the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Coupled hydrological and hydrodynamic modelling application for climate change impact assessment in the Nemunas River watershed–Curonian Lagoon–south-eastern Baltic Sea continuum
Rasa Idzelytė
Natalja Čerkasova
Jovita Mėžinė
Toma Dabulevičienė
Artūras Razinkovas-Baziukas
Ali Ertürk
Georg Umgiesser
Abstract. We analyse the cumulative impacts of climate change in a complex basin-lagoon-sea system continuum, which covers the Nemunas River basin, Curonian Lagoon, and the south-eastern part of the Baltic Sea. A unique state-of-the-art coupled modelling system, consisting of hydrological and hydrodynamic models, has been developed and used for this purpose. Results of four regional downscaled models from the Rossby Centre high-resolution regional atmospheric climate model have been bias-corrected using in situ measurements, and were used as forcing to assess the changes that the continuum will undergo until the end of this century.
Results show that the Curonian Lagoon will be subjected to higher river discharges that in turn increase the outgoing fluxes into the Baltic Sea. Through these higher fluxes, both the water residence time and saltwater intrusion event frequency will decrease. Most of these changes will be more pronounced in the northern part of the lagoon, which is more likely to be influenced by the variations in the Nemunas River discharge. The southern part of the lagoon will experience lesser changes. Water temperatures in the entire lagoon and the south-eastern Baltic Sea will steadily increase, and salinity values will decrease. However, the foreseen changes in physical characteristics are not of the scale suggesting significant shifts in the ecosystem functioning, but are expected to manifest in some quantitative alterations in the nutrient retention capacity. However, some ecosystem services such as ice fishing are expected to vanish completely due to the loss of ice cover.
Rasa Idzelytė et al.
Status: open (until 21 Apr 2023)
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RC1: 'Comment on egusphere-2023-303', Anonymous Referee #1, 10 Mar 2023
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Review of
“Coupled hydrological and hydrodynamic modelling application for climate change impact assessment in the Nemunas River watershed– Curonian Lagoon–south-eastern Baltic Sea continuum”
by Idzelyte and co-authors,
The study investigates the effects of climate change on discharge, water temperature, and salinity in the Coronian lagoon as part of the Baltic Sea. They use a new developed high resolution hydrodynamic finite element model setup for the lagoon and part of the southeastern open Baltic Sea. This was coupled to a hydrological discharge model set up for the Nemunas river drainage basin. They demonstrate a freshening of the lagoon due to lower future salt intrusions from the open Sea and increased river discharge which controls the outflow out of the lagoon. The magnitude of these changes dependent on the chosen climate scenario.
The results provide new and important information about the local effects of climate change in the study area which may be of interest also for other marginal lagoons. However, there are a number of shortcomings in the presentation of results, which makes it somewhat difficult for the reader to follow and that should be improved before publication.
General comments
Actually, the results and the discussion remain somewhat descriptive. A lot of detailed information is given but it is difficult for the reader to set this information into a broader scientific context.
Such a context could be given in the Introduction which in it’s present form mainly informs about general issues of climate change in the Baltic. May be it’s feasible to introduce also to the role of lagoons in the ecosystem of marginal seas to attract researchers from outside the Baltic Sea / Curonian lagoon. What are the specific problems and research questions in the Curonian lagoon (erosion, retention, ventilation was mentioned somewhere later in the text)? Also may be some information about the choice of the climate scenarios. Are they chosen to be most realistic or to be well suited to study strong warming effects etc? Comparing RCP45 and RCP85 allows to assess the effect of moderate climate mitigation. Information about scenarios can be found in e.g.
Thomson AM, Calvin KV, Smith SJ et al (2011) RCP4.5: a pathway for stabilization of radiative forcing by 2100. Clim Change 109:77. https://doi.org/10.1007/s10584-011-0151-4
Riahi K, Rao S, Krey V et al (2011) RCP 8.5—a scenario of comparatively high greenhouse gas emissions. Clim Change 109:33. https://doi.org/10.1007/s10584-011-0149-y
The model description is not sufficient. This makes it difficult for the reader to interpret the results later. The meteorological forcing data for the hydrological model should be listed and it should be clear what are the prognostic output variables. It is only stated that temperature and discharge were extracted. Are there more variables of interest (e.g. evaporation which is mentioned in some places of the results). Actually, mainly the setup of SWAT and it’s boundary input data are mentioned.
It would be easier to follow if the model descriptions (section 2.3) would be given before section 2.2 boundary data.
As I understand a transient simulation from 1970 to 2099 was carried out. I wonder if time series plots could be shown to display the temporal evolution and the variability of some integrated variables (e.g. discharge data, sea level etc) that also would allow to get information about that ratio of the climate signal to it’s internal variability.
Likewise, the authors may consider to show some maps of salinity (or their future minus historical anomalies) could be shown. As stated, water temperature is quite homogenous in the lagoon (probably due to the constraint by meteorological air temperature). But are there notable gradients in salinity which would demonstrate the added value of the high resolution FE model?
Specific comments
Abstract
line 18: salt water intrusion into the lagoon?
Introduction
Please give a few more info about the role of lagoons and/or the Curonian lagoon for ecosystem, freshwater supply, nutrient retention, recreation etc) to make this topic interesting for a broader readership and to demonstrate the significance of this area for economy and ecosystem services
line 33: „Additional climate change impacts…“, What is meant with additional. Please give some examples
I also recommend to more introduce to the problems of coupling hydrology and hdrodnamic models in climate applications, and demonstrate why a coupled hydrological model is important in this regard, see e.g.
Hagemann S, Stacke T and Ho-Hagemann HTM (2020) High Resolution Discharge Simulations Over Europe and the Baltic Sea Catchment. Front. Earth Sci. 8:12. doi: 10.3389/feart.2020.00012
Material and methods:
Please consider section 2.1 to move to the Introductions.
Section 2.2.1. Climate projection data.
Please indicate somewhere that the scenario you use are from CMIP5 suite of global projections to avoid any confusion with CMIP6 models. Something like:
“The projections are derived according to two Representative Concentration Pathway (RCP) scenarios: RCP4.5 and RCP8.5 of the Coupled model Intercompariosn project phase 5.
line 107. Please include Dieterich et al., 2019
Dieterich, C.; Wang, S.; Schimanke, S.; Gröger, M.; Klein, B.; Hordoir, R.; Samuelsson, P.; Liu, Y.; Axell, L.; Höglund, A.; Meier, H.E.M. Surface Heat Budget over the North Sea in Climate Change Simulations. Atmosphere 2019, 10, 272. https://doi.org/10.3390/atmos10050272
2.3.1 Hydrological model
The description of the hydrological model is very sparse. What are the prognostic variables the model simulates. Which processes are included?
Please indicate which variables were used as forcing for the hydrological models to simulate the discharge to the lagoon and where these data are derived from. Which are the prognostic variables (if there are more than discharge to the lagoon)?
Please explain what is a “Hydrological Response Unit” of which your setup contains so many. And how is the discharge from SWAT implemented in SHYFEM (is it on one grid cell or distributed over more in accordance to the two nearest basins of the horological model).
2.3.3 Coupling of models
It hard to figure out the coupling from the given information. Could you indicate what variables/integrated parameters are exchanged between SWAT and SHYFEM? SWAT delivers discharge I guess ?
Also it’s not clear how these models can overlap. SHYFEM is a hydrodynamic model covering the Balti Sea and the lagoon. Is the lagoon also included in the hydrological model? Is it updated by SHYFEM and evaporation is calculated in SWAT?
line 185: “...although here we observe that the climate models have their own internal dynamics, thus the correlation with measurements is weak (~0.25). “
please consider removing this statement.
Climate models produce their own variability on synoptical timescales (days to week) for both air temperature and precipitation (as well as for all other prognostic variables). In case of daily temperature the day to day variability is usually much lower than the seasonal amplitude. Therefore, correlation for temperature is expected to be high, which mainly demonstrates the climate model simulates low temperatures in winter and high temperatures in summer.
In case of precipitation the day to day variations are much higher than the seasonal amplitude. Therefore I guess, if you would have taken daily sums of precipitation you would have get no correlation at all. By using monthly sums of precipitation you smooth out the synoptical variability which “extracts” the seasonal cycle and consequently you get a somewhat higher correlation.
line 194/195
“The differences between the scenarios in the short-term are smaller than in the long-term by an average of 49% under RCP4.5 and ~123% under RCP8.5. “
please rephrase. What does average mean? average over the four climate models?
line 201.
Please add a short sentence highlighting in what the context of this information is important. What is the role of land - sea difference?
Table 2 contains a lot of numbers but is discussed only very briefly. Perhaps a figure instead of a table would provide more info about changes in seasonallity etc. Also, yearly changes could be pointed out as this could be of interest for the long term changes in the freshwater inventory.
line 214: what is meant by elements?
line 217ff: You could discuss the general pattern in a broader context. For example, the opposite seasonal trends, i.e. the higher discharge during winter, and the reduced discharge during summer well reflect the changes in the water cycle over eastern Europe for which dryer conditions during summer and wetter conditions during winter are projected with a strong surplus of precipitation in the yearly sum. (e.g. Jacob et al., 2014, Gröger et al., 2021, Christensen, 2022).
Jacob, D., Petersen, J., Eggert, B., Alias, A., Christensen, O. B., Bouwer, L. M., Braun, A., Colette, A., Déqué, M., Georgievski, G., Georgopoulou, E., Gobiet, A., Menut, L., Nikulin, G., Haensler, A., Hempelmann, N., Jones, C., Keuler, K., Kovats, S., Kröner, N., Kotlarski, S., Kriegsmann, A., Martin, E., van Meijgaard, E., Moseley, C., Pfeifer, S., Preuschmann, S., Radermacher, C., Radtke, K., Rechid, D., Rounsevell, M., Samuelsson, P., Somot, S., Soussana, J.-F., Teichmann, C., Valentini, R., Vautard, R., Weber, B., and Yiou, P.: EURO-CORDEX: new high-resolution climate change projections for European impact research, Reg. Environ. Change, 14, 563–578, https://doi.org/10.1007/s10113-013-0499-2, 2014
Gröger, M., Dieterich, C. & Meier, H.E.M. Is interactive air sea coupling relevant for simulating the future climate of Europe?. Clim Dyn 56, 491–514 (2021). https://doi.org/10.1007/s00382-020-05489-8
Christensen, O. B., Kjellström, E., Dieterich, C., Gröger, M., and Meier, H. E. M.: Atmospheric regional climate projections for the Baltic Sea region until 2100, Earth Syst. Dynam., 13, 133–157, https://doi.org/10.5194/esd-13-133-2022, 2022.
line 226 and previous occurrences:
what is meant by study domains ? Does it refer to SWAT hydrological model and the hydrological SHYFEM models?
Figs 6 & 7: I am not sure how to interprete these figures. Please denote what is the reference volume from where water flows in or out. Considering section 4 (LT-RU-border), and provided SHYFEM conserves the water volume, the net water exchange across this section would be close to zero. But Figure 6 shows a deficit, since there is much more outflow than inflow. So where is this deficit balanced? Is it the riverine water input from the hydrological discharge model?
Table 3: Please give the some explanation how to interpret the results. Do out and inflow have to be balanced?
Section 3.4 Salinity
The results for salinity should be discussed in the context of uncertainty. As I understood, lateral open sea boundaries for were prescribed from results of Gröger et al. 2019. These scenarios do not contain the effects of global sea level rise. This should be discussed. See literature for example Meier et al. 2021. In short: rising global see levels would lead to higher salinities in the Baltic Sea due to higher volume inflow from the North Sea.
Markus Meier, H.E., Dieterich, C. & Gröger, M. Natural variability is a large source of uncertainty in future projections of hypoxia in the Baltic Sea. Commun Earth Environ 2, 50 (2021). https://doi.org/10.1038/s43247-021-00115-9
line 269: Please rephrase the sentence. When there are no water intrusions from the Baltic Sea to the lagoon, how can salt intrusions be calculated?
Section 3.5 Water temperatures
Please consider to give changes in temperature units instead of relative % increases. It is not surprising that the temperature is homogenous as the water surface is tightly constraint by the atmospheric temperature (in particular when the lagoon water body is small). But how does this look like for salinity? SHYFEM has extremely high resolution which is a prerequisite to represent steep salinity gradients.
section 3.6 Water level
line 313: what is the reference level and where do these values come from? From the model, literature, observations? Are extreme values included like e.g. during storms?
Table 6: again what is the reference level. Is it perhaps the border between the lagoon and the open sea?
What is the idea of averaging 5 stations in the Curonian together with area average of the SE baltic Sea? How can this be interpreted?
Section 3.7 Water residence time
how were these data derived? Usually I would assume you need a total volume of the water body and water fluxes into/out of it. Are the values in Table 7 for the entire SHYFEM domain, only for the lagoon or for the open SE Baltic Sea?
4 Discussion
line 364: I am puzzled here. Before I got the impression the results were derived with the coupled SHYFEM – SWAT model which used input data from the downscaled global models.
4.2 Water flow
line 388. It is clear that outflow of the lagoon dominates when so much river discharge takes place. This is also consistent with the reduced water residence times. Therefore you may consider merge section 4.2 with 4.6
line 405. The higher risk for flooding in winter is a nice result which could be highlighted in the abstract as well.
4.3 water level dynamics
last sentence: why leads a fixed boundary to the Baltic Sea to an overestimation of the increase in sea level.
4.4 Salinity dynamics
Salinity in the open SE Baltic is much controlled by the lateral boundary data which do not include the effect of global mean sea level rise. Therefore the salinity drops in the future is likely overestimated. Please discuss the uncertainties for salinity in the light of lacking effect of global mean sea level rise (Meier et al., 2021).
4.5 Water temperature dynamics
This paragraph is mainly descriptive. Please consider to move it to the results section.
4.6 Residence time
line 448: Isn’t the increased outflow ultimately rather a result of the increased discharge from the rivers to the lagoon?
4.7. Ice thickness
Indeed, it is very likely that the ice season is affected. Is it possible to analyze the length of the season with ice presence out from the model outputs?
4.8 ecosystem impact
“The foreseen salinity changes in the mostly freshwater ecosystem are not of the scale suggesting significant shifts in the ecosystem functioning even in the northern part of the lagoon “
This is a strong statement. Please consider to formulate this weaker.
lines 469 to 447: You state that the results of Ivanauskas, 2022 are in contrast to predictions of Baltic Sea fishery. But it’s not clear why. And can your results be used to explain this contrast. I do not understand what is the point with this paragraph.
lines 475ff. Again, you mention the shortening of the ice cover period. This is exactly what would be expected in warming scenarios and not very surprising. Can you give give a more quantitative measure for this, like e.g number of days with sea ice or shifts in the start/end of the ice season for the lagoon etc? You use a model which is well suited for the lagoon and so this would yield an added value to the current knowledge.
To reach a broader readership it would be good to be a bit more verbose about the speculations on retention and nutrient cycling. What are the processes that determines retention and how do the drivers change according to your results and what can be expected.
5 Conclusions
line 491: What kinds of ecosystem functioning shifts do you refer here?
you should also mention here that you did not consider mitigation scenarios, i.e. the RCP2.6 scenario which is in agreement with the goal to limit global mean climate warming to 2.0 degree compared to the preindustrial level. See e.g.
van Vuuren DP, Stehfest E, den Elzen MGJ et al (2011) RCP2.6: exploring the possibility to keep global mean temperature increase below 2 °C. Clim Change 109:95. https://doi.org/10.1007/s10584-011-0152-3
line 495: This sentence could be removed if no information is given what kind of extensive sensitivity analyses are envisaged.
As I understand, the residence time in the lagoon is mainly controlled by river discharge implying the changes in wind driven circulation are less important (contrary to the open sea). This might be also of interest for other lagoons that may suffer under episodic hypoxic events and for which decreasing discharges are predict under future scenarios (even if this is not a problem in the Curonian lagoon).
Citation: https://doi.org/10.5194/egusphere-2023-303-RC1 -
RC2: 'Comment on egusphere-2023-303', Anonymous Referee #2, 13 Mar 2023
reply
egusphere-2023-303
The ms entitled “Coupled hydrological and hydrodynamic modelling application for climate change impact assessment in the Nemunas River watershed–Curonian Lagoon–south-eastern Baltic Sea continuum” by Idzelyte et al. focuses on the analysis of the effects of climate change on a Baltic coastal region using modelling tools. The approach is particularly interesting because it does not focus only on one water body, as is usually the case, but, using coupled hydrological and hydrodynamic models, covers the land-sea gradient, analysing the expected changes in the catchment, the Nemunas River, the Curonian Lagoon and the adjacent coastal Baltic sea, in an integrated way, considering the interactions between them.
The work is well laid out and the data used to feed and set the boundary conditions of the model are well documented and presented in a detailed way. The two coupled models used, SWAT and SHYPHEM, are extensively tested in their respective applications, the former for catchment and runoff models and the latter in several coastal lagoons, especially in the Mediterranean, but also in the Curonian lagoon.
Perhaps, one of the weaknesses of the work would be that the future scenarios consider only climatic conditions, but not the evolution of the drainage basin in accordance with them or socio-economic development. These changes will undoubtedly affect land use, water requirements for different uses and, in general, surface runoff and the functioning of aquifers. Although this involves that the predictions cannot be those that will actually occur, this does not invalidate the work since it gives the frame of reference in which they will occur, and can make the article a necessary reference for more socio-economic predictive models.
Perhaps, these expected changes, or in which direction can they affect, should be discussed in the Discussion chapter from the knowledge of how SWAT model responds to such variables although assuming that it must be just a series of considerations just to take it into account, more than a real analyses if there are no real data, to avoid to be speculative.
One of the aspects included in the discussion is 4.8 Impacts on the ecosystem structure and functions. To consider this is important for having a complete view of the real consequences of the climate change and to design management actions to avoid negative consequences for the biodiversity and human uses as fisheries. Authors assume that it is more complicated to predict but they state that “However, the foreseen changes in physical characteristics are not of the scale suggesting significant shifts in the ecosystem functioning, but are expected to manifest in some quantitative alterations in the nutrient retention capacity”. This statement should perhaps be qualified and discussed.The modelling results show that the Baltic Sea saltwater intrusions into the Curonian Lagoon will likely decrease in the future and seasonally, the highest change of saltwater intrusion days is observed during winter and especially spring.
The authors address here the consequences on pelagic production and fisheries, and nutrient fluxes in sediments, but do not consider the implications on lagoon-sea connectivity and migratory movements, or larval exchanges between the lagoon and sea. There is already some papers demonstrating the importance of connectivity between lagoons and sea in the structure of ichthyoplanktonic assemblages and genetic population structure of the fauna. Although going into these aspects would require Lagrangian models, a discussion on the implications of changes in residence times might be possible. It is evident that since the results show a decrease in marine intrusions in the future and that the greatest changes materialize during the spring (typical time of larval production), these changes must have consequences on the genetic structure of the populations and on the community structure.
Citation: https://doi.org/10.5194/egusphere-2023-303-RC2
Rasa Idzelytė et al.
Data sets
The computation results of coupled hydrological and hydrodynamic modelling application for the Nemunas River watershed – Curonian Lagoon – South-Eastern Baltic Sea continuum Rasa Idzelytė, Natalja Čerkasova, Jovita Mėžinė, Toma Dabulevičienė, Artūras Razinkovas-Baziukas, Ali Ertürk, and Georg Umgiesser https://doi.org/10.5281/zenodo.7500744
Rasa Idzelytė et al.
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