干旱指数在山西逐日监测中的适用性研究

摘要

The meteorological composite drought index (CI)and the standard precipitation index (SPI)are usually used as effective daily drought monitoring indexes in the whole country.However,there are many uncontinuous enhancement of drought(UED)in devel-opment of drought process at many stations,because daily precipitation in certain period are calculated by equal weighted coefficients in the contribution of present drought development.In fact,the contribution of precipitation today is bigger than that of precipitation twen-ty nine days ago.In this paper the standard precipitation index (SPI)and the meteorological composite drought Index (CI)were both modified.Modified SPI(SWI)was based on weighted average precipitation (WAP),and modified CI (CI_new)was calculated by u-sing linear decreasing unequal weight method.Taking an example of Shanxi,overall distribution of UED,daily evolution of typical drought events,and correlations with soil moisture were compared between CI and CI_new,SPI and SWI in order to analyze the moni-toring ability of four drought indexes in daily evolution of drought in Shanxi.The results are as follows:(1 )The frequency of uncontinu-ous enhancement of drought (UED)monitored by CI_new and SWI decreased obviously than that monitored by CI and SPI,respective-ly,and the occurrence times of UED monitored by SWI was least among the four drought indexes;(2)The correlation coefficients be-tween CI_new,SWI and 10 cm,20 cm soil moisture were improved than that between CI,SPI and soil moisture,which confirmed that CI_new and SWI had better accord with soil moisture.Above all,the analysis found that the results monitored by CI_new and SWI are close to actual drought,so CI_new and SWI are suitable for daily drought monitoring in Shanxi.%综合气象干旱指数（CI）和标准化降水指数（SPI）在逐日监测中往往会出现干旱突然加重的现象，这是由于某时段内每日降水量对当前干旱的发展贡献是等权重的。本文基于线性递减非等权重的方法对CI进行了修正，同时对加权降水量（WAP）进行了标准化（Standard WAP Index，SWI）。以山西为例，通过对比CI 修正前后，即CI 和CI_new（CI 修正后），与SPI 和SWI 在不连续加重现象（UED）的总体分布、典型事例干旱演变特征以及与土壤湿度相关性等方面的差异，分析了4种干旱指数对山西逐日干旱演变的监测能力。结果表明：1）CI_new出现UED的次数较CI有了明显下降， SWI出现UED的次数也比SPI有了大幅的减少，且SWI在这4种指数中是出现UED次数最少的指数；2）CI_new和SWI较CI和SPI与同期土壤湿度的相关性均有所提高，表明修正后的CI_new和SWI更加符合土壤湿度的变化，更能反映土壤干旱的演变规律。针对干旱发展过程中不连续加重的现象，通过非等权重方法有效地减少了该现象的发生。