|Second Review of Liebl et al., 2022 |
The current version of the paper has not changed significantly from the previous version, so I refer to my previous review with regards to my synopsis of the paper and the good job the authors have done in terms of awareness of the field, being thorough and careful in their benchmarking project, and setting their work in proper context. I find that the authors have generally addressed the concerns of the reviews and made changes where needed. The authors have accurately pointed out that my concerns about the treatment of width refer to an already published model and therefore fall squarely outside the scope of this paper.
I just have a few remaining points in the text where I think an inaccurate statement was made, and should either be clarified or corrected.
The lines 174-176 describe the differences between the glacier erosion model of Deal and Prasicek and OpenLEM:
“In contrast to iSOSIA and to the glacial stream-power law proposed by Deal and Prasicek (2021), the version implemented in OpenLEM assumes that the entire ice flux arises from sliding along the bed and neglects the contribution of deformation, which becomes increasingly relevant with increasing ice thickness (Cuffey and Paterson, 2010).”
There are more differences between the two models. Deal and Prasicek carefully track the ice surface slope and upstream ice accumulation. The ice flux term Ai equivalent in Deal and Prasicek depends intrinsically on the upstream ice surface and surface slope. This connection is not implemented here in OpenLEM, the actual evolution of the ice surface slope is also not tracked, but a fixed depth to width ratio is used. There are some consequences of these differences - for example the model of Deal and Prasicek does not exhibit advective behaviour, and therefore behaves diffusively and doesn’t have any knickpoints.
Line 96: “In contrast to rivers, glaciers are typically not much smaller than their valleys, so that assumptions about their width and the rates of erosion across the valley have to be made.”
It is not really accurate to say that because rivers are much smaller than their valleys that assumptions about their width do not have to be made. The river width exerts a first order control on the relationship between volumetric water flux, Q, and the resulting shear stress that is applied on the river bottom. The assumption that is typically made for rivers is that width increases approximately as Q^(1/2), or some exponent relatively close to 1/2. If this empirical relation is not assumed, the erosive power of rivers cannot be estimated. In this sense, rivers are a sub-grid feature but still have a real width. The sub-grid part just means that the actual river width is handled not on the grid, but in the equations that calculate erosion rate and are then applied to the cardinal flow line. This ensures that no matter what the grid resolution is, the erosive power of a given discharge, Q, is calculated properly. This is as true for rivers as glaciers; the scale of the river has no bearing on the importance of width.