|Review of “Effects of climate anomalies on low flows in Switzerland” by Floriancic et al.|
Many of my previous comments were acknowledged and I find the focus shift towards the assessment of the shaping of the low flows worthwhile.
I went through the revised version and found still some points that the authors might consider before publishing:
• Now the shaping of low flows focuses on summer low flows, which excludes all the catchments that have winter low flows. While I find it important to explicitly distinguish between summer and winter low flow due to the processes driving, I would suggest looking also at the development and drivers of the low flows in the cold season. Here as well there might be a pattern occurring from simply lack of precipitation and the built up of snowpack or a combination of the two. This analysis could be done in a similar manner as the one for summer low flows using PET and lack of precipitation, for instance using low air temperature (or since available estimates of SWE).
• While the authors made efforts in the revision to make the terminology used less ambiguous and more consistent, I am not convinced of every choice made here. To me anomaly for the cumulated sum of differences between actual an average value the preceding period is not a good idea. It suggests already that the variable (PET or PRECIP) was anomalous before even looking at it, at the start of the analysis only the streamflow is low, but it might not be at all. I disagree also on the definition of anomaly for low flows with the authors arguing that within the year the streamflow deviates from the “norm”. In my opinion on the contrary it most of the times is not anomalous since it occurs very reliable in the same season of the year.
• The authors have with their data set the chance to show which of the preceding periods for which driver or combinations of drivers was anomalous but the study at state does not take that chance. It would require rewording and calling only anomalous conditions anomalies and then second it would make the study much stronger if there was a quantification of the drivers and clear communication of what a combination causes in terms of low flow or not.
• Rather than only arguing with correlation coefficients it would be very interesting to see a quantification f the effects. To keep it comparable between the catchments that could be expresses in percentage. But precipitation deficit in the preceding period as well as PET compared to mean PET in the season could be compared to Qmin. This would make a stronger argument for the study.
• Snow in the preceding winter was found to be only weakly related to summer low flow, this was based on the SWE of the 1st of March. I am not convinced of taking the SWE of a specific date, that might be already in the melting season for many lower elevated catchments and may not be the maximum amount for the highest elevations. I would suggest testing another metric maybe the maximum SWE in the winter period or something else that can be considered representative for the snowpack that could (or not) contribute to summer low flow for each catchment.
• Most results show only % of the low flow of the catchments show correlation with anomaly xy. I suggest to add more comparison between the catchments (where is the correlation stronger, weaker? can this be attributed to a region, geology, elevation range?) for more than only the map of low flow occurrence, such material could help follow up studies and they could be for instance be placed in a supplemental material.
• Statistical tests and quantification of process importance: Why the original data set was reduced first to selected warm season flow? Why not first select this period and then determine Qmin for all years and all catchments in this period? Were there really some that have only 5 years available? Are these then only the drought years?
• The expression “annual late summer and autumn low flow period” could be simply introduced as warm season/period, as is also used already sometimes by the authors (why would May be late summer?).
• The colors of the figures are not readable in black and white print. While I am aware these are also electronically available at least the figures with blue and red lines should be distinguishable in b&w, that could be by choosing a different line type or a different color choice. Best would be if also the colors of the maps could be distinguishable then e.g. Jun and Oct would not look the same. There are palettes that have 12 colors that can be also distinguished in b&w e.g. viridis.
Detailed comments line by line
L18 In the abstract 2011 is still mentioned as summer low flow despite occurring in spring
L21 what is meant by characteristics and where is that picked up in the study?
L35 I am not convinced that event is the right term for low flow; for an event I would expect that there is a threshold involved defining he start and end of it. The reference discussed droughts and I guess the authors intend to write also here “drought” as suggested by the follow-up sentence
L37 One key reference here is Price (2011)
L41 I guess here drought is intended again (at least the references point there)
L53 add Staudinger et al. (2017) for storage in Swiss catchments
L55-62 see general comment above: I am not convinced of the terminology and would not call every low flow preceding period “climate anomaly”, I would rather call the deviating periods among the preceding periods anomalies.
L65 “likely” sounds like it could be also not true? Why would that be?
L70 add something on that the lag time is dependent on the storage behavior that is different for each catchment; limited-substantially they express the opposite please clarify
L73 But there is also snow involved in these changes which could cause a less seasonal than we are used to effect anyhow?
L75-76 please revise and be more precise: Temperature is not depleting soil moisture storage. PET is also influenced by wind and not solely by temperature. T is a good indicator for PET, but AET will be very much dependent on how much a storage (soil) is wet or dry. E.g. with high PET but zero water in the soil, the soil will not be depleted and AET accordingly will be low, same PET on a wet soil will cause the depletion described and AET is high.
L103 which are the few studies? Please add.
L194 What is “tend” intended to express? The majority? And after which climate anomalies this is not the case? Please, clarify or reformulate.
L196 remove “tend”
L197 affect -> affects
L203 at the same time? or in the same period in series or in parallel -> rephrase
L213 remove Particularly
L245 “below-threshold precipitation … above-threshold PET” which threshold is that? From the methods I got that it is about the entire cumulated variable (difference actual value and average) preceding the low flow period. Please, clarify
L243-248 I would argue that it is not only because of the duration but when the “anomaly” starts, because together they form somehow an intensity of the anomaly. The authors argue that duration of high PET is stronger correlated than duration of low precipitation. This might be because of the seasonal character rather than the duration implicitly considered. This is for precipitation much lower than for PET. And this might change again whether the anomaly starts say in May and lasts for a month or whether it starts in September and lasts for a month.
L249/250 If a single precipitation event can exceed the “threshold” maybe the definition of anomaly is ill-posed?
L286/287 exhibit clear growth -> clearly increase
L317 “explains most of the predictability” should that be variability? Please clarify
L362 What does “relatively comparable” mean?
Generally, all figures have long captions including partly interpretation that is already in the text. Please, remove the redundant parts and only leave the necessary elements in the captions. E.g. Figure 1 drop the last sentence, that is literally repeated in the main text.
Figure6 in black and white the lines cannot be distinguished either use different line type or colors with different hue. Some methodological questions: 1) are the anomalies in fact cumulated sums over the preceding period for PET and precipitation differences, respectively 2) if these are anomalies shown, does each point consider a different number of data pairs 3) are here all catchments included, i.e. also the high elevation catchments?
Figure 7 b) it would be good to see next to an average how different that is for the dry years, please add the lines for each single year
Price, K. (2011). Effects of watershed topography, soils, land use, and climate on baseflow hydrology in humid regions: A review. Progress in physical geography, 35(4), 465-492.
Staudinger, M., Stoelzle, M., Seeger, S., Seibert, J., Weiler, M., & Stahl, K. (2017). Catchment water storage variation with elevation. Hydrological Processes, 31(11), 2000-2015