|This manuscript presents a description of the superconducting gravimeter installation at Zugspitze and some preliminary data. The authors have responded thoroughly and adequately to the many review comments. I agree with the decision to publish as a Technical Note. The major shortcoming of the paper is the lack of overlapping spring discharge and gravity signal, but I don’t think that should prevent publication at the present time. |
I find the discussion of instrumental problems to be useful, as we all know these installations each have unique problems, but it also has the effect of scaring away those not already familiar with superconducting gravimeters. I might caution that many of the issues present with older OSGs may not be present in the newer iGravs.
Minor comments follow.
It may be worth mentioning the significant primary limitation of superconducting gravimeters, the requirement for AC (mains) power.
L27: Should “gravimetric methods” be “gravity residuals”? I think you can delete this sentence altogether.
L33: What are “integral insights”?
L52: change “regarding” to “during”
L54: Does “special geological karst situation” mean that there is an impermeable layer below the karst that forces groundwater discharge at the spring? That would be worth mentioning. If there is significant subflow not discharging at the spring
Fig. 1 caption: It looks like the alpine catchments are delineated by black lines, not white?
L150: delete “after abnormal drift was observed”
L154: What does “nominal” mean here?
L156: I see the absolute-gravity measurements are in the AGrav database, is that worth mentioning?
L188: Throughout the manuscript, “has been” can be replaced by “was”
L200: What does “on the full signal” mean? Can it be deleted?
L219: I don’t understand “50 % overlap”?
L230: should be “estimated to be”
L230: The very low drift is a nice result. At that level of drift probably uncertainty in the scale factor also limits the ability to estimate drift.
L240: Although I realize this section and Table 2 were added in response to reviewer comments, I find the tide analysis to be unimportant for a hydrologic audience.
L256: The barometric admittance factor doesn’t come from tidal analysis, does it?
L320: “significant” and “only a fraction” contradict each other. Seems like a fairly small effect given the amount of snow.
Fig. 3: Panels a and b could be deleted to save space if needed.
L376: On line 357 you state the SWE measurements are representative.
Fig. 5: This figure is much improved and responds to my comments in the original manuscript.
L399: SWE, not snowpack
L402: Regarding the 393 nm/s2 observed gravity signal, because the gravity residual doesn’t go to zero in fall 2019 (presumably because of groundwater/soil water storage), the observed SWE gravity change would be about 30 nm/s2 less (i.e., the wintertime maximum minus the fall (snow-free) minimum)
L456: If the spring is at 1430 m, what is significant about the 1440 m elevation? Interesting to note during this early snowmelt period that most of the water isn’t leaving the catchment, but is leaving the gravimeter’s region of sensitivity.
L470: Suggest “water storage changes” instead of “water level changes”. If it is purely karstic and storage changes are happening in large voids, it may not make sense to discuss groundwater-level changes and aquifer porosity (i.e., it’s not the matrix porosity that’s significant but rather the secondary karst porosity)
L474: suggest groundwater storage, not groundwater height.
L519: In addition to more sophisticate modeling, which is useful, I think it would also be useful to look at the nonlinear nature of the SWE admittance factor – does it change with depth of snowpack? Presumable as SWE increases, the gravitational effect of additional snow likely increases as there is less runoff/ET.