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The Modification of Meteorological Drought Composite Index and Its Application in Southwest China
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摘要: 以我国西南地区为例，在用加权降水量 (WAP) 改进标准化降水指数的基础上，对综合气象干旱指数 (IC) 进行了修正，定义为ICW。通过对比指数修正前后在干旱频率、年干旱强度和干旱发展过程中的不连续加重、以及与同期土壤湿度相关性的差异，分析了ICW在修正后的改进效果及在西南地区的适用性。结果表明：修正前后的综合气象干旱指数在多年干旱频率和年干旱强度两方面没有显著差异，但ICW减少了干旱发展过程中的不连续加重现象，且与同期土壤湿度有更好的相关性，即ICW比IC更加接近实际干旱的演变规律，ICW比IC更适合在西南地区实时干旱监测业务中使用。然而，由于该研究未对IC中的相对湿润度指数进行修正，而相对湿润度指数的突变同样可以导致不连续加重的出现，因此，ICW只是在一定程度上减少了不连续加重的影响。
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关键词:
- 综合气象干旱指数IC;
- 加权降水量WAP;
- 西南地区
Abstract: The meteorological drought composite index (IC) is recommended as an effective operational drought monitoring index from National Standard and has been applied by National Climate Center to monitor drought conditions across the whole country in recent years. However, uncontinurous enhancement of drought is frequently observed in the development of drought process at many stations, where uncontinurous enhancement of drought means that daily change is greater than or equal to one drought degree (-0.6) and the IC value is equal to or less than 0.6 a day before. In order to optimize IC, modified standard precipitation index (Z65) by weighted average precipitation (WAP) is introduced. Due to different weighted coefficients of daily precipitation, modified IC(ICW) can decrease uncontinurous enhancement of drought to the most extent. Taking Southwest China as an example, daily IC and ICW(modified IC) from 1 January 1961 to 10 May 2010 is calculated, and the differences between IC and ICW in frequency of every drought degree and monthly variability of drought frequency are analyzed. Since formula and value range of ICW are both similar to IC, the same drought degree is adopted. It indicates that there is no significant difference between ICW and IC in frequency of drought. Moreover, the annual intensity of drought from 1961 to 2009 is also analyzed. The results indicate ICW is also similar to IC very much and there is no significant trend of annual intensity. Furthermore, as stable index of drought, uncontinurous enhancement of drought of IC and ICW are counted. It shows that ICW has less uncontinurous enhancement of drought than IC at all four stations, so ICW has improved the stability of drought according to different weighted coefficients of daily precipitation. Take soil moisture observed on 8, 18 and 28 as representative, the correlation between IC, ICW and soil moisture is analyzed from January 1993 to May 2010. The results show that ICW has greater correlation coefficients with soil moisture of 10 cm at all stations than IC, and ICW has greater correlation coefficients with soil moisture of 20 cm at three stations than IC, showing that ICW is more consistent with soil moisture.Above all, the analysis shows that ICW is close to the characteristics of actual drought and suitable for daily drought monitoring in Southwest China. However, there is still some uncontinurous enhancement of drought existing in ICW due to great uncontinuous change of M30. -
图 1 1961—2009年云南陆良逐日IC与ICW不同旱涝等级和干旱频率分布
(a) 不同旱涝等级频率分布, (b) 不同月份偏旱频率分布
Fig. 1 Frequencies of different flood and drought categories and monthly variation based on daily ICW and IC at Luliang of Yunnan from 1961 to 2009
(a) frequencies of different flood and drought categories, (b) monthly frequencies of drought
图 2 1961—2009云南陆良年干旱强度变化
Fig. 2 Annual drought intensity at Luliang of Yunnan from 1961 to 2009
图 3 2010年1月18日—2010年2月26日广西百色ICW与IC逐日变化
(a)IC, ICW与降水量，(b)Z30，Z90，Z65和M30
Fig. 3 Daily evolution of ICW and IC at Baise of Guangxi from 18 Jan 2010 to 26 Feb 2010
(a)IC, ICW and precipitation, (b)Z30, Z90, Z65and M30
图 4 西南四省IC，ICW的逐日演变及前30日的降水量分布
Fig. 4 Daily evolution of IC, ICW and precipitation ahead of 30 days in Southwest China
表 1 近期逐日降水量的权重
Table 1 Daily precipitation weights of recent 65 days
相距日数 权重 0 0.100 1 0.090 2 0.081 3 0.073 4 0.066 5 0.059 6 0.053 7 0.048 14 0.023 30 0.004 43 0.001 65 0.0001 
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表 2 ICW干旱等级的划分
Table 2 ICW categories based on ICW values
等级 类型 ICW范围 1 无旱 -0.6＜ICW 2 轻旱 -1.2＜ICW≤-0.6 3 中旱 -1.8＜ICW≤-1.2 4 重旱 -2.4＜ICW≤-1.8 5 特旱 ICW≤-2.4
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表 3 1961—2009年云南陆良各年干旱强度排名
Table 3 Ranking of annual drought intensity at Luliang of Yunnan from 1961 to 2009
年份 ICW IC 1961 41 40 1962 24 30 1963 1 1 1964 36 39 1965 42 37 1966 7 7 1967 27 15 1968 47 49 1969 2 2 1970 39 31 1971 48 46 1972 25 24 1973 38 41 1974 9 14 1975 26 25 1976 35 43 1977 19 11 1978 14 21 1979 8 6 1980 21 20 1981 49 45 1982 16 17 1983 34 32 1984 11 13 1985 43 23 1986 33 33 1987 17 18 1988 6 10 1989 32 19 1990 22 35 1991 46 47 1992 4 3 1993 15 16 1994 37 38 1995 31 27 1996 12 8 1997 20 42 1998 45 36 1999 23 34 2000 44 48 2001 3 4 2002 40 44 2003 10 12 2004 30 28 2005 28 9 2006 13 26 2007 18 29 2008 29 22 2009 5 5
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表 4 1961年1月—2010年5月上旬干旱发展过程中的不连续加重
Table 4 Uncontinuous enhancement of drought during the development process of the drought from Jan 1961 to the first ten days of May 2010
地点 日变化加重达1个等级 日变化加重达2个等级 ICW IC ICW IC 云南陆良 58 134 1 12 四川西昌 53 208 0 17 贵州六盘水 89 198 10 26 广西百色 40 150 0 8
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表 5 1993年1月—2010年5月上旬逐旬10 cm和20 cm土壤湿度与IC和ICW的相关分析
Table 5 Correlation coefficients between soil moisture of 10 cm, 20 cm and IC, ICW from Jan 1993 to the first ten days of May 2010
土壤湿度 云南陆良 四川西昌 贵州六盘水 广西百色 IC ICW IC ICW IC ICW IC ICW 10 cm 0.516 0.526 0.668 0.732 0.368 0.404 0.331 0.456 20 cm 0.454 0.442 0.640 0.684 0.392 0.404 0.484 0.553
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