*3.3. Temporal Trends and Cyclic Characteristics of ET0*

Based on the 1 km high-resolution regional climate data generated by the delta statistical downscaling and multi-model ensemble, annual *ET*<sup>0</sup> trends were estimated for the YRB from 1901 to 2100 (Figure 8). The annual *ET*<sup>0</sup> of the basin under the different future emission scenarios (2022–2100; SSP126, SSP245, SSP370, and SSP585) substantially increased with time relative to the historical period (1901–2014), and all passed the 99% confidence test. Among all scenarios, the SSP585 scenario had the most pronounced upward trend in *ET*0, increasing at a rate of 22.9 mm/10a, reaching 1170.39 mm in 2100. It was followed by the SSP370 scenario, which increased at a rate of 16.6 mm/10a, reaching 1120.42 mm in 2100. However, SSP245 and SSP126 scenarios had relatively small trends in *ET*0, increasing at rates of 10.4 and 3.3 mm/10a, reaching 1062.71 and 987.68 mm in 2100, respectively. In general, the annual *ET*<sup>0</sup> in the YRB will rapidly increase with increasing levels of radiative forcing in the future.

**Figure 8.** Interannual variations in *ET*<sup>0</sup> in the Yellow River Basin over the historical period (1901–2014) and under different future emission scenarios (2022–2100; SSP126, SSP245, SSP370, and SSP585) (*p* is an indicator of significance obtained by the F-test).

To comprehensively understand the temporal *ET*<sup>0</sup> variations in the basin, Morlet wavelet analysis was used to investigate the cyclic variations in *ET*<sup>0</sup> in the YRB under the four future emission scenarios (Figure 9). In the SSP126 scenario, three distinct peaks were observed in the wavelet variance of *ET*<sup>0</sup> in the YRB, corresponding to the time scales of 52, 35, and 8 years, with the most pronounced periodic oscillation of approximately 52 years and a cyclic pattern of 34–38 years for *ET*<sup>0</sup> in the basin under this time scale (Figure 9a,b). In the SSP245 scenario, the *ET*<sup>0</sup> in the YRB considerably oscillated for approximately 53 years, and there was a cyclic pattern of approximately 34 years in the basin *ET*<sup>0</sup> at this time scale (Figure 9c,d). In the SSP370 scenario, the *ET*<sup>0</sup> in the YRB considerably oscillated for approximately 60 years, and there was a cyclic pattern of approximately 39 years in the basin *ET*<sup>0</sup> at this time scale (Figure 9e,f). In the SSP585 scenario, the *ET*<sup>0</sup> in the YRB substantially oscillated for approximately 48 years, and there was a cyclic pattern of approximately 27–32 years in the basin *ET*<sup>0</sup> at this time scale (Figure 9g,h).
