Streamflow Reconstructions Using Tree-Ring-Based Paleo Proxies for the Sava River Basin (Slovenia)

Round 1

Reviewer 1 Report

Comments and suggestions for authors

Streamflow Reconstructions using Tree-ring based Paleo Proxies for the Sava River Basin (Slovenia)

Manuscript ID: hydrology-2433692

 

Summary

 

The subject tackled by the manuscript is novel, interesting and in addition it is fitting well in the scope of the Hydrology journal. The issues taken up by the authors are very topical in the context of dendroclimatology. It has a clear layout and appropriate methodology. However, I suggest modifying the Abstract part, providing the more comprehensive and most important information about the manuscript as are the main results of the presented study. This information needs to be included in the Abstract. Please describe in detail the results achieved in this section to make the Abstract clearer and to attract the readers. I am sure that it will improve the manuscript's attractiveness.

Author Response

Reviewer #1

The subject tackled by the manuscript is novel, interesting and in addition it is fitting well in the scope of the Hydrology journal. The issues taken up by the authors are very topical in the context of dendroclimatology. It has a clear layout and appropriate methodology. However, I suggest modifying the Abstract part, providing the more comprehensive and most important information about the manuscript as are the main results of the presented study. This information needs to be included in the Abstract. Please describe in detail the results achieved in this section to make the Abstract clearer and to attract the readers. I am sure that it will improve the manuscript's attractiveness.

We appreciate the reviewer’s support of the manuscript. The authors agree and appreciate the reviewer identifying the need to include important information regarding low flow (i.e., droughts) and high flows (i.e., pluvials) in the abstract. Please refer to the revised manuscript as the authors have added information to improve the manuscript.

Revised abstract: The reconstructions were evaluated, and both low flow (i.e., drought) and high flow (i.e., pluvial) periods were identified for various filters (five-year to 30-year). When evaluating the most recent ~20 years (2000 to present), multiple low flow (drought) periods were identified. For various filters (five-year to 15-year), the 2003 end-year consistently ranked as one of the lowest periods while the 21-year period ending in 2012 was the lowest flow period in the ~2,000-year reconstructed-observed-historic period of record. The ~30-year period ending in 2020 was the lowest flow period since the early 6^th century. A decrease in pluvial (wet) periods was identified in the observed-historic record when compared to the paleo record, again confirming an apparent decline in streamflow. Given the increased activities (construction of water control structures) impacting the Sava River, the results provide important information to water managers and planners.

Reviewer 2 Report

Authors are commended for a very well written and presented paper. The research and material should be very helpful to both researchers and water planners as it supplements the knowledge base of using tree-rings to extend use of historical hydrology.

The first comment below is to provide some context to a reader who may not be familiar with area hydrology. The remainder are suggested mainly editorial to improve readability of the paper.

Line 112: The Section could benefit a general reader not familiar with hydrology in Slovenia if there was a summary on the precipitation distribution (monthly and magnitudes) during an average year. i.e., what is the precipitation season (months), and average monthly values.

Line 185: In Table 1, “…(scPdSI proxy cell number in parenthesis)” does not show. Did the authors mean the “#122”, for example, as the station number? Maybe labelling the station as S₁₂₂ rather than #122 would be clearer to a reader.

Lines 186 and 188: Differentiating the blue and green colored lines is somewhat difficult. Blue and red colors, for example are easier to see.

Line 203: In Figure 4, the two timeseries (red and black) overlap and are somewhat difficult to distinguish.  Would adding a scatter plot help identify any biases?

Lines 234-235: Same as the comment above for Table 1 (Line 185).

Author Response

Reviewer #2

Authors are commended for a very well written and presented paper. The research and material should be very helpful to both researchers and water planners as it supplements the knowledge base of using tree-rings to extend use of historical hydrology.

The authors appreciated the support from the reviewer. Thank you for the kind words.

The first comment below is to provide some context to a reader who may not be familiar with area hydrology. The remainder are suggested mainly editorial to improve readability of the paper.

Line 112: The Section could benefit a general reader not familiar with hydrology in Slovenia if there was a summary on the precipitation distribution (monthly and magnitudes) during an average year. i.e., what is the precipitation season (months), and average monthly values.

Agree. The authors added the following line in the manuscript: The SRB average annual precipitation is ~1,350 mm with May through October being the wettest months (~100 to ~150 mm per month) and the winter (December through March) being the driest months (~65 to ~100 mm per month).

Line 185: In Table 1, “…(scPdSI proxy cell number in parenthesis)” does not show. Did the authors mean the “#122”, for example, as the station number? Maybe labelling the station as S122 rather than #122 would be clearer to a reader.

Agree. To address the reviewer’s comment, we have added scPDSI cell numbers (#) retained in the reconstruction models to Figure 3.

Lines 186 and 188: Differentiating the blue and green colored lines is somewhat difficult. Blue and red colors, for example are easier to see.

We appreciate the reviewer’s comment. We selected blue to represent observed flow, green (as in trees or vegetation) to represent the tree-ring based scPDCI proxy reconstruction, and black to represent the bias-corrected reconstructed flows.

Line 203: In Figure 4, the two timeseries (red and black) overlap and are somewhat difficult to distinguish.  Would adding a scatter plot help identify any biases?

We appreciate the reviewer’s comment. Yes, the challenge with this figure is the length (~2,000 years) of the reconstruction. We hope the reviewer will accept that the figure shows that the temporal behavior of the two gauges is very similar and that the figure is adequate.

Lines 234-235: Same as the comment above for Table 1 (Line 185).

Agree. To address the reviewer’s comment, we have added scPDSI cell numbers (#) retained in the reconstruction models to Figure 3.

Reviewer 3 Report

This is an interesting contribution which is worth publishing. However, content should be better integrated into previous studies. How robust is the statement in lines 33-35, that last 100 years were driest of last 1000 years? This needs to be compared with other palaeohydrological papers, e.g. Ionita et al. 2021:  https://www.nature.com/articles/s43247-021-00130-w

Additional comments:

Abstract: need to mention length of reconstruction in years 

 

line 63: correlation of what?

 

Material and Methods: Why was data from the Global Streamflow Database not used for calibration?

https://www.bafg.de/GRDC/EN/Home/homepage_node.html

 

Introduction should discuss natural variability of precipitation and hence streamflow. See Müller-Plath et al. for a good overview: https://www.nature.com/articles/s41598-022-14028-w

Rainfall in Slovenia is e.g. affected by the West Mediterranean Oscillation Index (WeMOI) and the North Sea Caspian Pattern (NCP)

 

Missing literature that should be integrated into teh study:

Kern et al. 2016: https://www.sciencedirect.com/science/article/abs/pii/S1040618216000306

 

Figure 4: Could the very low streamflow in the last 100 years be an artefact, e.g. due to bias correction? There is nothing special for the last 100 years in figure 5. Please also discuss alternative interpretations.

Author Response

Reviewer #3

This is an interesting contribution which is worth publishing.

The authors appreciate the support of the reviewer. Thank you!

However, content should be better integrated into previous studies. How robust is the statement in lines 33-35, that last 100 years were driest of last 1000 years? This needs to be compared with other palaeohydrological papers, e.g. Ionita et al. 2021:  https://www.nature.com/articles/s43247-021-00130-w

Agree. The authors added the reference and reviewed the suggested paper. This excellent research identified two drought periods (~15^th century and late 18^th / early 19^th century). Our research, when evaluating longer periods (~20 to 30-years) found the early 6^th century to be the driest period. A possible explanation of these differences is the current study focused on a specific region (upper Sava River) and seasonal (AMJJAS) streamflow. Per the reviewer’s kind suggestion, the authors added this in the revised manuscript.

Additional comments:

Abstract: need to mention length of reconstruction in years

Agree. This was added to the manuscript.

line 63: correlation of what?

Agree. The correlation of Danube River (DR) streamflow and the newly processed tree-ring chronology.

The highest correlation value relating DR streamflow to the newly developed tree-ring chronology was for the November (previous year) to July (current year) streamflow season [4].

 Material and Methods: Why was data from the Global Streamflow Database not used for calibration?

https://www.bafg.de/GRDC/EN/Home/homepage_node.html

Great question. When available, we prefer using streamflow gauge records in lieu of this database. No “hate” at all on this data ? We have used it in past studies but the streamflow gauge records in Slovenia were robust.

 Introduction should discuss natural variability of precipitation and hence streamflow. See Müller-Plath et al. for a good overview: https://www.nature.com/articles/s41598-022-14028-w. Rainfall in Slovenia is e.g. affected by the West Mediterranean Oscillation Index (WeMOI) and the North Sea Caspian Pattern (NCP).

Agree. We added the reference and the following to the Introduction. The SRB average annual precipitation is ~1,350 mm with May through October being the wettest months (~100 to ~150 mm per month) and the winter (December through March) being the driest months (~65 to ~100 mm per month) and past research [2] revealed SRB precipitation is influenced by the West Mediterranean Oscillation Index (WeMOI) and the North Sea Caspian Pattern (NCP).

Missing literature that should be integrated into teh study:

Kern et al. 2016: https://www.sciencedirect.com/science/article/abs/pii/S1040618216000306

Agree. A statement was added to the Introduction and the reference was added to the revised manuscript.

Figure 4: Could the very low streamflow in the last 100 years be an artefact, e.g. due to bias correction? There is nothing special for the last 100 years in figure 5. Please also discuss alternative interpretations.

Great observation. If you go back and look at Figure 2b (Catez gauge) and focus on the last, say, ~20-years, the bias-corrected streamflow (black line) exceeds (as in, results in a lower value) when compared to the observed flow (blue line) only for ~1993. For the other, multiple, low flows years (droughts) during this time-frame, the bias-corrected flows are “above” the observed-historic flow. So, it appears the bias-correction, when focusing on recent low-flows (droughts), does not “over-estimate” so to speak the droughts. Note, Figure 5 does not include the most recent 100-years (only reconstructed flows). The observed-historic extremes (blue and red lines) are provided to compare the reconstructed flows to observed-historic.