6.1 Peatland Drainage

Most peatland disturbances are related to activities that drain or otherwise desiccate the peat. This can result in subsidence because of the increased effective stress associated with a lower water table; such subsidence is only partially reversible upon rewetting. Additional subsidence occurs with lowered water tables as a result of enhanced decomposition by aerobic microbes, which is non-reversible.

Both causes of subsidence alter the structure, thus hydraulic properties, of the peat; peat collapse reduces its porosity and permeability and increases its water retention capacity, thus decreases hydrological connectivity between the old cutover peat and the new moss layer in restored systems (Gauthier et al., 2018). In addition to peat structural changes, subsidence of the peat surface can result in a large-scale change in the groundwater relationship to and interactions with adjacent ecosystems because the lowered surface can induce increased surface and groundwater inflows from adjacent ecosystems.

In peatlands, the drainage efficacy increases with ditch depth and decreases with ditch spacing. Drainage efficacy is greatest adjacent to the ditch and diminishes with distance, such that common spacings range from 30 to 50 m, depending on the land management goals. To a certain extent, this is a self-regulating process since peat subsidence associated with drainage reduces the hydraulic conductivity of peat, thus its potential to drain. However, on steeply sloped peatlands, such as blanket peatlands common in the British Isles, drainage may be contoured to catch overland flow, increasing the efficiency of peatland drainage (Holden et al., 2006). Drainage directly alters the water balance by promoting seepage to the drains, but the lower water table can limit evapotranspiration losses. However, the connectivity of the surface to water stored in the vadose zone and below the water table is enhanced (Price, 1996) by the higher water retention capacity associated with more decomposed drained peat, and by surface subsidence that keeps the water table closer to the surface than it otherwise would be. The higher connectivity of the surface to stored water in this setting helps maintain soil evaporation.

Draining peatland for agriculture or forestry is common in Europe, but less so in North America. In addition to the impacts of drainage on peat physical structure noted above, the addition of fertilizers or nutrient-rich water for agriculture can preferentially enhance the decomposition of surface peats (Liu et al., 2017). This can create an inversion of the typical peat profile, where the densest peat is at the surface and the peat becomes less dense with depth. As decomposition occurs in these surface peats, a greater proportion of overland flow occurs, similar to those drained for horticultural peat extraction. Drainage of peatlands for forestry often results in even lower water tables due to enhanced water loss from transpiration. As the root systems of trees often penetrate much deeper into the phreatic zone than shallow—rooted shrubs common in peatlands, there is a greater loss of water directly from below the water table than with shrubs alone. The lowered water table may enhance local groundwater discharge or reduce groundwater recharge.

In tropical peatlands, especially in South East Asia, peatland degradation is driven by logging, drainage, large-scale plantations, and recurrent fires (Dohong et al., 2017). The extremely high hydraulic conductivity found in tropical peat domes exacerbates the impact of ditching, resulting in water tables >1 m below the surface, promoting rapid decay of organic material (Baird et al., 2017), thus subsidence-causing topographic irregularities. This makes uniform rewetting difficult following ditch blockage (Dohong et al., 2018). Wösten and others (2008) suggested water table levels more than 40 cm below ground surface result in degradation, thus these systems are very sensitive to change.

It is worth noting that peatland degradation in all settings can result inadvertently from inappropriately-situated roadways, having the same effect on peat properties as drainage on the down-gradient side. These effects may be partially reversible in the case of temporary roadways, depending on the extent and duration of disturbance (Elmes et al., 2021).