*3.2. Calculation of SPI*

The weights estimated from Section 3.1 are used to calculate the SPI for 14 stations over the whole VG-TB. It is noted that the weights are applied for both scenarios of RCP4.5 and RCP8.5 during 2021–2050 with an assumption of no change in future. The SPI is designed to project the precipitation deficit for multiple timescales with a weighted ensemble of multiple climate models in future. SPI values are computed for the timescales of 1- (SPI-1), 3- (SPI-3), 6- (SPI-6), 9- (SPI-9), 12- (SPI-12) and 24- (SPI-24) month for 14 stations. For instance, the temporal variation of drought for the timescales of 1 to 24months under RCP4.5 (Figure 4) and RCP8.5 (Figure 5) at Danang station from 2021 to 2050 is displayed.

According to the classification of SPI values [56], generally, there are no events classified as "extremely wet" and "extremely dry" for the Danang station under both scenarios of RCP4.5 and RCP8.5 during 2021–2050 for considered multiple timescales. Under these scenarios, events classified as "near normal" hold more than 90% out of considered months for all timescales of 14 stations. In particular, events classified as "extremely wet" and "extremely dry" are not found out for the all stations under both scenarios of RCP4.5 and RCP8.5 during 2021–2050 for SPI-1, SPI-3, SPI-6, SPI-9, SPI-12 and SPI-24 indices. More importantly, under RCP4.5 scenario, events classified as "very wet" and "severely dry" are lower than 0.6% (mainly in stations located in the east of basin

as Caulau) (see Supplementary Material). Meanwhile there is a little change (less than 2.8%) in events classified as "very wet", "moderately wet", "moderately dry" and "severely dry" for SPI-1 for Ainghia, Camle, Giaothuy, Danang and Hien stations under the RCP8.5 scenario. Under this scenario, the remaining stations have no change in drought and wet events for all considered multiple time scales (see Supplementary Material). This indicates only little changes in drought and wet events in the eastern domain of basin under RCP8.5 for considered time scales. "Moderately wet"events for the SPI-1 range from 1.4% (Hien, Hoian and Khamduc stations) to 2.8% (Danang station) under the RCP4.5 scenario. Hence, an emphasis of events classified as "moderately wet" and "moderately dry" for multiple timescales over the whole basin under the RCP4.5 scenario during 2021–2050 is documented in Table 5.

**Figure 4.** Standardized precipitation index (SPI)for multiple timescales under RCP4.5 scenario (2021–2050) for Danang station.

As shown in Table 5, the maximum of moderately dry event is 6.5% for SPI-24, followed by 6.3% and 6.0% for SPI-12 and SPI-9, respectively. The median of moderately dry is projected to get the maximum value of 4.2% for SPI-24, followed by 4.0% and 3.7% for SPI-9 and SPI-12, respectively. Importantly, the maximum of moderately wet event is 5.9% for SPI-24, followed by 3.7% and 3.4% for SPI-12 and SPI-9, respectively. It is noteworthy that the values reported from Table 5 and Tables S1–S6 are assigned by the percentages of drought and wet events under considered multiple timescales. In other words, these values are defined as the percentage of number of months suffering wet and drought events among all months during a 30-year period under considered scenarios. Under the RCP4.5 scenario, in general, timescales are longer, intensity of moderately dry is stronger. In the same timescale, occurrence of moderately wet events is smaller than that of moderately dry under RCP4.5 scenario during 2021–2050.

**Figure 5.** SPI for multiple timescales under RCP8.5 scenario (2021–2050) for Danang station.

**Table 5.** Percentages of drought and wet events under considered multiple timescales under RCP4.5 scenario over basin during 2021–2050 (%).

