更多文献参考：
[1]Are droughts becoming more frequent or severe in China based on the standardized precipitation evapotranspiration index: 1951-2010?
[2]World drought frequency, duration, and severity for 1951-2010
[3]Study of the temporal and spatial patterns 41 of drought in the Yellow River basin based on SPEI
[4]Long-term spatiotemporal variation of drought patterns over the Greater Horn
of Africa
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