Vegetation-Modulated Erosion in Badland Catchments

Badlands are highly erosive environments, where vegetation colonization and growth is strongly limited by the intensity of erosion. However, when vegetation is able to survive in such terrain, it may strongly reduce erosion by acting on infiltration, runoff, or soil cohesion. For instance, in Draix-Bleone Observatory (French Alps, OZCAR), observed sediment yield from a reforested catchment is 100 times lower than in its mostly denuded neighbor. Studying such environments in a critical zone perspective requires accounting for the interactions between the physical processes of erosion on one hand, and the biological processes of vegetation growth on the other hand. As a first step, the present work aims at identifying the impact of vegetation on badland erosion and topographic evolution.

We build a landscape evolution model to describe sediment fluxes and topographic evolution on badland catchments using Landlab library. The model couples two elements: first, a non-linear diffusion component is used to account for soil creep, shallow landsliding and debris-flow. Secondly, a stream-power law with conservation and entrainment of alluvium is used to describe sediment transport in the stream network.

We calibrate the model parameters based on a 30-year data set of annual sediment flux for two contrasted catchments: the Laval is mostly denuded while the Brusquet has been completely reforested at the end of the XIXth century. We use a leave-one-out-calibration procedure to calibrate and test at the same time the model prediction efficiency. The model is forced with observed flood time series from both catchments.

Results indicate that the hillslope diffusion process is highly sensitive to vegetation cover whereas the transport in the stream network is not. Moreover, the difference in simulated sediment yields for the Laval and the Brusquet is explained essentially by a difference in soil erodibility rather than by differences in the runoff regime. This suggest that reforestation has resulted into a strong reduction of soil erodibility in the Brusquet catchment.