Rainy Season Precipitation Changes and Projections in the Water Source Area of the South-to-North Water Diversion Middle Route Project

Extreme precipitation events have significant implications for the water resource security of the South-to-North Water Diversion Project. This study first analyzes the characteristics of extreme precipitation events during the rainy season (May to October) in the water source area using daily precipitation data from 1961 to 2021. Subsequently, future projections for the near term (2026-2050) and long term (2071-2100) were conducted using daily precipitation estimates from 12 models under three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5) of the Intergovernmental Panel on Climate Change (IPCC) Coupled Model Intercomparison Project Phase 6 (CMIP6). Considering model precipitation biases, quantile delta mapping (QDM) bias correction algorithm was applied before projections. The results show: (1) In the past 61 years, precipitation frequency (RD) decreased, while heavy rain days (Rr25) and extreme precipitation (R95PTOT) increased. Precipitation intensity (SDII) significantly increased at a rate of 0.19 mm/day/decade, especially in Ankang and its upstream regions between the Daba Mountains and the Qinling Mountains. Moisture budget diagnosis indicates that the vertical moisture advection term makes the dominant contribution to the increase in SDII, with the largest contribution from the dynamic term, followed by the nonlinear term; (2) The heavy rain days (Rr25), maximum 1-day precipitation (Rx1day), extreme precipitation (R95PTOT), very extreme precipitation (R99PTOT), and precipitation intensity (SDII) experienced an interdecadal shift in the late 1970s, followed by a significant increase during the 1980s; (3) Compared to the reference period (1961-2014), under low (medium, high) emission scenarios, heavy rain days (Rr25) increased by 10.8% and 19.0% (5.0% and 19.2%; 7.9% and 31.8%) for the near term and long term, respectively, and extreme precipitation (R95PTOT) increased by 8.8% and 8.7% (1.1% and 12.7%; 8.6% and 25.1%). Spatially, extreme precipitation increases are most pronounced in the Funiu Mountains and the Danjiangkou Reservoir. In summary, based on the analysis of observational data over the past 61 years and future projections from CMIP6, the total precipitation during the flood season in the water source area shows an increasing trend, which is beneficial for the sustainable benefits of the project. However, the increase in extreme precipitation poses challenges for water resource management and engineering safety.