18.5.1.1 Ground Water Level (GWL)
A proper hydrology and water management study should be done in the beginning to
design a drainage system that can maintain the ground water level (GWL) at the
desired level. Failure to do so would be detrimental to oil palm growth/yield and
estate operation (Pupathy 2018). Subsequent work to repair the damage may incur
higher cost. Higher GWL (e.g. <40 cm from peat surface) may reduce yields but
would reduce GHGs emissions and subsidence as well as increase the lifespan of a
plantation that could over time reach an undrainable situation or expose acid
sulphate as subsoil.
A good water management system for oil palm on peatland is one that can
effectively maintain an average GWL of 40–50 cm average (range of 30–60 cm)
as measured by a groundwater piezometer reading. It should be able to remove
excess surface and sub-surface water quickly during wet seasons and retain water for
as long as possible during dry spells. The moist peat surface at this GWL level will
also help to minimize the risk of accidental peat fire and to minimize subsidence.
In droughts, the GWL can drop 0.5–1 cm per day. During a 1 month drought, the
GWL may fall 15–30 cm if there is no input from surface or sub-surface flow.
Therefore, in areas with extended dry seasons, it is recommended to raise the GWL
in the plantation as high as possible at the end of the wet season—so that in
subsequent dry periods the GWL can be maintained at 40–50 cm in the field.
Such good water management can most easily be realized in regions that rarely
suffer from prolonged drought periods; during which rainfall deficits occur
(i.e. evapotranspiration exceeds rainfall). In drought-prone regions, GWL tends to
fluctuate severely and may often fall below 0.6 m below peat surface. In such
conditions, more efforts will be needed to retain additional water at the end of wet
season to reduce GWL drop in the dry season, than are needed in regions where dry
seasons are mild.
A well planned and executed water management system with water control
structures should be used for drainage and effective water management (Parish
et al. 2019a). Water gates and/or weirs should be installed at strategic locations
along the main and/or collection drains for effective control of the GWL at an
optimum level (see Fig.18.2). Flap gates are usually installed at the main outlets in
coastal regions, which may be subject to tidal or seasonal variations. It is generally
not recommended to install permanent water management structures (made of
concrete) as subsidence will ruin the system. It is most appropriate to use natural
materials such as wood or sandbags for constructing weirs/stop-offs.
A cascade of closely-spaced control structures is needed to maintain relatively
constant, high GWL in the drain during the dry season (Ritzema 2007). Weirs or
stop-offs should be placed at appropriate intervals to ensure that the drop-off across
each weir is about 20 cm (i.e. 5 weirs are needed for a drop of 1 m—with a spacing of
200–400 m between the blocks—depending on the slope (Parish et al. 2019a).
Good water management is that which can ensure that the in-field GWL is
maintained at an average of 40–50 cm below the surface. This translates to a level
18 Role of the Roundtable on Sustainable Palm Oil (RSPO) in Tropical Peatland. . .
517
A proper hydrology and water management study should be done in the beginning to
design a drainage system that can maintain the ground water level (GWL) at the
desired level. Failure to do so would be detrimental to oil palm growth/yield and
estate operation (Pupathy 2018). Subsequent work to repair the damage may incur
higher cost. Higher GWL (e.g. <40 cm from peat surface) may reduce yields but
would reduce GHGs emissions and subsidence as well as increase the lifespan of a
plantation that could over time reach an undrainable situation or expose acid
sulphate as subsoil.
A good water management system for oil palm on peatland is one that can
effectively maintain an average GWL of 40–50 cm average (range of 30–60 cm)
as measured by a groundwater piezometer reading. It should be able to remove
excess surface and sub-surface water quickly during wet seasons and retain water for
as long as possible during dry spells. The moist peat surface at this GWL level will
also help to minimize the risk of accidental peat fire and to minimize subsidence.
In droughts, the GWL can drop 0.5–1 cm per day. During a 1 month drought, the
GWL may fall 15–30 cm if there is no input from surface or sub-surface flow.
Therefore, in areas with extended dry seasons, it is recommended to raise the GWL
in the plantation as high as possible at the end of the wet season—so that in
subsequent dry periods the GWL can be maintained at 40–50 cm in the field.
Such good water management can most easily be realized in regions that rarely
suffer from prolonged drought periods; during which rainfall deficits occur
(i.e. evapotranspiration exceeds rainfall). In drought-prone regions, GWL tends to
fluctuate severely and may often fall below 0.6 m below peat surface. In such
conditions, more efforts will be needed to retain additional water at the end of wet
season to reduce GWL drop in the dry season, than are needed in regions where dry
seasons are mild.
A well planned and executed water management system with water control
structures should be used for drainage and effective water management (Parish
et al. 2019a). Water gates and/or weirs should be installed at strategic locations
along the main and/or collection drains for effective control of the GWL at an
optimum level (see Fig.18.2). Flap gates are usually installed at the main outlets in
coastal regions, which may be subject to tidal or seasonal variations. It is generally
not recommended to install permanent water management structures (made of
concrete) as subsidence will ruin the system. It is most appropriate to use natural
materials such as wood or sandbags for constructing weirs/stop-offs.
A cascade of closely-spaced control structures is needed to maintain relatively
constant, high GWL in the drain during the dry season (Ritzema 2007). Weirs or
stop-offs should be placed at appropriate intervals to ensure that the drop-off across
each weir is about 20 cm (i.e. 5 weirs are needed for a drop of 1 m—with a spacing of
200–400 m between the blocks—depending on the slope (Parish et al. 2019a).
Good water management is that which can ensure that the in-field GWL is
maintained at an average of 40–50 cm below the surface. This translates to a level
18 Role of the Roundtable on Sustainable Palm Oil (RSPO) in Tropical Peatland. . .
517