Surface and near-surface runoff processes in peatland catchments.

Blanket peatlands are the most extensive peatland environments in the UK and exhibit flashy responses to rainfall (Evans et al., 1999; Acreman and Holden, 2013). Near-surface and surface flows dominate, produced by saturation-excess and percolation-excess mechanisms, resulting in rapid runoff. Previously, coupled models with explicit representations of subsurface and overland flow, have been used to simulate peatland hydrological processes. These models assume a clear boundary between the subsurface and surface; however, unlike most mineral soils, peatlands lack a distinct surface. This study suggests that the surface constitutes more of a transition zone, comprising a tangle of porous vegetation, meaning that flow is perhaps akin to stormflow in the upper peat, rather than true overland flow. Thus, it is unclear if flow here is Darcian or should be described by a surface flow equation such as the Chézy equation. Laboratory flume experiments were undertaken on two peat samples to simulate flow in the near-surface region, using the data collected to empirically test the Darcian model, DigiBog_Hydro, and determine if a Darcian approach can be used to represent these rapid flows. The flume experiments showed that for the scenarios with higher water levels, a flashier hydrograph was produced indicating that flow in the near-surface region is responsible for the rapid response of peatlands. DigiBog_Hydro was successful in producing reasonable replications of the observed data, with discharge hydrographs that were comparable in terms of magnitude and shape, and the modelled water tables showing similar patterns of input attenuation as the flume. The differences in the data (lags and absolute water-table heights) are thought to be a result of model ‘short-circuiting’ and initial model draining, respectively. These data show that the incorporation of a Darcian overland flow layer within DigiBog_Hydro is effective at modelling the rapid quasi-overland flow in the peat near-surface, indicating that a single representation of flow could be used in future and scaled up to entire peatland catchments.