Response to Reviewer #1

Comment 1: Water isotope data have long been recognized to have potential value to hydrological process understanding. However, its potentials have seldom been explored in high mountainous areas, where proper understanding on the complicated hydrology is of critical importance for future projection on hydrological conditions. This paper tries to investigate the value of water isotope data on improving process understanding in a glacierized catchment on the Tibetan Plateau. The authors developed a tracer-aided hydrological model and adopted a long time series of water isotope data to feed the model to quantify the runoff components and MTT/MRT. The results shows that the developed tracer-aided hydrological can substantially reduce the simulation uncertainty with the aid of precipitation and streamflow water isotope data. At the same time, the method adopted in the paper can estimate the MTT/MRT in a reasonable accuracy, which opens a new window to understand the hydrological processes in the plateau area. The paper is well structured and the language is well written. I strongly recommend its publication on HESS after properly addressing the following comments.

Response 1: Thank you very much for your comment. We will revise the manuscript according to your suggestions.

Comment 2: MTT and MRT are useful terms to understand hydrological processes in a catchment. The authors discuss the values of MTT/MRT. More discussions on the influencing factors are encouraged.

Response 2: Thanks for your suggestion. We analyzed the influence of the meteorological factors and wetness condition on the MTT and MRT. We found that the MRT is controlled by the soil water content. The backward MTT is controlled by both soil water content and the precipitation amount during dry season and wet season, respectively. The forward MTT has a strong correlation with temperature, which controls the fraction of snowfall and the rate of evaporation. We will add the discussion about this in the revision version.

Comment 3: The seasonal contribution of water sources is an important result, but the values are quite xxx to identified from Fig. 5, better to show the result by a table. It would also be better to show the result of seasonal contribution of surface and subsurface runoff to better understand the seasonal runoff regimes.

Response 3: Thanks for your suggestion. We will present the seasonal contribution of water sources and runoff component in a table in the revision version, and describe the characteristic of seasonal contribution of runoff component (surface and subsurface runoff) in the 3.2 section.

Comment 4: In the multiple-objective calibrations, the NSEdis and MAESCA, MAEiso were added directly. Better to clarify the influence of calibration objective function on the result.

Response 4: Thanks for your comment. This is indeed an important issue, and the weight of each calibration objective should be carefully determined when developing a general calibration strategy. But this study aims to illustrate the benefit from the calibration of isotope, and putting the three objective functions together is used to demonstrate that sound simulation for the three objectives can be produced simultaneously. Our result showed that when three objectives were all simulated well, the uncertainty of parameter and runoff component contribution was significantly reduced compared to the condition when only one objective was satisfied. To this end, the influence of weight of each objective is beyond the scope of this study although it is an important topic, and we will clarify this in the revision version.

Comment 5: The abstract must be concise, summaries the research aims, methodology, results and discussion, and conclusions. This lacks in your abstract.

Response 5: Thanks for your suggestion. We will revise the abstract to make it more concise and contain all the important things in aims, methods, results and discussion.

Comment 6: The conclusions should be brief and more informative. Please cut down the conclusion to a short paragraph.

Response 6: Thanks for your suggestion. We will simplify the conclusion part to one paragraph.

Comment 7: May consider the subscribe “N” to “S” to represent snow for quicker understanding.

Response 7: Thanks for your suggestion. We will change the subscribe of snow to “S” in the revision version.

Response to Reviewer #2

Comment 1: This paper is very interesting by coupling the isotopic tracers and hydrological model, which has the potential to solve the problem of hydrograph separation in a large-scale catchment. The authors get reasonable results with their model. So I accept the paper after a minor revision.

Response 1: Thank you very much for your comment. We will revise the manuscript according to your suggestions.

Comment 2: Could the authors add some precious work to compare with their results on the hydrograph separation as well as the MRT and MTT in the discussion section?

Response 2: Thanks for your suggestion. The result of hydrograph separation has been compared with another work conducted in the same catchment in the limitation section. We will add a section to discuss the MTT and MRT in two aspects. One is to compare the estimated MTT and MRT with other studies conducted in snow-influenced catchments, and to explain the reason for the differences. The other is to analyze the influence factor of MTT and MRT, and compare the result with previous studies.

Comment 3: Page 2: Method: how to define the snow-melt and glacier-melt? And how to obtain their isotope values? In my view, it is really hard to differentiate them in the filed work because they are always mixed when do the sampling.

Response 3:

Thanks for your question. As a work focusing on quantifying the contribution of water sources, the simulation of water sources and their isotope composition is indeed need clarify.

The snowmelt and glacier melt were mainly differentiated according to the glacier coverage from the Inventory data. The melt water in glacier covered region is glacier melt, and the melt water in non-glacier region is snow melt. The two kinds of water sources are melting with different DDF. We simulated the variation of snow cover area using the method described in 2.2. For model simplification, the evolution of glacier thickness and area was not simulated.

As for the isotope values, the snowpack was regarded similarly with other hydrological simulation units, thus the isotope composition was simulated similarly by Eq. 8. The isotope composition of glacier meltwater was assumed to be constant, adopting the value reported in published paper (Gao et al., 2009).

We will clarify the above issues in the Method section in the revision version.

Comment 4: Page 10 Lines 353 -360 The calibration is quite interesting. The finding ‘The single-objective calibration produced good performance for the simulation of discharge, but had an extremely poor performance for the simulations of SCA and δ18O’ means without the tracers, even the calibration is accepted, the model may still bring large uncertainty. Is my understanding correct?

Response 4:

Yes, you are right. According to our result, even when the model produces good discharge simulation with high NSE_dis, the internal processes can be very different, because discharge is only an external characteristic of catchment. This phenomenon has also been highlighted in some previous modelling works (such as Birkel et al. 2011, Campell et al. 2012, Chen et al. 2017). This indicates that the parameter cannot be constrained well solely by the behavior of discharge simulation. Consequently, involving more datasets such as snow, glacier, isotope is helpful for reducing the parameter equifinality.

Comment 5: Page 11 Lines 391-393 Please reconsider your explanation on the river O-18. The temperature effect is kind of a statistical result, while the effect of southwest monsoon is more likely a reason to cause the temperature effect, and thus it is not suitable to put them together.

Response 5:

Thanks for your suggestion. We will change the explanation by attributing the reduced δ¹⁸O to the effect of monsoon in the revision version.

Comment 6: Figure 3 Black circles and red line in sub-figure b are same to Fig.2?

Response 6:

No. They are same to sub-figure a of Fig. 3 (i.e., the black circles and red line represent the isotope composition of rainfall and snowmelt). We will clarify this in the revised version.