Future changes in annual runoff and hydroclimatic extremes in the upper River Basin

The upper Yangtze River Basin (UYRB) provides almost half of the total runoff volume of the entire Yangtze River and is of great importance for the population living in the upstream and downstream regions. The recent streamflow decline and more frequent hydroclimatic extremes have raised concerns about the water sustain -ability and water security of the UYRB in a changing climate. Here we examine the hydrological changes in the UYRB during the historical period (1961-2020) and the future period (2021-2070) using a physically-based hydrological model. Based on the simulation results, the future changes in water resources and hydroclimatic extremes in the UYRB were analyzed. The results show that the mean annual runoff volume in the UYRB during the future period will further decrease by 20.4 (SSP1-2.6), 17.7 (SSP2-4.5) and 22.0 (SSP5-8.5) billion m3 compared with the historical mean value. The agricultural drought severity will increase in all sub-regions, which tends to be more severe than the meteorological droughts, and Wujiang is going to experience the largest increase in drought severity. Averaged across the UYRB, the unprecedented agricultural droughts and pluvials will occur more than once per decade on average in the future, indicating that the most extreme events in the historical period would become the new normal in the future. The design floods with different return periods at all major hydrological stations in the UYRB are projected to increase, and in the future there will be more frequent drought-pluvial seesaws at the major hydrological stations. These findings highlight the impor-tance of adaptive water resources management to tackle the elevated risk of water shortage and more frequent and severe hydroclimatic extremes in the future UYRB.