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. An under recognized issue is that this leads to soil subsidence, or lowering of the peat soil, which causes increased flooding risks in low lying peatlands all over the world. In the tropics this process is very fast due to high temperatures.
Wetlands International raises attention for the problems of peat soil subsidence and solutions to come to sustainable peat landscapes.
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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. 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
Wetlands International works to raise awareness on this issue in South-east Asia, where as a result of the tropical circumstances subsidence and flood risks will become an even bigger issue than elsewhere in the world. Peat soil subsidence is faster due to higher temperature and flood risks are more severe because of the high precipitation.
Radical land use change needed
In South-east Asia the land conversion of peatswamp forests started relatively recently, mostly for palm oil and pulp plantations and other drainage dependent agricultural land-uses. If land use on peatswamps is not changed radically, subsidence and flooding of lowland peatlands will become unavoidable with tremendous land loss and subsequent socio-economic consequences. In particular coastal areas of eastern Sumatra and substantial parts of Borneo will be heavily affected.
Research results on subsidence in the tropics
Recent research has shown that in the tropics subsidence occurs very rapidly: in the first years after drainage at a 60-80 cm water table, peatland subsidence can be up to 50 cm per year. In the subsequent years this will stabilize to a constant 4.5 - 5 cm per year, resulting in a subsidence of 1.5 meters within 5 years and 4-5 meters within 100 years.
Looking at the medium to longer term, current drainage-based land-use in Indonesia and Malaysia will lead to extensive flooding in most of their lowland peatland areas. Within decades, the peatland landscape and plantations will face increasing flood risks and may ultimately result in the loss of arable land.
Problems will first occur during periods of high rainfall in combination with high tide extremes, causing floods in coastal areas. As the freshwater retention capacity of the peat swamps will also decrease, these same areas will face increased risks of salt water intrusion in the dry periods. This will impact coastal agriculture, communities and biodiversity.
Eventually, the peat layer may disappear altogether as a result of the oxidation process, exposing the underlying soils. In many cases these are infertile soils such as white sands or potential acid sulphate soils 9PASS), unsuitable for agriculture. These PASS soils turn acid when exposed to oxygen affecting productivity and freshwater habitats downstream.
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The impact of subsidence: can peatland drainage be sustainable in the long term?
Marcel Silvius, Programme Head Climate-Smart Land Use