| 30 Sep 2025
ConcentrationTracker: Landlab components for tracking material concentrations in sediment
Abstract. We present a set of new Landlab numerical model components that allow users to track sediment properties across a landscape grid. The components use a mass-balance approach to partition the mass concentration of each property based on sediment fluxes calculated by various Landlab flux components. The methods are generic, allowing the user to assign any sediment property that can be expressed as a mass, volume, or number concentration (for example, mass of magnetite, volume of quartz, number of zircons, number of radiogenic 10Be atoms, "equivalent dose" of luminescence). Several properties can be tracked at once, each with concentration tracked in both sediment and bedrock at every location on the grid. Two ConcentrationTracker components have been formulated; one for distributed, space- and time-varying hillslope regolith movement and another for transport in fluvial networks, allowing for interaction between sediment in the water column and on the channel bed. These components can be used individually to study a single process or coupled to study the interactions of multiple processes acting on a dynamic landscape. We present two examples that illustrate the diverse uses of the ConcentrationTracker components: colour banding in hillslope regolith and provenance tracking of fluvial sediments.
The manuscript by Roberge et al. presents a new implementation in Landlab that allows the concentration of a tracer in moving sediments to be traced. This development is exciting, and I fully share the initial motivation behind this study. We do indeed need this type of model to link field measurements of provenance, for example, with the corresponding landscape evolution.
The implementation of the model is clearly presented, and all the equations needed to understand and reproduce this implementation are provided. The examples are illustrative and give a good idea of the potential applications of this model. I particularly like the example of the ‘diffusion’ of colour bands in the landscape, which can correspond to different types of rock. This example could also illustrate another application of the model that does not seem to have been highlighted, namely the study of erosion laws. The link between the dispersion of a tracer and different erosion laws in a specific case could help to justify or calibrate these laws, which remain uncertain in landscape evolution models.
That said, I think there is a simple experiment missing that would demonstrate the validity of the model, which I proposed in my 2016 paper: placing tracers on a pixel at the top of an inclined plane with a constant slope and only diffusion (and no uplift). In this case, we have a simple analytical solution that links the evolution of the spatial standard deviation of the concentration with the diffusion coefficient and time (Einstein's formula). By comparing the theoretical predictions with the Landlab results, you could show that the model gives consistent results, and perhaps independent of the model's time step and space step. It would be useful to discuss the dependence of the new module on these two parameters.
Specific comments
Line 27 «few LEMs account for the storage, fate, and transport of other sediment properties” : which one ?
Line 29-30 I agree, this was my motivation for the grain tracers in Cidre in the 2016 paper.
Line 205 In the equation how Cxpi^t+1 is known ? and in the line bellow it is written that the “remaining unknown is 𝐶𝑋𝑠𝑡+1 on both sides of the equation” but I do not see it. Is there a tipo?
Line 250. Could you explain just a bit more how to obtain this equation?