Droughts and the Impacts of Dry Spells in North of Iraq

Different sets of dry spell length such as complete series, monthlymaximum, seasonal maximum, and annual maximum are applied andmodeled with different probability distribution functions (such as GumbelMax, generalized extreme value, Log-Logistic, generalized logistic, inverseGaussian, Log-Pearson 3, generalized Pareto) to recognize in whichduration, dry spells cause drought. The drought situation and temporalanalysis in the North of Iraq region were done using the SPI index andby software of DrinC at a time scale of 3.6 and 12 months. Because ofapplicability, availability of data and the aim of the study, SPI is selectedto analyze the dry spells in this study. Based on the maximum length ofthe available statistical period, the statistics for the years 1980 to 2019were used from nine meteorological stations for analysis. The results of thestudy showed the severity of drought during the study period which relatedto dry spells. The results of this research confirm the variation of droughtoccurrence with varying degrees in different time and different dry spellscondition in Iraq.

Validation of AOGCMs Capabilities for Simulation Length of Dry Spells under the Climate Change in Southwestern Area of Iran

Identification and extraction length of dry spells in arid and semi-arid regions is very important. Thus, the use of climate change prediction models for study the behavior of the climatic parameters in the future time is inevitable. With recognition of the spatial and temporal behavior variables such as precipitation, we can prevent from destructive effects. In this research, the performance of Atmosphere-Ocean General Circulation Models (AOGCMs) was evaluated for simulation length of dry spells in the south-western area of Iran. The results show that the length of dry spell is relatively decreased in cold seasons (autumn and winter) and increased in the warm season (spring and summer) in both A2 and B2 Scenarios. The length of the dry spell on monthly scale for scenario A2 is 6% (equivalent to 2 days) and for scenario B2 is 9 percent (approximately 2.4 day) increased compared to the baseline period. For assess the uncertainty, AOGCMs were weighting. The results show that the best model for simulation of dry spells is HADCM3 and GFCM2.1, because the results have a less error. On the other hand, NCCCSM have the lowest weight for simulation dry spells in both scenarios.

A LONG DRY SPELL

A severe drought lingers in south and southwest China,resulting in huge economic losses and a hard life for millions of people

金沙江流域1961—2020年干湿日时空演变特征分析

准确地观测降水数据在揭示干湿日指数时空演变特征及影响因素等方面极其关键。金沙江流域是气候变化最为敏感的地区之一。由于风速、湿润和蒸发损等可能导致标准雨量器观测的降水量远小于实际降水量,而降水资料的准确与否直接影响金沙江流域水文过程分析和模拟研究。利用修订的Mann-Kendall、空间场显著性检验等,识别金沙江流域1961—2020年降水校正前后干湿日指数时空演变。结果表明:1)观测降水(Po)和校正降水(Pc)识别的干湿日指数值差异明显,但其空间分布近似一致;相较于Po,Pc识别的干湿日指数趋势在金沙江上段主要表现为减少,中下段主要表现为增加,雅砻段则表现为干日指数减少,而湿日指数增加。2)空间场显著性检验结果表明,Po和Pc识别的干湿日指数显著变化趋势主要由气候系统内部变率引起,仅Po和Pc识别的平均干日事件持续时间趋于显著增长的站点比例存在明显差异。这表明该流域干湿日指数变化趋势主要由气候变化主导,因而对气候变化十分敏感。综上,Po和Pc识别的金沙江流域干湿日指数存在明显的空间差异。因此,为准确在该流域进行水文研究和模拟,应考虑矫正降水资料的应用;同时极端干湿日的变化可能对金沙江流域农业生产和生态系统造成极其严重的威胁,应引起足够重视;基于Po和Pc识别的干湿日演变特征的相关研究,能为该流域滑坡、泥石流的预防,水资源安全规划与水土保持工作部署等,提供一定理论参考。

An Assessment of the Projected Future Intra-Seasonal Rainfall Characteristics in Uganda

Rainfall is a key climate parameter that affects most operations that affect human life, especially in the tropics. Therefore, understanding the various factors that affect the distribution and intensity of this rainfall is important for effective planning among the different stakeholders in the weather and climate sectors. This study aimed at understanding how intra seasonal rainfall characteristics, especially Consecutive Dry Days (CDD) and Consecutive Wet Days (CWD), in the two major rainfall seasons will change under two future climate scenarios of RCP4.5 and RCP8.5 in Uganda, covering two future periods of 2021-2050 and 2051-2080. The results indicate a high likelihood of reduced consecutive rainfall days, especially over the Northeastern regions of the country, for both 2021-2050 and 2051-2080. However, the trends in the entire country for the two major rainfall seasons, March to May and September to November, are not significant. Nonetheless, the distribution of these days is important for most agricultural activities during different stages of crop growth. The consecutive dry days show a fairly increasing trend in the eastern part of the country, particularly in the second season of September to November. An increase in consecutive dry days implies more frequent dry spells in the midst of the growing season, potentially affecting some crops during critical growth stages.

中国北方夏半年极端干期的时空变化特征

利用1951—2004年我国北方348个测站的夏半年逐日降水资料,首先整理出各站点的极端干期日数和无雨总日数两个序列,得到极端干期日数的空间分布,并将北方划分为A、B、C 3个区,其中A、B 2个区相对湿润,C区则相对干旱和半干旱,这种分布与地理环境有密切的联系。然后建立这3个区夏半年极端干期以及无雨总日数的时间序列,采用经验模态分解(EMD)方法,分析极端干期序列的多尺度振荡结构以及多年变化的总趋势。结果显示,北方各区夏半年极端干期变化主要由前2个较高频的本征模态函数(IMF)分量构成,这表明3～4年和6～7年的变化对整个极端干期的变化起主要作用。对较低频分量和总趋势的分析表明,近54年来东北和华北大部分相对湿润的地区极端干期延长,有变干的趋势;北方主要干旱半干旱地区的极端干期变短,干旱有所缓解。近10～20年来整个北方极端干期日数增加,干旱化加剧,但西北局部干旱地区的极端干期在缩短,干旱有所缓解。这说明,分析的时—空尺度不同,得到的结果会有所不同。

1962—2007年广东干湿时空变化特征分析

利用广东省74个气象站点1962—2007年的月降水与气温数据,计算多时间尺度的标准化蒸散发指数,采用旋转经验正交函数(REOF)、Mann-Kendall趋势检验和小波分析等方法分析广东近50年来的干湿时空变化特征。研究结果表明:①广东20世纪70年代以来干旱发生事件随时间持续增多,空间范围扩展;②根据REOF时空分解的前6个空间模态,可以将广东划分成6个干湿特征区域,分别位于珠江三角洲、韩江流域及东江流域上游、西江流域及北江中下游流域、粤东沿海区域、北江上游区域和粤西沿海区域;③广东干湿发展具有明显的东西部差异性,其中西江流域和北江中下游流域、雷州半岛为主的粤西沿海流域存在着显著的干旱趋势;④6个分区干湿变化普遍具有2～8年的振荡周期,但最强振荡周期有所差别。

Soil mulching can mitigate soil water defi ciency impacts on rainfed maize production in semiarid environments

Temporally irregular rainfall distribution and inefficient rainwater management create severe constraints on crop production in rainfed semiarid areas. Gravel and plastic film mulching are effective methods for improving agricultural productivity and water utilization. However, the effects of these mulching practices on soil water supply and plant water use associated with crop yield are not well understood. A 3-yr study was conducted to analyze the occurrence and distribution of dry spells in a semiarid region of Northwest China and to evaluate the effects of non-mulching （CK）, gravel mulching （GM） and plastic film mulching （FM） on the soil water supply, plant water use and maize （Zea mays L.） grain yield. Rainfall analysis showed that dry spells of ≥5 days occurred frequently in each of 3 yr, accounting for 59.9-69.2% of the maize growing periods. The 〉15-d dry spells during the jointing stage would expose maize plants to particularly severe water stress. Compared with the CK treatment, both the GM and FM treatments markedly increased soil water storage during the early growing season. In general, the total evapotranspiration （ET） was not significantly different among the three treatments, but the mulched treatments significantly increased the ratio of pre- to post-silking ET, which was closely associated with yield improvement. As a result, the grain yield significantly increased by 17.1, 70.3 and 16.7% for the GM treatment and by 28.3, 87.6 and 38.2% for the FM treatment in 2010, 2011 and 2012, respectively, compared with the CK treatment. It＇s concluded that both GM and FM are effective strategies for mitigating the impacts of water deficit and improving maize production in semiarid areas. However. FM is more effective than GM.

近60a来西秦岭及周边地区降水的分布格局

基于西秦岭及周边地区15个气象站点的降水、气温等月值、年值资料,采用相关统计分析及检验的方法,研究了1951年以来该区域近60 a干湿变化的时空特征。结果表明:西秦岭及周边地区1951年以来降水量呈下降趋势,秋季降水量减少趋势最明显,速率为-18.6 mm·(10 a)-1;而近60 a年平均气温呈上升趋势,升温速率为0.28℃·(10 a)-1。对比气温和降水要素,西秦岭及周边地区年平均温度每升高1℃,则年降水减少37 mm,表明该区近60 a由冷湿向暖干转变。同时将气候要素与Ni?o3.4指数进行相关分析,结果显示在厄尔尼诺事件发生当年该区降水少,气温高,容易发生干旱。利用改进的经验正交函数法分析西秦岭及其周边地区15个气象站点的气候要素,发现该区年降水距平百分率的第一模态解释方差为49.0%,整个区域呈同向变化,而年平均温度距平第一模态解释方差为78.8%,在整个区域内亦呈现同向变化。对比两个要素第一模态显示西秦岭近60 a东部地区与西部地区相比,呈现降水减少幅度大,气温升高速率小的分布格局。

南京地区城郊降雨差异特征分析

利用2009—2013年南京市71个加密自动气象站逐时降水观测资料,对南京地区城郊降雨差异的特征进行了分析。结果表明:南京地区城市"雨岛效应"存在明显的季节差异,城市"雨岛效应"集中出现在6—8月,尤其是7月和8月,而南京地区其他季节城郊降雨无明显差异。对于不同量级的降雨,南京地区小雨、中雨和大雨的降雨量城郊差异均不显著;而城市化使南京地区6—8月暴雨和短时强降水的城郊差异较明显,城区暴雨发生频次和强度及短时强降水的发生频次均高于周边郊区,易形成城市洪涝灾害。城市化进程对一般性降雨的城郊差异影响较小,但城市化使城区夏季暴雨和短时强降水等灾害性降水事件明显加强。

基于干湿期的随机天气发生器

为了按不同的应用需求生成可信的任意长序列逐日天气数据,为作物天气系统研究提供数据支持,该文描述了一个以干湿期随机模型为基础,组合了日降水量、温度和辐射变量随机模型的逐日天气发生器WGDWS(Weather Generator based on Dry and Wet Spells)。它分为两部分:以干湿期为独立随机变量的干湿期模型部分,和依赖第一种模型生成其余天气变量的模型部分;其天气要素的生成主要分2个步骤,即首先根据月经验分布值产生一个干期或湿期长度,然后生成干期或湿期的逐日值。利用代表中国不同地理区域的9个站点1973-2003年的逐日气象资料对天气发生器WGDWS进行了检验,并与基于干湿日开发的DWSS天气发生器进行了比较。结果表明两者性能基本相近,并且WGDWS模拟干湿期的效果更好。因此,WGDWS天气发生器用于生成逐日天气序列是可靠的,同时作为一个JAVA组件,还可以方便地嵌入作物模型系统。

Trends in Seasonal Precipitation over China during 1961-2007

Trends in six indices of precipitation in China for seasons during 1961-2007 were analyzed based on daily observations at 587 stations. The trends were estimated by using Sen's method with Mann-Kendall's test for quantifying the significance. The geographical patterns of trends in the seasonal indices of extremes were similar to those of total precipitation. For winter, both total and extreme precipitation increased over nearly all of China, except for a small part of northern China. Increasing trends in extreme precipitation also occurred at many stations in southwestern China for spring and the midlower reaches of the Yangtze River and southern China for summer. For autumn, precipitation decreased in eastern China, with an increasing length of maximum dry spell, implying a drying tendency for the post-rainy season. Wetting trends have prevailed in most of western China for all seasons. The well-known 'flood in the south and drought in the north' trend exists in eastern China for summer, while a nearly opposite trend pattern exist for spring.

内蒙古近53年连续无雨期的时空变化特征

利用我国内蒙古自治区1960—2012年46个气象站点的逐日降水数据,分析了该地区连续无雨期的时空变化特征。计算了3个连续无雨期指数:连续无雨期次数(NDS)、连续无雨期平均日数(MDS)和最长连续无雨期日数(MLDS),并通过线性倾向估计法和MK趋势检验法研究连续无雨期指数的变化特征。结果表明:MDS,MLDS呈现自西向东的梯度递减,而NDS则呈现相反的空间分布。连续无雨期指数的空间分布表明内蒙古西部的干旱强度要高于其他区域。年尺度下,内蒙古大部分站点的NDS,MLDS呈现不显著的减少趋势,而多数站点的MDS呈现不显著的增加趋势。月尺度下,内蒙古7月、8月连续无雨期的变化特征较显著。研究区7月、8月的NDS呈显著减少趋势,而MDS,MLDS表现为显著增加趋势。连续无雨期的季节变化表明内蒙古夏季干旱化程度正逐步加重。

长江中下游夏季降水及其与全球热通量的关系

统计分析了1951—2002年52 a长江中下游夏季降水的年际和年代际变化特征。结果显示:长江中下游夏季降水在20世纪60、70年代处于少雨期,80年代旱涝相间,90年代处于多雨期,而且年际异常和年代际异常的配置决定了旱涝的强弱。在此基础上,分析了长江中下游夏季旱涝年的前期潜热和感热通量在年际和年代际尺度上的异常合成场,以及夏季旱涝年同期大气环流场及风场的异常合成场。结果表明:中北太平洋西部(T区)和日本以东洋面(R区)当年春季热通量的异常分布形势是长江中下游夏季旱涝的一个前期讯号。

我国南方夏半年干湿的年代际变化分析与转换机理探讨

用我国南方58年(1951—2008年)夏半年的总降水量、极端干期以及干湿指数综合表现干湿特征,进而用经验模态分解方法分析南方干湿的年代际变化。结果表明,58年内有3个干期(1961—1965、1985—1990和2005—2008年)以及3个湿期(1952—1954、1970—1973和1994—1998年)。为了探讨南方干湿的转换机制,采用与南方干湿年代际变化相关联的夏季风和西太平洋副热带高压等年代际环流因子作为结点构建网络,分析同步和耦合与干湿气候主要转换之间的关联。网络平均距离的3个局部最小值(即同步),都伴之以耦合的增强,对应着我国南方由湿期向干期的转换。

黔东南地区各季节极端干期日数的时空分布特征

利用1961—2009年黔东南地区16个地面气象站的逐日降水资料,分别统计了四季最大连续无雨日数(日降水量<0.1mm)的时间序列,采用正交函数分解、Mann-Kendall突变检验和线性倾向估计等方法,分析了各季节极端干期日数的空间结构和时间演变规律。结果表明,黔东南地区各季节极端干期日数的时空分布存在较大差异,极端干期日数最多出现在秋季,最少是春季;夏季黔东南北部的极端干期日数多,春、秋季南部多,冬季南、北部多,西部四季相对较少,年内非均匀性特征显著。在大尺度天气系统控制下,四季极端干期日数事件的步调基本一致,黔东南中东部发生异常的频次较高,不同季节的天气系统对黔东南各区域的影响具有明显的局地性和阶段性;近49年中各季节出现极端干期日数典型多的年份比典型少的年份多;进入21世纪以后,春、秋、冬季极端干期日数均呈显著的增多趋势,而夏季变化的特征不明显。

干湿持续期随机模拟

该文应用数据建模技术,实现干湿期随机建模。主要包括:利用历史气象资料,从中采集干湿期数据;应用实测数据,创建干湿期经验分布函数;应用Monte Carlo方法和经验分布参数,随机生成干湿期序列,通过和Markov链模型输出的对比分析,讨论生成序列的统计误差,测试结果显示,与两状态Markov链方法相比,所建模型性能更好。

Response of Seasonal Vegetation Dynamics to Climatic Constraints in Northeastern Burundi

The climate crises in East Africa (EA), particularly in Burundi, have affected vegetation which, in turn, plays a key role in the climate system by modifying the terrestrial water and energy balance. Consequently, it is vital to understand vegetation dynamics and its response to current and projected climate conditions to support the design of climate resilient land management strategies. The objective of this study was to study the dynamics of vegetation cover over the Northeastern Burundi (NEB) in response to climatic constraints. The methodology used consisted of the interpretation of satellite images along with the analysis of data collected through rain-gauge stations. The data sets used include time series composite moderate resolution imaging spectroradiometer (MODIS) normalized difference vegetation index (NDVI) data collected between February 2000 and December 2017;long term (1986-2017) rainfall data acquired from two meteorological stations throughout the Northeastern provinces in Burundi and precipitation and mean temperature data (1986-2017) from Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) and ERA5 Daily aggregates for the study area. The study provides an assessment of the vegetation trends in NEB using the NDVI time series. In addition, regression analysis is applied to assess the relations between NDVI and precipitation, air temperature, potential evapotranspiration, as well as with growing season characteristics (onset, cessation, and length). Results show that the rate in vegetation productivity is persistently gradual between 2000 and 2011 despite fluctuations from the mean position, followed by a lower growth rate over the period 2011-2017. There has been trend variation in precipitation, neither the temperature was constant. The temperature over the region has increased while the precipitation has decreased. The onset of the growing season and air temperature also show a significant influence on seasonal vegetation dynamics in the region. Drought-induced plant stress observed at the onset of the rainy season was the most important contributor to the subsequent less greening of vegetation especially over the area near the northern lakes.

Agroclimatological Characteristics of Rainy Seasons in Southwestern Burkina Faso during the 1970-2013 Period

The different drought phases observed during the 1970-2010 period have underlined important weaknesses of West African agricultural systems. The droughts resulted in important decreases in crop production, triggering a significant deficit in food availability. Many studies have identified changes in rain events seasonal patterns as the key drivers of agricultural production failure during these drought phases. In this study, seven agriculturally-relevant intra-seasonal rainfall characteristics (i.e., annual rainfall amount, onset and cessation of the rainy season, dry spells, extreme rain events, hot spells, and strong winds) and associated constraints to crop growth are described for the main cereals (maize, millet, and sorghum) in southwestern Burkina Faso. These characteristics are calculated or determined using daily climate data from a local network of 16 weather stations spanning the 1970-2013 period. A computation of the intensity and the occurrence of these phenomena during the rainy seasons helped to draw the rainy seasons’ nomenclature. Findings suggest that the rainy seasons during the drought phases are characterized by low annual rainfall amount, late onset, early cessation and more frequent long dry spells (>7 days). Furthermore, the long dry spells mostly occurred during the most sensitive phases of crop development: germination at the beginning of the rainy season and flowering at the end of the rainy season. Also, the intensity and the probability of occurrence of the other extreme events (hot spells and strong winds) during rainy seasons are very high in the establishment phase. Thus, adaptation strategies to mitigate these unfavorable climate conditions include a selection of short-cycle crop varieties combined with supplementary irrigation systems during long dry spells.

近58 a我国西北地区干期与湿期变化特征

利用西北地区1960-2017年68个站点逐日降雨气象数据,分别将日降雨小于(大于等于)0.1 mm和1 mm定义为旱日(湿日),从干期和湿期变化特征的角度分析西北地区雨日年内分配变化。结果表明:西北地区东部年降雨量变化不明显,降雨频率下降,平均降雨强度增加;西北地区西部年降雨量、降雨频率和平均降雨强度均呈现增加趋势,平均降雨强度增加主要是由于降雨量增加速率快于降雨频率增加速率。结合干期和湿期变化特征,发现西北地区东部虽然干期旱日总数增加,但干期平均长度、干期次数和最长干期旱日数变化不明显,同时湿期湿日数和次数减少,说明西北地区东部在降雨量不变情况下,降雨更加集中。在西北地区西部,干期次数增加,但干期旱日总数、干期平均长度以及最长干期旱日数减少,湿期湿日数和湿期次数增加,湿期平均长度不变,西北地区西部在降雨量和降雨频率增加过程中,干期持续时间缩短,对该区域农业生产和生态环境有利。另外,使用不同阈值会影响特征值变化趋势大小及其显著性,甚至会得到相反的变化趋势,说明选择合理阈值对于研究降雨、干期以及湿期变化十分重要,需要结合区域气候特征进一步研究。