Mechanisms of Suprapermafrost Groundwater Recharge Streamflow in Alpine Permafrost Regions: Insights From Young Water Fraction Analysis.

This study investigates the temporal processes of suprapermafrost groundwater (SPG)‐supplied streamflow in alpine permafrost regions, aiming to fill the gap in understanding this process from a water‐age perspective. Precipitation, streamflow, and SPG samples were collected from the Three‐Rivers Headwaters Region (TRHR). We defined the physical meaning of Fyw (the young water fraction) of the SPG and calculated it for the first time. The results showed that in the TRHR, the SPG mean travel time (MTT) was 159 days, and approximately 46.4% of SPG was younger than 77 days, whereas the streamflow MTT was 342 days, and approximately 12.2% of the streamflow was younger than 97 days. The correlation analysis revealed that various climatic factors played dominant roles in the recharge time variations of the SPG‐supplied streamflow within the TRHR. The SPG recharge rate did not significantly affect the streamflow Fyw; however, the thickness of the active layer ultimately controlled the SPG transit time distribution. Regression analysis further demonstrated the nonlinear impact of precipitation, average temperature, and average freezing days on SPG Fyw, which is closely related to seasonal freeze–thaw heat conduction and groundwater heat advection in the active layer. During the initial ablation period, the streamflow was primarily recharged by young SPG, resulting in a short‐tail travel time distribution. Our findings provide valuable insights into runoff generation and concentration processes in permafrost regions and have important implications for water resource management. Plain Language Summary: This article investigates the temporal process of suprapermafrost groundwater (SPG) (groundwater within the active layer above the permafrost) recharging streamflow in alpine permafrost regions, aiming to fill the gap from this perspective. We collected samples of SPG, streamflow, and precipitation from the Three‐Rivers Headwaters Region and calculated the water age of SPG. We found that approximately half of the SPG contributing to streamflow is young water, however, the young water fraction of the streamflow is lower than the global average for rivers. We analyzed the environmental factors controlling water age and identified climatic conditions, active layer thickness, and relief amplitude as the primary influencing factors in permafrost regions. Based on analyses of permafrost thermal and hydrological conditions, we determined that streamflow was primarily recharged by the young SPG during the initial ablation period. Our results provide important insights for studying the runoff generation and concentration processes in permafrost regions and for water resource management strategies. Key Points: The mean travel time of suprapermafrost groundwater (SPG) in study area was 159 days, and ∼46.4% of the water was younger than 77 daysClimate conditions, active layer thickness, and relief amplitude are the primary controlling factors for the SPG ageThe recharge of streamflow by young SPG primarily occurs during the initial ablation period [ABSTRACT FROM AUTHOR]