Hydrological response to rewetting of drained peatlands – case study of three raised bogs in Norway

The presence of water in a peatland determines its proper functioning and is a prerequisite for its provision of ecosystem services other than water retention. Since the majority of degraded peatlands were drained for agriculture through the construction of ditches, the most common first step in the restoration of drained peatlands is rewetting through drain-blocking. The aim of this study was to analyze the hydrological response of three independent drained raised bogs in Norway (Aurstadmåsan, Midtfjellmosen and Kaldvassmyra) to ditch-blocking. The hydrological response to rewetting as well as the drain-blocking efficiency were assessed based on groundwater level monitoring conducted from 2015 to 2021 as a BACI design (Before-After-Control-Impact). The data was retrieved from water level loggers installed in piezometers placed in several locations at each of the sites. Rewetting technique used in the study sites included blocking the ditches draining the mires with peat dams. In each of the sites points with increased mean groundwater levels after rewetting were observed. It was also found, that the differences in precipitation before and after rewetting had no significant effect on groundwater levels. Both in Aurstadmåsan and Midtfjellmosen most of the piezometers reported an increase in average groundwater levels after rewetting. In Kaldvassmyra, 3 out of 8 piezometers reported an increase in mean groundwater levels. Even though in all sites precipitation was very similar before and after performed rewetting actions, comparison in Kaldvassmyra shows that the period after the implementation of restoration measures was noticeably drier. This might have inhibited the rewetting role of the dams in that site, which shows in the results. Considering the data from all impact piezometers, the groundwater levels increased by an average of 0.062 m. The same value for control piezometers was -0.003 m. The influence range of the ditch-blocking was 12.7-24.8 m, with the average of 17.2 m. Obtained results show that ditch-blocking might be an effective tool in restoring the hydrological conditions of peatlands, although it might be limited by meteorological factors, such as low precipitation. Assessment of the success of restoration should be integrated with the analyses of other conditions, including changes in vegetation cover or gas emissions (CO₂, CH₄, N₂O).