Simulating carbon fluxes in boreal catchments: WSFS-Vemala model development and key insights

Abstract. Lakes and streams play an important role in the global carbon cycle through carbon sedimentation and evasion. The development of carbon processes in the water quality model WSFS-Vemala (Vemala) presents a significant advancement in simulating carbon dynamics, particularly in capturing both total organic (TOC) and inorganic (TIC) carbon processes and their contributions to carbon retention and emissions through a river/lake network. The model was tested in the Vantaanjoki catchment, located in southern Finland and covering an area of 1680 km². The model's ability to simulate TOC and TIC loading across various land use and soil types aligns closely with reported literature values. The addition of organic acids to the total alkalinity definition improved pH simulations and thus the simulation of CO₂ emissions in the acidic and organic rich waters of Finland. Annual CO₂ emissions of 25 gC m² yr^-1 were simulated from lake Tuusulanjärvi, the largest lake in the catchment, and 223–260 gC m^-2 yr^-1 from the river network, while only 3 gC m^-2 yr^-1 was simulated as organic carbon burial in the lake sediments. The model's performance in estimating CO₂ emissions shows good correlations with established ranges for lakes as well as good correlation with TOC and TIC loads across the river network. The inclusion of sedimentation and mineralization processes in the lake carbon budget underlines the necessity of accounting for both organic and inorganic pathways in carbon modelling. This improved representation of the carbon cycling in Vemala, linked with the phytoplankton growth and nutrient cycling, allow to distinguish between carbon losses to the atmosphere and long-term carbon storage in the sediments of inland waters. Overall, the enhanced Vemala model provides a robust foundation for understanding carbon cycling and supporting sustainable, integrated water resource management and scenario assessments from sub-catchments to the national scale.