### scPDSI in different regions

Since 2002, a drought period began in the north of 21 °N, especially the Lancang basin where the driest period in the last 100 years has appeared (Figure 1).

Figure 1 scPDSI drough index in the last 100 years

The spatial distribution of drought is tripolar (the middle is strong, north and south weak).

Lancang River: drought trend and frequency are obvious in the middle and lower reaches, but the frequency in the upper reaches is not obvious

Mekong River: the drought trend is obvious, but the wet frequency is the most in the lower reaches (Figure 2).

Figure 2 scPDSI drought index in different reaches

A resonance between two periodic spectra of drought:

In the Lancang River basin the two periodic (11- and 30-year cycles) energy resonated since 2000.

Figure 3 Scale-averaged wavelet power spectrum of the scPDSI index

Change in extreme events :

Since 2002, the frequency of drought in Lancang River basin increase d obviously. The drought index of fitting maximum probability decreased by 0.63 to -0.92, and the corresponding probability increased from 21% to 24%.

The most significant increase was in moderate drought of -1.5 and -1.0, the total probability of which increased from 15% to 33%. The frequency of drought in the Mekong River basin has decreased since 2002 (Figure 4).

Figure 4 Change in probability distribution of scPDSI drough index