Abstract
Abstract. Accurate monitoring of shallow subsidence in cultivated peatlands is a great challenge. Peat compaction by loading and peat oxidation by groundwater level lowering are two important processes contributing to shallow subsidence in cultivated peatlands, causing an overall increase in soil wetness over time and hence a lower soil-bearing capacity and agricultural production. Peat oxidation also causes emission of CO2 and other greenhouse gasses. Rigorous monitoring techniques are urgently needed to spatially and temporally map the amount and rate of subsidence and to monitor effects of measures to reduce subsidence and its negative impacts on livestock farming and agriculture. Monitoring shallow subsidence in peatlands is particularly challenging, because subsidence is a slow and spatially complex process, with average rates in the order of mm yr−1 but with higher rates possibly occurring on shorter timescales. The desired monitoring system must be able to capture this temporal and spatial variability, and preferably the contribution of different processes to total subsidence. The system needs to be applicable (technically- and financially-speaking) at regional scales, without severely impacting daily farming activities. To help design and test a subsidence monitoring system for cultivated peatland areas, four methods to measure subsidence are applied and assessed in a cultivated peatland in Overijssel (NL), namely spirit levelling, extensometery, LiDAR, and InSAR. In this paper, we focus on the levelling and extensometery methods and measurements. Subsidence was measured since October 2018 at eight livestock farms once every three months by levelling. In the same period, extensometers have measured vertical movement of (sub)surface levels hourly at two livestock farms. In addition, phreatic groundwater levels are continuously monitored. Preliminary results show vertical movements (up and down) in the order of centimeters on the timescale of weeks. These movements seem to be related to groundwater level fluctuations, but also evapotranspiration is expected to contribute to additional subsidence during the summer period. Because long term net subsidence is a slow process, additional data collection is needed to assess the different methods and the temporal and spatial fluctuations in subsidence on longer timescales. This is vital information to design the optimal method for monitoring subsidence in cultivated peatlands on large spatial scales, and to help in selecting effective measures to reduce subsidence and greenhouse gas emission in peatlands.
Highlights
A large part of the coastal plain of The Netherlands contains as much as several meters of peat in the subsurface (Erkens et al, 2016)
Like many other coastal plains worldwide, the Dutch coastal plain is subject to land subsidence from both anthropogenic and natural causes (Erkens et al, 2016; Van Asselen et al, 2018, and references therein)
In the Dutch coastal plain, subsidence has especially been caused by peat oxidation, peat compaction and peat mining in the Holocene sequence, starting about 1000 years ago (Erkens et al, 2016)
Summary
A large part of the coastal plain of The Netherlands contains as much as several meters of peat in the subsurface (Erkens et al, 2016). One measure proposed to reduce subsidence and emission of CO2 in peat areas is the application of submerged drainage systems (Pleijter and van den Akker, 2007) This drainage technique is designed to enhance infiltration of ditch water to the parcels to elevate groundwater levels in dry (summer) periods, but at the same time to enhance drainage from the parcels to the ditches in wet (winter) periods to prevent too wet soils. The use of LiDAR and InSAR for measuring land subsidence in peat areas is promising but still experimental and the accuracy of the measurements needs testing These two remote sensing techniques may result in timeseries of maps with spatial coverage, which is needed to monitor the temporal and spatial variability of land subsidence and the effects of applied mitigation measures. Both methods are described and assessed, and preliminary results and conclusions are presented
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