Declining runoff sensitivity to precipitation following permafrost degradation: Insights from event-scale runoff response in the Yellow River source region

Frozen ground, including permafrost and seasonally frozen ground (SFG), is a critical component of the cryosphere. Rapid atmospheric warming has accelerated the degradation of frozen ground, profoundly altering hydrological processes in cold regions and affecting downstream water resources. Here we investigate the impacts of frozen ground degradation on event-scale runoff responses to daily precipitation in the source region of the Yellow River (SRYR) on the northeastern Tibetan Plateau using ensemble rainfall-runoff analysis (ERRA). ERRA is a data-driven, nonparametric, and model-independent method that quantifies dynamic, nonlinear, and spatially heterogeneous linkages between streamflow and precipitation. Applying ERRA, we evaluate changes in daily precipitation-streamflow coupling within the permafrost region, where frozen soil depth has decreased by ~0.1 m per decade, and within the SFG region, where frozen soil depth has remained relatively stable, declining by only 0.03 m per decade. Between 1979–1998 and 1999–2018, the permafrost zone experienced a 47 % reduction in peak runoff response per unit precipitation and a 32 % decrease in the 25-day runoff coefficient. By contrast, no substantial changes in runoff response were observed in the SFG region. Rising temperatures and increased active layer thickness in the permafrost zone have substantially reduced streamflow sensitivity to precipitation, particularly under higher precipitation intensities. Specifically, for daily precipitation intensities exceeding 10 mm d-1, peak runoff response in the permafrost zone declined by 73 % and the 25-day runoff coefficient declined by 72 % between the two periods. These changes likely result from increased hydraulic connectivity and water storage capacity within the thawing active layer, facilitating increased infiltration and subsurface storage. Our findings underscore the effectiveness of data-driven methods in capturing hydrological regime shifts and offer critical insights for drought mitigation and flood risk assessment in permafrost-affected regions amid ongoing climate warming.