Maintenance of high peatland water levels by subsurface water regulation

About 30% (0.4 Mha) of German peatlands are located in Lower Saxony and about 65% of these peatlands are used for agriculture, mainly grassland. These peatlands are drained for agricultural use, which creates huge GHG emissions. Grasslands on carbon rich soils are responsible for seven percent of the GHG budget of Lower Saxony. Raising the annual water level to 30 cm below surface or higher should substantially reduce peat oxidation and GHG emissions from such sites, while allowing grassland management or other ways of peatland utilization under wet conditions. Such water levels, however, may be difficult to achieve by high ditch water levels alone, because the low hydraulic conductivity of the degraded peat does not allow sufficient water movement to compensate for evapotranspiration in summer. We hypothesize that subsurface water regulation may allow constant high peatland water levels, because the applied submerged drains form conduits from ditches into the peat that should improve the water exchange.

We tested subsurface water regulation at 1 ha plots on a fen and bog grassland in NW-Germany. Both sites included three treatments: (1) blocked ditches with subsurface water regulation, (2) blocked ditches without subsurface water regulation, and (3) conventional drainage (control). Ditches in treatments (1) and (2) were filled with surface water up to 15 cm below land surface during the growing season using a solar pump. Over a period of three years, we monitored ditch and peatland water levels along transects. We analyzed effects of treatments, ditch water levels, climatic water balance, and saturated water conductivity (kf) on peatland water levels and changes of surface elevation.

Our results show that subsurface water regulation allowed for a better control of peatland water levels as compared to ditch blocking and conventional drainage. In the winter, subsurface water regulation improved drainage, so that water levels within the site were not much higher than the ditch water levels. In the summer, subsurface water regulation allowed to maintain peatland water levels of 30 to 40 cm below surface, more than 20 cm higher compared to both other treatments. Furthermore, subsurface water regulation reduced subsidence. However, despite a narrow drain spacing of four to five meters, it was difficult to maintain the target peatland water levels during very dry summer months albeit the tested years were atypically dry and hot. The differences between ditch water levels and peatland water levels were closely related to the climatic water balance, and the slope of the linear function depended on saturated water conductivity (kf) of the peat. Based on climatic water balances, weir adjustment can be optimized to achieve high and stable peatland water levels. The results help in understanding and analyzing the hydrology of degraded peatlands. This information will prove extremely useful for planning water management measures, which are necessary to reduce the GHG emissions from drained peatlands.