Modeling seasonal sediment dynamics and landscape evolution in a marly badland catchment, Draix-Bléone Critical Zone Observatory, SE France

Badlands are particularly sensitive components of the critical zone where weathering, erosion and transport processes can be observed on human time-scales. Within the Draix-Bléone Critical Zone Observatory (CZO), SE France, water and sediment (both bedload and suspended load) fluxes and climatic drivers have been recorded since more than 35 years, making it an ideal natural laboratory to develop a landscape-evolution model (LEM) for badland evolution calibrated with field data. Based on these records and existing knowledge on the sediment dynamics of these marly catchments, the aim of this study is to develop a LEM that is able to reproduce the observed intra-annual sediment-flux variability, in particular the transition from transport-limited to supply-limited conditions that occurs during summer. Our model predicts soil thickness and sediment export at monthly timescales, thereby providing potential links between “classical” LEM that run at long time scales and event-scale models, and simulating the physical processes driving the sediment dynamics in these catchments at their relevant timescale. We use the annual hysteresis cycle between rainfall and sediment export recorded in the Draix catchments as a quantitative indicator of the adequacy of the model. First, we model the supply-limited regime observed in the second half of the year in the badlands, illustrated by an clockwise loop in the annual hysteresis pattern, using depth-dependent hillslope regolith production and erosion laws. Next, we express the impact of rainfall intensity, identified as the main trigger of sediment motion both on hillslopes and in the drainage network, in order to reproduce the observed non-linear relation between sediment export and rainfall during the first part of the year (illustrated by an initial anti-clockwise loop in the hysteresis cycle). Parameter calibration is performed using average annual sediment export and soil depth in specific compartments of the catchment. The model successfully reproduce the hysteresis pattern but further work is needed on the calibration to obtain consistent magnitudes of sediment export. This new landscape evolution model appears to be a relevant tool to model observed annual morphology changes in badlands and to predict badland evolution in a context of climate change.