Effectiveness of Current Farming
Systems in the Control of Dryland
Salinity
Leakage
Leakage
below farming systems
Leakage
varies throughout the basin
Leakage
below farming systems
Careful soil physical measurement and analysis combined
with techniques to measure water balance have given
us the capacity and confidence to measure the relative
magnitude of water leakage beneath our agricultural
systems compared to that under native vegetation.
However, leakage is difficult to measure, particularly
when comparing how much it varies under different farming
systems and management strategies. Leakage is often
only a relatively small component of the overall water
balance, and can vary dramatically between seasons.
Nevertheless, a substantial set of measurements clearly
show that our current agriculture is very leaky, that
is, its leakage values are very high compared to those
of our native forests and woodlands. CSIRO has collated
the final results of many of the leakage studies undertaken
throughout Australia. Figure 1 shows the high leakage
under annual pastures and crops in comparison to leakage
under native vegetation. It also illustrates that although
the leakage under perennial agriculture is generally
lower than for annuals, it is still much higher than
for native vegetation.
Figure 1. This figure combines the results of
many leakage studies throughout Australia to show
the relationship between annual rainfall and the
amount of leakage for three types of vegetation:
annuals, perennials and trees. Generally, while
leakage under perennials is lower than for annuals,
it is significantly higher than for trees.
The high variability and complexity of factors
affecting leakage mean that we need long-term estimates
over decades to have confidence in the ability of
current farming systems to control leakage sustainably,
and hence salinity. The use of computer models that
simulate the behaviour of plants, soils and water
use over these much longer time scales can help reveal
the complexity, episodicity and relative magnitude
of the effect of changes in farming systems on the
amounts of long-term leakage.
One example is the results of a simulation study
of the annual leakage amounts under three different
farming systems—annual pasture, perennial pasture
and trees—at Hamilton in Victoria. The modelling
shows (Figure 2) the great variation in leakage between
years. It also highlights that although there is
generally less leakage under well managed perennials
than under annuals, there is much more leakage under
both systems than there is under trees alone. In
other words, the leakage under pasture systems remains
many times greater than leakage under the original
vegetation or the discharge capacity of the landscape.
Figure 2. This figure shows the annual amounts
of leakage simulated for three vegetation types
over 26 years. While the amount of leakage varies
considerably between years, leakage under perennials
is generally less than it is under annuals, while
leakage under trees is significantly less than
it is under annuals or perennials.
While gains are made in moving from annual to well
managed perennial pasture, our agriculture can
not sustainably put more water into the landscape
than the groundwater systems can discharge into
rivers and lakes. Once our agriculture exceeds
the discharge capacity of the landscape, water
tables will rise. In most Australian landscapes
this will eventually result in salinisation.
The following results and case studies can help us
understand the interactions between soils, water
and vegetation, they are good tools for comparing
the relative differences between various management
options. The overall results of the studies provide
greater insight into the long-term behaviour of leakage
to groundwater under agricultural systems.
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Grazing systems
The Murray-Darling Basin has extensive areas of
grazing systems over climatic conditions that range
from the high rainfall high input systems in the
east and south to the semi-arid pastoral systems
in the west. In the high rainfall (>600 mm)
regions, leakage under farming systems can be high
(90—100+ mm/year). There are significant differences
between the water use of trees and agricultural
systems. Work carried out by researchers from the
Department of Natural Resources and Environment
at sites near Rutherglen (Victoria) estimated that
the deep drainage under perennial grasses ranged
between 50 and 120 mm per year depending on grazing
management and nutrition. This is far more than
the 5—10 mm of leakage estimated for the woodland
replaced by these pastures. Hence pasture management
options could make only small reductions in leakage
to groundwater. A high proportion of trees will
need to be incorporated into the landscape to achieve
a significant reduction in leakage, should salinity
be a problem in those areas. In this case, where
to plant the trees becomes the issue.
In the medium rainfall (400—600 mm) zone the
differences in water use between trees and agriculture
are less distinct. As a result, the variation in
leakage rates between different grazing systems
is important, as is the variable nature of the
rainfall. Studies have shown that in some situations,
perennial systems are controlling leakage in these
areas to around 10 mm/year. In most of these cases,
perennials reduce leakage by around 20—50%
when compared to annuals, except for lucerne grown
continuously, which can reduce leakage by up to
90%. While perennial systems reduced leakage rates,
the rates remained two to three times greater than
under the woodland vegetation that the pastures
replaced.
In the low rainfall (<400 mm) regions of the
western and southern Murray Basin (Upper South-East
and Cooke Plains areas of South Australia), the
use of deep rooted lucerne has been shown to reduce
leakage rates to the level of natural Mallee vegetation
(less than 1 mm/year). In the western lands of
New South Wales, a study found that clearing trees
for grazing seemed to cause little or no increase
in leakage rates for well managed systems (which
are not over grazed) with heavier soil types (less
than 1 mm/year), and also for sandy soils (2 mm/year).
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Cropping systems
Cropping systems are generally found in a rainfall
belt that is narrower than that of areas used for
grazing alone (250—500 mm in the southern
Basin; 400—600 mm in the northern parts).
Leakage amounts vary from a few millimetres per
year in the Mallee to more than 80 mm/year in the
higher rainfall cropping areas. Removing the long
fallow period has had the biggest impact in reducing
leakage in these cropping systems, accounting for
a possible reduction in leakage of around 20—40%.
Traditionally, the long fallow has been used to
enhance water storage before cropping and as a
disease break. Incorporating lucerne as part of
a long-term or shorter-term rotation is another
strategy to reduce the leakage under cropping.
However, variability in rainfall reduces the effectiveness
of lucerne in rotations, leading to deep drainage
when lucerne is not part of the rotation.
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Agroforestry systems
Researchers and farmers are increasingly
interested in the potential benefits of agroforestry
systems. These systems can range from small blocks
of trees, to belts of trees, to scattered trees in
paddocks. It has been suggested that a small percentages
of trees over the land area can reduce leakage over
the entire area. Two general possibilities exist:
tree belts and break of slope plantations.
- The roots of tree belts extend laterally
into the cropping systems to use water that
is excess to crop requirements.
- Break of slope plantations intercept
the shallow lateral movement of groundwater
before it discharges at the surface.
In the low rainfall Mallee region, a University
of Adelaide study of several rainfall areas has
shown that mature trees can control leakage 20—50
m into the cropped area. This suggests that in
such low rainfall areas, a typical agroforestry
system (using 10 m tree belts at 100 m spacing)
would halve annual leakage from 20 mm to 10 mm.
This is still much greater than the estimated
average leakage of only 0.6 mm/year under native
vegetation. Having said this, close (<50 m)
tree belt spacings could be expected to completely
control leakage, although crop production will
be severely reduced. In some types of catchments
it is possible to intercept shallow local groundwater
flows before they reach the more saline aquifers
by placing belts of trees across the flow path
and using them as pumps. The ability of trees
to reduce leakage will vary dramatically, depending
on specific catchment characteristics. However,
the groundwater systems in many catchments in
Australia have low permeability. In these cases,
the ability of trees to use water before it becomes
leakage is limited to water lying directly beneath
them.
Back to top
Plantation forestry
Leakage rates under mature plantations in the
low to medium rainfall areas (<600 mm/year)
are close to zero in most cases. This means that
for any particular catchment, the volume of leakage
should reduce in accordance with the increase in
plantation area. Although, for the first few years
of a plantation, leakage may be greater.
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Agriculture on saline
land
It has been a popular belief that plants are able
to use large volumes of groundwater when a shallow
groundwater table is present. Several studies have
shown that this is not true, even where the groundwater
is not particularly saline. In some cases there
may be seasonal recharge and discharge. This means
that recharge occurs quickly during the wetter
season then is used by the plants over the dry
season. However, generally the net effect has been
only small.
A possible role for vegetation in groundwater
discharge areas is to minimise recharge so as not
to exacerbate the salinity problem. Often, much
of the water that leads to salinity problems is
in the vicinity of the saline land. Reducing this
recharge, while not preventing the fundamental
cause of the problems, may reduce the likelihood
of them becoming worse.
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Leakage varies throughout
the Basin
The Murray-Darling Basin encompasses a range of
rainfall and climate conditions over a large area.
This means that while similar patterns of leakage
behaviour would be expected across the Basin, we
would not expect all areas to behave in exactly the
same way.
The following results and case studies illustrate
the consistent and widespread increases in leakage
throughout the Murray-Darling Basin as a result of
agricultural practice. Most of these studies are
based on detailed field measurements supported by
modelling scenarios. The overall results of these
studies provide greater insight into the long-term
behaviour of leakage into groundwater under agricultural
systems.
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