干旱事件重现期计算问题研究

2017-01-06 12:40梁忠民卜慧刘和昌胡义明郦
南水北调与水利科技 2016年6期

梁忠民卜慧++刘和昌++胡义明郦于杰

摘要:干旱重现期大小是用于评价干旱事件严重程度的重要指标。干旱重现期的计算涉及给定阈值下干旱过程划分(识别)、样本系列分布函数拟合等关键环节,其中干旱阈值的确定是前提。提出以干旱事件的最长调查期为约束条件确定干旱阈值的思路,即根据样本计算的干旱事件最大重现期不应超过最长调查期,以此为据确定干旱阈值并从样本序列中识别干旱事件。同时,针对因干旱历时样本经验点据“平台式”过度集中而导致的频率曲线适线困难问题,建议采用基于游程理论的游程长度分布函数估计干旱历时概率分布。以青海民和县1932年-2010年的月降雨资料为例,对上述方法进行了应用研究,结合Copula函数计算了干旱事件的重现期。

关键词:干旱重现期;干旱阈值;干旱历时;干旱烈度;Copula函数

中图分类号:P333文献标志码:A文章编号:

16721683(2016)06000105

Return period calculation of drought events

LIANG Zhongmin1,2,BU Hui1,LIU Hechang3,HU YiMing1,LI Yujie1

(1.Collge of Hydrology and Water Resources,Hohai University,Nanjing 210098,China;2.National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety,Nanjing 210098,China;3.Technical Advisory Center of Pearl River Water Resources Commission of the Ministryof Water Resources,Guangzhou 440100,China)

Abstract:[JP]The concept of return period is commonly used to evaluate the severity of drought events.The calculation of the return period for a given drought contains drought recognition and selection of distribution function and so on,which relies on the determination of drought threshold.This paper proposed to use the maximum survey period of historical drought as a reference to determine the drought threshold,in other words,the resulting return period based on frequency analysis should not exceed the maximum survey period of historical drought.Besides,considering that the drought duration samples tend to concentrate on several values and lead to the platformconcentrated phenomenon of empirical points,the distribution function of run length was suggested to fit the drought duration samples.The monthly precipitation series of Minhe County in Qinghai province covering the period from 1932 to 2010 was used to demonstrate the proposed method,where the Copula function was employed to build a bivariate joint distribution of drought duration and drought severity to compute the return period of drought event.[JP]

Key words:return period of drought;drought threshold level;drought duration;drought intensity;copula function

干旱是我国的主要自然灾害。干旱事件包括干旱持续时间(干旱历时)、干旱强度大小(干旱烈度)、干旱影响范围(受旱面积)等特征,对一场干旱的严重程度可以采用干旱重现期这一综合性指标进行度量[13],其中,利用Copula函数构建干旱历时、烈度、面积等干旱特征变量的多维联合分布函数,进而计算干旱事件的重现期,是目前该领域的最新研究进展[48]。

估计干旱事件的多维联合分布需要确定干旱特征变量样本系列,其前提是对历史干旱事件进行识别,即对研究的水文变量(如降雨、径流等)时间序列,根据指定的某一阈值,从历史时间序列中筛选出哪些是干旱事件,进而构建干旱特征(历时、烈度、面积等)样本系列,以供频率分析之用。因此,如何正确地确定干旱阈值是干旱事件识别及干旱重现期计算合理性的关键。但在现有的研究与应用中,对干旱阈值的选用缺乏统一标准,额外增加了干旱事件识别及重现期计算的不确定性。为此,本文借鉴水文频率分析中样本重现期调查分析的概念,提出以干旱事件最长调查期为约束条件,通过试算确定干旱阈值的方法。即,类似于洪水调查及重现期的确定,先进行历史干旱调查并确定其样本系列的最长重现期;假定一个干旱阈值,按此阈值划分干旱事件、确定干旱特征,再根据干旱特征的样本进行干旱重现期计算,使得计算得到的干旱事件的最大重现期与调查的干旱重现期一致,否则,重新假设干旱阈值。根据青海民和县的长系列降水资料进行示例研究,其中,针对以月(旬)为时间尺度进行干旱识别及重现期计算时,干旱历时样本经验点据呈“平台式”集中而导致的频率曲线适线困难,建议采用基于游程理论的游程长度分布函数[9]对干旱历时样本进行拟合,以提高分布函数估计的可靠性,再与干旱烈度分布函数一起构建copula联合分布函数并估计干旱事件重现期。

4结论

(1)针对干旱事件重现期计算过程中干旱阈值(截取水平)的选取问题,本文提出以干旱事件调查期为约束条件来确定干旱阈值的观点,即根据样本计算的干旱事件最大重现期不应超过样本系列的调查期,以此确定干旱阈值并从样本时序资料中划分干旱过程。

(2)针对由于干旱历时样本点据“平台式”的过度集中而导致的频率曲线适线困难问题,建议采用基于游程理论的游程长度分布函数公式估计干旱历时的概率分布函数。

(3)以青海民和县月降水资料为据,提供了本文方法的应用示例。

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