the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jun 2023
Snowmelt-mediated isotopic homogenization of shallow till soil
Abstract. The hydrological cycle of sub-arctic areas is dominated by the snowmelt event. Understanding the mechanisms that control water fluxes during high-volume infiltration events in sub-arctic till soils is needed to assess how future changes in the timing and magnitude of snowmelt can affect soil water storage dynamics. We conducted a tracer experiment with deuterated water to irrigate a plot on a forested hilltop in Lapland, tracked water fluxes of different mobility and monitored how the later snowmelt modifies the labelled soil water storage. We used lysimeters and destructive soil coring for soil water sampling, and monitored and sampled the groundwater. Surface water flow during the tracer experiment was largely controlled by fill-and-spill mechanism. We found that labelled water remained in deeper soil layers over the winter, but the snowmelt event gradually displaced all deuterated water and fully homogenized all water fluxes at the soil-vegetation interface. The conditions required for the full displacement of the old soil water occur only during snowmelt with a persistently high groundwater table. We propose a conceptual model where infiltration into the soil, and eventual soil water replenishment, occurs in three stages. First, unsaturated macropore flow is initiated via surface microtopography and is directed towards the groundwater storage. The second stage is characterized by groundwater level rise through the macropore network, and subsequent pore water saturation and horizontal connectivity of macropores. Shallow subsurface lateral fluxes develop in more permeable shallow soil layers. In the third stage, which materializes during a long period of a high groundwater table and high hydrological connectivity within the soil, the soil water is replenished via enhanced matrix flow and pore-water exchange with the macropore network.
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RC1: 'Comment on egusphere-2023-884', Anonymous Referee #1, 22 Jul 2023
Summary: A “double” irrigation experiment was undertaken where first an experimental plot was irrigated with waters enriched in deuterium, followed by isotopically depleted snowmelt infiltration under natural conditions. Through soil water, groundwater, and soil sampling, a conceptual model of macropore and matrix storage/flow was developed which brings new insights into the physical processes of infiltration and the role of snowmelt on vadose zone hydrology. The quality of the writing and extensive literature review incorporated throughout the manuscript is excellent. I have just a few general and specific comments for consideration.
General:
1. It’s unclear how the tree response data is supportive of the overall theme of the paper, which largely focuses on macropore infiltration processes and soil storage dynamics. Presently it feels a bit out of place. It may be worth asking, “What does this information add to the story?” If this isn’t clear, consider removing. While I recognize the importance of plant uptake as a pathway for removing soil water, I think additional efforts should be made to tie this piece together with the other components of the manuscript if it is to be kept. This includes:
- Abstract
- The introduction: The manuscript is well positioned in the literature from a macro-pore/soil storage perspective, but considering vegetation dynamics are part of the research questions, this should be better introduced.
- Discussion: Greater discussion on the importance of the tree uptake in the homogenization effect or other new/critical insights gained
- Conclusions: There should be a conclusion for each research question/objective and this is not present in the conclusions
2. Although in this case, the soil temperatures remained above freezing, this will not be the case for many cold regions throughout the winter. How do you expect your conceptual model to change in years where seasonal frost develops at this site and for other sites with frozen soils?
Line Specific:
Figure 1: The subscripts in 1a should be enlarged for clarity. The co-ordinates around the border of 1b should be removed or enlarged so they can be read. They are presently illegible, even zoomed in.
Line 72: Some more background on the potential responses previously reported might help set up this research question a little better. At present it reads a bit disconnected from the other questions and introduction.
Line 116: What area do the sprinklers cover and where were they positioned relative to your sample locations? locations could be added to Figure 1.
Line 126: I think this information would be more helpful presented with the soil core sampling. I was left wondering when you sampled the cores after reading this section, but then realized these detailed were in 2.3.
Line 143: Since it is common to include Supplementary Data in a supporting document/appendix, I suggest renaming Section 2.4 to avoid confusion.
Figure 2 and 3: include all data in the legend, not just in the caption. It is difficult to interpret as is. Suggest switching the colours between 30 and 35 cm so there is a gradation of colour from light to dark = shallow to deep. Also need a legend for (2d).
Citation: https://doi.org/10.5194/egusphere-2023-884-RC1 - AC1: 'Reply on RC1 and RC2', Filip Muhic, 12 Sep 2023
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RC2: 'Comment on egusphere-2023-884', Anonymous Referee #2, 08 Aug 2023
GENERAL COMMENTS
The manuscript presents an interesting study using deuterated water to observe the response of a forested hillslope to an application of a large quantity of water mimicking a snowmelt event, and to understand the interaction between soil matrix and macropore network. The field experiment is designed well and the data are of high quality. The manuscript is well organized and written in most parts, and the methods are clearly described. The study has strong potential to make a unique and significant contribution to hydrological science. However, I see a substantial room for improvements in the analysis and discussion of the results regarding the soil water dynamics. The manuscript could use more careful and quantitative interpretation based on the principles of soil physics. As it is written, some of the data interpretation is speculative and may not support the main conclusions. I will elaborate more in my specific comments below.
SPECIFIC COMMENTS
Line 27. Unsaturated (vadose). The unsaturated zone and the vadose zone are not interchangeable. Please select the term that is most suitable for the context of this study and use it consistently throughout.
Figure 1. Please delineate the actual area of irrigation using a polygon or rectangle. Open white squares and light gray squares are shown in Fig. 1a but not explained. Please include them in the legends and explain them in the figure caption. Please indicate latitudes and longitudes in Fig. 1c. Please use appropriate font sizes in these figures keeping in mind that Figure 1 may be published in a reduced size.
Line 90-91. Please pay attention to the number of significant digits in comparison to the accuracy of measurements.
Line 117. 0.24. This is reported as a unitless number, but it should be either density (kg/m3) or snow water equivalent (mm).
Line 120. 163.6 mm. How was this determined? Was it measured using one precipitation gauge (Fig. 1a)? If so, how representative is it of the entire irrigated area? Unless the sprinkler system has a uniform rate of application over the entire area, one would normally need multiple precipitation gauges to estimate an average. Please explain the uncertainty in this value. Was there an attempt to calculate the application rate from the volume of water going through the sprinkler system?
Line 123. 600 kPa. This is roughly 6 bar. What was the bubbling pressure of the suction cup? How was this value chosen? Does it represent the actual condition of soil pores? Please explain.
Line 156. Bulk soil water. Does this mean the bulk of an entire core? Or, does it refer to individual core sections? What does ‘bulk’ mean in the context of this study? Please explain.
Figure 2c. This figure is difficult to comprehend. Symbols and colours are difficult to differentiate, and there are too many data series in one graph. Please improve the presentation. At least, more clearly distinguishable symbols and colours need to be used. Pale yellow is not easy to see on the white background. Also, it will be easier for the reader if Profile 2 and Profile 3 are presented in separate graphs.
Figure 2d. This figure is also difficult to comprehend. Please improve the presentation.
Line 186. This sentence describes the response of 35-cm depth. However, I see that the 60-cm sensor responded before the water table started to rise, but this sampler was located far above the water table. This seems contrary to the sentence. Please explain. Overall, this paragraph could use a clearer writing that is consisted with the data presented in figures.
Line 190. Orange and yellow are difficult to distinguish. Please use different colour schemes.
Line 192. Brown vertical lines. These lines look more purple than brown. Please include this in the graph legends, instead of describing it verbally.
Line 202. What was the date of another irrigation study? Was it before or after the study in the manuscript?
Line 203. What was the deuterium value of the tap water? Please indicate it here.
Line 220. Soil moisture levels. Pale yellow lines are difficult to see on white background. Please use a different colour.
Line 238. Reference plot. This is described as ‘natural soil’ in the legend of Fig. 5d. Please use a consistent term.
Line 284. Infiltration capacity of surficial soil matrix is exceeded. This is an example of statement that is lacking quantitative consideration. Infiltration capacity of surface soil is quite high when it is unsaturated and decreases as the soil becomes saturated. The lower limit of infiltration capacity is the saturated hydraulic conductivity, which is on the order of 10^-6 to 10^-5 m/s (Line 90-91). These were determined using a falling-head permeameter on soil samples, implying the soil matrix conductivity. The intensity of irrigation was generally less than 10 mm/h or 3 x 10^-7 m/s (Figure 2a). Given this, it is not clear how the infiltration capacity of the surficial soil matrix can be exceeded. Please present a convincing explanation.
Line 285-286. Macropore flow is unsaturated. What is the evidence of unsaturated condition? The soil moisture sensors measure soil matrix, and are insensitive to macropores that occupy relatively small volume. Again, please present a convincing explanation.
Line 288. Groundwater rises toward towards the surface. The rise of the water table is indeed shown in Fig. 2b. However, Fig. 2d shows constant moisture content at 60 cm in Profile 2, which is located adjacent to the water-table monitoring well. This implies that the soil at 60cm was saturated before the water table rose to 60cm. This does not seem likely because the capillary fringe cannot be as thick as 50cm in near-surface soil. I feel that something is missing here. Please present more careful interpretation of soil water dynamics based on the principles of soil physics.
Line 289. Groundwater exfiltration. The term exfiltration specifically means that groundwater is discharging at the ground surface. It seems unlikely that exfiltration was happening while the water table was below the ground surface (Fig. 2b). Was exfiltration visually observed? Please present an explanation.
Line 308. Dashed red lines. The red lines indicate 30cm, not 5cm, in this figure. Please be consistent between the texts and figures.
Line 311. Please refer the reader to Fig. 3.
Line 313. (Smaller pores in the soil matrix) is filled first. Figure 2d indicates only 5% increase in water content at 5-cm depth. This seems inconsistent with a major shift in isotopic composition depicted in Fig. 3. Does it make sense in terms of mass balance consideration? Please explain.
Line 323. Does lysimeters preferentially sample macropores? They are subjected to high magnitude of matric potential (600 kPa). Please interpret the data more carefully considering the actual function of lysimeters.
Line 345. Inability to infiltrate deeper. Many of the papers cited in this paragraph were on frozen soil infiltration, but this experiment was conducted under unfrozen condition. What prevents infiltration when the irrigation rate was much below the saturated hydraulic conductivity of surface soil? Please present a convincing explanation.
Line 354-355. This sentence compares the rate of water-table rise and the soil saturated hydraulic conductivity. I cannot understand the logical connection between the two quantities. The rate of water-table rise does not indicate groundwater flux. The water table can rise quickly with a small addition of water if the soil is nearly saturated in the capillary fringe. The rise in the water table can occur when the flow direction is downward. Please revise the conceptual model of the water table dynamics and reinterpret the data based on the principles of soil physics.
Line 364. Upward water flow. This requires the matric potential gradient in excess of 1. Is there direct evidence of such a gradient? Please present the data.
Line 378. Stumpp and Hendry (2012). This study was not conducted in a sub-arctic catchment. Please read the paper carefully and revise the sentence.
Line 397. Rothfuss et al. (2015). What were the findings, and where was the study conducted? Please add the information, so the reader can understand the context.
Line 407. Michelon et al. (2023). Where was this study conducted?
Line 431. Net radiation. Please indicate the unit in the graph.
Citation: https://doi.org/10.5194/egusphere-2023-884-RC2 - AC2: 'Reply on RC2 and RC1', Filip Muhic, 12 Sep 2023
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