Continuously measuring gravity changes in a high altitude Alpine site is a première, and these integrative measurements will provide new information on the ice-snow-water mass balance. More specifically, the combination of continuous, relative measurements with the superconducting gravimeter and occasional ones with an absolute instrument should provide valuable information on long-term changes informing about erosional processes, and ice and water mass changes.

Such a study is in principle worth publishing in HESS, but presently, it suffers from different shortcomings.

1. The bibliography is not complete.
2. Some parts are wordy; the information could be better structured. Often the authors beat around the bush, go back and forth, making it difficult to focus on the main project that is, I presume, constraining the snow mass model. Overall, this paper rather looks like a field report, a plea for funding or in the best case, a feasibility study, which is not the purpose of a scientific paper. For example, do we need the details on the way the instrument was moved?  Moreover, some parts are probably interesting for an audience of geodesists, but here, one should not forget that the paper will be mostly read by hydrologists.
3. It is very necessary to dispose at the very least of two absolute gravity measurements to determine the drift of the superconducting gravimeter and incidentally, confirm the calibration factor. The authors and colleagues have the skills and material to do the job, so, I urge the authors to perform the absolute measurements that will provide the healthy and solid foundations required to support their case. Without this, the inferred gravity changes will remain speculative, especially when those changes are discussed at the nm/s² level.
4. What is exactly the scientific question? Installing an SG in Alpine context? This is not a scientific goal. Evidencing the ability of an SG to monitor changes in water mass balance? Building a terrain model of the Newtonian effect of snow? This has been done in numerous studies. Instead, you should modify the title and focus on the investigation of the snow and water mass balance, and the novel results one could infer from it. But, this can only be achieved with a longer time series, appropriate modelling (e.g., the authors propose to build a model as SNOWPACK -do it!) and appropriate ancillary measurements (e.g. LIDAR for snow thickness).

See also my numerous comments in the annotated pdf.

I recommend rejection as presently, much more work should be done but I would be pleased to revise an improved version of this work when (1) the absolute gravity measurements are performed and (2) a more comprehensive investigation of the snow and water mass balance is done. Of course, more time is needed but this is often the case in geodesy, where one looks at slow processes.

I found the presentation of preliminary data from the ZUGOG observatory to be an interesting extension of the microgravity method into alpine environements. To my knowledge it is the first deployment of its kind in the high alpine environment and presents unique considerations concerning the spatial sensitivity of the instrument. I do no share the novelty or significance concerns of RC1; I think the preliminary presentation is inline with other publications (e.g., https://doi.org/10.5194/essd-11-1501-2019) and rapid publication is of greater benefit to those who might consider similar deployments (myself included). 

The paper hints at the fact that a mountain-top deployment is both an advantage and disadvantage - increase in sensitivty to distant mass change is accompanied by an (unwanted) increase at near distance. Perhaps the ideal location would be in a flatter region atop a mountain (of course in most cases it's limited by infrastructure). I agree with the proposal to collect gradient data between meters to better define the region of sensitivity.

I agree that a second send of AG measurements will greatly improve the manuscript by constraining the drift rate. 

Creutzfeldt (2013) is an important reference for the discussion on the relation between spring discharge, storage, and gravimetry.

Creutzfeldt, B., Troch, P., Güntner, A., Ferré, T. P. A., Graeff, T., & Merz, B. (2013). Storage-discharge relationships at different catchment scales based on local high-precision gravimetry. Hydrological Processes, 28(3), 1465–1475. https://doi.org/10.1002/hyp.9689

Does the very high correlation between the residuals and SWE have implications for the uniformity of the snowpack? I think that's an important aspect of this deployment, that because the gravity meter integrates over a large area, it inherently "smooths" the heterogeneous snowpack. But probably there is a level of heterogeneity where the assumption of uniform snowpack is not valid?

I encourage the authors to explore the relation between SWE-corrected residuals and spring discharge (e.g., in a figure). Although the discussion of recession constant is useful for bounding the thickness of groundwater storage, that could also be done by integrating spring discharge and dividing by area.

The figures are well prepared and relevant to the text.

Fig. 5: Although the figure is useful for showing the asymmetry of the gravimeter footprint, the magnitude of the contribution to g of each individual prism isn't useful, and the log colorbar-scaling can be misleading. Suggest showing instead the cumulative sensitivity contours—i.e., the region within which 30% of the signal originates, 60%, 90%, etc. 

490: Do 0.27 and 0.56 m refer to thicknesses of free-standing water (i.e., terrestrial water storage)? "Groundwater heights" would imply the values depend on the porosity of the porous media, as measured in a monitoring well.

495: Why is the precipitation admittance factor (90 µGal/m) so much higher than the SWE admittance factor (29.8 µ/m)? It may be interesting to discuss how the former is much higher than the infinite slab, and the latter lower.

505: "Problematic" is unclear.

526-529: This sentence is unclear (its difficult to tell how the first part justifies the latter).

572-582: I found this paragraph to be vague and of limited importance.