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1、标准化降水指标与Z指数在我国应用的对比分析袁文平 周广胜3(中国科学院植物研究所植被数量生态学重点实验室,北京 100093)摘 要 利用分布于我国不同气候区的7个气象台站19511995年的月降水资料,比较分析了标准化降水指标(Standardized precipitation index ,SPI)和在我国已成熟应用的Z指数。结果表明,SPI计算简单,资料容易获取,而且计算结果与Z指数有极好的一致性。同时,由于SPI是通过概率密度函数求解累积概率,再将累积概率标准化而得,具有稳定的计算特性,消除了降水的时空分布差异,在各个区域和各个时段均能有效地反映旱涝状况,优于在我国广泛应用的Z指数。
2、此外,SPI还可以计算不同时间尺度的指标值,能够满足不同水资源状况分析的要求。以北京为例,探讨了应用4种时间尺度的SPI值反映19511995年期间的旱涝事件,发现SPI能准确地反映北京45年间的旱涝趋势,对于旱涝灾害有着良好的预测作用,可用于我国的水资源评估以及不同时间尺度的干旱监测。关键词 标准化降水指标 Z指数 干旱COMPARISON BETWEEN STANDARDIZED PRECIPITATION INDEX ANDZ- INDEX IN CHINAY UAN Wen-Ping and ZHOU Guang-Sheng3 (Laboratory of Quantitative V
3、egetation Ecology , Institute of Botany , Chinese Academy of Sciences , Beijing100093,China)Abstract Droughts are the worlds costliest natural disasters , causing an estimated6 -8 billion in global damages annually and affecting more people than any other form of natural disaster. Given the conse2 q
4、uences and pervasiveness of droughts , it is important to assess the specialized indices that are used to assess drought severity. The standardized precipitation index (SPI) has several characteristics that are an improve2 ment over previous indices , including its simplicity and temporal flexibilit
5、y that allow it to examine both short term and long term drought conditions. Computation of theSPIinvolves fitting a gamma probability density function to a given time series of monthly precipitation totalsfor a weather station. The resulting parameters are then used to find the probability of a par
6、ticular precipitation event over a given time scale. This probability is then converted to the standard normal random variableSPIindex value. In this article , 1-monthSPIare cal2 culated and compared with theZ-index , the most widely used index in China. The results demonstrated that 1- monthSPIcalc
7、ulated for 7 observational stations are greatly consistent withZ2index , but that theSPIrarely relates to distributing on precipitation , avoiding some of the irregularities associated with theZ-index. Thus , theSPIis superior to theZ-index in its application. We also investigated drought and flood
8、eventsfrom 1951 - 1995 for Beijing in greater detail. By using the 24-monthSPI(SPI24) , three well-defined drought and flood events were identified from the data series. In general , the same drought and flood events were observed using the 12-monthSPI(SPI12) as theSPI24, al2 though there were some
9、interruptions where theSPI12values approached zero or became negative for short peri2 ods. For 3-month periods (SPI3) , theSPIvaluesfrequentlyfell above and below zero. These results highlighttheSPIcharacteristics at different time scales. As the time scale increases from 1 to 24 months , theSPIre2
10、sponds more slowly to short-term precipitation variation , and the cycles of positive (wet conditions) and nega2 tive (drought)SPIvalues become more visible. The possibility of calculating theSPIfor different time scales enhances its analysis capacity , since it allows the estimation of different an
11、tecedent conditions in the soils. Whereas the shortest scales (1 to 3 months) quantifies superficial soil water , which bears a direct significance for agriculture , the longest accumulation scales (12 to 24 months) indicate the state of subsoil moisture as well as other surface and subsurface water
12、 resources. The joint consideration of differentSPIscales in the analysis contributed to a satisfactory explanation of risk conditions before each flood event reported. These results indi2 cate that theSPIis an effective index for assessing drought conditions at different time scales and should be a
13、dopted for use in China. 收稿日期: 2003207222 接受日期: 2004202228基金项目:国家重点基础研究发展规划项目(973项目) (G1999043407)、 中国科学院创新工程项目(KZCXI-SW-01-12 , KSCX2-1-07)和国家自 然科学基金项目(40231018 , 30070642 , 30028001 , 49905005 , 39730110 ,30300049)3 通讯作者Author for correspondence E-mail :zhougs public2. bta. 植物生态学报 2004 , 28 (4)523
14、529 Acta Phytoecologica SinicaKey words Standardized precipitation index ,Z-index , Drought干旱作为一种气象灾害,长期困扰着工农业生 产。据测算,每年因干旱造成的全球经济损失高达6080亿美元,远远超过了其它气象灾害(Wilhite ,2000)。我国平均每年受旱面积约2 159. 3万公顷, 约占各种气象灾害面积的60 % ,每年因旱灾损失粮食100亿公斤。随着经济的迅速发展、 人口增长及 由此引起的以气候变暖为标志的全球变化的发生, 干旱有进一步加重的趋势,从而必将对我国社会经 济及人民生活造成严重的
15、影响。同时水分也是制约 陆地生态系统的关键因子。每年全球有一半以上的陆地生态系统面临着干旱的威胁(Felix ,1997)。 目前,关于干旱指标已有大量的研究(Richard 19701972年3年连旱,年均 降水量仅为508 mm ,造成200多万亩作物减产,最 后依靠大量打井超采地下水得以度过;20世纪80 年代初连续5年干旱,水库水位大幅度下降,农业受灾面积达300多万亩。在这45年期间发生过最大 规模的洪涝灾害是在1959年。1959年7月下旬至8 月下旬降水量比同期增长70 % ,累积平均降水量达 到1 051.8 mm ,致使农田大面积受灾,城区进水(李 裕宏,1999)。通过计算
16、12个月和24个月时间尺度的SPI值(分别标记为SPI12和SPI24) ,可以明确地标示出3 次旱灾和1959年的涝灾(图2c、d)。SPI12与SPI24的柱状图相比有较大的正负波动。不同时间尺度的SPI值对于降水量的敏感性不同,时间尺度越小,则 对于一次降水变化越显著,其值会发生较大变化,甚 至是正负波动。相反,时间尺度越大则对于一次降 水的反映并不显著,只有持续的多次降水才会使之发生波动。这对于监测长时期的水分状况是合理的 (Seileret al.,2002)。SPI12和SPI24都显示出虽然在1957年出现了短暂的轻微水分亏缺,但是总体而 言,从1954年起北京的水分充足,说明下层土壤水 分、 径流量、 地下水以及水库蓄水量丰富。因而,1959年7、8月间的大量降水不能有效地经由土壤下 渗、 地表径流和水库蓄水分担,造成了北京45年间 最严重的涝灾。长期的水分富足状态(SPI12和SPI24保持较大的正值)和短时间大量的降水(SPI1)可以很好地成为指示1959年涝灾的信号。SPI12和SPI2
