General comments:

In the manuscript by Liu et al. the authors aimed at quantifying how changes in the snow-rain proportion affect stream flow and groundwater recharge in a snowmelt-fed river system. Study site was the mid Merced River catchment, representative for the central and southern Sierra Nevada.

In extensive field campaigns hydrologic and meteorological data of precipitation, snow depths, daily mean discharge were collected, as well as water samples from the main (Merced) River and tributaries, springs, groundwater, snowpits, and glacier melt. Isotope data were available from stream water, groundwater, and springs weekly to biweekly (to monthly) from 2005-2008.

With their comprehensive study they try to better understand the processes or factors that control the spatiotemporal variability of isotopic composition in precipitation, stream water, and groundwater and how such information could be used to advance the understanding of hydrometeorologic and hydrologic processes. The manuscript is very well structured and nicely written. The topic fits well to the scope of the journal and appears to be of interest in the catchment hydrology and alpine hydrology community.

Based on the hydrologic and meteorological data they could show that less snow and earlier snowmelt lead to a shift in peak river runoff toward late winter and early spring, away from summer when water demand is highest. Based on the isotope data, they implemented a catchment characteristic isotopic value (CCIV) in order to elucidate hydrometeorologic processes over seasons – an interesting approach which seems to be quite appropriate for snowmelt-fed catchments.  

However, changes or systematic shifts in the snow-rain transition zone due to climate change couldn’t really be proved since the observation period was too short and further, during their 3-year observation period they stated that one of the years was very wet and one very dry. Therefore, only relying upon the data from these extreme years seems to be questionable or it might at least be difficult to draw reliable conclusions, especially long-term conclusions on climate change. It would thus be great to continue the time series in the future.

Specific Comments:

P7, Table 1: D-Ex data are missing – perhaps due to temperature issues for δ18-O during isotope analysis with the DLT-100?

P9, l. 196 please insert standard deviation after 1σ - in order to define this acronym in the first instance

P9, l. 246 DEM - define this acronym in the first instance

P9, l. 255 WY - define this acronym in the first instance

P16, l. 389 LMWL - be careful when establishing a LMWL based on snow samples because of isotopic fractionation.

P28, Figure 10b and text p. 29 l. 683ff: I’m not sure that evaporation in Yosemite is much stronger… True, it plotted further right but the slope seems to be steeper than for Tenaya.

The study conducted intensive sampling for isotopic compositions in precipitation, stream water and groundwater to help understand the hydrological processes in a typical snowmelt-fed catchment. The topic is very interesting and valuable and the manuscript is well organized.

However, my major concern for this study is that the objective is very vague. I couldn't catch what are the key points of the study. The elevational control of isotopic composition is a basic law for isotope hydrology and the phenomenon has been widely reported. The effect of evaporation effect makes stable isotope of streamflow changes among tributaries is not unusual. Thus, it should further focus on more specific issues and new findings or contributes to hydrology from the detailed samplings instead of presenting the form of the research as a form of case study. I suggest put more attention on the identification the recharge zones of groundwater and streamflow and their changes or the impact of snowpack decline on the recharge of groundwater and streamflow.

Specific comments：

Lines 5-10. The title is too long. “Altitudinal” should be “elevational”.

Lines 41-43 “flow and flow duration of Yosemite Creek are much more sensitive to temperature increase due to a strong evaporation effect caused by waterfalls, suggesting possible prolonged dry-up period of Yosemite Falls in the future.” The conclusion seems arbitrary.

The Section of Introduction seems too general and the part should be re-organized to review the state-of-the-art methods on identifying the streamflow and groundwater sources and recharge zones, or how the stable isotopes could be used to improve our understanding on the certain hydrological process in complex catchment.

Section 2. What’s annul runoff depth of the catchment and are there significant variation of precipitation and runoff depth with elevation? What is the temperature lapse rate? Besides, I suggest presenting the time series of precipitation. It may be added in Fig 2.  

Figure 2. The annual cumulative precipitation is large at Gin Flat, but the snow depth is small. Is it a mistake?

Figure 4. I suggest using a different line type to show PSF to make the figure more clearly.

Lines 464-468. It's not clear how the evaporation effect correction is conducted.

In Figs 5 and 8, the significance test is needed.

Lines 682-685, the slope of lines for Tenaya Creek seems lower than Yosemite Creek in Fig 10. It’s hard to say the evaporative effect is more intensive in Yosemite Creek.