Manuscript EGUsphere-2022-1512

Long-term monitoring (1953-2019) of geomorphologically active sections on LIA lateral moraines under changing meteorological conditions

 by Altmann et al.

General comments:

The preprint of an ESurf manuscript open for interactive discussion constitutes an interesting and important study and addresses one of the important research geomorphology currently focuses on. Among many aspects of modern and future 'Global Environmental Change', the development of recently deglaciated glacier forelands and connected morphodynamic processes in mountain regions is surely of considerable significance. Despite the related concept of the 'paraglacial period' is now well established for several decades, existing work often suffers from the lack of detailed long-term observations. Some conceptual facets and underlying assumptions would certainly highly benefit from such data.

Gully formation on the slopes of lateral moraines exposed by successive glacier retreat since the 'Little Ice Age' maximum extension is a prominent example of paraglacial processes and characteristic for many glacier forelands worldwide. The current study presents such detailed and accurate data from a total of twelve active gully systems on lateral moraines in five glacier forelands, all located within the Eastern European Alps. Several surveys over a rather long timeframe from 1953 to 2019 allow a detailed investigation of gully system development and sediment yield over a comparable long time and 3 separate time periods within this interval. The authors apply a highly developed methodology related to both the acquisition of aerial imagery, DEMs, LiDAR scenes etc. and their subsequent morphometric analysis. All results are well presented and all data are of high quality and accuracy. All individual methodological steps within the data analysis and well explained alongside all necessary information on the data base. It is, therefore, no surprise that the results of the truly long-term study are impressive and a valuable contribution to the topic.

In many aspects, the majority of the investigated sites confirm the validity of the established 'paraglacial period concept', for example with the highlighted decrease of sediment yield over time during the targeted time period (i.e. the 'sediment-exhaustion' model). On the other hand, an interesting result is that the gullies have still not been fully stabilised and some morphodynamic activity is still recorded during the final (youngest) time period. This is in disagreement to some existing studies, but also in agreement with other ones. Alongside a newly developed 'sediment activity concept' this finding and two sites that do, as exceptions among the total data set, not fully follow the expected conceptual decreasing sediment yield for 'paraglacial processes' are finally discussed. But here, a deeper discussion on basis of published work from other regions would likely improve the high quality of the manuscript. The new concept is introduced and discussed too briefly to convince that the observations justify the introduction of a new concept. An extension of the discussion chapter should serve to interpret the results of the study in more detail and highlight the differences from the established concept and assumption to underline its significance.

It seems only a minor points given the scientific value of the preprint, but an excessive use of acronyms and abbreviations for terms where they are neither necessary nor established negatively affects fluent reading, to an extent where it is annoying. It would be acceptable for a technical report, but for a journal article this should be avoided. Because it surely constitutes no major effort to make related changes in the text alongside some final editorial and typographic changes, the authors are recommended to consider such changes (some examples are indicated in the technical comment section below).

Summarising, the long-term study and its well presented results constitute a valuable contribution to a wider audience within the targeted scientific community. The only scientific room for improvement is a recommended extension of the discussion section by adding some depths. In my specific comments below I will address this in more detail.

Specific comments:

I feel that the authors should extent the discussion chapter by exploring some of their most interesting findings, for example that their investigated active gully systems still are active and show, despite a decrease of sediment yield in most cases, no stabilisation. As correctly stated, this finding is different to previously published work from other regions, for example Western Norway. Perhaps the authors should present a hypothesis or some possible reasons for this, simply because it is to some extent contradictors to the established sediment-exhaustion concept for the development of gullies. Regional different conditions of gully development need, however, to be taken into account with the discussion of this apparent discrepancy. Among those are the different geomorphological setting (typical Alpine-type lateral moraines vs. debris-covered slopes of different origin in Norway) and the sedimentological properties of the lateral moraines related to their genetic origin etc. Factor other than the morphometric properties have to be taken into account.

The 'sediment activity concept' developed by the authors is only comparatively briefly introduced. With a limited number of study sites and - as least this is my (potentially wrong) assumption - mainly based of two exemptions from the trend the basis for developing such an innovative concept is rather small. And with the 'ice release' not included and a limited temporal validity (see lines 556 ff.) the authors need to properly elaborate is this constitutes a significant new and valuable concept - in other words justify that their observations support such a step instead of accept that exceptions from other established concepts always may exist. Perhaps it would strengthen the value and depths of the discussion chapter if the authors focus more on the investigation of potential reasons for the deviating date of these two gully systems instead of developing a new and obviously limited concept.

In their discussion section 5.4 'meteorological drivers' the authors present an interesting review on potential meteorological drivers for the current morphodynamic activity of the gully systems investigated. In contrast to what some readers may have expected based on frequently emphasised (popular)scientific statements, there seems to be major increase in the frequency or magnitude of heavy-precipitation events in the study areas. They accurately describe the differences of simulated vs. observed meteorological data what is good and provides good insights. But as this aspect of the study is even included in the title, it seems that some summarising conclusion (or assumptions) regarding the potential influence on climate change nerd to be provided. These could be along the lines 'paraglacial period'/'sediment exhaustion' concept vs. development of geomorphological activity and morphodynamics in times of Global Change. And to throw in just a provocative hypothesis: Could future climate change lead to increased morphodynamic activity (erosion) and disturb the 'normal' decrease of sediment yield as predicted by the established paraglacial period concept? 

I am confident that by investing some effort to extend and strengthen some sections of the discussion chapter will substantially increase the overall scientific value of the comprehensive and important research presented by the authors. The overall goal should be to place the significant long-term approach and its results better in a general context, also beyond the Eastern European Alps. 

Technical corrections:

The manuscript is mostly well structured and written, with the important exception of excessive use of (to a considerable extent) unnecessary acronyms/abbreviations that can make reading a pain. A few typographic/editorial changes should, however, be addressed during the revision:

• I feel that in the title the acronym 'LIA', despite well established, should be written in full. Also, I think it should be '...sections of ...' and, given the time period investigated, perhaps better 'climatic conditions'.
• Line 13: Add 'European' to Alps (only once in abstract and general text).
• Line 19/20: I recommend to consider different expressions for 'areas of interests' and 'entire areas of interest'. As I understand it, this refers to the active gully system and the entire lateral moraine. Why making it unnecessarily complicated with an excessive use of the term 'area of interest'? 'Sites' or gully systems allow the reader to read the abstract more fluently
• Line 21: 'Can be shown'
• Line 21: 'Epochs' is a wrong term in the context.
• Line 21: This sentence should be re-written as it is a bit unclear.
• Line 34: Better: 'with influence of dead ice over decades'
• Keywords: A quite high number, are all necessary? Paraglacial process system should be added
• Line 42: There is no defined or general 'end' of the Little Ice Age. The authors could well be more specific and relate it to the Eastern European Alps (if they wish).
• Line 44: Better: 'extending'
• Line 61: Better: 'and subsequently'
• Line 97: A recent comparative study could be worth being cited in this context: Eichel, J., Draebing, D., Winkler, S. & Meyer, N. (2023): Similar vegetation-geomorphic disturbance feedbacks shape unstable glacier forelands across mountain regions. Ecosphere 14(2), e4404.
• Lines 104, 111: The inflationary use of acronyms/abbreviations in this chapter (ans subsequent ones) makes it a bit hard to read the text fluently. Whereas for very established and complex term (DEM, SfM, LiDAR) it is all fine, abbreviating 'historical aerial imagery' and in particular 'sediment yield' goes over the top. The space saved does not compensate for poor readability with so many acronyms.
• Line 124: Explain the acronym AOI the first time it is used in the general text - or much better avoid this term at all.
• Line 124: See comment to 'epoch' above, why not 'period' or 'time period'?
• Line 128: Add 'European'
• Line 138: Capital letters for 'Main Alpine Divide'
• Line 153: 'Sparse' for 'low'
• Figure 1: Although the term is 'Gletschervorfeld' in German, the appropriate term is 'foreland' not 'forefield'. It would be good to (a) mention 'glacier outline' or 'glacier margin' in the legends as well. Please use the same colour for the same glacier extent (e.g. not green for the 1953 margin at Gepatschferner and blue for the same margin at Weißseeferner). Better to only use one colour for one data on all
• Line 180: Is it necessary to give such a detailed information? The same applies for the full project titles on Table 3.
• Line 245 ff.: This sentence could be made clearer by re-wording it.
• Line 365/366: Find a different solution for the two subsequent brackets and re-word the sentence
• Line 368/369: This sentence is a good example what I highlighted as excessive use of acronyms. Apart from those for the different active gully sections in the sentence above, no other acronym is necessary here, neither AOI nor SCA or SY.
• Line 501/502: If the authors would like to find a citation for this assumption, they could use [Jäger, D. & Winkler, S. (2012): Paraglacial processes on the glacier foreland of Vernagtferner (Ötztal Alps, Austria). Zeitschrift für Geomorphologie N.F. Supplement Bd. 56 (4): 95 – 113.] where this influence is described from another glacial foreland in the region.

This study analyzes the complex long-term geo-morphological dynamics (about 60 years) that took place in in five glacier forelands within the Eastern European Alps, by means of a data-analysis approach. Specifically, the main challenge of the work, if I understood correctly, is to derive a procedure based on acquisition of aerial imagery, DEMs, LiDAR scenes, laser scanner images, etc., to monitor the sediment yield and the volume production over a time horizon of 6 decades. Analyses of the meteorological drivers are also added.

The work is within the scope of the journal, and may provide an important contribution to the specific field of geomorphological active gully systems on Little Ice Age, although other similar efforts were published by the same authors for the same area (see Altmann et al., 2020 or Betz-Nutz et al., 2023).

I argue the choice of the title ‘under changing meteorological conditions’ for the reasons I will explain in the following. Additionally, I found the reading particularly tough and, to my opinion, the clarity of the presentation can be improved.

Please read in the following my general and specific observations.

General observations:

• My main concerns regard the analyses of the meteorological drivers. (i) WRF simulation data are normally used to make forecasts. I don’t’ see the need to use such data here, unless justified. In fact, the use of the two climate products (from reanalysis and from WRF) for two different time horizon can be dangerous, because they are affected by different uncertainties and different bias errors, which are normally adjusted by means of bias correction method, and that can affect the retro-respective assessment. I would have used ground observations, given also the very small areas of interests. Please explain the choice and identify eventual meteorological stations. (ii) In fact, an additional concern regards the spatial resolution of the climate model products, that are characterized by a large resolution. Which is the overlap with the areas of interest? Provide a figure and discuss.

• There is no real analysis of the link between morphodynamics processes and climate drivers; the study provides a screening of the meteorological series, that is fine. Therefore, the statement of analyses ‘under climate changes conditions’ is to strong, in my opinion. Again, there are no analyses of impacts of climate forcing on the physical processes but only a retrospective assessment. Also, the literature about the changes in precipitation is wide. Please, have a look to Libertino et al., 2019, Caporali et al.,2021. Within this context, how trends are assessed? Why the choice of those precipitation classes? An example of classification is given by Alpert et al., 2002.
• Uncertainty assessment: the section describes mainly data statistics.
• Clearly, all the conclusion derived for the meteorological drivers are affected by the modeling uncertainties of the used data. I suggest to smooth the title. 

Observations and technical suggestions to improve the manuscript:

• Please, at the end of the introduction, state very clearly which are the scope and the research questions of the work, avoiding to refer to previous works (L126-127).
• Since different methods based on data are implemented, I strongly suggest to include a flow chart to clearly describe the entire work flow.
• Include a list of acronyms (they are impressively a lot!!)
• Sometimes figures and tables captions are not explicative. Please, explain the content of the figure/table in a concise manner.
• Figure 1: include letters (or number) to refer to the specific subplot figures. At first sight the figure results confusing (AOIs not clearly localized).
• Figure 2, 4, 5: detail captions
• Figure 6: try to synthesize the results by plotting the slope and intercepts for all the AOIs.
• Regression is a very simple tool that correlates dependent and independent variables. Please, state clear the problem in L252 to L256! Also lines from L257 to L262 are a bit confusing, I suggest to include a figure (a scheme) to describe the analyzed variables.
• L348: provide a reference. Often, other definitions are used.
• L102: you may have a look to the contribution Noto et al., 2017 for the literature review.
• L124: define AOI
• L194: define HAI
• I found some content of the section 5.3.1 (including figure 15) more opportune for the introduction; this concept was not clearly introduced.

Alpert, P., Ben-Gai, T., Baharad, A., Benjamini, Y., Yekutieli, D., Colacino, M., Diodato, L., Ramis, C., Homar, V., Romero, R., Michaelides, S., and Manes, A.: The paradoxical increase of Mediterranean extreme daily rainfall in spite of decrease in total values, Geophys. Res. Lett., 29, 1–31, doi:10.1029/2001GL013554, 2002.

Altmann, M., Piermattei, L., Haas, F., Heckmann, T., Fleischer, F., Rom, J., Betz-Nutz, S., Knoflach, B., Müller, S., Ramskogler, K., Pfeiffer, M., Hofmeister, F., Ressl, C., and Becht, M.: Long-Term Changes of morphodynamics on Little Ice Age Lateral Moraines and the Resulting Sediment Transfer into Mountain Streams in the Upper Kauner Valley, Austria, Water, 12, 3375, https://doi.org/10.3390/w12123375, 2020.

Betz-Nutz, S., Heckmann, T., Haas, F., and Becht, M. 2023. Development of the morphodynamics on LIA lateral moraines in ten glacier forefields of the Eastern Alps since the 1950s, Earth Surf. Dynam.  https://doi.org/10.5194/esurf -11-203-2023

Caporali E, Lompi M, Pacetti T, Chiarello V, and Fatichi S (2021) A review of studies on observed precipitation trends in Italy. Int. J. Climatol., 41, E1–25, TS7. https://doi.org/10.1002/joc.6741

Libertino A, Ganora D, Claps P (2019). Evidence for increasing rainfall extremes remains elusive at large spatial scales: The case of Italy. Geophysical Research Letters, 46: 7437– 7446. https://doi.org/10.1029/2019GL083371

Noto, L.V., Bastola, S., Dialynas, Y.G., Arnone, E., Bras, R.L., 2017. Integration of fuzzy logic and image analysis for the detection of gullies in the Calhoun Critical Zone Observatory using airborne LiDAR data. ISPRS J. Photogrammetry Remote Sens. 126, 209–2