Peatswamps play an important role in preventing floods and droughts in surrounding landscapes due to their high water retention capacity. The drainage of peat soils lowers the water table and the peat dries and decomposes (rotting process), leading to high carbon emissions. This also leads to soil subsidence, or lowering of the peat soil. In the tropics this process is very fast due to high temperatures and may result in increased flooding risks and possibly in production loss in agriculture and silviculture in low lying peatlands.
Wetlands International collaborates with Deltares to better quantifying the peatland subsidence and flooding problem in SE Asia and raises attention for mitigating this problem. The insights in peatland drainage impacts and subsidence rates are largele taken from Hooijer et al. (2010 and 2012).
Quick link: What can we do about it.
What is soil subsidence exactly?
Subsidence is the lowering of the soil surface as the result of compaction, consolidation and loss of volume (carbon) due to oxidation and erosion. Peat soils comprise of 10% accumulated organic materials and 90% of water. They conserve large amounts of carbon through the accumulation of dead roots and other tree material in the peat soil. When drained, the peat oxidises and all the peat above the drainage level will eventually be lost. In areas with polders and pump-operated drainage the soil will continue to subside below the gravity-drainage limit.
Subsidence and flooding
Subsidence and the related flood risk is a well-known and inevitable phenomenon in all places in the world where lowland peatlands have been converted to drainage-dependent land-uses, such as in the UK (Somerset), USA (New Orleans, Everglades), northern Germany, Denmark, and for example the Netherlands where a large part of the highly populated west is situated below sea-level as a result of soil subsidence.
Subsidence in South-east Asia
In South-east Asia, subsidence and flood risks may become an even bigger issue than elsewhere in the world as a result of the higher temperature and flood risks are more severe because of the high precipitation.
Due to its origin in mangrove swamps and coastal river floodplains, the bottom of tropical peat domes is usually close to sea level. The reason these areas are still land, and not sea or lakes, is because tropical peatlands have developed as huge domes on top of the mineral soil, often reaching over 10 metres in height. In western Indonesia (Sumatra and Kalimantan) and Malaysia combined, many of the former peat swamp forests have been converted to palm oil and pulp wood plantations.
Research results on subsidence in the tropics
Research by Deltares (Hooijer et al., 2010 and 2012) has shown that in the tropics subsidence occurs very rapidly: in the first years after drainage peatland subsidence is typically 1 to 2 metres. In subsequent years this stabilizes to a constant 3 to 5 cm per year, resulting in a subsidence of up to 1.5 meters within 5 years and 4-5 meters within 100 years.
Some lowland peatlands in Indonesia and Malaysia are already facing flooding due to such subsidence, and many other and more extensive areas will follow if this issue remains unaddressed.
The impact of subsidence: can peatland drainage be sustainable in the long term?
Marcel Silvius, Programme Head Climate-Smart Land Use
Partner: Deltares, Gadjah Mada University, Puslitbang Air, Puslitbang Kehutanan (Forest Research and Development Agency - FORDA, Ministry of Forestry Indonesia), Forest Research Institute Malaysia (FRIM)