Historical Evolution and Future Scenario Prediction of Hydrological Drought in the Upper Reaches of Xin’an River

Predicting future hydrological drought characteristics can assist relevant departments in taking proactive measures to mitigate drought losses. Based on the SWAT model and the Sixth International Coupled Model Comparison Program, this study employs an improved Mann–Kendall test, cumulative anomaly method, and continuous wavelet transform to investigate future runoff and hydrological drought characteristics in the upper reaches of the Xin’an River under different Shared Socioeconomic Pathways (SSPs). The SSPs scenario consists of three typical paths. SSP126 represents the sustainable development path (low carbon emissions, ecological protection first), SSP245 is the intermediate balance path (equal emphasis on economic growth and environmental protection), and SSP585 is the fossil fuel-intensive path (high emissions, high development intensity). The results indicate that from 2000 to 2020, under the influence of ecological compensation policies, the upper reaches of the Xin’an River transitioned from hydrological drought to hydrological wetness in 2012. Under the three future scenarios, runoff volumes increased by 41.72%, 40.74%, and 40.72% compared to the historical period, respectively, with peak runoff occurring in May, June, and July, alleviating hydrological drought conditions. Under the SSP245 and SSP585 scenarios, drought characteristics were more pronounced, with the number of drought-free months increasing by 21 and 30 months, respectively, compared to the SSP126 scenario, and the number of extremely dry months increased by 9 months and 17 months, respectively. The standard runoff index in the SSP126 scenario exhibits two oscillation cycles of 400 months and 359 months, respectively, while SSP245 and SSP585 both exhibit an oscillation cycle of 835 months. After discussion, it was concluded that ecological compensation policies can improve hydrological drought conditions. Drought characteristics become increasingly pronounced as carbon emissions intensify. This research can provide theoretical references for water allocation and drought prevention in river basins.