Estimating the lateral transfer of organic carbon through the European river network using a land surface model

Abstract. Lateral carbon transport from soils to the ocean through rivers has been acknowledged as a key component of global carbon cycle, but is still neglected in most global land surface models (LSMs). Fluvial transport of dissolved organic carbon (DOC) and CO₂ has been implemented in the ORCHIDEE LSM, while erosion-induced delivery of sediment and particulate organic carbon (POC) from land to river was implemented in another version of the model. Based on these two developments, we take the final step towards the full representation of biospheric carbon transport through the land-river continuum. The newly developed model, called ORCHIDEE-C_lateral, simulates the complete lateral transport of water, sediment, POC, DOC and CO₂ from land to sea through the river network, the deposition of sediment and POC in the river channel and floodplains, and the decomposition of POC and DOC in transit. We parameterized and evaluated ORCHIDEE-C_lateral using observation data in Europe. The model satisfactorily reproduces the observed riverine discharges of water and sediment, bankfull flows and sediment delivery rate from land to river, as well as the observed concentrations of organic carbon in rivers. Application of ORCHIDEE-C_lateral for Europe reveals that the lateral carbon transfer affects land carbon dynamics in multiple ways and omission of this process in LSMs may result in significant biases in the simulated regional land carbon budgets. Overall, this study presents a useful tool for simulating large scale lateral carbon transfer and for predicting the feedbacks between lateral carbon transfer and future climate and land use changes.