Agriculture Reference
In-Depth Information
Trunk shrinkage, stem water potential, leaf water potential, stomatal resis-
tance and the rate of apple fruit growth have been used with varying success
as physiological parameters relevant to irrigation scheduling (Bravdo,
).
Leaf shrinkage has been found to be the most sensitive indicator of water stress
in other tree fruits. It can be measured as a change in leaf thickness which is
linearly related to leaf turgor.
Traditional depletion or maintenance irrigation may be by surface systems
or sprinklers. Surface irrigation is largely into basins around each individual
tree. This is a very efficient way of supplying water to young, widely spaced
trees. Furrow irrigation is also used but has been largely replaced by overhead
sprinkler irrigation because of its ease of management and flexibility of use.
Sprinklers apply water over most or all of the orchard surface and so can be
wasteful. They are most valuable when multi-purpose, e.g. when also used for
frost protection or tree cooling.
Trickle irrigation
Trickle irrigation (Elfving,
) differs from conventional bulk water applica-
tion in that smaller volumes are applied to the plant at more frequent intervals
and the application is to only a limited part of the soil surface area. Water is
piped through plastic tubing and is released only at selected points through
emitters (drippers). Typically the tubing will run on the soil surface down the
centre line of a hedgerow, or to either side of it, with emitters at regular inter-
vals. The more frequent the emitters, the less water is applied through each
and the less the risk of water loss through drainage.
The shape of the irrigated soil volume under the emitter varies with the
hydraulic characteristics of the soil. The wetted 'bulb' is always deeper than
it is wide but heavy soils have a higher horizontal to vertical ratio of bulb
dimensions than lighter (e.g. sandy) ones.
As discussed in Chapter
, roots proliferate in the wetted volume to give
a much higher rooting density, especially of fine roots, in that volume than is
characteristic of rainfed or sprinkler-irrigated orchards. There is some lateral
water movement from roots in the wetted bulb to roots outside this (Bravdo,
). This may contribute to the overall annual efficiency of root systems in
areas with alternation of wet and dry seasons, or even just periods, and to tree
anchorage, root hormone production, etc.
The irrigation water requirement is calculated much as for standard main-
tenance irrigation but application is based on high frequency, which maintains
the wetted bulb at a high water potential, and relies much less on the soil as a
water reservoir. The normal frequency in hot climates is once every
days,
but in extreme hot weather and on sandy soil may be a few times each day
(Bravdo,
-
). In humid areas where trickle irrigation is used to supplement
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