the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2024
Effects of subsurface water infiltration systems on land movement dynamics in Dutch peat meadows
Abstract. Large-scale drainage and cultivation of peat soils over the last centuries, occurring worldwide, has resulted in substantial CO2 emission and land subsidence caused by peat decomposition by microbial activity, shrinkage and soil compaction. In addition, seasonal reversible vertical soil movement is caused by shrink and swell in the unsaturated zone and by poroelastic deformation in the saturated zone. To reduce CO2 emission and land subsidence in drained peat soils, subsurface water infiltration systems (WIS) are expected to be a suitable measure. In this study, effects of WIS on seasonal vertical soil movements are evaluated, based on field measurements from five locations in Dutch peat meadows, for the years 2021 and 2022. For one of these locations, a 4-years timeseries was available, allowing to make a first estimate of the rate of multi-year land subsidence. At each study location, vertical soil movement has been measured using spirit levelling and extensometers, both in a parcel with a WIS and in a nearby reference parcel without any measure. Phreatic groundwater level fluctuations are found to induce soil volume decreases and increases in both the saturated and the unsaturated zone, which cause vertical land movement dynamics of up to 10 cm in the dry summer of 2022 at a location with a relatively thick (6 m) peat layer. Poroelastic deformation processes in the deeper saturated soil contribute substantially to surface level movement. In peat meadows, subsurface water infiltration systems, if correctly applied, reduce seasonal vertical soil movements while (potentially) reducing soils’ resilience to drought-induced volume losses. Seasonal vertical soil surface dynamics are about an order of magnitude higher than longer term (years to decades) land subsidence rates, which are commonly in the order of mm yr-1 in the Dutch drained peat areas. Therefore, multi-year data series are needed to filter out variations in seasonal dynamics, which are mainly introduced by annual variations in weather conditions, and more accurately estimate land subsidence.
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RC1: 'Comment on hess-2024-152', Anonymous Referee #1, 25 Jul 2024
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The work presented by van Asselen et al. investigates the effects of subsurface infiltration systems in subsoil vertical dynamics. Five study sites in the Dutch peat meadows are considered where the installation of extensometers in parcels with and without Water Infiltration Systems (WIS) enables the continuous measurements of the vertical dynamics. Campaigns of spirit leveling are also employed to measure vertical movements of soil and compared to extensometers. The extensometers have different anchor depths to gain deformation of the soil at different levels.
The main outcome of the study is the correlation between water level fluctuations (from a previous study) and surface vertical movements, showing the capability of WIS to increase the phreatic groundwater level thus reducing seasonal land surface fluctuations.
The paper is well-written and well-organized. The concepts are clearly explained as well as the experimental setup.
I think minor revisions are necessary to clarify/improve/discuss the following points:
- As the author stated, there is a discrepancy between the spirit levelling and the extensometers. Dynamics from the extensometers are often lower than the dynamics measured by the spirit levelling. The authors state that this may be due to the deformation of the top 5 cm of soil which is not taken into account in the extensometers. I am not convinced this is the reason. Could be linked to the anchoring system of the extensometers? How is the horizontal plate anchored to the ground?
- The reader is expected to see the results of the layers deformation at different depths. However, the analysis of these data is postponed to a future study. However, I think this is useful information to understand and unravel the physical processes leading to the reduction of deformation with the WIS implementation.
- Is there any way to relate the soil lithology to the high/low result in decreasing the seasonal soil fluctuation? The lithological data available in the study have not been used to interpret the results.
- The results from observations of vertical land movements are presented in terms of elevation change. To understand the mechanisms driving the reduction of deformation (poroelastic effect, shrinkage/swelling), I think it could be beneficial to plot the deformations from the surface (-0.05 m) to the -0.8 m anchor. Which is the behavior in time? Example from Fig. 11 (ASD reference but also with AWIS). Why the Anchor 0.06 and 0.79 have an overlapping behavior until March 2022 (no deformation from 0.06 to 0.79)?
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Citation: https://doi.org/10.5194/hess-2024-152-RC1 -
AC1: 'Reply on RC1', Sanneke van Asselen, 29 Aug 2024
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Thank you for the comments. Below, we reply to the four comments of the reviewer (RC1):
Reviewer (1): As the author stated, there is a discrepancy between the spirit levelling and the extensometers. Dynamics from the extensometers are often lower than the dynamics measured by the spirit levelling. The authors state that this may be due to the deformation of the top 5 cm of soil which is not taken into account in the extensometers. I am not convinced this is the reason. Could be linked to the anchoring system of the extensometers? How is the horizontal plate anchored to the ground?
Reply (1):Â The top anchor is a square perforated stainless-steel plate (0.5 x 0.5 m, 8 mm perforation, about 40% open area) that is dug into the soil at a depth of circa 5 cm (the top 5 cm soil are removed before installation and are put back on top of the plate after installation). A displacement sensor is attached to the plate. The setup is carefully designed to measure vertical movement in soft soil.Â
Firstly, lower dynamics obtained from extensometer measurements, as compared to levelling, are only observed when extensometer dynamics are calculated based on averages of measurements on de levelling days (four times a year). When the entire extensometer data series are considered, extensometer dynamics are usually higher than dynamics calculated based on levelling. The most likely explanation for the observed (structural) differences is deformation caused by shrinkage and swelling of the top 5 cm, as indicated in paragraph 3.6.Â
Reviewer (2): The reader is expected to see the results of the layers deformation at different depths. However, the analysis of these data is postponed to a future study. However, I think this is useful information to understand and unravel the physical processes leading to the reduction of deformation with the WIS implementation.
Reply (2): This paper focuses on effects of water infiltration systems on land movement dynamics, for which it is not necessary to include all anchors. In a separate paper focusing more on different processes contributing to land movement and subsidence and the relation to groundwater level dynamics and lithology, all anchor levels will be included (including these in this paper would make it too extensive). This explanation will be added in the methods section 2.4.Â
Reviewer (3): Is there any way to relate the soil lithology to the high/low result in decreasing the seasonal soil fluctuation? The lithological data available in the study have not been used to interpret the results.
Reply (3): It is still difficult to do this because there are many factors influencing vertical soil movement. We will add some (speculative) remarks about this in paragraph 3.6. Â Â
“…Zegveld is also the location with the thickest peat layer (ca 6 m thick) of all sites considered in this study. It is likely that a thicker peat layer results in higher vertical soil dynamics, because a thicker layer is available especially for poroelastic deformation. But, also in Assendelft relatively high soil dynamics have been observed. Here, the peat layer is ca 2 m thick, but below this layer a ca 10 m layer consisting of marine clay and sand is present. This thick Holocene layer is probably also subject to poroelastic deformation. The relation between lithology and soil dynamics will be further investigated in a follow-up paper.”
Reviewer (4): The results from observations of vertical land movements are presented in terms of elevation change. To understand the mechanisms driving the reduction of deformation (poroelastic effect, shrinkage/swelling), I think it could be beneficial to plot the deformations from the surface (-0.05 m) to the -0.8 m anchor. Which is the behavior in time? Example from Fig. 11 (ASD reference but also with AWIS). Why the Anchor 0.06 and 0.79 have an overlapping behavior until March 2022 (no deformation from 0.06 to 0.79)?
Reply (4): This will also be part of the follow-up paper mentioned earlier. Including it here will make the paper too extensive.Â
Citation: https://doi.org/10.5194/hess-2024-152-AC1
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Cited
3 citations as recorded by crossref.
- Rewetting drained peatlands through subsoil infiltration stabilises redox-dependent soil carbon and nutrient dynamics S. Harpenslager et al. 10.1016/j.geoderma.2024.116787
- Monitoring long-term peat subsidence with subsidence platens in Zegveld, The Netherlands H. Massop et al. 10.1016/j.geoderma.2024.117039
- Groundwater level effects on greenhouse gas emissions from undisturbed peat cores E. Blondeau et al. 10.1016/j.geoderma.2024.117043