Declining autumn zero-curtain duration in the Headwater Area of the Yellow River (2011–2024)

The zero-curtain period (ZCP), occuring during seasonal freeze‒thaw cycles, plays a crucial role in energy and water exchanges, biogeochemical and hydrological cycles, and ecosystem dynamics. However, its temporal variations and controlling factors remain poorly understood, particularly in high-altitude regions of the Qinghai‒Xizang Plateau (QXP). This study investigates the ZCP within seasonal freeze‒thaw cycles in the central Headwater Area of the Yellow River (HAYR), using high-precision soil temperature observations from two seasonally frozen ground sites (TCM-2 and ZLH-WS) in 2011–2024. Results demonstrate that at TCM-2, the thawed duration generally decreases with depth, ranging from 163.2 d at 200 cm to 183.6 d at 20 cm, with no significant temporal trends. At ZLH-WS, the thawed duration was 13.2–33.8 d longer than that at TCM-2 but exhibits a distinct decreasing trend in 2016–2023. ZCP duration exhibits strong, albeit contrasting, correlations with soil water storage (SWS) at both sites, negative at TCM-2 (R = −0.86, p < 0.01) and positive at ZLH-WS (R = 0.83, p < 0.01). Notably, the highest recorded soil unfrozen water content in 2019 corresponded to an extended ZCP duration. ZCPs were observed during both thawing and freezing periods. The duration of the freezing period ZCP showed a marked temporal decline, particularly near the maximum frost penetration (MFP) depth and becoming more pronounced at greater depths. A substantial reduction in ZCP duration was detected in the middle and lower portions of the MFP at both sites, with decreasing rates of −1.86 d per year (p < 0.001) at TCM-2 and −2.7 d per year (p < 0.001) at ZLH-WS. These findings suggest that the shortening ZCP altered subsurface water phase dynamics, potentially leading to reduced water retention capacity within the shallow frozen ground profile. This highlights the significant implications of ongoing frozen ground degradation for subsurface hydrology in seasonally frozen ground.