,研究分析了水源区不同时间尺度和分期(汛期、非汛期和全年期)干旱过程动态变化特征。分析表明:降水量呈现不显著的下降趋势,气温总体呈现显著性上升趋势;短时间尺度SPEI对降水和气温变化更加敏感,随着时间尺度增大,SPEI值变化更加平缓;非汛期和全年期水源区发生严重干旱的概率增大,不同气象站点3个不同时期发生特旱或重旱的年份与历史发生干旱的资料相一致;干旱严重程度从上游到下游逐渐衰减,整个汉江上游和西北部地区干旱发生频次略高于下游和南部地区;对比标准化降水指标(SPI)计算结果,SPEI指数较SPI指数补充考虑了降水和气温对干旱过程的综合影响,可作为SPI指数的一个重要补充分析手段,可应用于描述南水北调中线水源区不同时间尺度的干旱特征。     

基于SPI的辽宁省庄河市近35年干旱演变特性研究

庄河市是辽南地区的重要水源地,分析其干旱演变特性有利于该地区水资源的合理开发与利用,对于防旱减灾、合理布局产业结构具有重要的现实意义。本文根据庄河市1984年1月1日至2018年12月31日的逐日降水数据计算了1个月、3个月、6个月、12个月共4个不同时间尺度下的标准化降水指数(SPI),并基于SPI对该地区近35年的年际与季节干旱演变特征进行了分析。结果表明:在年际变化方面,庄河市的SPI在2000年以前波动幅度较小,说明该地区的降水在时间上分布较为均匀;但自2000年以来,SPI序列出现较大的波峰与波谷,说明降雨的分布不均匀性有所增强;SPI的时间序列趋势线斜率为负值,说明该地区整体上呈现偏旱趋势,因此其水资源压力不断增大。在季节演变特征方面,夏季干旱和秋季干旱呈现增强趋势;秋旱的发生频率最高,而夏旱的出现频率最低。本研究结果可为庄河地区的抗旱减灾与水资源规划提供科学依据。     

不同干旱指数在唐山地区旱情评价中的应用

基于唐山地区1957—2011年的气象资料,比较分析标准化降水指数（S SPI）、标准化降水蒸散发指数（S SPEI）和帕默尔水文干旱指数（P PHDI）3种不同干旱指数的相关性,并评价了各个指数在唐山地区连旱期间的适用性。结果表明：各时间尺度（年、季、月）的S SPI和S SPEI均显著相关;P PHDI与年尺度的S SPI和S SPEI均显著相关。P PHDI对长期干旱判断效果明显优于短期干旱。年尺度S SPEI和S SPI对干旱事件识别效果比较接近,二者对长期干旱判断偏涝,对短期干旱判断效果明显优于P PHDI。在气温偏低的年份,季尺度的S SPI能很好地反映旱情的严重程度,在气候变暖的背景下,季尺度S SPEI比S SPI更能准确地判断和评价旱情。     

Influence of Precipitation Changes on the SPI and Related Drought Severity. An Analysis Using Long-Term Data Series

Drought indices, such as the Standardized Precipitation Index (SPI) are used to quantify drought severity. Due to the SPI probabilistic and standardized nature, a given value of SPI computed in distinct time periods or locations indicates the same relative drought severity but corresponds to different amounts of precipitation. Thus, the present study aims at contributing for a comprehensive analysis of the influence of long-term precipitation variability on drought assessment by the SPI. Long records of monthly precipitation, spanning from 1863 to 2007 in several locations across Portugal, were divided into 30 years sub-periods and the SPI with 12-month time scale (SPI-12) was computed for each sub-period and for the entire period of records. The probability distributions adjusted to precipitation in those different time periods were compared envisaging to detect the SPI sensitivity to the reference period and, therefore, to changes in precipitation. Precipitation thresholds relative to the upper limits of SPI-12 drought categories were obtained and the influence of the time period was investigated. Results have shown that when SPI values derived from the full data record for a recent time period are lower/higher than the SPI values derived from data of the considered time period a recent downward/upward shift of precipitation has occurred. Coherently, a common pattern of drought aggravation from the initial until the more recent period was not detected. However, in southern locations, lower precipitation thresholds of the SPI drought categories were generally found in the more recent period, particularly for more severe drought categories, whereas in the northern locations Porto and Montalegre, an increase was detected. The impacts of the reference period on the computed SPI drought severity and frequency are shown, bringing to discussion the need for updating ´normal´ conditions when long term precipitation records are available and precipitation changes are observed.     

基于SPI指数的内蒙古地区干湿气候特征

探究气候的干湿格局及其演化趋势对旱涝灾害监测防御具有重要意义,围绕北方内蒙古生态功能区重要定位,依据1970-2020年45个气象监测站点月尺度降水资料,采用标准化降水指数(SPI),运用线性趋势、突变检验及Morlet小波分析等方法,探究内蒙古地区多时间尺度下气候的干湿特征及周期演化规律.结果 表明:1970-202...     

Fifty Years of Precipitation: Some Spatially Remote Teleconnnections

In the present paper the authors analyse the drought occurrence over the European region by using the NCEP/NCAR reanalysis precipitation rates covering the period from 1948 to 2000. The drought assessment is based on the Standardized Precipitation Index (SPI), which has been proposed as an indicator of drought condition. At variance with other fields derived from precipitation, the SPI is, by construction, a Gaussian field. Thus, the understanding of its covariance structure exhausts the study of the associated density distribution. A method allowing a factorisation of a multivariate Gaussian distribution is the one known as Principal Component Analysis (PCA) or Kauman-Loeve decomposition. Therefore, a PCA is used to study the main spatial patterns and the time variability of drought first over Europe and then over the Northern Hemisphere. The analysis reveals a downward trend for the index over most of central Europe and the Mediterraneanbasin, implying an overall decrease of precipitation in the above mentioned regions. Moreover, the scores associated with the PCA covariance decomposition, besides the aforementioned trend, show few long-term periodicities.Similar drought analyses have been performed by considering the Palmer Drought Severity Index (PDSI).A preliminary comparison between the SPI and PDSI obtained by using the previously discussed data set is presented. It is shown that the indices compare favourably in assessing drought variability. Finally, when the SPI analysis is extended to the Northern Hemisphere some interesting spatially remote teleconnnections linking the Tropical Pacific with the European area are shown.     

Spatial and Temporal Variability of Precipitation and Dryness/Wetness During 1961–2008 in Sichuan Province,West China

Changes in precipitation exerts a huge impact on human beings and it is of vital importance to study the regular pattern of meteorological and hydrological factors. In order to explore the changing patterns of precipitation in Sichuan province in west China during 1961–2008, several precipitation related indices were analysed by the Mann–Kendall test. For monthly precipitation, significant increasing trends are mainly found during January, March and June, while significant decreasing trends mostly are observed during July, September and October. Most of extreme precipitation indices are decreasing. Especially the annual total precipitation in wet days and maximum number of consecutive wet days show significant negative trends. Furthermore, the spatial and temporal variation of dryness/wetness has been assessed by Standardized Precipitation Index (SPI) and principal component analysis (PCA) on 24-month time scales. The results demonstrated noticeable spatial patterns with several sub-regions characterized by different trends: a remarkable dry tendency prevails in central and east Sichuan, while the other areas are dominated by a wet tendency.     

Spatial Patterns and Temporal Variability of Drought in Western Iran

An analysis of drought in western Iran from 1966 to 2000 is presented using monthly precipitation data observed at 140 gauges uniformly distributed over the area. Drought conditions have been assessed by means of the Standardized Precipitation Index (SPI). To study the long-term drought variability the principal component analysis was applied to the SPI field computed on 12-month time scale. The analysis shows that applying an orthogonal rotation to the first two principal component patterns, two distinct sub-regions having different climatic variability may be identified. Results have been compared to those obtained for the large-scale using re-analysis data suggesting a satisfactory agreement. Furthermore, the extension of the large-scale analysis to a longer period (1948–2007) shows that the spatial patterns and the associated time variability of drought are subjected to noticeable changes. Finally, the relationship between hydrological droughts in the two sub-regions and El Niño Southern Oscillation events has been investigated finding that there is not clear evidence for a link between the two phenomena.     

Stochastic Prediction of Drought Class Transitions

This paper aims at the stochastic characterization of droughts applying Markov chains modeling to drought class transitions derived from SPI time series. Several sites in Southern Portugal having updated data on precipitation available were considered. The drought class probabilities, the expected residence time in each class of severity, the expected time for the transition between drought classes and the drought severity class predictions 1, 2, or 3 months ahead have been obtained. Those predictions are then compared with observed drought classes for the recent drought periods of 2003–2006. In addition, the estimation of the cumulated precipitation deficits, amount of monthly precipitation needed to decrease drought severity, and foreseen SPI values depending on different precipitation scenarios are also presented as complementing the prediction of drought class transitions.     

Long-Term Climatic Variability in Calabria and Effects on Drought and Agrometeorological Parameters

In recent decades a general change in climate has been documented in several locations over the world. Such changes could have significant effects on various environmental scenarios, including water resource management, agriculture, hydrology and ecosystems. The complex topography and coastlines of Mediterranean regions influences the climatic regime exhibiting substantial fine-scale spatial variability. In Italy, the climate is generally becoming warmer and drier, with quite large differences depending on the site and data treatment. In this study a historical set of meteorological data (110 precipitation and 28 temperature series), collected over 1921–2007 in the Calabria region (Southern Italy) was analysed. Several meteorological and agrometeorological indices were selected for whether they could evaluate the potential effects of climate change on water availability for natural vegetation and cultivated plants. The significance of the analysed time series (monthly, seasonal and annual time scales) was evaluated by using statistical trend analysis (Mann-Kendall and t-test). Moreover, the intensities of drought events were determined using the Standardized Precipitation Index (SPI) for the time scales of 3 and 6 months. The analysis highlighted a general decrease in annual precipitation and an increase in drought intensity. At a regional level, yearly precipitation decreased by almost 318 mm/100 years (representing almost 30 % of the yearly mean precipitation in the region). Temperature changes were more complex. On a regional scale, yearly mean minimum temperatures increased by 0.9 °C/100 years and maximum and mean temperatures decreased by 1 °C/100 years and 0.8 °C/100 years, respectively. Due to the asymmetric behaviour of temperatures, there was a decreasing impact on evapotranspiration.     

Large-Scale Assessment of Drought Variability Based on NCEP/NCAR and ERA-40 Re-Analyses

The impacts of different spatial resolutions and different data assimilation schemes of the available re-analysis data sets (NCEP/NCAR and ERA-40) on the assessment of drought variability are analysed. Particular attention has been devoted to the analysis of the possible existence of a linear trend in the climatic signal. The long-term aspects of drought over the globe during the last forty years have been evaluated by computing the Standardized Precipitation Index (SPI) on 24-month time scale. The SPI, in fact, seems to be a useful tool for monitoring dry and wet periods on multiple time scales and comparing climatic conditions of areas governed by different hydrological regimes. To unveil possible discrepancies between the analyses carried out with the two data sets, we studied the leading space-time variability of drought by applying the principal component analysis (PCA) to the SPI time series. Results suggest that on the global scale, the two re-analyses agree in their first principal component score, but not in the associated loading: both re-analyses capture a linear trend, though the areas where this feature should be most likely observed are not uniquely identified by the two data sets. Moreover, while the ERA-40 unveils the presence of a weak net “global” trend towards wet conditions, the NCEP/NCAR re-analysis suggests that the areas in the world characterised by positive/negative trends balance to zero. At large regional scale, a good agreement of the results with those obtained from the observations are found for the United Stated, while for the European sector the two re-analyses show remarkable differences both in the first loading and in representing the timing of the wet and dry periods. Also for these areas a linear trend, superposed on other short-term fluctuations, is detectable in the first principal component of the SPI field.