Box 3 – Tropical Peatland Hydrology
Where tropical peatlands are common, rainfall can be extreme (~3800 mm/y; Wösten et al., 2007). Consequently, groundwater inflow is typically a minor contributor to the water balance, although in some peatlands it can be important geochemically (Grundling et al., 2015). Moreover, many tropical peatlands accumulate substantial layers of peat and become domed, similar to boreal bogs—thus ombrotrophic and moderately acidic (Baird et al., 2017). Water table fluctuations are primarily driven by rain events; by comparison, losses by groundwater outflow or evapotranspiration are much less variable. The water table commonly rises above the surface in wet seasons and can fall substantially in the dry season.
Where they have a domed profile, tropical swamps shed water. Groundwater outflow can be substantial, facilitated by extremely high hydraulic conductivity of the upper peat layers (470 m d-1; Baird et al., 2017). These rates are similar to that in unconsolidated gravel. However, horizontal flow is tempered by the generally low hydraulic gradients typical of extensive tropical swamps. These gradients approximately follow the topographic slope created by the peatland dome. Such gradients range from 0.01 near the margins to 0.001 at the top (Page et al., 2006). Deeper peat hydraulic conductivity can be two orders of magnitude lower than that near the surface, so the transmissivity feedback mechanism described in the main text of this book also operates in tropical peatlands.
Ditching of vast areas of tropical peatland for Palm oil production results in water table lowering, commonly to ~1 m or more, compared to ~0 m at undrained areas outside the dry season. Drainage ditches increase local hydraulic gradients so that groundwater outflow, via discharge to the ditch network, can be substantial. Drainage also causes peat subsidence, in part from the loss of water pressure that buoys the peat but also from peat decomposition. Drainage of a South East Asian swamp resulted in nearly 1.5 m of subsidence in the first five years, mainly attributable to primary consolidation. Thereafter, surface subsidence of 0.05 m y-1 was attributed to peat decay (Hooijer et al., 2012).
Reclamation of these areas is problematic because primary consolidation is only partially reversible and subsidence caused by peat decay is non-reversible. Ditch blocking can be effective at raising water tables during the wet season, but flooding may ensue. During the dry seasons, the water table is often below the base of the ditch, so ditches have little effect (Putra et al., 2021).
Peat fires are a common outcome of drainage activities in tropical swamps and are exacerbated by seasonal dry periods and longer-term cycles such as El Niño. Page and others (2002) estimated that the 1997 El Niño peat fires in Indonesia released a mass of carbon equivalent to 13 to 40 percent of that generated by annual global fossil fuel use. Fire also lowers the peat surface, and as a consequence extensive flooding can follow in wetter periods. Burning reduces the hydraulic conductivity of near surface peat (Holden et al., 2014).
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