Development of a process-based distributed water-carbon coupling model integrating terrestrial and aquatic systems in permafrost region

The Tibetan Plateau (TP), also known as the Asian water tower, becomes warmer and wetter in recent decades. This has led to drastic permafrost degradation, increased soil erosion and higher greenhouse gas emissions from water bodies, profoundly altering the regional carbon cycle. With continued climate change, there are ongoing debates on whether the TP will undergo a transformation from a carbon sink to a source. Currently, the magnitudes of carbon fluxes transferring from terrestrial to aquatic systems are highly uncertain due to the unique hydrothermal conditions of permafrost region. This uncertainty arises because very few studies have comprehensively quantified the full range of carbon fluxes, including vertical carbon fixation, respiration and lateral carbon transport in different forms, i.e., DOC, POC and DIC. Here, we develop a process-based distributed water-carbon coupling model (GBEHM-C) applicable for permafrost region, which integrates the vertical water-heat-carbon fluxes between atmosphere, vegetation and soil, the lateral water-carbon fluxes transported from hillslopes to the river channels, as well as the water-carbon dynamics in river networks along the river routing process. The model is then applied in the Yellow River Source Area (YRSA) in the northeastern TP which helps quantify the net ecosystem carbon budget (NECB) at the catchment scale. According to the simulation results, the NECB of the YRSA was 4.27 Tg C/yr on average, and showed an increasing trend during 1960-2019. The lateral carbon fluxes accounted for 16.8% of the NECB and should not be overlooked. It’s also found that the alpine steppe ecosystem performs as a net carbon source in the YRSA. The future risk of carbon source-sink transformation mainly depends on the net carbon fixation by vegetation, carbon release from permafrost, and the intensity of lateral carbon transport driven by hydrological processes. Our study provides critical insights into the dynamics of water and carbon fluxes in the TP and offers valuable guidance for water resource and ecological management in alpine river systems.