Impact of Changing Cryosphere on Streamflow Seasonality of Three River Basins in Southwest China

Three River Basins (TRB) in southwest China: the Jinsha, Lancang and Nu River, which originate from the Qinghai–Tibet Plateau (QTP), are distributed with cryosphere elements such as snow, permafrost, and glacier. The upstream and midstream areas of the TRB show a trend of warming and wetting, but the downstream regions with the temperature rising shows a trend of decreasing precipitation under climate change. This has led to the vanishing cryosphere and changes in the hydrological process of the TRB. How the changes in a single meteorological forcing or cryosphere element influence the amount and seasonality of streamflow remains unclear. This study simulated the different streamflow components and analyzed the amount and seasonality of streamflow in all sub-basins using a distributed hydrological model by controlling the changes of cryosphere elements and meteorological forcing during 1961-2020. The results showed that the contribution of snowmelt to streamflow in the midstream sub-basins was relatively high during April-June with a decreasing trend; the increasing glacier meltwater contributed to the streamflow in the source areas from June to September, especially in the Jinsha and Nu River; groundwater affected by permafrost degradation exhibited an increasing trend in the downstream reaches of the TRB. The spring rise timing was advanced and the recession timing was delayed with the reduction of snowfall fraction and the degradation of permafrost, and this effect gradually weakened from the upstream to the downstream areas of the TRB. Less glacier generally delayed the timing of summer streamflow in all reaches of the study basins. The seasonal variation of streamflow in the TRB decreased with the vanishing cryosphere. In addition, results of two experiments by using the multiyear mean precipitation and air temperature as forcing manifested that precipitation was the dominant factor causing changes in annual runoff and seasonal variation of streamflow, and increase in air temperature played a significant role in reducing the runoff and streamflow seasonality of the TRB. These findings shed light on the difference of changes in the streamflow process between sub-basins under climate change and provide a useful reference for water resource management in future for the TRB in southwest China originating from the QTP.