the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Young and new water fractions in soil and hillslope waters
Abstract. The transport processes and corresponding times scales of water's infiltration into, and percolation through, the shallow subsurface are poorly understood. Here we characterize the transport of recent precipitation through a forested hillslope, using a continuous three-year record of O and H stable isotopes in precipitation, streamflow and soil waters from various depths. We found that the fractions of recent precipitation decreased with depth, both in waters extracted using suction-cup lysimeters and in waters extracted from bulk soil samples using cryogenic distillation. Not surprisingly, fractions of recent precipitation found in soils and streamflow were much larger with wet antecedent conditions, showing that wet landscapes can transmit recent precipitation quicker than dry landscapes. Approximately 18 % of streamflow was younger than 2–3 months, 11 % was younger than three weeks and 7 % was younger than one week; these new water fractions were similar to those seen in 20 to 80 cm deep soils. Mobile soil waters below 2 m depth contained much less recent precipitation (1.2±0.4 % younger than two weeks) than streamflow did (12.3±2.1 %), indicating that they are not the dominant source of streamflow. Instead, streamflow must be generated from a mixture of deep subsurface waters, with very little isotopic seasonality and short-term variability, and shallow soil waters, with more pronounced isotopic seasonality and short-term variability. This study illustrates how flow, storage, and mixing processes linking precipitation to streamflow and evapotranspiration can be constrained by measuring isotopic variability across different hillslope positions, subsurface depths, and time scales.
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RC1: 'Comment on egusphere-2024-437', Anonymous Referee #1, 19 Mar 2024
Overview: The authors presented a very interesting experimental design to estimate the water-ages of streamflow and soil-water. They gathered a high temporal resolution stable water isotope dataset over 3 years from different hydrological compartments like precipitation and streamflow as well as soil-water and ground water depth profiles. By fitting sinusoidal curves to the observed signatures and comparing the curves amplitude, they showed that the fraction of young and new water decreases with depth. They also showed that streamflow in their catchment was a mixture of deeper and more shallow soil water.
Overall, I found the paper pleasant to read and belief this is a great contribution to the field of catchment hydrology and travel-time assessment. The application of the developed method by Kirchner (2016a; 2016b) on such a high resolution stable water isotope dataset, especially with the focus on soil-water, is novel and will be a great addition to the current literature. I think the writing could be slightly improved. Please see comments below.
Line by line comments:
L 26: Leave out Switzerland and only focus on the global distribution of forests in your first introduction sentence
Figure 1: Add elevation contour lines so reader gets a better sense of the plots topography
Figure 2a: Could change streamflow from point to line plot
L 197: You introduced “piston flow” as “translatory flow”; decide on one term for consistency
Figure 3: While a great colorblind color-palette, the point transparency makes it hard to distinguish between points in (a); maybe try solid points?
Figure 3 Caption: “… in mobile (a) and bulk soil waters (b) of 10, 20, 40 and 80 cm depth and in deep mobile waters (c) collected in boreholes of 2 to 6 m depth.”
Also add that the percentages below the depths indicate Fyw, otherwise the reference in Line 238 makes no senseSection 3.3, Figure 4 as well as Table 1 repeat a lot of information, especially information that is plotted in Figure 4 is already described in text, plot and table. Here I would advocate for less redundancy and maybe push the table back to the supplementary material.
Literature
Kirchner, J. W. (2016a). Aggregation in environmental systems – Part 1: Seasonal tracer cycles quantify young water fractions, but not mean transit times, in spatially heterogeneous catchments. Hydrology and Earth System Sciences, 20(1), 279–297. https://doi.org/10.5194/hess-20-279-2016Kirchner, J. W. (2016b). Aggregation in environmental systems – Part 2: Catchment mean transit times and young water fractions under hydrologic nonstationarity. Hydrology and Earth System Sciences, 20(1), 299–328. https://doi.org/10.5194/hess-20-299-2016
Citation: https://doi.org/10.5194/egusphere-2024-437-RC1 -
RC2: 'Comment on egusphere-2024-437', Anonymous Referee #2, 25 Mar 2024
General comments:
The authors present an impressive isotope dataset collected from precipitation, different reservoirs of a hillslope’s soil water and the adjacent stream. They use these data to derive the young and new water fractions of soil and stream water. Such a detailed investigation is certainly of interest for the hydrological community and I recommend publication after moderate revision.
Formally, the authors decided to combine the result and discussion section which unfortunately tempted them to neglect a proper description of some of the results. This made recalculation of the presented young and new water fractions impossible. The authors should at least present the equations of the fitted sinusoidal cycles prior to discussing their amplitudes and phase shifts.
Further, I have issues with some of the presented isotope data. There appears to be a notable shift in bulk water isotope data which potentially must be attributed to extraction artefacts. While this may not necessarily affect the interpretation of sinusoidal amplitudes and phase shifts, I feel that the drawn conclusions pointing at certain seasons of replenishment need to be discussed again. Also, I am unhappy with the authors’ definition of the mobile water phase. It was obtained with 0.7 bar applied suction. This translates to a pF value of 2.8 which exceeds the commonly accepted pF 1.8-2.1 for threshold between mobile and immobile water. Additionally, the manuscript seems to suggest that mobile and bulk water are two separate reservoirs and not mobile water being part of the bulk water reservoir.
I provide a list of specific comments below.
Specific comments:
L14: “not surprisingly” seems to contradict the “poorly understood” from L11.
Figure 1: What is the slope and exact position of this site? Can it be considered representative of the entire stream catchment regarding hydrological response?
L135: 0.7 bar applied suction translates to pF 2.8. Why was such a high suction applied and why was it the same for all depths regardless of the different resulting water columns assuming that all sample collectors were placed on surface level?
L137: How much soil was sampled for extraction, i.e. from what depth ranges was soil obtained using the auger? Did that ranges match the lengths of the suction cup tips?
L143ff: What were the elevations of the boreholes or rather the observed groundwater tables relative to the adjacent stream water level? Did you observe a gradient and (how) did it change over time?
L154: Why was 95% recovery rate considered sufficient? Araguas-Araguas et al. (1995, doi.org/10.1016/0022-1694(94)02636-P) demonstrated that a recovery rate of better than 98% is necessary to obtain accurate isotope data with their accepted measurement uncertainty being higher than is this study. Isotope values flawed by incomplete extraction tend to be lower thus erroneously pointing at water from colder seasons.
L157: You report the precision of the analyser being derived from measurement replicates. What was the accuracy of your method, i.e. the deviation from target values?
L167: How much is “much”? Without numbers ideally presented together with the equations of all fitted sinusoidal cycles this statement cannot be quantitatively retraced.
L171: Why “median” and not (depth-weighted?) mean? Further, the difference between the reported numbers is only a fraction of the reported measurement precision. Why do you bother discussing such small differences?
L175: Also here, “ratio” is lacking numbers.
Figure 2: Does relative streamflow refer to runoff (in volume unit per time unit)?
L184: Please move the description of how sinusoidal cycles were fitted to the obtained isotope data to the method section. Please also include a statement of the degree of freedom you allowed. I assume, the frequency was pre-set to 365 days and only amplitude and phase shift were fitted?
L186ff (and other figure captions): I feel that these interpretations and descriptions unnecessarily inflate the figure caption. The respective statements already appear in the main text.
L196: Statements regarding phase shifts are only meaningful when the underlying numbers are available.
L205: To me, the isotope values of bulk water being lighter seems to be an artefact resulting from incomplete extraction (see also comments on Figure 6). Therefore, I would be careful drawing the conclusion regarding winter precipitation.
L208f: At some time of a year winter precipitation is actually the recent precipitation. Why would bypass only occur in summer?
L232f: Do you happen to have soil water content data (e.g. via relative post-extraction soil sample weight losses) and (how) would they make your claim more plausible?
L250 (and L252): Isn’t it remarkable that the 3-week fraction which INCLUDES the 2-weeks fraction is actually smaller than the latter alone?
L261: Are the uncertainties derived from the scatter of isotope data, i.e. the deviations from the sinusoidal cycles?
L275: Why did you exclude data from 80 cm from this figure?
L283: Is it Floriancic et al., 2023 a or b?
L294: Please provide the numbers this statement is based on.
L287: Delete “assumed to be”
L305 (and elsewhere): By “evaporation”, do you mean entire loss a certain fraction (with no effect on isotopic composition and deuterium excess on the remaining reservoir) or partial loss from the total reservoir (resulting in the aforementioned effects)?
L307ff: This statement seems to suggest that mobile water and bulk water are two separate reservoirs. But isn’t mobile water part of bulk water? The extraction process will certainly drive out not only the more tightly bound water, right? Assuming that one is part of the other, can you make a statement regarding the relative proportions under wet vs. dry antecedent conditions?
L325: These prerequisites (“all samples have the same bias”) seem to be contradicted by Figure 6 where data scatter more in the case of bulk soil water isotope data. In Figure 6 it also appears to me that not only are isotope data shifted towards the lower left but also the centers of gravity seem to be shifted at a slope < 8 above the LMWL. If so, this would indicate artefacts resulting from incomplete extraction. While that may not so much flaw interpretations of sinusoidal amplitudes and phase shifts, it certainly makes interpretations of the season of bulk soil water replenishment questionable.
L332ff: Is the significance and non-significance a result of the differences in scatter?
Figure 6. Would it be possible to also present precipitation and stream water data in dual isotope space?
L344: Please move the site description to the method section. Please provide the underlying numbers before discussing the differences in variation.
L355: Please move this sentence to the method section.
Citation: https://doi.org/10.5194/egusphere-2024-437-RC2
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