General comments

The main objective of the study was to improve our understanding of runoff response to year-to-year variations in precipitation phase and magnitude at the rain-snow transition zone. The authors addressed two main research questions related to 1) spatial and temporal distribution of surface water inputs (SWI), defined as sum of rainfall and snowmelt at the rain-snow transition zone and its variation between particularly wet, dry, rainy or snowy years and 2) the response of stream discharge to the above variations in SWI.

Authors used observations from eleven weather stations and snow depths measured using lidar survey to set up a distributed snowpack model iSnobal/Automated Water Supply Model in a semi-arid, headwater catchment of Reynolds Creek, Idaho, USA. Authors found that runoff in a snow-rich year was almost twice as high as in a rainy year, despite similar SWI, although they did not find any relation to annual snowfall fraction. In addition, dry-out date of the catchment was positively correlated to the melt-out date.

In my opinion, authors did an interesting work. I certainly agree that the focus on the rain-snow transition zone is important since this zone might extend to higher elevations due to climate change which will influence the water balance and timing of catchment runoff. Although the results are not surprising as they mostly confirm our existing knowledge, I found the study important and particularly novel (see also comments below), thus appropriate for HESS. Since the study investigated the effect of SWI on runoff just for one experimental catchment and few years, the ability of results generalization to different regions and climate is limited. Therefore, I have some comments listed below, which should be addressed before I can recommend the manuscript for publication.

Specific comments

In my opinion, the novelty of the study should be better described. I agree that the focus on the rain-snow transition zone is important and particularly novel, but I would encourage authors to better highlight research gaps and how the study goes beyond to what has been done in the past. Therefore, some additional justification can be added to introduction section (e.g., after research questions).

Although authors used frequently applied iSnobal/AWSM model, which is well enough described in the literature, it would be good to provide the reader with more specific information about generating snowmelt runoff, which is specifically important for SWI calculation. For example, how does the model calculate snowmelt? For rain-on-snow situations, is the rainwater directly added to SWI at the specific time or is it temporarily stored and delayed in the snowpack? Does model account for refreezing? Does model consider sublimation from snowpack and canopy interception? These details are not fully described in the current manuscript, but I think they might help the reader with better understanding of how the SWI were calculated.

L 197: As authors correctly stated, the use of only one lidar survey to describe the snowpack spatial distribution for all study years brings some uncertainty. I see the point that the topography is the main control of snowpack variability. Nevertheless, the meteorological controls might be important as well, such as wind speed and direction influencing snow redistribution and accumulation on leeward sites of slopes. What is the prevailing wind direction? And was it same for all years during snowfall events (and thus likely causing same snowpack distribution)? I would like to see a bit more discussion related to the topic.

Table 2, Fig. S4: The model performance for north-facing stations and in the “Upper region” (Table 2) in the water year 2011 is relatively poor when comparing simulated and observed SWE values. In addition, even for one single station, simulations for some years are well enough, while this is not the case for another years (e.g., jdt1 and jdt4). Is there any explanation for both temporal and spatial differences in model performance? How confident are observed SWE data for individual stations?

The conclusion that the snowfall fraction is not correlated to annual runoff or day of stream drying is certainly important, but maybe not such surprising. The snowfall fraction does not contain the information about total amount of snowfall, but only its relation to the total amount of precipitation. It means, that a year with high snowfall fraction is not necessarily the year with overall high snowfall. Therefore, it would be maybe interesting to select more characteristics describing the snow conditions in different years (such as amount of snowfall during cold season, annual maximum SWE, amount of snowmelt in spring etc.) to better show whether or not the cold season snowfall could positively influence the stream drying compared to the same amount of rain. Perhaps, the results can be shown in some table (heatmap) of paired correlations between individual characteristics.

L 286-288: This part would maybe deserve a bit more attention since it touches the important issue of catchment storage and its “memory effect”. I found this partial analysis interesting (despite the fact that results did not confirm an effect of “previous water year precipitation”). Therefore, I suggest some extension of the related text.

L 297-300: For day of stream drying, would it make more sense to account for sum of SWI preceding the day of stream drying instead of annual sum of SWI?

Although, I found the reasoning presented in results and discussion sections correct, the supporting illustrations are, in my opinion, less informative and I am not sure whether they fully support all the results and interpretation. For example, one of the main conclusions is that temporal distribution of SWI is more important than its total amount. While I agree with that, it is difficult to me to clearly see this in figures which mostly shows only time series (Figs. 4 and Fig.5). I do not have any clear suggestion how to make figures more informative and supporting the results, but I would encourage authors to reconsider their illustrations and perhaps add another figure which would better show how the timing of SWI influence the runoff response.

Technical corrections

L 116: The decrease in streamflow should be expressed in mm/decade to be comparable with other characteristics.

L 138: “stage height-discharge relationship”. Maybe more common term “rating curve” would be better.

L 193: “Trujillo et al. (2019, manuscript in preparation)”. As it seems from references, this paper has been already published.

Fig. 6a: The annual discharge is related to the precipitation at jdt125 climate station. Why not to show catchment mean precipitation instead? If I understood correctly, the model interpolates stational data to a catchment scale using some kind of elevation dependency. Therefore, to show catchment precipitation in Fig. 6a makes more sense to me to make it better comparable to catchment runoff.

Fig. 6b: What the triangles represent? Maybe, there is a mistake in the figure as they represent “other years”, but different symbol is used in the legend.

HESS Review - Kiewiet et al.

In this study, the relation between precipitation input and discharge output is analyzed for a small headwater catchment located in the rain-to-snow transition zone in a semi-arid climate in the US. Specifically, the study aims to examine how differences in the partitioning of precipitation into snowfall and rainfall affects the spatial and temporal distribution of the sum of rainfall and snowmelt (SWI) and subsequently, how this affects discharge quantity and timing of dry-out of the stream. The analyses were done using a combination of a snowpack model and discharge and snow observations, and selecting four different hydro-climatological years and investigating their differences.

The paper covers an interesting topic. It focuses on snowmelt and discharge processes in the rain-to-snow transition zone and is therefore suitable for HESS. The paper is well structured and I found the existing figures well presented. However, I do have a few comments before I can recommend the paper for publication.  

Main finding and figures: I agree with reviewer #1 that the text at some places in the results and discussion sections read clear and sound, but the figures do not always support the conclusions or findings. As mentioned by reviewer 1 especially the conclusion on the temporal distribution of SWI is not easy to extract from the figures. Figure 4 and 5 show SWI and discharge, but because the years differ in so many aspects (ratio of rain and snowmelt, timing of SWI, variability of SWI, Q), it is hard to tell which process caused the discharge response from the timeseries, i.e. to see a clear link between temporal distribution and discharge. The text describes these different aspects, but how to generalize these results more? Maybe some measures related to the timing of the center of volume for rainfall and snowmelt, antecedent conditions before spring or number and timing of melt/rainfall events could give some insights. Could also some measure on spatial and temporal distribution be combined? Probably the authors know best how they drew this particular conclusion and could use that to focus on that aspect in the results/figures more explicitly.

Study setup: While going through the manuscript I was wondering why only four years were selected. Because of the many processes that influence the discharge signal, a larger sample of years may have provided stronger evidence how processes relate, i.e. avoid that for example the dry year that was analyzed had many rainfall events. From the data description it is a bit unclear to me what the maximum possible amount of years could have been for analyzing. The decision may have to do with the runtime of the model? At least I would expect some description how the selected years deviate from the mean hydro-climatology of the catchment. Maybe the discussion/limitations section could elaborate on the selection of the years and the intertwined processes when looking at observations and possibilities for future model experiments, isolating some of these aspects (for which discharge would needed to be simulated as well) – but this last point as the authors see fit.

Argumentation in introduction: Partly related to the comment of reviewer 1 on a better description of the novelty in the introduction, I think that the line of thoughts for this study and the research gap can be better described. In my opinion, the introduction mixes 1) changes in snowmelt generated streamflow, 2) differences between catchments seasonally snow covered and in the rain-to-snow transition zone, 3) rain-to-snow zones as a space-for-time substitution of catchments that are now seasonally snow covered and 4) changes that have occurred in the rain-to-snow transition zone and may occur in the future. Although all of these aspects may be important to put the study into context, I would suggest to clearly identify the research gap (how do yearly variations in rainfall and snowmelt influence discharge, relation with snowfall fraction not yet clear, rain-to-snow zone suitable to analyze ‘extremes’, i.e. snowy and rainy) and explain the implications for future changes and relations to observed changes in different type of catchments in a more structured way.

Methods and data description: Here I missed some details regarding the available data, the model and the choice of years. As indicated above, it is not mentioned how the four climatologically different years were selected. I was also a bit confused by the numbers in table 1, how come that in a rainy year, the SWIsnow is higher than in a snowy year? Are numbers switched here? And without knowing the range of snowfall fractions over a longer time period it is difficult to interpret the values of the different years. It would also be helpful to explain the reasoning and possible hypotheses of selecting rainy and snowy years and wet and dry years. Could temperatures also be given for the years? Regarding the data and model, what is needed as input for the model? And which of the stations do have this data available for which time period.

Minor and technical corrections:

Title + abstract: ‘Snowfall fractions’ – since you only clarify in the introduction, maybe another term could be used here, e.g. ratio of snowfall to precipitation. Regarding the title, maybe it needs to be adjusted depending on the changes, e.g. temporal distribution and total input? Or specify what is meant with temporal distribution. Stream discharge – Annual (stream) discharge.

 L13 ‘..spatial and temporal distribution of precipitation’ – add phase of precipitation?

L68 which catchments?

L71-72: on an annual time-scale is this so different, apart from the effects of snow redistribution? Is this something interesting to show for you analyses, i.e. spatial distribution of rainfall and spatial distribution of snowmelt?

L94: ‘However’ – where does this refer to?

L116-117: did increased ET played a role here?

L195-196: ‘this uncertainty…. Patterns’ – double with few sentences above

Section 3.5 How do catchment precipitation and discharge compare? Are there estimations for ET?

L223 ‘this pattern was masked by the effects of other processes’ – what is meant here? In general in the results section it would be helpful to indicate better when observations or when simulations are described.

L236-237 ‘differential melt-out patterns’ – what was compared for that?

L267 ‘As a result, average daily SWI rates were higher’ – as a result of what?

L274 ‘whereas roughly 30% of SWI….’ – are delays taken into account, or is meant here the comparsion between SWI from month x to month y and discharge from month x to month y? Are the events where Q is higher than SWI also of interest?

L279 Have you tried plotting % of SWI translated into discharge against temperature (annual, or during growing season?)

Section 5 – the subsections have no numbering

L357 ‘This highlights the importance of the temporal distribution of SWI’ – also the importance of total water input?

L360 ‘events’ – throughout the manuscript when using ‘event’ please check if it is clear why event is meant? Precipitation, rainfall, snowmelt, discharge?

L369 ‘catchment’ – sub-catchment?

Discussion on simulated snow depts – could it be extended with a description of the reasons for varying performance for individual years and maybe a hypothesis how such ‘bad’ simulated years potentially could have influenced the results?

L419 ‘…, which influences’ – should it be, which may influence? As for example one of your conclusions is that the spatial distribution of SWI stays rather stable over time?

L428 -429 Could a short explanation/hypothesis be added why Q was much higher in 2010?

Figure 2e – what do the light coloured pixels mean? Was there no snow cover in the simulations while there was around 0.5 in the satellite observations? Because of the comparison of different years?

For all figures it may be good to not only indicate the year but also its characteristic (i.e. snowy, rainy, wet and dry) in the figure itself instead of the legend.