the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 May 2025
Assessing the impact of rewetting agricultural fen peat soil via open drain damming: an agrogeophysical approach
Abstract. Open drainage ditch (i.e. open drain) damming aims to raise the water table in agricultural grassland peat soils thereby reducing greenhouse gas (GHG) emissions. A current knowledge gap is how to examine the spatial and temporal effectiveness of such an action i.e., assessing the behaviour of the water table in the adjoining field. To address this gap, at a drained agricultural grassland site with shallow fen peat soils (ranging from 0 to 2 m depth), water level in an open drain was raised by installing a dam. Associated changes to the water table depth (WTD) were monitored using two nests of dip wells installed at two locations (Rewetted and Normal areas) in the adjoining field. Soil profile volumetric water content (VWC) data were obtained in these two areas in addition to the temperature, salinity, pH, and electrical conductivity signature of the water in the open drain. These data were integrated with geophysical (electromagnetic induction (EMI)) survey data conducted during summer and winter. Results from the dip wells (located > 20 m from dam) indicated that no measurable change in WTD occurred due to the dam installation, aligning with previous studies suggesting limited spatial influence in agricultural fen peat soils. VWC profiles, while consistent with peat physical properties, showed no deviation attributable to drain damming. The EMI results identified a distinct zone with electrical conductivity values similar to those of open drain water, suggesting localised water infiltration within ~20 m of the dammed drain during summer. This spatial impact was less evident during winter, likely due to increased precipitation and regional groundwater influence. This study demonstrates that EMI surveys, shown here in combination with other high-resolution data capture, can detect rewetting effects when combined with neural network clustering and Multi-Cluster Average Standard Deviation analysis, highlighting its value for rapid site assessment. Moreover, the results underscore the importance of survey timing, as summer measurements provided clearer evidence of drain damming impact than winter measurements.
Competing interests: The lead author (Dave O’Leary) is a member of the guest editorial board for this EGU SOIL Special Issue on AgroGeophysics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1966', Anonymous Referee #1, 11 Jun 2025
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AC1: 'Reply on RC1', David O Leary, 28 Aug 2025
Many thanks to this reviewer for their insightful comments.
Note that we have responed to both reviewers in a single response, which you can find attacehd to this reply, which we formatted to highlight the responses and any chances to be made to the revised article.
Please refer to this attachment for all responses.
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AC1: 'Reply on RC1', David O Leary, 28 Aug 2025
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RC2: 'Comment on egusphere-2025-1966', Anonymous Referee #2, 16 Jul 2025
This manuscript presents a multi-method assessment of the hydrological effects of drain ditch damming on fen peat soils. The authors combine well-based water table observations, volumetric water content (VWC) profile measurements, and electromagnetic induction (EMI) surveys to characterize spatial changes following dam installation. The manuscript is well-structured and flows nicely, with a particularly accessible introduction that provides useful context for readers unfamiliar with peatlands.
The study demonstrates the potential of EMI surveys in combination with clustering analysis to distinguish hydrological zones within the field site and to derive vertical electrical conductivity (EC) profiles through inversion (based on the use of multiple coil spacings, and the clustering). This approach enables better spatial coverage than traditional point measurements. However, the data from wells and soil moisture probes located upstream (W / "Rewet") and downstream (D / "Normal") of the dam showed no measurable difference in water table depth or soil moisture content, suggesting that the hydrological influence of the dam is spatially limited, likely confined to a few meters around the drain itself.
Overall, the manuscript effectively illustrates the added value of EMI in identifying spatial hydrological patterns, the potential for repeated EMI surveys, and the derivation and interpretation of vertical EC profiles. Nonetheless, several points require clarification or further discussion.
Some general comments or questions that I have after reading the manuscript:
- The EMI surveys are said to be conducted in “summer” and “winter,” yet the dates provided are 26/06/24 and 10/12/24. The former seems more representative of late spring, and the latter of late autumn/fall rather than winter. This mislabeling could mislead readers about hydrological conditions. When I think of winter, I think of very high water tables, filled drain ditches, and saturated soils in January/February. I recommend adjusting the terminology, e.g., late spring and fall.
- The study would benefit from a brief overview of meteorological conditions in the study region during 2024. Was the year, spring, summer particularly wet or dry? This context could help readers interpret the water table, EMI and VWC findings.
- The height of the dam and the corresponding change in water level in the drain ditch (before vs after installation, upstream vs downstream of the dam) are not reported. These data are essential for understanding the potential impact of the dam.
- The manuscript refers to two 1.2-meter deep VWC profiles, but their locations are not clearly indicated in Figure 1 or the text. From the results (Figure 3), one appears to be in the Rewet area and the other in the Normal area, but this should be clearly stated and shown in the figure legend or map annotation.
- It is unclear how VWC is defined in the study. In standard usage, volumetric water content is the volume of water divided by total soil volume (m³/m³ or %). In Figure 3, values up to 100% are shown, which seems too high, even for peat. If this is a normalized or relative VWC (e.g., relative to porosity or saturation), that should be clearly defined throughout the text and on all relevant figures. Without clarity on this, interpretation of Figure 3 becomes difficult.
- Both EMI surveys were conducted after the dam was already in place. Given this, I find it unclear how the EMI data can be used to infer the impact of damming unless a clear temporal difference is observed that can be linked to the dam. Without baseline (pre-dam) EMI data, attributing changes solely to damming remains speculative, I would say.
The observed zone of elevated electrical conductivity (ECa) near the dam is interpreted as a result of (increased) soil moisture from the drain ditch due to damming. However, could this signal also result from other factors such as soil compaction, increased iron or salt concentration, or historical management effects? It would be helpful if the authors could discuss alternative explanations and, if possible, provide supporting data (e.g., soil chemistry or structure observations). - The discussion does mention the potential use of EMI prior to dam installation to optimize rewetting strategies. While this is an interesting idea, it is not demonstrated or supported by the current study. If this is a forward-looking statement, I suggest clearly framing it as a suggestion for future work, rather than a direct conclusion from the current results. I also do not fully understand how prior knowledge from EMI measurements might optimize the rewetting?
- There are some inconsistencies in capitalization. Please ensure consistent formatting throughout.
- I assume the abbreviations D & W originally come from 'dry' and ‘wet’ while the ‘Normal’ (D) areas are not really dry. D is a weird letter for ‘Normal’ areas; consider calling it the control (C) area?
I have included further detailed comments, questions, and suggested edits in an attached pdf version of the manuscript. E.g., some figures are not colorblind-friendly, and are difficult to read due to a small font size.
In summary, this manuscript addresses an important question regarding the spatial effectiveness of peatland rewetting through drain ditch damming. The integration of EMI into such assessments seems promising, but needs more clear statements on how and why. I hope the authors will find my comments constructive and take them into consideration during revision. I look forward to seeing the updated version.
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AC2: 'Reply on RC2', David O Leary, 28 Aug 2025
Many thanks to this review for their insightful comments. Note that we have responed to both reviewers in a single response, which you can find attacehd to this reply, which we formatted to highlight the responses and any chances to be made to the revised article.
Please refer to this attachment for all responses.
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The manuscript entitled „Assessing the impact of rewetting agricultural fen peat soil via open drain damming: an agrogeophysical approach” by O’Leary et al. evaluates how the implementation of open drain damming affects the hydrology of a fen peat site. It is shown that the interpretation of sparse information from wells and SWC monitoring stations provides little insights, but that the spatially continuous nature of electromagnetic induction (EMI) measurements (an important agrogeophysical tool) provides important insights on the limited extent of the rewetting impacts of such damming activities. This is achieved through an advanced cluster analysis of the EMI data, followed by an inversion to obtain typical EC profiles with depth for the identified clusters. Overall, I found this to be an interesting case study highlighting the added value of agrogeophysical measurements in a peat hydrology context . Below I have provided specific comments that should be addressed in a revised version. Although not considered in my evaluation, I would say that the quality of the writing can also still be improved. I recommend to avoid very short paragraphs, and I would like to ask the authors for a careful proofreading before submitting the revised manuscript.
SPECIFIC COMMENTS
Figure 1. I wonder whether this figure is not too basic for the readership of SOIL? It seems like textbook material to me. Is it critically important for the narrative to explicitly address the different types of bog?
Line 87. Remove “… that …”.
Line 110. There is a range of studies dealing with time-lapse EMI measurements. The perspective seems to be a bit too narrow here.
Line 136. Are the 10 wells only open at the bottom (piezometer), or are they filtered along the entire length of the tube.
Line 137. Please provide type and manufacturer of these SWC probes (if presented later in the manuscript). I also could not find the location of these sensors in Figure 2a.
Line 164. I recommend to not use bulk in this context. It is typically reserved for the electrical conductivity of a mixture of materials (here: water, air, organic matter). I would prefer the introduction of the classical terminology here (i.e. “apparent electrical conductivity”).
Line 230. This statement only makes sense if your tubes are only open at the bottom. If the tubes are filtered (have slits), this would not make much sense to me. Please note that tubes that are open at the bottom do not indicate the position of the water table but instead the pressure potential at the opening. Please clarify your situation.
Figure 3. A volumetric water content of 100% is confusing. Please clarify what is reported here. Does 100% indicate pure water here. Or do you mean saturation in terms of filled pore space? This would not be a volumetric water content anymore.
Table 2. Please clarify whether the reported EC has already been corrected to a standard temperature. If not, can the difference in EC be explained by temperature only? Typically, 2% per degree is assumed for water, which would be a difference by 14%. The measured difference seems to be bigger. However, the salinity seems to be constant. What is then the cause of the remaining difference in EC? I think some more discussion and reflection is warranted here.
Line 320. At some point, I would like to see a clear statement that relates the clusters to the area affected by rewetting measures.
Line 379. The challenge with inversion is that EMI measurements need to be calibrated to obtain consistent inversion results. How was this addressed here?
Line 385-391. This paragraph needs to be improved. Argumentation currently is not fully clear to me.
Line 403. Should only water content be considered here, or should the electrical conductivity of the pore water also be considered? I am not sure that it can safely be assumed that the water in the open drain matches the pore water in the soil. A more in-depth reflection would be appreciated here.