Revealing the Long‐Term Catchment‐Scale Carbon Budget Dynamics on the Permafrost‐Dominated Eastern Tibetan Plateau

Abstract The Tibetan Plateau (TP) has become warmer and wetter, which has led to permafrost degradation, increased soil erosion and higher CO 2 emissions from water bodies. There are ongoing debates on whether the TP will transition from a carbon sink to a source, but relevant analysis accounting for both terrestrial and aquatic carbon processes at the catchment scale remains lacking. Here, we develop a process‐based distributed water‐carbon coupling model applicable for the permafrost region GBEHM‐Carbon, which integrates the vertical water‐heat‐carbon fluxes through the soil‐vegetation‐atmosphere continuum, the lateral water‐carbon fluxes transported from hillslopes to river channels, as well as the water‐sediment‐carbon dynamics in river networks along the river routing process. The model is then applied in the Yellow River Source Region (YRSR) on the eastern TP. The results show that the ecosystem carbon budget of the YRSR is 3.3 Tg C/yr on average, with an increasing trend during 1960–2019. Lateral carbon fluxes played a substantial role, accounting for 30.1% of the net ecosystem production (NEP) on average, with their relative contribution rising to 37.8% during the 2010s. Regions undergoing permafrost degradation are identified as potential hotspots for carbon sink‐to‐source transitions, primarily driven by reduced vegetation productivity and enhanced heterotrophic soil respiration. In addition, riverine carbon fluxes show distinct spatial patterns associated with stream order and are strongly modulated by hydrological conditions. This study provides critical insights into the long‐term variations of water and carbon fluxes in the TP headwater catchments, and offers valuable guidance for water resource and ecological management in alpine river systems.