Insights into runoff changes in the source region of Yellow River under frozen ground degradation

The source region of the Yellow River (SRYR) in the northeastern Tibetan Plateau is critical for supplying water resources to downstream areas. However, streamflow in the SRYR declined despite a slight increase in precip-itation during the past few decades. The SRYR experienced significant frozen ground degradation with climate warming, but how frozen ground degradation influences runoff remains unclear. This study investigated the changes of the precipitation-runoff relationship using the double-mass curve method and examined the impact of long-term spatiotemporal changes in frozen ground on the water balance components using the geomorphology -based eco-hydrological model (GBEHM). The results showed that the precipitation-runoff relationship changed significantly since 1989 in the SRYR from 1960 to 2019. In the same period, the areal mean value of the maximum thickness of seasonally frozen ground (MTSFG) decreased by 0.10 m/10a and the areal mean active layer thickness (ALT) of permafrost increased by 0.06 m/10a. Besides, 21.0 % of the entire SRYR has degraded from permafrost to seasonally frozen ground (SFG). Runoff decreased mainly in the region with elevation below 4200 m, where the evapotranspiration increase exceeded the precipitation increase. Frozen ground degradation significantly altered the hydrological processes, which is reflected by the increased subsurface runoff and the decreased surface runoff. The total water storage increased by 2.9 mm/a in the permafrost region due to the increase in active layer thickness and by 5.7 mm/a in the degradation region where permafrost completely thawed during 1960-2019. The runoff seasonality was also altered, being indicated as an increase in winter runoff. These findings help provide a better understanding of the runoff change under climate warming in permafrost-affected regions and provide insights into future water resources management in the Yellow River basin under the climate warming.