Altitudinal Control of Isotopic Composition and Application in Understanding Hydrologic Processes in the mid Merced River Catchment, Sierra Nevada, California, USA

Liu, Fengjing; Conklin, Martha H.; Shaw, Glenn D.

doi:https://doi.org/10.5194/hess-2023-230

Preprints

https://doi.org/10.5194/hess-2023-230

Preprints

02 Nov 2023

| 02 Nov 2023

Status: a revised version of this preprint was accepted for the journal HESS and is expected to appear here in due course.

Altitudinal Control of Isotopic Composition and Application in Understanding Hydrologic Processes in the mid Merced River Catchment, Sierra Nevada, California, USA

Fengjing Liu, Martha H. Conklin, and Glenn D. Shaw

Abstract. Mountain snowpack has been declining and more precipitation has fallen as rainfall than snowfall, particularly in the US West. Isotopic composition in stream water, springs, groundwater, and precipitation was examined to understand the impact of declining snowpack on hydrologic processes in the mid Merced River catchment (1,873 km²), Sierra Nevada, California, USA. Mean isotopic values in small tributaries (catchment area < 122 km²), rock glacier outflows and groundwater from 2005–2008 were strongly correlated with mean catchment elevation (R² = 0.96 for δ²H, n = 16, p < 0.001), with an average isotopic lapse rate of -1.9 ‰/100 m for δ²H and -0.22 ‰/100 m for δ¹⁸O in meteoric water. The lapse rate did not change much over seasons and was not strongly affected by isotopic fractionation. A catchment-characteristic isotopic value was thus established for each sub-catchment based on the relation between isotopic composition and the mean catchment elevation to elucidate hydrometeorologic and hydrologic processes. Compared to Tenaya Creek without water falls, flow and flow duration of Yosemite Creek are much more sensitive to temperature increase due to a strong evaporation effect caused by waterfalls, suggesting possible prolonged dry-up period of Yosemite Falls in the future. Groundwater in the Yosemite Valley (~900–1,200 m) was recharged primarily from the upper snow-rain transition zone (2,000–2,500 m), suggesting its strong vulnerability to shift in snow-rain ratio. The information gained from this study helps advance our understanding of hydrologic responses to climate change in snowmelt-fed river systems.

Received: 25 Sep 2023 – Discussion started: 02 Nov 2023

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Fengjing Liu, Martha H. Conklin, and Glenn D. Shaw

Status: closed

RC1:
'Comment on hess-2023-230', Anonymous Referee #1, 07 Dec 2023

General comments:
In the manuscript by Liu et al. the authors aimed at quantifying how changes in the snow-rain proportion affect stream flow and groundwater recharge in a snowmelt-fed river system. Study site was the mid Merced River catchment, representative for the central and southern Sierra Nevada.
In extensive field campaigns hydrologic and meteorological data of precipitation, snow depths, daily mean discharge were collected, as well as water samples from the main (Merced) River and tributaries, springs, groundwater, snowpits, and glacier melt. Isotope data were available from stream water, groundwater, and springs weekly to biweekly (to monthly) from 2005-2008.
With their comprehensive study they try to better understand the processes or factors that control the spatiotemporal variability of isotopic composition in precipitation, stream water, and groundwater and how such information could be used to advance the understanding of hydrometeorologic and hydrologic processes. The manuscript is very well structured and nicely written. The topic fits well to the scope of the journal and appears to be of interest in the catchment hydrology and alpine hydrology community.
Based on the hydrologic and meteorological data they could show that less snow and earlier snowmelt lead to a shift in peak river runoff toward late winter and early spring, away from summer when water demand is highest. Based on the isotope data, they implemented a catchment characteristic isotopic value (CCIV) in order to elucidate hydrometeorologic processes over seasons – an interesting approach which seems to be quite appropriate for snowmelt-fed catchments.
However, changes or systematic shifts in the snow-rain transition zone due to climate change couldn’t really be proved since the observation period was too short and further, during their 3-year observation period they stated that one of the years was very wet and one very dry. Therefore, only relying upon the data from these extreme years seems to be questionable or it might at least be difficult to draw reliable conclusions, especially long-term conclusions on climate change. It would thus be great to continue the time series in the future.

Specific Comments:
P7, Table 1: D-Ex data are missing – perhaps due to temperature issues for δ18-O during isotope analysis with the DLT-100?
P9, l. 196 please insert standard deviation after 1σ - in order to define this acronym in the first instance
P9, l. 246 DEM - define this acronym in the first instance
P9, l. 255 WY - define this acronym in the first instance
P16, l. 389 LMWL - be careful when establishing a LMWL based on snow samples because of isotopic fractionation.
P28, Figure 10b and text p. 29 l. 683ff: I’m not sure that evaporation in Yosemite is much stronger… True, it plotted further right but the slope seems to be steeper than for Tenaya.

Citation: https://doi.org/10.5194/hess-2023-230-RC1
- AC1: 'Reply on RC1', Fengjing Liu, 26 Dec 2023
  
  Thanks for your time reviewing our manuscript and general support on our study! We accepted most of your specific comments/suggestions for edits, which are not included in this discussion. Below are our responses to your primary concern.
  The future shift of more rainfall relative to snowfall was not part of this study, but the trend in the US West has been well documented by many other studies such as those cited in our manuscript (e.g., Mote et al., 2005; Knowles et al., 2006; Stewart et al., 2004). Under this backdrop, two objectives were specified, as described in the text, one to understand the factors that control spatial variation of isotopic values in stream flow and groundwater and the other, which depends on the first, to demonstrate the applications to improve our understanding in hydrology and hydrometeorology and implications to infer climate change impacts on stream flow and groundwater recharge using space-for-time concept. Our study aimed at dealing with a general impact, rather than with specific years. We agree that it would be invaluable to continue the monitoring and extend the time series of the data. With extended time series of the data, we may be able to examine trends in real time and quantify the actual rate of changes in groundwater recharge and evaporation effects.
  
  Citation: https://doi.org/10.5194/hess-2023-230-AC1
RC2:
'Comment on hess-2023-230', Anonymous Referee #2, 10 Dec 2023

The study conducted intensive sampling for isotopic compositions in precipitation, stream water and groundwater to help understand the hydrological processes in a typical snowmelt-fed catchment. The topic is very interesting and valuable and the manuscript is well organized.
However, my major concern for this study is that the objective is very vague. I couldn't catch what are the key points of the study. The elevational control of isotopic composition is a basic law for isotope hydrology and the phenomenon has been widely reported. The effect of evaporation effect makes stable isotope of streamflow changes among tributaries is not unusual. Thus, it should further focus on more specific issues and new findings or contributes to hydrology from the detailed samplings instead of presenting the form of the research as a form of case study. I suggest put more attention on the identification the recharge zones of groundwater and streamflow and their changes or the impact of snowpack decline on the recharge of groundwater and streamflow.
Specific comments：
Lines 5-10. The title is too long. “Altitudinal” should be “elevational”.
Lines 41-43 “flow and flow duration of Yosemite Creek are much more sensitive to temperature increase due to a strong evaporation effect caused by waterfalls, suggesting possible prolonged dry-up period of Yosemite Falls in the future.” The conclusion seems arbitrary.
The Section of Introduction seems too general and the part should be re-organized to review the state-of-the-art methods on identifying the streamflow and groundwater sources and recharge zones, or how the stable isotopes could be used to improve our understanding on the certain hydrological process in complex catchment.
Section 2. What’s annul runoff depth of the catchment and are there significant variation of precipitation and runoff depth with elevation? What is the temperature lapse rate? Besides, I suggest presenting the time series of precipitation. It may be added in Fig 2.
Figure 2. The annual cumulative precipitation is large at Gin Flat, but the snow depth is small. Is it a mistake?
Figure 4. I suggest using a different line type to show PSF to make the figure more clearly.
Lines 464-468. It's not clear how the evaporation effect correction is conducted.
In Figs 5 and 8, the significance test is needed.
Lines 682-685, the slope of lines for Tenaya Creek seems lower than Yosemite Creek in Fig 10. It’s hard to say the evaporative effect is more intensive in Yosemite Creek.

Citation: https://doi.org/10.5194/hess-2023-230-RC2
- AC2: 'Reply on RC2', Fengjing Liu, 26 Dec 2023
  
  Thanks for your time reviewing our manuscript and general support on our study! We accepted most of your specific comments/suggestions for edits, which are not included in this discussion. Below are our responses to your primary concern.
  We have two specific objectives, as described in the text, one to understand the factors that control spatial variation of isotopic values in stream flow and groundwater and the other, which depends on the first, to demonstrate what we can do to improve our understanding in hydrology and hydrometeorology. The two objectives were strongly connected. It is well known that elevation exerts a significant control on isotopic variation in a region with a significant relief, but that relationship is not always guaranteed. Particularly, the quality of that relationship is critical to determine how accurate the results are in hydrologic applications if the impacts of other factors are not properly considered. For example, how can we determine the isotope-elevation relationship (e.g., using precipitation samples vs stream samples as we discussed in the study)? Does isotopic fractionation affect the relationship? How does the relationship vary over seasons and years? How does evaporation affect the relationship? These are important questions we have to examine before we apply the relationship in our and any studies as discussed in the introduction. However, it is hard to put these specific questions especially in the introduction a priori. Rather, we adopted more general statements to open up the study in the introduction. We believe that in the revision we could add more details or cite some of the questions as examples to open up the study in the last paragraph of the introduction.
  
  Citation: https://doi.org/10.5194/hess-2023-230-AC2

Status: closed

RC1:
'Comment on hess-2023-230', Anonymous Referee #1, 07 Dec 2023

General comments:
In the manuscript by Liu et al. the authors aimed at quantifying how changes in the snow-rain proportion affect stream flow and groundwater recharge in a snowmelt-fed river system. Study site was the mid Merced River catchment, representative for the central and southern Sierra Nevada.
In extensive field campaigns hydrologic and meteorological data of precipitation, snow depths, daily mean discharge were collected, as well as water samples from the main (Merced) River and tributaries, springs, groundwater, snowpits, and glacier melt. Isotope data were available from stream water, groundwater, and springs weekly to biweekly (to monthly) from 2005-2008.
With their comprehensive study they try to better understand the processes or factors that control the spatiotemporal variability of isotopic composition in precipitation, stream water, and groundwater and how such information could be used to advance the understanding of hydrometeorologic and hydrologic processes. The manuscript is very well structured and nicely written. The topic fits well to the scope of the journal and appears to be of interest in the catchment hydrology and alpine hydrology community.
Based on the hydrologic and meteorological data they could show that less snow and earlier snowmelt lead to a shift in peak river runoff toward late winter and early spring, away from summer when water demand is highest. Based on the isotope data, they implemented a catchment characteristic isotopic value (CCIV) in order to elucidate hydrometeorologic processes over seasons – an interesting approach which seems to be quite appropriate for snowmelt-fed catchments.
However, changes or systematic shifts in the snow-rain transition zone due to climate change couldn’t really be proved since the observation period was too short and further, during their 3-year observation period they stated that one of the years was very wet and one very dry. Therefore, only relying upon the data from these extreme years seems to be questionable or it might at least be difficult to draw reliable conclusions, especially long-term conclusions on climate change. It would thus be great to continue the time series in the future.

Specific Comments:
P7, Table 1: D-Ex data are missing – perhaps due to temperature issues for δ18-O during isotope analysis with the DLT-100?
P9, l. 196 please insert standard deviation after 1σ - in order to define this acronym in the first instance
P9, l. 246 DEM - define this acronym in the first instance
P9, l. 255 WY - define this acronym in the first instance
P16, l. 389 LMWL - be careful when establishing a LMWL based on snow samples because of isotopic fractionation.
P28, Figure 10b and text p. 29 l. 683ff: I’m not sure that evaporation in Yosemite is much stronger… True, it plotted further right but the slope seems to be steeper than for Tenaya.

Citation: https://doi.org/10.5194/hess-2023-230-RC1
- AC1: 'Reply on RC1', Fengjing Liu, 26 Dec 2023
  
  Thanks for your time reviewing our manuscript and general support on our study! We accepted most of your specific comments/suggestions for edits, which are not included in this discussion. Below are our responses to your primary concern.
  The future shift of more rainfall relative to snowfall was not part of this study, but the trend in the US West has been well documented by many other studies such as those cited in our manuscript (e.g., Mote et al., 2005; Knowles et al., 2006; Stewart et al., 2004). Under this backdrop, two objectives were specified, as described in the text, one to understand the factors that control spatial variation of isotopic values in stream flow and groundwater and the other, which depends on the first, to demonstrate the applications to improve our understanding in hydrology and hydrometeorology and implications to infer climate change impacts on stream flow and groundwater recharge using space-for-time concept. Our study aimed at dealing with a general impact, rather than with specific years. We agree that it would be invaluable to continue the monitoring and extend the time series of the data. With extended time series of the data, we may be able to examine trends in real time and quantify the actual rate of changes in groundwater recharge and evaporation effects.
  
  Citation: https://doi.org/10.5194/hess-2023-230-AC1
RC2:
'Comment on hess-2023-230', Anonymous Referee #2, 10 Dec 2023

The study conducted intensive sampling for isotopic compositions in precipitation, stream water and groundwater to help understand the hydrological processes in a typical snowmelt-fed catchment. The topic is very interesting and valuable and the manuscript is well organized.
However, my major concern for this study is that the objective is very vague. I couldn't catch what are the key points of the study. The elevational control of isotopic composition is a basic law for isotope hydrology and the phenomenon has been widely reported. The effect of evaporation effect makes stable isotope of streamflow changes among tributaries is not unusual. Thus, it should further focus on more specific issues and new findings or contributes to hydrology from the detailed samplings instead of presenting the form of the research as a form of case study. I suggest put more attention on the identification the recharge zones of groundwater and streamflow and their changes or the impact of snowpack decline on the recharge of groundwater and streamflow.
Specific comments：
Lines 5-10. The title is too long. “Altitudinal” should be “elevational”.
Lines 41-43 “flow and flow duration of Yosemite Creek are much more sensitive to temperature increase due to a strong evaporation effect caused by waterfalls, suggesting possible prolonged dry-up period of Yosemite Falls in the future.” The conclusion seems arbitrary.
The Section of Introduction seems too general and the part should be re-organized to review the state-of-the-art methods on identifying the streamflow and groundwater sources and recharge zones, or how the stable isotopes could be used to improve our understanding on the certain hydrological process in complex catchment.
Section 2. What’s annul runoff depth of the catchment and are there significant variation of precipitation and runoff depth with elevation? What is the temperature lapse rate? Besides, I suggest presenting the time series of precipitation. It may be added in Fig 2.
Figure 2. The annual cumulative precipitation is large at Gin Flat, but the snow depth is small. Is it a mistake?
Figure 4. I suggest using a different line type to show PSF to make the figure more clearly.
Lines 464-468. It's not clear how the evaporation effect correction is conducted.
In Figs 5 and 8, the significance test is needed.
Lines 682-685, the slope of lines for Tenaya Creek seems lower than Yosemite Creek in Fig 10. It’s hard to say the evaporative effect is more intensive in Yosemite Creek.

Citation: https://doi.org/10.5194/hess-2023-230-RC2
- AC2: 'Reply on RC2', Fengjing Liu, 26 Dec 2023
  
  Thanks for your time reviewing our manuscript and general support on our study! We accepted most of your specific comments/suggestions for edits, which are not included in this discussion. Below are our responses to your primary concern.
  We have two specific objectives, as described in the text, one to understand the factors that control spatial variation of isotopic values in stream flow and groundwater and the other, which depends on the first, to demonstrate what we can do to improve our understanding in hydrology and hydrometeorology. The two objectives were strongly connected. It is well known that elevation exerts a significant control on isotopic variation in a region with a significant relief, but that relationship is not always guaranteed. Particularly, the quality of that relationship is critical to determine how accurate the results are in hydrologic applications if the impacts of other factors are not properly considered. For example, how can we determine the isotope-elevation relationship (e.g., using precipitation samples vs stream samples as we discussed in the study)? Does isotopic fractionation affect the relationship? How does the relationship vary over seasons and years? How does evaporation affect the relationship? These are important questions we have to examine before we apply the relationship in our and any studies as discussed in the introduction. However, it is hard to put these specific questions especially in the introduction a priori. Rather, we adopted more general statements to open up the study in the introduction. We believe that in the revision we could add more details or cite some of the questions as examples to open up the study in the last paragraph of the introduction.
  
  Citation: https://doi.org/10.5194/hess-2023-230-AC2

Fengjing Liu, Martha H. Conklin, and Glenn D. Shaw

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Short summary

Mountain snowpack has been declining and more precipitation falls as rain than snow. Using stable isotopes, we found flows and flow duration in Yosemite Creek are most sensitive to climate warming due to strong evaporation of waterfalls, potentially lengthening the dry-up period of water falls in summer and negatively affecting tourism. Groundwater recharge in Yosemite Valley is primarily from the upper snow-rain transition (2,000–2,500m) and very vulnerable to shift in the snow-rain ratio.


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