Freeze–Thaw Controls on Dissolved Organic Carbon Export: A Numerical Investigation of Hydrological Mechanisms in a Hillslope

The lateral export of dissolved organic carbon (DOC) is a critical yet poorly constrained flux in warming permafrost carbon budgets. The governing mechanisms remain unclear, particularly how freeze-thaw dynamics alter subsurface flow paths and thereby control DOC mobilization and export. To bridge this gap, we applied a process-based, integrated hydro-thermal-geochemical model to a two-dimensional representative hillslope in the Hulugou Catchment, Qinghai-Tibet Plateau. This high-resolution, process-based model simulates seasonal freeze-thaw dynamics and quantifies how these processes influence lateral DOC export at the hillslope scale. Model results demonstrate that DOC flux follows groundwater (supra-permafrost water in the active layer) discharge patterns, whereas DOC concentration is governed primarily by the depth of dominant flow paths. Seasonal timing differs between concentration and flux, with peak DOC concentration at the hillslope outlet occurring in April and maximum DOC flux in September. Under future warming scenario, permafrost thaw deepens the active layer and lengthens flow paths, redistributing discharge toward deeper flow paths, thereby driving a 16% reduction in DOC flux and a decline in DOC concentration over 40 years. These findings underscore the pivotal role of freeze-thaw dynamics in DOC export and provide valuable insights into carbon processes in permafrost-affected regions.