Fig.A4 (a)-(b), same as Fig.1a-b, bur for the results derived from SPEI_PRE. (c) The ratio of the linear trend of SPEI_PRE to that of SPEI_All. (d) the ratio of the linear trend of extreme drought occurrence derived from SPEI_PRE to that derived from SPEI_All. The original SPEI with both changes in precipitation and ET is referred as SPEI_All. SPEI_PRE is similar to SPEI_All, but calculated from the detrended ET for 1901-2018 and the original precipitation (Cook et al. 2014). When the ratio in (c)-(d) is higher than 0.5, it indicates the contribution of precipitation changes is higher than that of ET.