the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Sep 2023
Technical Note: two-component Electrical Conductivity-based hydrograph separaTion employing an EXPonential mixing model (EXPECT) provides reliable high temporal resolution young water fraction estimates in three small Swiss catchments
Abstract. The young water fraction represents the fraction of water molecules in a stream that have entered the catchment relatively recently, typically within 2–3 months. It can be reliably estimated in spatially heterogeneous and nonstationary catchments from the amplitude ratio of seasonal isotope (δ18O or δ2H) cycles of streamwater and precipitation, respectively. It has been found that young water fractions increase with discharge, thus reflecting increased direct runoff with wetter catchment conditions. This so-called discharge sensitivity of the young water fraction (S*d) can be useful for describing and comparing catchments’ hydrological behaviour; however, the estimation of S*d can be highly uncertain and unreliable when the streamwater isotope data are sparse and don’t capture the entire flow regime.
Here, we present a new method that can increase the temporal resolution of the young water fraction estimates, and thus better constrain the estimation of S*d. Our so-called EXPECT method is built upon three key assumptions: 1) the two-component hydrograph separation technique can be used to obtain the portion of young water and old water in a stream by considering EC as a proxy of the water age, 2) the EC value of the young water endmember (ECyw) is lower than that of the old water endmember (ECow), and 3) the mixing of young water and old water fractions is described assuming an exponential decay of electrical conductivity with increasing young water fraction. We calibrate the two endmembers, ECyw and ECow, by constraining the time-weighted and flow-weighted average young water fraction achieved with hydrograph separation to be equal to the same quantities obtained from seasonal isotope cycles.
We test the EXPECT method with data from three small experimental catchments in the Swiss Alptal valley by using two different temporal resolutions of Q and EC data: sampling-resolution (i.e., we only consider Q and EC measurements during dates of isotope sampling) and daily-resolution. By leveraging high-resolution and low-cost EC measurements and bi-weekly isotope data, the EXPECT method has provided reliable young water fraction estimates at bi-weekly and daily resolution, from which S*d could be determined with higher accuracy compared to the existing method that uses only bi-weekly isotope data. For proper use of the EXPECT method, we further highlight its main limitations that may vary in their relevance depending on the characteristics of the catchments under study.
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RC1: 'Comment on egusphere-2023-1797', Anonymous Referee #1, 05 Oct 2023
I appreciate the effort of authors in developing this interesting field. However, the assumptions of the proposed method are in my opinion incorrect:
- Hydrograph components, i.e. the event and pre-event water, do not represent young and old water fractions defined by the young water fraction concept.
- The conceptual problem in my opinion is that if the young water fraction is defined as a characteristic (metric) calculated from the seasonal variability of a tracer, it is attributed to the seasonal time scale. Thus, it is not meaningful to “improve” the temporal resolution of such a characteristic for finer scales (hydrographs) if the tracer streamflow data is sparse.
- EC is not a conservative tracer although it is generally true that the longer the water stays in the catchment, the more ions it contains. However, EC varies also with air (water) temperature despite temperature compensation employed in the sensors and if an event water meets very soluble minerals, it can have high EC as well.
- Exponential relationship between EC and discharge (increasing event water contribution) appears when the long-term data (combining many events) is analysed. However, measurements I have seen did not document such a relationship for individual runoff events (hydrographs). While generally the EC does decrease with increasing discharge for many events, it does not happen so for each event. In fact, in my experience a decrease in EC with increase in discharge was evident only during larger events. Thus, it is in my opinion incorrect to apply the general relationship emerging from the long-term data for all individual events.
- I agree that catchments with a greater number of runoff events (i. e. more frequent higher discharge) likely have higher young water fraction. Therefore, I appreciate the idea of calibrating separated event/pre-event water components for all the events of period used to determine the young water fractions to estimate the proportion of “young water fractions” (see comment 2) in individual events. However, as I argue above, the EC does not provide the reliable information. Furthermore, I think that isotopes (atoms of elements forming water) and EC (ions of compounds saluted in water) may not provide the same (compatible) information about the streamwater sources. Last but not least, catchment runoff response is nonlinear. It is therefore questionable to assume that the young water fractions of individual events are proportional to distribution of the event water fractions.
I regret to conclude that, based on the above, I cannot recommend the manuscript for publication.
Citation: https://doi.org/10.5194/egusphere-2023-1797-RC1 -
AC1: 'Reply on RC1', Alessio Gentile, 19 Oct 2023
Dear referee #1,
We thank you for your appreciation about our effort in developing a multi-tracer method for advancing the knowledge of young water in catchment hydrology. We thank you for your comments, also if, in our opinion, the method has not been fully understood. We think that many of the criticisms made should be supported by scientific results in the literature to bring substance to the discussion. Moreover, many problems that have been listed have already been addressed by the authors along the manuscript.
Please find in the Supplement a point-by-point response to your comments in which we cite published works that support our assumptions and methodology.
Perhaps, we will have to clarify some points better and we will be glad to do so in the revised version of the manuscript if we receive a positive editor’s response.
Accordingly, we kindly ask to reconsider the decision of not-recommending the manuscript for publication in light of our answers.
With kind regards,
The Authors
-
RC2: 'Comment on egusphere-2023-1797', Anonymous Referee #2, 29 Oct 2023
This article presents an interesting method for estimating the young water fraction based on high-resolution EC measurements. My only two major concerns are: 1) the authors may consider providing more evidence or referencing literature to support their three main assumptions for the method; and 2) the authors could discuss how their method can be applied to other basins beyond their experimental watersheds. I also have some smaller comments as follows:
-
Lines 52-55, readers may seek more detailed descriptions for the terms 'unweighted,' 'flow-weighted,' and 'time-weighted.'
-
Lines 85-86, what do you mean by the 'uncertainty of the discharge sensitivity of the young water fraction'?
-
Table 2, are the numbers of 18O samples and EC samples the same?
-
Eqs. 2.1-2.2, I would appreciate more details on the estimation of As, A*s and Ap.
-
Figure 5, could you explain what Qmed and Q50/50 represent?
-
Figure 6, is the variable snow depth represented as HS in the figure? Please specify the years in each of the panels.
-
Figure 7, why not include a scatter plot for Fyw and P, which might better illustrate the correlation?
-
Line 395, 'significantly reduced the uncertainty of'—how can we observe this reduction in uncertainty from the results section? Please provide more details in the text.
Citation: https://doi.org/10.5194/egusphere-2023-1797-RC2 -
AC3: 'Reply on RC2', Alessio Gentile, 28 Nov 2023
Dear Anonymous referee #2,
Thank you for your care during your reading of the manuscript, your positive remarks and your comments that will help to improve the work. Please, find here attached the responses to all your comments.
We will take into account all your constructive feedback in the revised version of the manuscript once we receive the editor’s response.
With kind regards,
The Authors
-
-
RC3: 'Comment on egusphere-2023-1797', Anonymous Referee #1, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1797/egusphere-2023-1797-RC3-supplement.pdf
-
AC2: 'Reply on RC3', Alessio Gentile, 28 Nov 2023
Dear Anonymous referee #1,
We thank you very much for your reply to our comments that further stimulates the discussion. We are pleased to note that the discussion has solved some possible misunderstandings and brought constructive comments and feedback to our manuscript.
Please, find below a point-by-point response to your comments.
We will incorporate all your constructive feedback in the revised version of our manuscript if we receive a positive editor’s response.
Sincerely,
The Authors
-
AC2: 'Reply on RC3', Alessio Gentile, 28 Nov 2023
Status: closed
-
RC1: 'Comment on egusphere-2023-1797', Anonymous Referee #1, 05 Oct 2023
I appreciate the effort of authors in developing this interesting field. However, the assumptions of the proposed method are in my opinion incorrect:
- Hydrograph components, i.e. the event and pre-event water, do not represent young and old water fractions defined by the young water fraction concept.
- The conceptual problem in my opinion is that if the young water fraction is defined as a characteristic (metric) calculated from the seasonal variability of a tracer, it is attributed to the seasonal time scale. Thus, it is not meaningful to “improve” the temporal resolution of such a characteristic for finer scales (hydrographs) if the tracer streamflow data is sparse.
- EC is not a conservative tracer although it is generally true that the longer the water stays in the catchment, the more ions it contains. However, EC varies also with air (water) temperature despite temperature compensation employed in the sensors and if an event water meets very soluble minerals, it can have high EC as well.
- Exponential relationship between EC and discharge (increasing event water contribution) appears when the long-term data (combining many events) is analysed. However, measurements I have seen did not document such a relationship for individual runoff events (hydrographs). While generally the EC does decrease with increasing discharge for many events, it does not happen so for each event. In fact, in my experience a decrease in EC with increase in discharge was evident only during larger events. Thus, it is in my opinion incorrect to apply the general relationship emerging from the long-term data for all individual events.
- I agree that catchments with a greater number of runoff events (i. e. more frequent higher discharge) likely have higher young water fraction. Therefore, I appreciate the idea of calibrating separated event/pre-event water components for all the events of period used to determine the young water fractions to estimate the proportion of “young water fractions” (see comment 2) in individual events. However, as I argue above, the EC does not provide the reliable information. Furthermore, I think that isotopes (atoms of elements forming water) and EC (ions of compounds saluted in water) may not provide the same (compatible) information about the streamwater sources. Last but not least, catchment runoff response is nonlinear. It is therefore questionable to assume that the young water fractions of individual events are proportional to distribution of the event water fractions.
I regret to conclude that, based on the above, I cannot recommend the manuscript for publication.
Citation: https://doi.org/10.5194/egusphere-2023-1797-RC1 -
AC1: 'Reply on RC1', Alessio Gentile, 19 Oct 2023
Dear referee #1,
We thank you for your appreciation about our effort in developing a multi-tracer method for advancing the knowledge of young water in catchment hydrology. We thank you for your comments, also if, in our opinion, the method has not been fully understood. We think that many of the criticisms made should be supported by scientific results in the literature to bring substance to the discussion. Moreover, many problems that have been listed have already been addressed by the authors along the manuscript.
Please find in the Supplement a point-by-point response to your comments in which we cite published works that support our assumptions and methodology.
Perhaps, we will have to clarify some points better and we will be glad to do so in the revised version of the manuscript if we receive a positive editor’s response.
Accordingly, we kindly ask to reconsider the decision of not-recommending the manuscript for publication in light of our answers.
With kind regards,
The Authors
-
RC2: 'Comment on egusphere-2023-1797', Anonymous Referee #2, 29 Oct 2023
This article presents an interesting method for estimating the young water fraction based on high-resolution EC measurements. My only two major concerns are: 1) the authors may consider providing more evidence or referencing literature to support their three main assumptions for the method; and 2) the authors could discuss how their method can be applied to other basins beyond their experimental watersheds. I also have some smaller comments as follows:
-
Lines 52-55, readers may seek more detailed descriptions for the terms 'unweighted,' 'flow-weighted,' and 'time-weighted.'
-
Lines 85-86, what do you mean by the 'uncertainty of the discharge sensitivity of the young water fraction'?
-
Table 2, are the numbers of 18O samples and EC samples the same?
-
Eqs. 2.1-2.2, I would appreciate more details on the estimation of As, A*s and Ap.
-
Figure 5, could you explain what Qmed and Q50/50 represent?
-
Figure 6, is the variable snow depth represented as HS in the figure? Please specify the years in each of the panels.
-
Figure 7, why not include a scatter plot for Fyw and P, which might better illustrate the correlation?
-
Line 395, 'significantly reduced the uncertainty of'—how can we observe this reduction in uncertainty from the results section? Please provide more details in the text.
Citation: https://doi.org/10.5194/egusphere-2023-1797-RC2 -
AC3: 'Reply on RC2', Alessio Gentile, 28 Nov 2023
Dear Anonymous referee #2,
Thank you for your care during your reading of the manuscript, your positive remarks and your comments that will help to improve the work. Please, find here attached the responses to all your comments.
We will take into account all your constructive feedback in the revised version of the manuscript once we receive the editor’s response.
With kind regards,
The Authors
-
-
RC3: 'Comment on egusphere-2023-1797', Anonymous Referee #1, 07 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1797/egusphere-2023-1797-RC3-supplement.pdf
-
AC2: 'Reply on RC3', Alessio Gentile, 28 Nov 2023
Dear Anonymous referee #1,
We thank you very much for your reply to our comments that further stimulates the discussion. We are pleased to note that the discussion has solved some possible misunderstandings and brought constructive comments and feedback to our manuscript.
Please, find below a point-by-point response to your comments.
We will incorporate all your constructive feedback in the revised version of our manuscript if we receive a positive editor’s response.
Sincerely,
The Authors
-
AC2: 'Reply on RC3', Alessio Gentile, 28 Nov 2023
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