基于洛伦茨曲线的陕北黄土高原降雨分布不均匀性分析

利用建立的基尼系数降水分布不均匀性模型,对延安市和榆林市1961—2010年的降水量、基尼系数和洛伦茨不对称系数的系列进行研究,并用Mann—Kendall方法趋势分析和检验。研究结果表明,近50年来榆林市和延安市的年降水量呈下降趋势,年平均基尼系数分别为0.61和0.56;大于1的洛伦茨不对称系数的年份占总年份的比例分别约为33%和16.7%。榆林市的基尼系数呈上升趋势,洛伦茨不对称系数呈下降趋势;延安市的基尼系数呈下降趋势,洛伦茨不对称系数呈上升趋势。榆林市的降水分布不均性增强,主要是由于降水量小的月份所占的比例较大;延安市降水分布趋于均匀,降水量大的月份所占比例增加。榆林市在1972、1986年和1997年的洛伦茨不对称系数有较大波动,这些年中降水不均匀性主要是由于降水量大的月份引起的,同期也是土壤干旱异常的年份。因此,洛伦茨曲线从一种新的角度来量化评价降水分布时间不均匀性及其成因,为研究土壤干旱和水土流失等自然灾害成因机理提供科学依据。

Study on temporal inequality of precipitation in the loess plateau based on Lorenz curve

Based on the model of precipitation distribution describing with Gini coefficient in Yan’an and Yulin,the annual precipitation,Gini coefficient and Lorenz asymmetry coefficient were analysized for the period of 1961—2010 and the Mann-Kendall test was employed to detect the upward or downward trend in precipitation,Gini coefficient and Lorenz asymmetry coefficient during the study period.The results showed that the annual precipitation in Yan’an and Yulin located in the loess plateau was in a decreasing trend during the past 50 years and mean Gini coefficient was respectively 0.56 and 0.61.The yearly Lorenz asymmetry coefficient(S>1) accounted for a high percentage,33.3% in Yulin and 16.7% in Yan’an.Yulin showed increasing Gini coefficient and decreasing Lorenz asymmetry coefficient while Yan’an indicated decreasing Gini coefficient and increasing Lorenz asymmetry coefficient,but these trends had no significance at a > 95% confidence level.Therefore,precipitation distribution was becoming more and more uneven in Yulin since greater percentages of the yearly total precipitations in a few very unrainy months.However,the monthly precipitation within one year in Yan’an had a uniform distribution and a greater percentage of the precipitation in a few rainy months.In the three years of 1972,1986 and 1997 in Yulin,the Lorenz asymmetry coefficient was greater than 1.It indicated that the precipitation inequality was caused by the heavy rainfall in some months of the year which was also the most abnormal year of drought.In conclusion, Lorenz curve from a novel perspective is an available way for evaluating quantitatively the temporal distribution of precipitation and analyzing its causes,which provided scientific basis for the analysis of mechanism of natural disasters like drought and soil loss.