《Table 1 Dryness/wetness categories according to the SPEI and the corresponding cumulative probabili

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《1961-2015年中国极端干旱时间演变特征（英文）》

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The calculated Di is aggregated at different time scales.It is recommended to use a three-parameter log-logistic distribution for standardizing the accumulative series Di to obtain the SPEI at a given time scale.It calculates drought indices by standardizing the series Di at different time scales.The SPEI can characterize the degree of dryness/wetness deviation from normal conditions.To capture the spatio-temporal variations in drought more typically，we defined extreme drought as SPEI≤–2（Table 1）which is more closely related to drought disaster（Kelley et al.，2015）.Drought duration（in months）was defined as a continuous period that SPEI is less than-1.0.By comparing the average annual SPEI and the areas of China’s crops affected by drought disasters from 1961 to 2015，the correlation coefficient of the SPEI at the 6-month time scale was higher than at other time scales.So，we selected 6-month time scale in all parts of the study except the analysis of critical precipitation as annual precipitation with 12 months scales of SPEI.