|The authors have carefully considered my comments and the issues I raised during the first round of review. I think that this revised version of the manuscript is acceptable for publication in HESS. |
I invite the authors to consider only the following considerations, that could lead to some minor revision of the Discussion and, maybe, to modify something also in the Conclusions.
The comparison between the landslide predictions carried out with the (coarse-gridded) physically based model, with those obtained with a conceptual hydrological model (with finer resolution), shows that the latter still outperforms the first one, although in this new version the Authors exploit in a better way the outcome of the physically based model and obtain some different indications, compared to the former analyses.
In the Discussion section, they ascribe the limits of the physically based analysis mainly to the coarse resolution (of both the model itself and of input data about geomorphological and geotechnical parameters).
I agree with this intepretation, but I would like the authors to consider also another point (and add some comment in this respect, if they see my point).
They state that the coarse-gridded physically based model cannot reproduce the (lateral) fluxes between cells, as they are too distant from each other (the model grid is 12.5x12.5 km), as the high number of events during which nothing happens to soil moisture seems to demonstrate. Differently, the (conceptual) hydrological model, providing soil moisture at 0.5x0.5 km resolution, gives more valuable information to predict landslides.
What I want to stress here is that in initially unsaturated (shallow) soil covers, the prevailing direction of water fluxes is close to the orthogonal to the ground surface (e.g. Lu et al., 2011), owing to the top (atmosphere) and bottom (whatever it is) boundary conditions, that make the orthogonal hydraulic gradient much much larger than the one parallel to the slope (because all the verticals share more or less the same hydraulic conditions, and so there is very little gradient along the slope (which is at least one order of magnitude longer than the thickness of the soil cover). Only when saturation is reached somewhere within the slope cover, then, in that saturated part, the orthogonal hydraulic gradient becomes of the same order of the one parallel to the slope (depending on slope and bedrock inclination), and so lateral fluxes become significant (leading to subsurface runoff generation). However, I guess that most of the conditionally unstable slopes would have already failed before saturation was reached. I don't expect this picture to change significantly if the model was run over a 0.5x0.5 km gird instead of the 12.5x12.5 grid.
Far from saturation the only way infiltrating water can be drained out of the slope cover is either evapotranspiration (a too slow process over the time scale of 1 to 6 days of rainfall events) or drainage through the soil-bedrock interface, which becomes the most delicate point of the physically based model, about which the authors do not give any information to the reader.
Although I did not look in the cited paper where the PREVAH model is described, I expect that water exchange mechanisms between the three storage modules are somehow introduced in such model, and if the model is somehow calibrated in order to provide reliable results, these mechanisms consider the water exchange between the two upper storage modules and the lower one. This could explain why the dynamics of soil saturation estimated by the PREVAH model better matches with the effects of single rainfall events.
So, at least this is my opinion, the physically based predictions are limited not only by resolution and/or accuracy of input data issues, but also by the unsuitability of thechosen model to correctly assess the effects of one of the (major) processes controlling the dynamics of soil moisture in the unsaturated zone, i.e. the leakage towards the underlying saturated zone.
N. Lu, B.S. Kaya, J.W. Godt (2011). Direction of unsaturated flow in a homogeneous and isotropic hillslope. Water Resources Research 47(2), https://doi.org/10.1029/2010WR010003