normally just below (5–15 cm) the peat surface (allowing the presence of a shallow,
oxygenated layer for the tree roots) and only above the surface following heavy rain
or in areas which are affected by flooding from adjacent river systems.
18.6 Sustainability of Long-Term Peatland Management
for oil Palm
18.6.1 Subsidence and Long-Term Impacts
Peat is 90% water by weight. Once a peatland is drained and water removed, the
remaining organic material will compact or subside. This subsidence will continue
through a combination of oxidation and shrinkage until all the peat is lost or it
subsides to a point where is no longer possible to drain.
The duration and severity of flooding will increase over time, as the peat surface
gets closer to the natural drainage limit. In the long term, sufficient drainage of a
peatland to enable crop production may become a challenge, particularly during wet
periods, because drainage by gravity is no longer possible, leading to serious
environmental and operational issues such as continuous flooding, saline intrusion,
accessibility issues and yield losses. If assisted drainage in the form of water pumps
is applied, increased operational costs will be incurred, possibly to the extent of
negative return of investment. In addition, pumped drainage will lead eventually to
total loss of the peat layer and permanent flooding when pumping becomes
non-viable or the concession period ends. It is, therefore, critical to stop drainage
before reaching a point of no return.
The drainage base inside the plantation is in most cases calculated in reference to
the water level in the closest receiving water body and on the distance to this water
body. In order to drain the plantation area, the drainage base inside the plantation
must be at higher elevation than the water level in the water body. This is because
there must be a difference in the GWL before the water can flow. A general rule of
thumb is that for each kilometre of distance into the plantation, the drainage limit
elevation increases by 20 cm relative to the mean water level in the adjacent river or
water body (Department of Irrigation and Drainage, Sarawak 2001). As shown in
Fig. 18.5, the water profile in the peat soil must have a minimum gradient (slope) of
1 in 5000 for the water to flow to the water body and subsidence can lead to
disruption of drainage.
18.6.2 Drainability Assessment
Historically, peatlands were continuously drained over time due to peat subsidence
to ensure the viability of the land for crop cultivation. This activity would continue to
the extent that perimeter bunding or mechanical pumping was required to prevent
18 Role of the Roundtable on Sustainable Palm Oil (RSPO) in Tropical Peatland. . .
529
oxygenated layer for the tree roots) and only above the surface following heavy rain
or in areas which are affected by flooding from adjacent river systems.
18.6 Sustainability of Long-Term Peatland Management
for oil Palm
18.6.1 Subsidence and Long-Term Impacts
Peat is 90% water by weight. Once a peatland is drained and water removed, the
remaining organic material will compact or subside. This subsidence will continue
through a combination of oxidation and shrinkage until all the peat is lost or it
subsides to a point where is no longer possible to drain.
The duration and severity of flooding will increase over time, as the peat surface
gets closer to the natural drainage limit. In the long term, sufficient drainage of a
peatland to enable crop production may become a challenge, particularly during wet
periods, because drainage by gravity is no longer possible, leading to serious
environmental and operational issues such as continuous flooding, saline intrusion,
accessibility issues and yield losses. If assisted drainage in the form of water pumps
is applied, increased operational costs will be incurred, possibly to the extent of
negative return of investment. In addition, pumped drainage will lead eventually to
total loss of the peat layer and permanent flooding when pumping becomes
non-viable or the concession period ends. It is, therefore, critical to stop drainage
before reaching a point of no return.
The drainage base inside the plantation is in most cases calculated in reference to
the water level in the closest receiving water body and on the distance to this water
body. In order to drain the plantation area, the drainage base inside the plantation
must be at higher elevation than the water level in the water body. This is because
there must be a difference in the GWL before the water can flow. A general rule of
thumb is that for each kilometre of distance into the plantation, the drainage limit
elevation increases by 20 cm relative to the mean water level in the adjacent river or
water body (Department of Irrigation and Drainage, Sarawak 2001). As shown in
Fig. 18.5, the water profile in the peat soil must have a minimum gradient (slope) of
1 in 5000 for the water to flow to the water body and subsidence can lead to
disruption of drainage.
18.6.2 Drainability Assessment
Historically, peatlands were continuously drained over time due to peat subsidence
to ensure the viability of the land for crop cultivation. This activity would continue to
the extent that perimeter bunding or mechanical pumping was required to prevent
18 Role of the Roundtable on Sustainable Palm Oil (RSPO) in Tropical Peatland. . .
529