the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Sep 2023
New water fractions and their relationships to climate and catchment properties across Alpine rivers
Marius G. Floriancic
Michael P. Stockinger
James W. Kirchner
Christine Stumpp
Abstract. The Alps are a key water resource for central Europe, providing water for drinking, agriculture, and hydropower production. Thus, understanding runoff generation processes of Alpine streams is important for sustainable water management. It is currently unclear how much streamflow is derived from old water stored in the subsurface, versus more recent precipitation that reaches the stream via near-surface quick flow processes. It is also unclear how this partitioning varies across different Alpine catchments in response to hydroclimatic forcing and catchment characteristics. Here, we use stable water isotope time series in precipitation and streamflow to quantify the young water fractions Fyw (i.e., the fraction of water younger than approximately 2–3 months) and new water fractions Fnew (here, the fraction of water younger than one month) in streamflow from 32 Alpine catchments. We contrast these measures of water age between summer and winter and between wet and dry periods, and correlate them with hydroclimatic variables and physical catchment properties.
New water fractions varied from 9.6 % in rainfall-dominated catchments to 3.5 % in snow-dominated catchments (mean across all catchments = 7.1 %). Young water fractions were approximately twice as large (reflecting their longer time scale), varying from 17.6 % in rainfall-dominated catchments to 10.1 % in snow-dominated catchments (mean across all catchments = 14.3 %). New water fractions were negatively correlated with catchment size (Spearman rank correlation rS = 0.38), q95 baseflow (rS = -0.36), catchment elevation (rS = 0.37), total catchment relief (rS = -0.59), and the fraction of slopes steeper 40° (rS = -0.48). Large new water fractions, implying faster transmission of precipitation to streamflow, are more prevalent in small catchments, at low elevations, with small elevation gradients, and with large forest cover (rS = 0.36). New water fractions averaged 3.3 % following dry antecedent conditions, compared to 9.3 % after wet antecedent conditions. Our results quantify how hydroclimatic and physical drivers shape the partitioning of old and new waters across the Alps, thus indicating which landscapes transmit recent precipitation more readily to streamflow, and which landscapes tend to retain water over longer periods. Our results further illustrate how new water fractions may find relationships that remained invisible with young water fractions.
- Preprint
(2174 KB) - Metadata XML
-
Supplement
(463 KB) - BibTeX
- EndNote
Marius G. Floriancic et al.
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2023-1854', Anonymous Referee #1, 24 Oct 2023
Comments on ‘New water fractions and their relationships to climate and catchment properties across Alpine rivers’ by Floriancic et al., 2023.
General comments：Understanding runoff generation processes in high mountain catchments is important as it is water tower for providing water for drinking, agriculture, and hydropower production. The main findings in this manuscript reveal that Alpine rivers tend to have larger new water fractions at low elevations, in flatter terrain and while in smaller catchments with large forest cover. The findings are interesting to scientific community, however, is also controversial in different perspective. As claimed by the authors, there seems less studies linking new water fractions to hydroclimatic drivers and physical catchment properties across small to very large basins. We know that the application of chemistry tracing methods is usually underlain by many basic assumptions, for instance, applicants must account for the heterogeneity in rainfall isotope referring to Pinder and Jones (1969). Hence, the methods adopted as well as the conclusions in this manuscript should be carefully discussed to justify the rationality of the methods and the reliability of this conclusions.
Moreover, as there are cross-correlations between many catchment properties, and hydroclimates potential drivers, the explanations that which may be a first-order control on new water fractions should be further verified with more cautions as indicated by the authors. For example, the authors found that high fractions of new water (Fnew) were more likely in small catchments, at low elevations, with small total relief and larger forest cover, and following months with high precipitation. However, they also found that Fnew tended to decrease downstream, from smaller headwaters to larger river basins, in which it can be inferred that altitude, relief, slope gradient, and even precipitation (see in Ménégoz et al., 2020) all decrease with altitude from smaller headwaters to larger river basins in Alps. A reasonable explanation to the issue is that it is the impact of water storage in lakes and reservoirs, as well as the potential effects of anthropogenic flow regulation when moving from the headwaters downstream. But it seems a little bit contradiction in the context. So, the authors should provide more deepen and persuasive discussions for this key conclusion in the manuscript. I wonder whether water stored as snow or glaciers in high altitude basins have led to relatively lower Fnew in headwaters, which is quite inconsistent with our intuition about runoff generations. Cause in alpine basins more areas with naked rocks or thin soils could be observed in high altitude regions as erosion rates increase with local relief (Heimsath et al., 2012), thus, more rainfall directly drains to drainage networks as new fractions in high altitude areas?
In addition, Fyw as defined by the authors is the ratio of seasonal amplitudes of sinusoidal fits to precipitation and streamflow isotope time series. However, as shown in Figure 2, the seasonal fluctuations in streamflow isotopes among all the basins are quite similar. Therefore, Is Fyw only related to the corresponding fluctuations in precipitation isotopes? We have known that interannual variations of precipitation isotopes have a significant impact on the young water fraction (Gou et al., 2023; Dai et al., 2022), which raises doubts about the results. Could it be consistent in the results obtained by using isotopic data with different time series lengths? Furthermore, the sampling frequency of precipitation isotopes also has a significant impact on the young water fraction (Gallart et al., 2020; Stockinger et al., 2016). So, is it reliable to estimate Fyw through using monthly-scale data?
In general, the manuscript needs more deepen discussions, and the authors should provide more persuasive evidences to clarify the first order control of runoff generation across headwaters to down streams for credible conclusions.
Specific comments：
L means lines
L121-131: added how precipitation varies with altitude.
L140-150: the accuracy of the monthly gridded precipitation isotope reanalysis database Piso.AI should be evaluated before adopted in the study regions.
L258: The catchment areas range from 29 km2 to 103’946 km2. Such huge difference in area would lead to quite different patterns of rainfall and discharge distribution in different catchments which dramatically impact isotope fractions in downstream rivers, and weaken the basic assumption for isotope estimations, and there is especially large spatial heterogeneity of rainfall and discharge concentration across a catchment with larger areas. So how do you calculate monthly precipitation isotope particularly in catchments with larger area (e.g., >1000 km2)?
Technical corrections:
(1) In figure 1 Gauge names should be provided and drainage areas should also be added and listed in table 1.
References:
Dai, J., Zhang, X., Wang, L., Luo, Z., Wang, R., Liu, Z., ... & Guan, H. (2022). Seasonal isotopic cycles used to identify transit times and the young water fraction within the critical zone in a subtropical catchment in China. Journal of Hydrology, 612, 128138.
Gallart, F., Valiente, M., Llorens, P., Cayuela, C., Sprenger, M., & Latron, J. (2020). Investigating young water fractions in a small Mediterranean mountain catchment: both precipitation forcing and sampling frequency matter. Hydrological Processes, 34(17), 3618-3634.
Gou, J., Qu, S., Guan, H., Shi, P., Zhang, Z., Yang, H., ... & Han, X. (2023). Seasonal variation of transit time distribution and associated hydrological processes in a Moso bamboo watershed under the East Asian monsoon climate. Journal of Hydrology, 617, 128912.
Heimsath, A., DiBiase, R. & Whipple, K. Soil production limits and the transition to bedrock-dominated landscapes. Nature Geosci 5, 210–214 (2012). https://doi.org/10.1038/ngeo1380.
Ménégoz, M., Valla, E., Jourdain, N. C., Blanchet, J., Beaumet, J., Wilhelm, B., Gallée, H., Fettweis, X., Morin, S., and Anquetin, S.: Contrasting seasonal changes in total and intense precipitation in the European Alps from 1903 to 2010, Hydrol. Earth Syst. Sci., 24, 5355–5377, https://doi.org/10.5194/hess-24-5355-2020, 2020.
Pinder, G., Jones, J., 1969. Determination of the ground-water component of peak
discharge from the chemistry of total runoff. Water Resour. Res. 5 (2), 438–445.
Stockinger, M. P., Bogena, H. R., Lücke, A., Diekkrüger, B., Cornelissen, T., & Vereecken, H. (2016). Tracer sampling frequency influences estimates of young water fraction and streamwater transit time distribution. Journal of hydrology, 541, 952-964.
Citation: https://doi.org/10.5194/egusphere-2023-1854-RC1 -
CC1: 'Comment on egusphere-2023-1854', Jiri Svatos, 03 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1854/egusphere-2023-1854-CC1-supplement.pdf
-
CC2: 'Review on egusphere-2023-1854', Arnaud Jansen, 03 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1854/egusphere-2023-1854-CC2-supplement.pdf
-
CC3: 'Comment on egusphere-2023-1854', Rinske de Ronde, 05 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1854/egusphere-2023-1854-CC3-supplement.pdf
-
RC2: 'Comment on egusphere-2023-1854', Anonymous Referee #2, 13 Nov 2023
The work of Floriancic et al. (2023) addresses the estimation of new water fractions with the ensemble
hydrograph separation in 32 catchments to investigate their relationships to climate and catchment
properties across Alpine rivers. The work is novel since, according to my knowledge, there are no previous
studies that have related “new” water fractions with hydroclimatic and physical properties. Nevertheless,
there are some major issues that should be taken into consideration before considering the manuscript for
publication in HESS.
Main comments:
1) You have used monthly streamflow isotope for 12 Austrian sites and 8 Swiss stations. Moreover, you
have used monthly gridded precipitation isotopes from Nelson et al. 2021. It is well known that the
sampling resolution affect the young water fraction estimates (Gallart et al., 2020; Stockinger et al.,
2016). When you compare your results with those of previous studies you should include the effect
of sampling resolution since some discrepancies between your results and previous results can be
also due to the low sampling resolution of isotope data used in your study.
Additionally, by considering this sampling resolution you consider as “new” some water that is
younger than 1 month. This threshold age is of the same magnitude of the typical threshold age for
young water (2-3 months), as also reflected by similar Fyw and Fnew for some catchments very close to
the 1:1 line in Figure 3. Accordingly, I would underline this when you state that similar relationship to
catchment properties is obtained with young water fractions. This happens since they have a similar
threshold age. I would add in the conclusion that in future studies is necessary to investigate the
relationship between new water fraction and climate/catchment properties by using high-resolution
(e.g., daily) isotope data (and accordingly much lower threshold ages).
Please, specify in the manuscript title the age of new water: as soon as I read the title I thought that
you have investigated event water with a threshold age of the order of few days.
2) About your conceptual scheme reported in Figure 10. I think that it is biased from the use of very
large catchments. Indeed, some catchments you have used in your dataset, due to their large
extension, cover the entire landscape (reported in Fig. 10) from high to low elevations (in some
catchments the elevation difference is higher than 4000 m) including both steeper and gentler terrain,
largely heterogenous land cover etc. Thus, the fraction of new water you estimate is the result of
hydrological processes occurring across the entire catchment area. Indeed, Fnew could depend more
on processes occurring at higher elevations or more on processes occurring in plain areas (e.g., with
influence of natural and/or artificial lakes). In this regard I would suggest looking at your results by
not considering very large catchments and to better detail your conceptual scheme by subdividing it
in two/three different elevation ranges (e.g., a possible elevation threshold could be 1500 m a.s.l.
since previous studies (e.g., Ceperley et al. 2020) have found a change in hydrological regime above
this threshold).3) The database for geology is very coarse, but I guess nothing is available. Is it possible to better discuss its implications?
4) In the paper you cited in relation to quaternary deposits, Gentile et al. (2023) in this same journal, it has been discussed the relation between baseflow and Fyw. Can you compare their results with yours, although the different size of the catchments?
5) Less important (the paper structure is a personal choice) : you have chosen to separate results from discussion.
Nevertheless, I think that in many parts of the “Results” section you have also discussed the results.
Moreover, in the “Discussion” section there are many parts that are redundant since you recall the
results or methods sections. Accordingly, I would suggest changing the paper structure in “Results
and Discussion”. This will shorten the manuscript length and improve its readability.
Specific comments:
Figure 1: please, add the catchment areas on the Map by indicating the Site code.
Lines 216-221: Please, explain better how you have split your dataset. It is not fully clear to me.
Lines 320-321: Have you considered a delayed input or a direct input (von Freyberg et al. 2018) for estimating
new water in snow-dominated catchments? I think that should be discussed more about the role of snowpack
on the estimation of the new water and what are the consequences of choosing a direct or delayed input on
new water fraction (similarly to what von Freyberg et al. 2018 did for young water fraction).
Lines 505-508: Are there previous studies that support this result? If not, please provide an explanation to
this counterintuitive result.
Table 3: Add the fields “min elevation” and “max elevation”
Figure 5b: Please add the site code in panel b. If possible, think about a possible alternative representation of
Figure 5 since it is really intricate.
Figure 7 & Figure 8: I would suggest to represent rainfall-dominated, hybrid and snow-dominated catchments
by using three different colors. This could lead to additional insights to improve the discussion about these
results.Citation: https://doi.org/10.5194/egusphere-2023-1854-RC2
Marius G. Floriancic et al.
Marius G. Floriancic et al.
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 337 | 115 | 21 | 473 | 22 | 6 | 13 |
- HTML: 337
- PDF: 115
- XML: 21
- Total: 473
- Supplement: 22
- BibTeX: 6
- EndNote: 13
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1