the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jul 2024
Trends in long-term hydrological data from European karst areas: insights for groundwater recharge evaluation
Abstract. Long-term observations of spring discharge provide an alternative to estimate the evolution of groundwater resources based on observational data at catchment scale. Karst springs can be found in large parts of Europe covering all climate zones of the mid latitudes. Continuous spring discharge measurements are holistic signals, representing both fast and slow flow components, typical of karstic environments. Due to relatively short response times, karst systems are focus points for enhancing the understanding of the impact of climate change on groundwater resources. This work analyses observational data (precipitation, temperature and discharge) of more than 50 springs spatially distributed over Europe to give a continental overview of changes in groundwater resources in karst areas. The work focuses on two different periods of 20 and 40 years, to identify any possible acceleration or moderation in changes. For both periods a trend analysis of the observational data, using Mann-Kendall and Sen’s slope, was performed on the entire time series and per season. Possible process changes were considered by analysing also trends in high and low flow values. Structural differences of the systems were considered by using two indices related to the storage and inertia of the system. In combination, these indices were able to i) highlight structural differences and ii) characterize karst systems accordingly. The results show that the sensitivity of karst aquifers to climate change is not controlled by their degree of karstification. Long-term trends in spring discharge calculated in this study follow the general pattern of river discharge found in literature, but the last 20 years deviate from this behaviour. During this period, increasing temperature plays a more important role in the evolution of spring discharge than changes in precipitation. These results are discussed in relation to the indirect influence of other drivers such as changes in land use or land cover, specific regional conditions but also changes in processes related to groundwater recharge and storage, providing insights for assessing groundwater recharge in the past and in the future.
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CC1: 'Comment on egusphere-2024-2078', Giacomo Medici, 17 Jul 2024
General comments
Excellent research in karst hydrology that just needs more detail before publication.
Specific comments
Line 13. Specify the European countries where you have studied the springs in the abstract.
Lines 25-88. The climate is affected by anthropogenic activities such input of CO2. You analyse exclusively the last 20 – 40 years that register other types of climate variations with different driving forces. This scientific point is clarified and discussed exclusively later in the text and not in the introduction. Please, consider to modify the introduction.
Line 65. “Rising temperatures and the consequent reduction of snow contribution”. Please, add more recent literature on snowmelt aquifer recharge in karst environments:
- Tracking flowpaths in a complex karst system through tracer test and hydrogeochemical monitoring: Implications for groundwater protection (Gran Sasso, Italy). Heliyon, 10(2).
- Snowmelt as a determinant factor in the hydrogeological behaviour of high mountain karst aquifers: The Garcés karst system, Central Pyrenees (Spain).
Science of the Total Environment, 748, 141363.
Lines 86-89. Very good that you specify the objectives of your research using bulletin points
Line 105. “Storativity” is clear, but not “inertia”. Please, provide explanation for this concept/terminology in karst hydrology.
Line 428. “First period”, specify four decades. You cannot recall the attention of the reader only for the for the second period.
Line 462. Please, integrate recent papers on karst hydrology.
General comments
Figure 1. The map can be much larger.
Figures 2, 4 and 5. Remove the underscore from the legend.
Figures 6 and 7. Please, provide more detail on the box plots in the captions. The red line? Median, or arithmetic average?
Citation: https://doi.org/10.5194/egusphere-2024-2078-CC1 - AC3: 'Reply on CC1', Markus Giese, 08 Nov 2024
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RC1: 'Comment on egusphere-2024-2078', Anonymous Referee #1, 15 Aug 2024
The manuscript entitled “Trends in long-term hydrological data from European karst areas: insights for groundwater recharge evaluation” presents an analysis of daily karst spring discharges across Europe and aims to identify connections between daily climate variables and karst discharge. I find this topic very interesting, and HESS is a suitable journal for this type of study. However, the manuscript has several weaknesses, and in my opinion, several major and minor revisions are necessary before considering this manuscript for publication. Please take into account the following points:
- To avoid confusion, it would be helpful to clearly differentiate between recharge, actual recharge (mentioned in line 32), and absolute GW recharge (line 80). What are the differences between these terms?
- In line 87: The phrasing of this question is unclear. What do you mean by "significant change"? What does "overall spring discharge" refer to? Are you discussing low and high flow conditions of what specifically?
- In line 88: To which karst storage properties are you referring? How can a property be sensitive to climate change?
- In its current form, the introduction does not clearly convey the study's objectives. Please restructure the introduction to improve readability and ensure it includes all the necessary information to understand the objectives. Additionally, it is important to cite studies that have conducted similar research, so that you can clearly highlight the novel aspects of your study and conclusions.
- Line 125: Why is it important to mention that some of the investigated areas lie along climate zone boundaries? This detail seems disconnected from the rest of the paragraph. Why is this not reflected in Table 1?
- Line 145: You mention that you used the R package BFI. Why did you choose this method over others? How does this package separate quick from slow flow components? How does the package calculate the BFI? How much do the results and conclusions of your study change if the value of alpha is different?
- Could you explain in Section 2.3 the meaning of a positive and negative Sen Slope? How are these slopes calculated?
- Line 160: Why do you analyze the information from only one cell? Is it possible that karst capture areas are larger in extent than 0.1 degrees (about 10 kilometers)? If so, what are the implications of this for your study?
- Line 177: With your data, you could evaluate how close or far your discharge distributions are from normal. I would recommend calculating this instead of simply assuming that your data is not normal.
- Line 183: You initially mentioned that 6 springs exhibit significant changes. Then, explained that 3 of them exhibit negative and 2 positive variation. What kind of variation does the remaining spring exhibit—positive or negative?
- Why do you include figure 2a if you never mention it in the text?
- Line 222: How can you justify this assumption? To me, it is critical that you disregard the effects of temperature variability when you are trying to quantify the impact of climate variables on karst spring discharge.
- The label for Figure 4 refers to subplots a and b, but only one figure with an inset is presented. Following Section 3.3 is complicated; it is unclear which figure corresponds to the 20-year period and which to the 40-year period.
- You begin the discussion section by arguing that precipitation trends are rare and hardly explain variability in karst discharge. However, I think the temporal scale should be considered. Daily precipitation trends might be rare, but hourly precipitation trends could be common and significant. Could you elaborate on this?
- In Section 4.1, you mention that karst spring flow follows a similar pattern to that depicted by streamflow. If precipitation variability is not important, how can you justify this?
- English usage can be improved throughout the entire document, especially in the introduction.
Please also consider the following minor comments:
Line 27: global change of what? Climate change?
Line 32: Is it possible to directly measure integrated (over space) recharge at regional scale?
Line 51: Why GW recharge “needs” to be divided into concentrated and diffuse processes?
Line 58: Efficient in which sense?
Line 73: Correlated to what?
Line 75: Which authors?
Line 84: Multi-decadal
Line 100: Which quantiles?
Line 112: How is this information combined?
Lines 118-122: Check correct use of word “respectively”. Do this in the rest of the document too.
Line 118: It is written “No further pre-processing was done prior to the analysis”. It is not clear what preprocessing you applied to the data. Do you refer to the data selection?
Also, you mentioned that data is available for four countries. Maybe it is a good idea to write their names, given that in Table 1 you only list abbreviations.
Line 169: Why the spring discharge?
Line 232: How many? Could you add this information to the figure?
Figure 1 label: I see orange symbols.
Citation: https://doi.org/10.5194/egusphere-2024-2078-RC1 - AC2: 'Reply on RC1', Markus Giese, 08 Nov 2024
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RC2: 'Comment on egusphere-2024-2078', Anonymous Referee #2, 08 Oct 2024
In this study, the authors use observational data of precipitation, temperature, and discharge of more than 50 karst springs spatially distributed over Europe to give a continental overview of changes in groundwater resources in karst areas. They perform a trend analysis using Mann-Kendall and Sen’s slope on two different periods of 20 and 40 years, stating that long-term trends in spring discharge follow the general pattern of river discharge found in literature, while the last 20 years deviate from this behavior, mainly influenced by the temperature increase. Possible process changes were assessed by analyzing also trends in high and low flow values, and structural differences of the systems were considered by using two indices related to the storage and inertia of the system. The results of the analysis of the observed trend of hydroclimatic and hydrological variables are discussed with respect to the indirect influence of other drivers such as changes in land use or land cover, specific regional conditions but also changes in processes related to groundwater recharge and storage, providing insights for assessing groundwater recharge in the past and in the future.
The scientific contribution of this paper falls within the scope of Hydrology and Earth System Sciences. The results are discussed in an appropriate and balanced way; the paper is well-written with a clear and well-organized structure.
I suggest considering just some minor revisions:
- A more mathematical and detailed explanation of how the Mann-Kendall test and Sen’s slope are performed could be included in paragraph 2.3 to help the reader better interpret the results of the study.
- The final connection to modeling approaches provided in the conclusions is not clear: can you explain better how these results impact the discharge modeling?
- The visualization of the results is crucial and I found all the figures suitable to convey the different messages about trends, changes, and relationships in the observed variables. However, I suggest improving the selection of the markers to make the plots more effective, clear, and straightforward to interpret. In particular, I have difficulties identifying bold symbols.
- Line 162: clarify the procedure reformulating the sentence “Daily values of precipitation and temperature were accumulated respective averaged to obtain monthly and seasonal values.”
- Fig. 4: the caption refers to panels a) and b) but in the figure no label is provided to identify the panels.
Citation: https://doi.org/10.5194/egusphere-2024-2078-RC2 - AC1: 'Reply on RC2', Markus Giese, 08 Nov 2024
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