the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
26 Jan 2021
26 Jan 2021
Technical note: Evaporating water is different from bulk soil water in δ2H and δ18O
- 1Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University, Yangling, Shaanxi Province 712100, China
- 2Department of Soil Science, University of Saskatchewan, Saskatoon SK S7N 5A8, Canada
- 3Gansu Provincial Department of Water Resources, Lanzhou, Gansu Province 730000, China
- 1Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University, Yangling, Shaanxi Province 712100, China
- 2Department of Soil Science, University of Saskatchewan, Saskatoon SK S7N 5A8, Canada
- 3Gansu Provincial Department of Water Resources, Lanzhou, Gansu Province 730000, China
Abstract. Soil evaporation is a key process in the water cycle and can be conveniently quantified with δ2H and δ18O in bulk surface soil water (BW). However, recent research shows that larger soil pore water evaporates first and differs from small pore water in δ2H and δ18O, which disqualifies quantification of evaporation from BW δ2H and δ18O. We hypothesize that BW has different isotopic compositions than evaporating water (EW). Therefore, our objectives are to test the hypothesis, and to evaluate if the difference alters the calculated evaporative water loss. We measured isotopic composition in soil water in two continuous evaporation periods in a summer maize field. Period Ⅰ had a duration of 32 days following a precipitation event and Period Ⅱ lasted 24 days following an irrigation event with a 2H-enriched water. BW was obtained by cryogenically extracting water from samples of 0–5 cm soil taken every three days; EW was derived from condensation water collected every two days on plastic film placed on soil surface. Results showed that when newly added water was heavier
than pre-event BW, δ2H of BW in Period Ⅱ decreased with the increase of evaporation time, indicating evaporation of heavy water; when newly added water was lighter
than pre-event BW, δ2H and δ18O of BW in Period Ⅰ and δ18O of BW in Period Ⅱ increased with increasing evaporation time, suggesting evaporation of light water. Moreover, relative to BW, EW had significantly smaller δ2H and δ18O in Period Ⅰ and significantly smaller δ18O in Period Ⅱ (p < 0.05). This suggests that evaporating water was newly added water, both of which were different from bulk soil water. Further, the newly added water may be in large pores, from which evaporation takes precedence. We also calculated soil evaporation losses from using water isotopes from EW and BW and they did not differ significantly (p > 0.05). Our results have important implication for quantifying evaporation process with water stable isotopes.
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Hongxiu Wang et al.
Status: open (until 23 Mar 2021)
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RC1: 'Comment on hess-2020-648', Anonymous Referee #1, 27 Feb 2021
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The “Technical note: Evaporating water is different from bulk soil water in d2H and d18O” describes an experiment to elucidate hysteresis of water isotopic signals during evaporation. The process described is known, but the experiment nicely shows the concept and the implication for deriving evaporative loss from isotopic signals. It is, however, a pity that the difference in d18O was not high enough to result in significant differences in evaporative loss. In this context, it would be beneficial to add more hypothetical calculations under which conditions (difference pre-event/event water) and soils this process might be important. The latter would strengthen the conclusions. It general, it would have been beneficial to have information on soil texture and eventually matric potential.
Another point that is not addressed yet is that evaporation of heavier water than bulk water evaporation loss cannot be calculated. The authors should comment on whether such replacement of heavier isotope occurs under natural conditions and which effect it could have to calculate evaporative loss for natural isotope abundances. Another main point concerns the description of the calculations. The equation and variables used should be introduced sequentially. The Figures are appropriate and relevant literature cited. However, the manuscript should be corrected by a native speaker (particularly the first part until Discussion). Finally, the title should be adapted since evaporating water is per se different from bulk soil water, and as such, the title does not reflect the process you seek to investigate.
L26 Please make clear that this is not a general statement but specific to the conditions of your experiment.
L28 Which important implications?
L41 “occupied” seems not the right term in this context.
L45 Please rephrase the sentence.
L54 Large pores instead of pore.
L37-59 This section should be moved to the methods.
L82-84 This should be moved to the method section.
L84 Rephrase: This study may help to ….
L98 Add values or signature in the sentence.
L107 “secondary” evaporation
L129-130 It is not clear whether the authors refer in this sentence to there own findings (in this case I would move the sentence to the results) or if there refer to other studies (in this case they should be cited). Moreover, the structure of the sentence is not clear and should be corrected.
L131: When are higher temperature needed? In case of higher clay content. This is not clear from the sentence. Could you provide soil texture information?
L147: “sub samples”
L159 Use the present tense for referring to Tables and Figures.
L167- : Why did you change the soil of the lysimeters. The reason is not apparent.
L222-228: Here, the introduction of the variables an equations is mixed up and difficult to follow. Please introduce each equation with its variables from top to bottom since this is an important aspect of your study.
L240: Is the variable n introduced?
L242. The article is missing: A general linear …
L270: Delete “was”.
L277: What is meant by newly added water? The irrigation water? Please use the same terminology as before.
L300-306: style: delete some “therefore”
L414: Could you explain how you estimated the value of 3.52 to result in significant differences.
L418: Do you mean matric potential?
L436: Please make clear that this statement refers only to small differences in isotopic signals.
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RC2: 'Comment on hess-2020-648', Anonymous Referee #2, 01 Mar 2021
reply
Evaporating water is different from bulk
soil water in 2H and 18O
Summary:
Wang et al. sought to determine the contribution of bulk water from cryogenic extraction to evaporation water using stable isotopes of water. The team used a clever and practical method to collect evaporated water in a corn field and compared this to extracted bulk water throughout the growing season. Additionally, the authors applied a deuterium labeled irrigation to improve endmember resolution. Following the label, the evaporation and bulk water appears to decrease in 2H through time in similar overall values, whereas the 18O signature increases through time with significant differences between these two sampling domains. The authors interpret this to mean that, in this system, evaporation shows a strong preference for new water residing in large pores and that the source of evaporation differs from that of cryogenically extracted bulk water.
General Comment
I think that both the aim and the results of this study are relevant an interesting. These kind of experiments are severely lacking in modern hydrological sciences, and are needed to force the field to think openly about flow and mixing assumptions. However, there are numerous instances where the presentation and interpretation of the results make it difficult to judge the merit of the experiment, overall. I detail these discrepancies below. I think most of the necessary analyses have been conducted but I find it hard to accept without a substantial change to the current presentation and interpretations.
Specific Comments
- Introduction to Evaporation Dynamics
Lines 40-51: This section is a bit unclear. How exactly are the initial evaporation phases preferentially expressing larger pores? Yes, the larger pores connecting the deeper (more positive pore water pressure) source water to the near-surface may require higher contribution from higher conductivity ("larger") pores to sustain evaporation. However, it is unclear if the source of water vapor at the evaporation front is distinctly associated with larger pores, as smaller pores are dominated by stronger capillary forces (capillary > gravity + viscous forces) that maintain the gradient that links surface evaporation to deeper layers.
I think that this section needs to be made clearer which appears to be a critical point of the manuscript. I suggest providing a more detailed link to the literature, especially as these references (e.g, Ohr and Lehman + Zhang et al) do not make such obvious pore-scale distinctions.
- Figures and Presentation
Generally, it is difficult for the reader to interpret results from most of these figures. The labels of the figures are sporadic with non-intuitive descriptions in figure captions. Having to flip back and forth between plots and timelines to attribute dates with important time periods does not help (maybe get rid of dates, use time, and intuitive descriptors for each key time period?). Overall the quality of figures is often lacking. The exception is figure 8 which is well done. Please see my specific comments below (and attached file).
Also regarding the fractional evaporation:
Line 325: This gets a bit confusing.
1) how are you expressing the fraction of evaporated water source from both pools if equation 10 requires input from bulk water (i.e., this should work for just BW)?
2) why are you only comparing EW vs BW for 18O in period 2 and not 2H (or period 1)?
3) Why make all of these sporadic comparisons and list one panel as not available.
These points really detract from the meaning meant to be conveyed here.
- Interpretation and Explanations
Here are some key points:
Line 361: This is quite puzzling. How could you expect a difference in detected source in 18O between evaporation and bulk water, when the there is such a stronger end member separation in 2H? ~ 80 delta 2H per mil divided by instrument precision 0.2 = 400 units of detection versus almost no separation for 18O.
If this finding is indeed true, I think its worth discussing how you would see this in one isotopic signature (2H) and not 18O. Is it possible that that the instrument precision of 2H was greatly reduced after the label (e.g., drift and memory effects) whereas we see a more correct version of 18O during phase 2? Would you have any data to calculate the precision of the analysis throughout the study period to confirm?
Lines 373-375: Here is where the soil physics perspective matters. As you mention in your introduction (Lines 53-54) when tighter pores are filled with water (e.g., field capacity or wetter) the likelihood of preferential flow increases, as high porewater pressures force more water into large pores. However, under dry conditions (e.g., your irrigation event on 8/22) infiltrating water will initially fill these small pores, due to high matrix flux potential or a strong potential gradient between wetting front and dry soil. As the infiltration event proceeds, hydraulic length increases (e.g., depth of wetting front) driving down the infiltration rate (low gradient), the pore water pressures increase such that the air-entry pressure of large pores is exceeded, and then macropore or preferential flow ensues. Under the later phase gravitational forces exceed capillary "pull" into he matrix, increasing the likelihood of dual domain flow and separation between small and large pores.
The main point here is that dry conditions would likely facilitate preferential wetting of smaller pores due to strong capillary forces during initial infiltration. Thus, dry conditions could result in greater continuity between small and large pores. Having said this, preferential flow is known to happen under dry conditions too (especially in cracks) yet these conditions could really reduce the separation between the two pore domains. Note also that your introduction covers this process of preferential filling of small pores under dry conditions on Lines 52-53.
Please consider this point in your interpretation.
Lines 381-382: Again, why exactly do you assume the small pores to only express old water? The average water content before irrigation was quite low (~ 0.15 in the upper 10 cm).
These 25 mm of irrigation could have filled ~7-10 cm of upper soil assuming a uniform wetting front and a conservative porosity of 0.45. Thus the signature of infiltrating water alone could have muted the pre-event evaporation water source by >70%.
Lines 388-393: See my comments about these stages in the introduction.
Lines 420-421: This is not consistent with Brooks et al. Brooks et al suggested that transpiration water and bulk soil were similar and that smaller pores with high residence time supplied this Ecohydrological flux.
Specific comments:
Line 10: This reads like you are referring to the pool of water as being larger. "soil water from larger pores" is more clear and direct.
Line 16: maybe distinguish this as "natural precipitation.." to be clear
Line 26: "…evaporation losses from .." from what?
Line: 27: "implicationS" (plural)
Line 28: "process" Remove or make plural.
Line 36: I do not think that these two previous sentences could be considered a full paragraph.
Lines 38-40: Why is this specific distinction relevant?
Lines 39-40: This sentence does not make sense as written. Also, it is not clear what you are trying to convey. Maybe you mean "minimum?"
Line 41: See earlier comment. Rephrase to water in smaller pores (or something like this). Please revise this throughout the manuscript
Line 45: Try to be clear with this term "depleted," as this is also a study of water isotopes (e.g., isotopic depletion). Maybe choose a different word (e. g., drained).
Lines 46-47: "capillary pumping" is never used in Or and Lehman (2019). This point is also unclear. Please specify.
Line 60: use "infiltration" not "invasion"
Line 71: "partitionING"
Line 74: Okay, I think that the authors have used this small versus large pores enough to warrant a more specific reference. I suggest giving a more specific example of small versus large pores, especially here where vacuum pressure matters.
Lines 77-78: Good point.
Lines 84: "improve our understanding" works better? Does not make sense as written.
Lines 133-135: Are these equations provided anywhere? Is the manuscript available for review. This seems to be an important detail.
Lines 156-158: What exactly was measured here and what was calculated? Please state explicitly here and in the Supplemental file.
Lines 170-176: Looks like you have 2 paragraphs with 2 sentences and no transition? Please fix this.
Line 175: Should use "instrument" not "machine."
Lines 201-202: Is it also possible that the plastic film itself can fractionate condensed water molecules? This point might be worth clarifying/considering at this stage.
Line 246: "mean values.." of what exactly?
Figure 4, Lines 258-259: This is very confusing . It looks like there are 4 periods. I suggest shading these these two areas with different colors or something similar.
Line 260: So the pink circles indicate when you compared bulk water versus evaporation water? Please clarify. Also were there no similar comparisons in Period 2?
Line 363: What is the porosity?
Line 265: Water contents can "jump"? :). please revise.
Line 266: Note that "Figure 4c" is not so clearly distinguished in the Figure. Would it be possible to move the letters e.g., "a)," "b)" to the left-hand side and increase the font size? Also, please refer to these sections directly in the figure captions.
Line 270: remove "was"
Line 277: "Therefore" ??
Line 278: "relatively" should be "relative"
Line 282: "resulting in.." this sentence has been cut off.
Line 290: BW 18O also increased? Looks like there is a missing section??
Line 292: still describing period 2? Specify
Line 306: Can you clarify why the period 1 EW and BW values are not shown together here? It looks like they would indicate a different source water for EW (minus one outlier)
Line 321: I would really suggest getting rid of the dates here and using some intuitive representation in time (e.g., before irrigation, after irrigation, early period 1 etc..) It is difficult for the reader to discern what the various times mean and their relevance is not mentioned in the Figure 6 caption.
Line 342: "preferentially evaporated" is more grammatical correct.
Line 354: "...THE evaporation period..”
Line 362: difference in what? Please also specify for clarity.
Line 365: "partitionING"
Line 372: "...in larger pores than in small.."
Line 408: difference did not make a difference?
Please also see my specific comments in the attached pdf, if needed.
Hongxiu Wang et al.
Hongxiu Wang et al.
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