The Rural Voice, 2001-03, Page 19from a continuously -flowing source
from an underground aquifer. It will
be cold but dean. Its temperature
therefore makes it unsuitable for
irrigation on growing crops. At
temperatures below I 2C growth
stops. Cold water may therefore
hinder crop growth but the low
temperature may be suitable for fish,
especially trout. Being continuously
fed, this type of pond requires an
overflow structure whose size and
cost depends entirely on the flow
volume during the maximum rainfall.
When water storage for
multiple uses is considered,
there are several needs
which must be met.
• I. Irrigation volume. For this figure
take your best estimate for maximum
crop needs with minimum rainfall
and double it.
• 2. Minimum depth of water required
to prevent growth of weeds and algae
or other aquatic growths.
• 3. Recreational desirability – Fish
farming, swimming, water clarity and
temperature.
Ignoring Type 1 Ponds as out of
the reach of the ordinary farmer, we
can look at the next two in some
detail.
The name gives us a clue to the
structure of a by-pass pond. It is
located near a stream — either
permanent or intermittent — and in
such a position that it can fill to the
maximum level of the supply stream
at the point of water withdrawal from
it and will retain at least 90 per cent
of its maximum when the peak of the
stream flow has passed. Excess above
the level of the low stream flow
returns to the stream. No overflow is
required for this pond. Each time the
level in the supply stream exceeds
that of the pond, water flows into the
pond and thus short -duration summer
flash floods will replenish this pond
to its high water level.
The fill -channel for this pond must
be protected from erosion, either by a
wide, shallow, grassed entry course
or by a rock -lined channel or a buried
pipe of adequate size. No overflow is
required. Pond depth should be a
minimum of two metres in order to
remain clean and suitable for
recreational purposes.
Type 3 ponds require the same
depth as Type 2 Ponds but will
require a means for filling. If
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MARCH 2001 15
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