I carefully read the contribution by Torres and co-authors relative to the development of a model predicting the evolution of the 14C content of organic carbon in river. This model will be useful to interpret field measurements in terms of storage of organic carbon within the watersheds. They show for instance that the age of organic carbon carried by rivers increases with the area of the catchments. They also show that the terrestrial biomarkers will correctly record any fluctuating environmental forcing function when the period of the input signal is much longer than the transit time of sediment within the catchment. Shorter periods of the forcing function result in the appearance of phase shift and dampening of the amplitude in the biomarker record.

I found this contribution interesting. The model for the calculation of the aging of organic matter as it is transported through the watershed includes a numerical model of the meanders, which seems to be an innovation. Pending on the model parameters, it is able to reproduce the dispersion of the field data. This dispersion appears to be fully dependent on the time required for the meander bend cutoff to occur, and on the reactivity of the organic matter. The paper is not an easy paper, but the equations look to me correct, and the method deserves publication.

I only have one concern: it seems to me that some results are obvious, such as the increase of age of the organic matter with the watershed size. Indeed, young reactive organic matter should be less abundant at the outlet of a large watershed compared to a small watershed, because of the repeated storage episodes. But I acknowledge that the present model can predict the numerical amplitude of the aging. Also regarding the periodic forcing: if the period is shorter than the transit time, the system mixes its own natural evolution with the forcing, hiding the original forcing. If you force a spring with a frequency well above its own natural resonance frequency, you ‘ll get almost no oscillation.

I may be wrong, but it seems to me that the most interesting content of the present contribution is the model itself, which will allow to decipher the way organic transfer is transported, stored within a catchment. As there are no mistakes in the formulation, I think this technical paper deserves publication.

Dear Mark and colleagues,
Thank you for submitting your manuscript to ESurf and for working with the Associate Editor and his referees to optimize the presentation of your interesting model. I am happy to follow the AE's recommendation and approve the publication of your paper, which, I trust, will provide a useful framework for future studies of the carbon dynamics of fluvial routing systems.
The copy editors of Copernicus Publishers will be in touch with practical details of the publication process. Your prompt actions in response will allow us to minimize the time publication.
Best wishes,
Niels Hovius