Fitting Drought Duration and Severity with Two-Dimensional Copulas

This study aims to model the joint drought duration and severity distribution using two-dimensional copulas. The method of inference function for margins (IFM method) is employed to construct copulas. Two separate maximum likelihood estimations of univariate marginal distributions are performed first, then followed by a maximization of the bivariate likelihood as a function of the dependence parameters. The drought duration and severity are assumed to be exponential and gamma distributions, respectively. Several copulas are tested to determine the best data fitted copula. Droughts, defined using the Standardized Precipitation Index (SPI), of Wushantou (Taiwan) are employed as an example to illustrate the proposed methodology. The copula fitting results for drought duration and severity are quite satisfactory. The bivariate drought analyses, including the joint probabilities and bivariate return periods, based on the derived copula-based joint distribution are also investigated to demonstrate the advantages of bivariate modeling of droughts.     

Multivariate Drought Frequency Analysis using Four-Variate Symmetric and Asymmetric Archimedean Copula Functions

In drought frequency analysis, as the number of drought variables increases, the joint behavior between these variables needs to be studied. Therefore, this study aims to develop a flexible four-variate joint distribution function of the regional stochastic nature of drought. Using run theory, drought duration, severity, peak, and inter-arrival time were abstracted from the Standardized Precipitation Evapotranspiration Index (SPEI) aggregated at six months, observed in mainland China between 1961 and 2013. As these drought variables showed significant dependence properties and followed different marginal distributions, we employed and compared six four-variate symmetric and asymmetric Archimedean copulas (i.e., Frank, Clayton, Gumbel–Hougaard). The best-fitting model for each region was carefully selected using RMSE, AIC, and BIAS goodness-of-fit tests. Results revealed that the empirical and theoretical probabilities of the symmetric Clayton in regions NE (Northeast), CS (Central and Southern China), EMC (Entire China), and symmetric Frank in regions NC (North China), SC (South China), IM (Inner Mongolia), NW (Northwest), TP (Tibet Plateau) agreed well. Symmetric Frank copula was considered the best-fit for station-based drought analysis in EMC. Based on these copulas, the drought probabilities and return periods for the occurrence of drought events over the next 5, 10, 20, 50, and 100 years in each region were hereby comprehensively explained, and the results shown here could be helpful in the appraisal of the adequacies of water supply systems under drought conditions in all regions. This study showed that a four-variate copula approach is a vital tool for probabilistic interpretation of hydrological and meteorological data in the different climatic region of mainland China.

     

Analysis of Rainfall Severity and Duration in Victoria,Australia using Non-parametric Copulas and Marginal Distributions

The analysis of joint probability distributions of rainfall characteristics such as severity and duration is important in water resources management. Deriving their distributions using standard statistical techniques are often problematical due to its complexity. Standard methods usually assume that the rainfall characteristics are independent or that their marginal distributions belong to the same family of distributions. The use of copulas based methodologies can circumvent these restrictions and are therefore increasingly popular. However, the copulas and marginal distributions that are commonly used belong to specific parametric families and their adoption could lead to spurious inferences if the underlying assumptions are violated. For this reason, we recommend a nonparametric or semiparametric approach to estimate the joint distribution of rainfall characteristics. In this paper, we introduce and compare several copula–based approaches, each involving a combination of parametric or nonparametric marginal distributions conjoined by a parametric or nonparametric copula. An empirical illustration of the different approaches using rainfall data collected from six stations in the state of Victoria, Australia, demonstrated that a nonparametric approach can often give better results than a purely parametric approach.     

Estimation of Drought Severity on Independent and Dependent Hydrologic Series

The drought severity is the most important parameter for the design of water storage systems in order to alleviate the water shortages during drought periods. The largest drought severity, i.e. severity of a worst drought at a desired truncation level and for the desired return period can be estimated using the truncated normal distribution of the deficits in individual drought years, Poisson distribution of the number of drought spells over a period of T years and geometric distribution of drought duration. The analysis can be done for random or Markovian structure of drought variable (annual rainfall or runoff sequences) distributed normally or lognormally. The study indicated that the severity at a high truncation level is larger than that at a low truncation level over any desired return period T. Severity is also larger in the case of autocorrelated drought variable. A frequency formula for the largest drought severity can be formulated analogous to the flood frequency formula commonly found in the hydrologic texts.     

基于双源蒸散与混合产流的Palmer 旱度模式构建及应用

选取半干旱地区老哈河流域为研究对象,基于双源蒸散发能力计算模型和混合产流模块,依据palmer旱度模式的思路,构建适用于我国北方半干旱地区的机理性旱度模式。利用该模式计算15个代表站点1957—2008年的旱度值,并与实际旱情记载以及降水距平百分率进行了对照检验。结果表明,该旱度模式计算的各地区的Palmer干旱指标与文献描述的干湿情况较为一致,能够反映所研究区域干旱程度的变化情况;与降水距平百分率相比,该模式综合考虑了水分亏缺量和持续时间因子对干旱强度的影响,可以反映流域下垫面特性及植被生理物候特性对干旱的影响机制,能够更好地表现干旱过程的持续性。此外,该模式还能合理给出旱情在空间上的发生和发展的变化情况。     

基于可变下渗容量模型和Palmer 干旱指数的区域干旱化评价研究

基于可变下渗容量(Variable Infiltration Capacity,VIC)模型的流域网格划分及其对径流、蒸散发和土壤含水量变化的模拟,对Palmer干旱指数各水量平衡分量的计算进行了优化,并采用分级修正的方式,改进了Palmer干旱指数中气候特征系数K的确定方法。在此基础上,建立了基于VIC模型和Palmer干旱指数的区域气候干湿变化评价系统,并采用该系统对黄土高原进行了研究。结果表明,近40年黄土高原气候正向暖干化趋势发展,1990年以后暖干化趋势尤为明显。研究区干旱发生频率在空间上呈由西北向东南递减趋势,不同季节干旱发生频率空间分布存在一定差异。该评价系统物理机制清晰,地区适应性较强,在实时监测和评估气候干湿变化及其时空分布上存在较大应用潜力。     

基于VIC模型和Palmer干旱指数的区域干旱化评价研究

基于VIC(可变下渗容量)模型的流域网格划分及其对径流、蒸散发和土壤含水量变化的模拟，对Palmer干旱指数各水量平衡分量的计算进行了优化，并采用分级修正的方式，改进了Palmer干旱指数中气候特征系数K的确定方法。在此基础上，建立了基于VIC模型和Palmer干旱指数的区域气候干湿变化评价系统，并将该系统在黄土高原进行了应用。结果表明，近40 a黄土高原气候正向暖干化趋势发展，1990年以后暖干化趋势尤为明显。研究区干旱发生频率在空间上呈由西北向东南递减趋势，不同季节干旱发生频率空间分布存在一定差异。该评价系统物理机制清晰，地区适应性较强，在实时监测和评估气候干湿变化及其时空分布上存在较大应用潜力。     

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.     

基于自适应帕默尔指数的 1961—2015 年全国干旱时空特征分析

干旱是我国频繁发生的自然灾害,全国平均每年因干旱灾害造成的损失会直接或间接地影响到社会经济的发展。为了揭示全国干旱的变化规律,利用 1961—2015 年全国 2 360 个气象站点的逐日气温、降水等观测资料,选择空间适应性较强的自适应帕默尔干旱指数( SC － PDSI) ,计算得到了55 年来月尺度的 SC － PDSI 序列,并通过干旱发生次数、历时、强度、频率和受旱面积等指标分析了全国干旱的空间格局和趋势变化。结果显示: 在 1961—2015 年全国范围没有明显的变干旱趋势,但局部地区存在显著差异,东北、内蒙古、西北、青藏以及东南部地区有明显的变湿趋势,华南和西南有变干的趋势,华北、华中基本保持不变; 干旱变化呈年代际波动,即 1961—1980 年代为干旱时期, 1981—2000 年代属于正常代,21 世纪以来干旱程度加剧。研究表明: 在 1961—2015 年间干旱化空间特征表现为带状区域,即自西南地区—黄土高原—海河平原形成了一个干旱化带。通过分析各年代干旱频率重心的变化得出,我国的干旱重心由西北部逐步转移到了西南部地区; 通过计算受旱面积的变化趋势,发现西南和华南地区的受旱面积有明显增加。     

基于ECMWF和WRF的嘉陵江流域面雨量预报精度评估

将降雨数值预报产品运用到水文预报中已经逐渐成为提高洪水作业预报的预见期的重要手段。为充分了解ECMWF(European Centre for Medium Range Weather Forecasts)和WRF(Weather Research and Forecasting model)2种数值天气预报产品对嘉陵江研究区面雨量预报的预报精度和误差分布,且为增强洪水预报精度的稳健性提供科学支持,采用TS评分、空报率、漏报率、正确率等指标,对嘉陵江地区7个气象分区内的2016年汛期面雨量预报结果进行了检验,分析了不同分区内各检验指标与预报时效的关系。结果表明:ECMWF数值预报产品和WRF数值预报产品均可用于该地区晴雨预报,且2种产品的预报精度随降水等级的增大呈增大趋势,随预报时效的增加呈减小趋势。综合而言,ECMWF数值预报产品对嘉陵江研究区的预报效果更好。     

新疆博斯腾湖二维水动力数学模型研究

风场是影响湖泊水动力的重要因素,它可以改变水体运动速度和方向,影响各种物质在湖泊内的输移扩散。通过分析博斯腾湖大湖区风场,准确估算出风力、风向变化情况,构建了博斯腾湖平面二维水动力模型。应用计算模型对博斯腾湖水动力进行模拟预测,分析不同风向、风速对湖泊流场结构的影响,为进一步研究博斯腾湖水动力和污染物输移扩散提供理论依据。     

Palmer指数和SPI指数对广西干旱的评估分析

利用广西地区2009-2010年20个气象站逐日气温和降水资料,计算逐月帕默尔指数(PDSI)和标准化降雨指数(SPI)。采用ANUSPLIN插值软件对降雨、气温和两种指数的空间分布进行差值分析,并分析了降雨量和温度对干旱指数的影响。研究表明,降雨量的大小和温度的高低影响着该地区干旱过程,在两种干旱指数中,PDSI指数更加符合广西的实际情况,更适合于广西地区的干旱评估。     

1D/1D及1D/2D耦合水动力模型构建方法研究

为实现高速率、精细化的洪涝模拟,以丹麦R市A区的排水系统为研究对象,构建和比较基于GIS空域建模技术及SWMM水动力耦合的1D/1D(双一维)排水模型与基于MIKE URBAN的1D/2D(一维管流和二维洪涝)耦合排水模型,对研究区的排水系统进行暴雨洪涝模拟及评估比较。研究两种模型的建模过程、模拟精度及计算效率,描述两种模型的优劣性和适用条件。结果表明:1D/1D排水模型可实现地表淹没的拓展分析,同时确保较高的稳定度和运算速度,且在较低重现期下具有较高的计算精度,适用于大尺度、应急管理且模拟精度要求相对较低的情景; 1D/2D排水模型能实现地表积水的双向流动计算,具有良好的精细度,但其信息处理和计算时间较长,适用于计算环境和模拟精度要求较高的情景。研究结果将为不同情景、不同尺度的洪涝模拟提供科学依据。     

2001年四季连旱带给我们的警示与思考

2001年,石门县遭受了罕见的冬、春、夏、秋四季连旱。据统计,全县19个乡镇、548个村、10.3万个农户受旱,8.5万人饮水发生困难。受旱作物75万亩,其中粮食作物51万亩,经济作物24万亩。2001年该县的干旱有4个特点:一是降雨少,且分布不平衡。4～9月,全县平均降雨625.1mm,较多年同期平均降雨量1090.1mm减少465mm,比1992年大旱同期降雨906.2mm减少281.1mm。降雨分布不均,沱水流域的太平、子良降雨较丰沛;道水流域降雨较少,其中蒙泉、夹山降雨稀少,有40天滴雨未下,为近50年来少见。二是蓄水少,且死水比例大,有效水位低。全县165座中小型水库,设计蓄…     

Hydrological Drought Class Transition Using SPI and SRI Time Series by Loglinear Regression

Loglinear models for three-dimensional contingency tables was used with data from 21 rainfall stations and 7 hydrometric stations in the Luanhe river basin, northeast China, for short term prediction of drought severity class. Loglinear models were fitted to drought class transitions derived from standardized precipitation index (SPI) and standardized runoff index (SRI) time series to find which series was more suitable for hydrological drought class prediction 1 and 2 months ahead, respectively. Expected frequencies for two consecutive transitions between drought classes were first calculated, and based on this the predicted drought classes 1 and 2 months ahead were obtained. The results showed that despite the contingency tables of drought class transitions presented the maintenance of the precedent drought class, results of three-dimensional loglinear modeling presented good results when comparing predicted and observed drought classes. Only for a few cases predictions did not fully match the observed drought class, mainly for 2-month lead and when the SRI values are near the limit of the severity class predicted by SRI time series. Based on the correlation analysis of SPI and SRI, we presented the well-known method of hydrological drought class prediction by SPI time series. It was found that, using loglinear regression method, the accuracy of predictions for 2-month lead predicted by SPI time series was higher than those predicted by SRI time series. When we divided the SPI and SRI time series into 2 sub-periods (pre- and post-1980 where land cover changed), we got the same drought class prediction as that predicted by the entire SPI and SRI time series, which illustrated that changes in land use did not affect predictions of hydrological drought classes in the Luanhe river basin. It could be concluded that loglinear prediction of drought class transitions is a useful tool for short term hydrological drought warning, and the results could provide significant information for water resources managers and policy makers to mitigate drought effects.     

二、三维 GIS 集成技术在水利中的应用

针对二维GIS技术的缺陷,将二维与三维GIS技术进行优势结合,可为水利信息化建设提供更好的服务与应用。阐述二维、三维集成技术在水利行业中的应用,从数据组织、功能集成设计、系统界面集成、系统特点4个方面对二维和三维技术的集成过程进行说明,结合集成技术在水利、河道洪水演进、三维空间分析及防洪抢险等方面给出相应的应用实例,如对抗洪地区进行可视化转换及对所获得的数据进行分析等。     

广东台山电厂二维潮流泥沙数学模型研究

运用建立的广东台山电厂二维潮流泥沙数学模型,在经过实测资料验证的基础上,研究了台山电厂不同取排水工况条件对电厂工程海域的海床地形演变以及电厂航道、港池的泥沙冲淤的影响,计算结果可以为台山电厂二期扩建工程的环境评价和工程优化提供技术支撑。     

陈雷在防汛抗旱会商会上的讲话

同志们: 这次防汛会商会是继国家防总第一次全体会议和全国水库安全度汛工作视频会议之后召开的又一次重要会议.2009年入汛以来,我国先后出现多次大范围降雨,局部地区出现强降雨过程,发生了严重的洪涝灾害,造成了一定的人员伤亡,同时,部分地区降雨严重偏少,发生了特大干旱,防汛抗旱形势严峻.     

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.     

基于小波分析的表面活性剂减阻流体二维流场流动特性研究

该文采用粒子图像测速仪对矩形槽道内减阻流体二维流场进行测量,采用小波分析方法将流场分解为平均速度场与7个不同尺度的脉动场.结果显示减阻流体横向速度脉动几乎被完全抑制,流向速度脉动略低于水,但从波数为2开始小尺度脉动被明显削弱.减阻流体流向速度脉动呈条带分布,同流向基本平行,并且具有较大尺度,统计结果曲线也在相应位置出现局部大值,显示高脉动层存在的普遍性.