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When seepage reduction via channel lining
and other engineering solutions is not cost-effective, trees
can often provide an economical means of managing the impacts
of seepage (Goulburn-Murray Water - unpublished, 1999). Tree
plantations do not prevent seepage, but intercept groundwater
seepage flow and hence control local groundwater discharge which
can degrade surrounding areas through waterlogging or salinity.
Significant tree planting was carried out in the 1990s in Boort,
Victoria, by providing financial assistance to landholders adjacent
to channels (Dunstone, 1998). Investigations are underway as
to the success of this scheme.
A number of trees are tolerant of high watertables and can transpire
a lot of the seepage from the channel. Establishing trees in
areas of seepage can reduce the watertable in a localised area
around the tree. Some shrubs or bushes such as phalaris are also
high-water-using plants (Wimmera Mallee Water, 1995).
Groundwater drawdown responses due to tree planting alongside
channels have been demonstrated in a limited number of studies
(Sonogan and Patto, 1985; Tagirov and Dosahmetov, 1970). The
establishment of vegetation adjacent to channels has been shown
to lower watertables associated with seepage. In doing so waterlogging
is reduced and a slowing, or elimination of the process of soil
salinisation and degradation can be achieved. However, it is
recommended that the planting of trees not be undertaken where
seepage rates are generally classified as high (Sonogan and Patto,
1985).
A study in the Shepparton Irrigation Region in northern Victoria
demonstrated that tree plantations may not be a viable alternative
to manage channel seepage for some sites. This study focused
on groundwater/tree interactions and found that salt accumulation
was occurring within the root zone in shallow watertable areas
(Heuperman, 1999). Salt accumulation would be expected to increase
to a threshold that restricts the water use and growth of the
tree plantation (Morris and Collopy, 1999).
Planting of appropriate tree species tolerant to shallow watertables
and saline conditions could assist with arresting the effects
of channel seepage. Red gums, melaleucas and other selected species
are planted near channels in northern Victoria and encouraged
in salinity management plans (Sinclair Knight Merz, 1998).
Tree planting alone is unlikely to provide a sufficient control
measure in the short term. Some other means, such as seepage
interception, would probably be required to provide early benefits
and enable the trees to grow to maturity. However, tree planting
could be a valuable part of an overall long-term seepage control
strategy (Sinclair Knight Merz, 1998).
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Groundwater intervention techniques
Core
trenching
Groundwater
pumping
Tile
drains |
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