Assessing Spatial Patterns of Carbon and Nutrient Dynamics in Catchments of Complex Topography

Abstract The topography of a landscape regulates the spatial distribution of water and energy fluxes, which are main drivers of vegetation and soil carbon and nutrient dynamics. Despite the recognized role of topography in mediating such processes, quantifying and predicting the spatial distribution of carbon and nutrient fluxes and stocks in highly heterogeneous landscapes remains challenging. The main limitations stem from the prevalence of largely decoupled modeling approaches which fail to concurrently account for ecohydrological and biogeochemical processes as well as the lack of adequate frameworks describing the links among topography, water and energy balances, and soil biogeochemical dynamics. Here, we extend the capabilities of the mechanistic ecohydrological model Tethys‐Chloris‐Biogeochemistry (T&C‐BG) by including a soil carbon and nutrient routing module in the distributed model version. The newly developed T&C‐BG‐2D model is validated against long‐term hydrological and biogeochemical measurements from the Hafren catchment in Wales (UK) and the Erlenbach catchment in the Swiss pre‐Alps. The model successfully captures carbon and nutrient concentrations and dynamics in these catchments, with relative differences between simulated and observed median values of between −4% and −0.3% for dissolved organic carbon, and between 1% and 20% for ammonia. A sensitivity analysis in the Erlenbach basin suggests that elevation explains over 80% of the observed spatial patterns, followed by topographic wetness index (12.6%), aspect (2.9%), and curvature (2.1%). These findings underscore topography's critical role in shaping water, carbon, and nutrient dynamics, which cannot be reflected in plot‐scale simulations neglecting spatial interactions and topographic effects.