The impact of ditch blocking on the hydrological functioning of blanket peatlands

Abstract

Ditch blocking in blanket peatlands is common as part of peatland restoration. The effects of ditch blocking on flow regimes and nearby water tables were examined in a field trial. After an initial 6‐month monitoring period, eight ditches had peat dams installed 10 m apart along their entire length (dammed), four of these ditches were also partially infilled through bank reprofiling (reprofiled). Four ditches were left open with no dams or reprofiling (open). These 12 ditches and the surrounding peat were monitored for 4 more years. An initial five‐fold reduction in discharge occurred in the dammed and the reprofiled ditches with the displaced water being diverted to overland flow and pathways away from the ditches. However, there was a gradual change over time in ditch flow regime in subsequent years, with the overall volume of water leaving the dammed and the reprofiled ditches increasing per unit of rainfall to around twice that which occurred in the first year after blocking. Hence, monitoring for greater than one year is important for understanding hydrological impacts of peatland restoration. Overland flow and flow in the upper ~4 cm of peat was common and occurred in the inter‐ditch areas for over half of the time after ditch blocking. There was strong evidence that topographic boundaries of small ditch catchments, despite being defined using a high‐resolution Light Detection And Ranging‐based terrain model, were not always equivalent to actual catchment areas. Hence, caution is needed when upscaling area‐based fluxes, such as aquatic carbon fluxes, from smaller scale studies including those using ditches and small streams. The effect of ditch blocking on local water tables was spatially highly variable but small overall (time‐weighted mean effect <2 cm). Practitioners seeking to raise water tables through peatland restoration should first be informed either by prior measurement of water tables or by spatial modelling to show whether the peatland already has shallow water tables or whether there are locations that could potentially undergo large water‐table recoveries.

An experimental study on the response of blanket bog vegetation and water tables to ditch blocking

“…As water tables fall next to the ditch they expose peat to rapid oxic decay and, in response, the peat may subside so that the peatland surface adjacent to ditches is lower than that midway between them. These possible changes have yet to be studied properly, but fine-scale topographic data from our study site in the Migneint are consistent with such processes occurring: we found the ground-surface midway between ditches was higher than the peat surface 2 m either side of the ditches which, in turn, was higher than the peat surface immediately next to the ditch (Holden et al 2016(Holden et al , 2017; in other words there was a clear surface slope perpendicular to the ditch (towards the ditch channel). The outcome of such changes in the peatland surface is a reduction in the apparent drainage effect of a long-established ditch when considered in terms of water-table depths.…”

“…We separated position (east or west of the ditch) in the vegetation analysis because of uncertainty about which side might show the greater drainage effect in two of the control ditches (Holden et al 2017). After ditch blocking, water could flow across some blocked ditches towards the control (open ditches).…”

Methane and carbon dioxide fluxes from open and blocked ditches in a blanket bog

“…The site description below follows closely that of Green et al (2016), Holden et al (2017) and . The study site was a~2 ha area of hillslope located approximately 500 m above sea level, drained by a set of parallel ditches running approximately downslope from a drainage divide at the hillslope crest (Fig.…”

“…Four of the 12 study ditches were left open as controls, four were dammed with peat (i.e., dams were constructed at intervals of a few metres along the length of the ditch -'dammed') and four were partially infilled with peat and dammed ('reprofiled') (see Holden et al 2017;Parry et al 2014). To avoid systematic bias, the assignment of the ditches to different treatments (Fig.…”

“…We did not measure water levels in the pools because the hydrological status and regime in these was clearly different from all other treatments (they were permanently ponded). In addition, we had a series of dipwells across the site fitted with automatic water-level recorders set to log every 120 min (see Holden et al 2017), and this network of automatic wells included the C S (controlSphagnum) treatment. The dipwells were made from 32 mm o.d.…”