Dear authors,

as you have seen, the three reviewers provided somewhat contrasting evaluations of your manuscript. while i share some concerns the reviewers have raised, such as the unclear presentation of results, the, in places, unclear objectives and methods, i nevertheless think that the manuscript could eventually become an interesting contribution to literature. Some work is, however, still required to bring the manuscript up to publication standard. i thus strongly encourage you to address all reviewer comments in detail in a revised version of the manuscript, which will be sent out for a second round of reviews.
best regards, markus hrachowitz

Dear authors,

The reviewers and myself agree that the revised manuscript has been significantly improved. There remain, however, several points that need to be addressed before it can be considered for publication. The reviewers point out that in particular the presentation of the results would benefit from a thorough revision. I would like to encourage you to address all the additional comments of the two reviewers in detail and I am looking forward to a revised version of the manuscript.
Best regards, Markus Hrachowitz


Reviewer #1:
Comments attached as .pdf

Reviewer #2:
Yde et al. present the oxygen isotope measurements from the currently quiescent surging glacier Kuannersuit and a river draining the Mittivakkat ice cap. The measurements are taken over several years and are used to illustrate differences in sub-glacial drainage configuration between the two systems. The observed differences between the two systems are indeed very interesting, but the value of the data would be greatly improved by clearer discussion and presentation. The data are collected over several seasons, yet it is not always clear which time period the discussion is referring to and conclusions derived from one time period are assumed to be valid for all the data. It needs to be clearly stated the discussion on Mittivakkat centres on 2004 and Kuannersuit on 2001. A figure illustrating all the data would be greatly beneficial, allowing all the data to be compared. It should be made clear that oxygen isotope trends are not constant in time i.e. diurnal variations are observed in Mittivakkat in summer but could well be absent early in the season. In addition, I would strongly encourage the authors to either publish the raw data in supplementary material or deposit it in a database such as EarthChem.

Hydrograph separation: I am not convinced this section (4.3) adds anything to the paper since the conclusion that ice melt contributes 82% of the water in the river can only be for a single point in time, and there is little in the way of comparison to separations conducted on other glaciers. Additionally, the uncertainty quoted seems remarkably low and it is not stated how this error is derived. Hydrograph separation models have been developed in recent years for glacial systems to account for the inherent uncertainties in the end-members e.g. Bayesian models (Cable et al 2011; Arendt et al 2015). If lines 18-33 are to be included then the abrupt change needs to be better illustrated in figure 6. Section 5.3 on uncertainties should be included here and should also cover fractionation e.g. Lee et al. 2010, Hindshaw et al. 2011.

Comparison with other rivers. δ18O values change in a predictable way with latitude and altitude. This factor has to be considered when exploring differences in values between different catchments. For example, the higher altitude of snowfall on the GrIS contributes to the lower δ18O values of rivers draining the GrIS compared to lower elevation glaciers at similar latitude (e.g. Kuannersuit).

P3 Line 3: change ‘ia’ to ‘is’.
P6 lines 1: If δD results were collected, why are they not reported?
P6 lines 6-13: The wording of this section could be understood to imply that δD results were collected but are not reported. Hydrograph separations use δD and δ18O to improve end-member characterisation so if δD results were collected they should be used!
P6 Line 19: State the instrumental precision.
P6 Line 20-24: Were the multi-sample tests done more-or-less simultaneously? Could the increased standard deviation in 2003 be partly explained by the increased number of samples taken over a slightly longer time period?
P8 Line 19: It would be more correct to state that the isotopic composition of river water was coincident with the values observed in snow.
P8 line 27: Which year?
P9 Line 20: There is diurnal variability but it is not a sinusoidal diurnal trend. It would be interesting to set this in context with other diurnal d18O data collected from glaciers. Is the standard deviation referring to the standard deviation of the mean of all the measurements taken over the 24 period? It might be more useful to report the amplitude (difference between the maximum and minimum value observed in 24 hours).
P9 Line 40: Is the three-hour time lag quoted valid for the whole time period? How was this derived? Literature references should be added to support the discussion of time lags.

P11 Line 21: should be a ‘5’
P11 Line 27: This is not strictly true as it is dependent on the time of year. Snow melting in the summer on an alpine glacier will reach the bed of the glacier just as fast as ice melt.
P11 Line 34-37: State the magnitude of the ‘largest diurnal amplitudes’.
I would put section 4.2 before section 4.1.

Table 1: The mean for 2008 seems markedly lower than in 2005, were environmental conditions different?
Table 3: Why not add in the data from Watson River which was in the earlier version of this manuscript? (I would not have removed this data, better to collect all the data in one place).
Fig 3: Highlight a diurnal cycle.
Fig. 5: Add in error bars derived from the multi-tests.
Fig. 6. Add on error envelopes


References.
Arendt et al. 2015. An open source Bayesian Monte Carlo isotope mixing model
with applications in Earth surface processes. Geochem. Geophys.
Geosyst., 16, 1274–1292.

Cable et al 2011. Contribution of glacier meltwater to streamflow in the Wind
River Range, Wyoming, inferred via a Bayesian mixing model applied to isotopic measurements. Hydrol. Process. 25, 2228–2236.

Hindshaw et al 2011. Hydrological control of stream water chemistry in a glacial catchment (Damma Glacier, Switzerland). Chem Geol. 285, 215-230.

Lee et al 2010. Isotopic evolution of snowmelt: A new model incorporating mobile and immobile water. Water Res. Res. 46, W11512.

Dear authors,

thank you for the detailed responses to the comments of the two reviewers. I think you have adequately addressed the points raised. the manuscript provides an interesting perspective of glacier dynamics and may raise the interest of some in the community.
i am glad to accept the manuscript for publication as is. congratulations!
best regards, markus hrachowitz