E1 – E 3
E
WATER MANAGEMENT
E1
Introduction
Plants obtain almost all the water they require from the soil. Of the enormous
quantities taken up, only a fraction of one percent is retained in the plant tissues.
Yet this minute fraction is all important for growth. Water is required for a
number of physiological functions, including manufacture of carbohydrates,
maintenance of hydration of the protoplast and for the transport of food and
nutrients to the various organs.
E2
Consumptive Use of Water
The consumptive use of water (evapotranspiration) of these crops ranges from
120-150 mm per month. To meet requirements, the monthly rainfall should equal
or preferably exceed this figure, failing which moisture stress would occur. The
rainfall in the UPB group ranges from 1600 to 2500 mm per year, with the
average being 2000 mm. Monthly distribution is reasonably uniform, but there
can be, in some estates, one to two months of dry period where the rainfall is less
than 100 mm.
E3
Effects of Moisture Stress
Moisture stress causes reduction in cell division and cell elongation – two
important growth processes. Its effect on oil palm and coconuts is summarized in
the Table below.
Oil Palm
i
Accumulation of unopened spears
ii
Reduced leaf production
iii
Drying and snapping of leaves
iv
Abortion of inflorescence
v
Affects sex differentiation favouring male flower production
vi
Reduced oil/bunch yield
Coconuts i
Premature nut fall
ii
Reduced nut yield
iii
Reduced nut size
iv
Reduced copra/nut
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E4–E5
E4
Weirs for Moisture Conservation
To conserve moisture during these periods, a series of weirs are constructed
across the collection drains to hold back water and raise the water-table to within
50-75 cm from the surface. The weirs are made of wood, concrete or they can
simply be sandbags. The concrete structures are either built on site or they are
precast and placed into position. To regulate the height of the watertable, wooden
planks are slotted into the desired level. Except for periodic flushing of acidic
water during the rainy seasons, the blocks are maintained at the predetermined
level at all times. The density of weirs varies with the soil type, slope, rainfall
and cropping system. On the average, one weir is provided for every 40 to 60
hectares or every 600-1000 m along the collection drain. Assisted by the water
gates at the discharge ends of the main drains, the weirs are very effective in
minimizing the adverse effects of the moisture stress.
E5
Moisture Conservation on Fragile Soils
Water management is particularly important on the acid sulphate and peat soils.
These soils are fragile and if overdrained, they will rapidly deteriorate. On the
acid sulphate soils, the water level should be maintained up to the jarosite layer,
submerging the pyrite (FeS2) and preventing it from oxidizing to sulphuric acid,
which can cause a steep drop in the pH.
Watergate
Weir with removable wooden planks
Irrigation using non saline river water should
also be actively pursued where ever possible
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