Water‐level dynamics in natural and artificial pools in blanket peatlands

Abstract

Perennial pools are common natural features of peatlands, and their hydrological functioning and turnover may be important for carbon fluxes, aquatic ecology, and downstream water quality. Peatland restoration methods such as ditch blocking result in many new pools. However, little is known about the hydrological function of either pool type. We monitored six natural and six artificial pools on a Scottish blanket peatland. Pool water levels were more variable in all seasons in artificial pools having greater water level increases and faster recession responses to storms than natural pools. Pools overflowed by a median of 9 and 54 times pool volume per year for natural and artificial pools, respectively, but this varied widely because some large pools had small upslope catchments and vice versa. Mean peat water‐table depths were similar between natural and artificial pool sites but much more variable over time at the artificial pool site, possibly due to a lower bulk specific yield across this site. Pool levels and pool‐level fluctuations were not the same as those of local water tables in the adjacent peat. Pool‐level time series were much smoother, with more damped rainfall or recession responses than those for peat water tables. There were strong hydraulic gradients between the peat and pools, with absolute water tables often being 20–30 cm higher or lower than water levels in pools only 1–4 m away. However, as peat hydraulic conductivity was very low (median of 1.5 × 10−5 and 1.4 × 10−6 cm s−1 at 30 and 50 cm depths at the natural pool site), there was little deep subsurface flow interaction. We conclude that (a) for peat restoration projects, a larger total pool surface area is likely to result in smaller flood peaks downstream, at least during summer months, because peatland bulk specific yield will be greater; and (b) surface and near‐surface connectivity during storm events and topographic context, rather than pool size alone, must be taken into account in future peatland pool and stream chemistry studies.

Carbon concentrations in natural and restoration pools in blanket peatlands

“…The dry summer and autumn of 2013 was associated with a long lasting pool level drawdown to around 30 cm below the peat surface at CL (Holden et al, 2018). We found that this summer period was associated with the highest DOC concentrations for the natural pools, but for the restoration, pools the same period produced DOC concentrations in line with those observed in the other two summers sampled during the study.…”

“…When pool water surface levels fell and water tables within the peat were deep, DOC concentrations tended to be greater in the pool water and the soil solution, reflecting increased DOC production in the more aerobic peat (Clark et al, 2009) or simply a concentration effect due to evaporation and lack of rainfall dilution. Holden et al (2018) found that mean peat water‐table depths were 4.7 cm for the peat around the natural pools sites and 3.7 cm for the peat around the restoration pools at CL but much more variable over time around restoration pools, possibly due to a lower bulk specific yield. We found a relationship that suggested that even when pools were full to the peat surface, DOC concentrations in restoration pools were likely to be much higher than in natural pools.…”

“…However, for all other periods, including summers 2014 and 2015 when pool levels dropped to ~15–20 cm below the peat surface, CH 4 ‐C concentrations were never above 40 μg L −1 . Since the peat at CL had median permeability at 30 and 50 cm depths of 1.5 × 10 −5 cm s −1 and 1.4 × 10 −6 cm s −1 respectively (Holden et al, 2018), there may have been little flow into and out of the pools through the main peat mass during summer 2013. The high summer 2013 CH 4 concentrations may have been due to a combination of warm temperatures (MacDonald et al, 1998), a lack of water turnover in the pools, and reduced water pressure on the pool floor, which may have allowed trapped CH 4 bubbles below the pool floor to expand and be released.…”

“…At CL, Holden et al (2018) continuously gauged all of the pool water levels and also monitored water‐table depths within the peat at a distance of 1 m from the edge of each pool. These hydrological data are used in this paper to provide background information on site hydrological functioning and to aid interpretation of the pool water carbon data.…”

Blanket Peat Restoration: Numerical Study of the Underlying Processes Delivering Natural Flood Management Benefits

“…Second, increase the total static storage capacity (as also suggested by Holden, Moody, et al [2018]) by creating more shallow open water pools in other parts of the catchment as well as in-channels and gullies, such that its volume is comparable to the large water volumes associated with flood relevant storms. Note that this may also increase kinematic storage volume by further reducing surface flow celerities, but comes with the caveat that since total storm-water volumes scale with catchment area, full catchment-scale implementation may be costly/unfeasible.…”