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Salt of the Basin-'Business as Usual' is not a Viable Option
Bill McCormick
Science, Technology, Environment and Resources Group
14 December 1999
Salinisation of the land and water resources of the Murray-Darling Basin
can be the result of salt stored in soils and/or groundwater being mobilised
by extra water provided by human activities such as irrigation or land
clearing. This extra water raises watertables. The water evaporates when
near the surface leaving the salt behind, causing land salinisation. The
mobilised salt can also increase surface water salinity when it moves
into watercourses(1). Both types of human-induced salinisation, dryland
salinity and irrigated land salinity, contribute to the salinisation of
the water courses in the Basin.
The consequences of salinisation and rising saline watertables include:
- declining river water quality,
- loss of productive land,
- damage to roads and buildings,
- damage to conservation reserves and remnant vegetation, and
- increased flood risk.
Dryland salinity was first reported in Victoria in 1853 and irrigation
salinity was noticed in the Kerang region in Victoria in the 1890s. The
national increase in human induced salinity from 1982 to 1989 was 9 per
cent per annum. By 1990, 798 000 hectares (ha) of Australia was affected
by dryland salinity and 156 000 ha by irrigation salinity(2). It has continued
to increase since then.
Salinity Audit
A Salinity Audit of the Murray-Darling Basin was commissioned by the
Murray-Darling Basin Ministerial Council in order to assess the salinity
hazard of the Basin. The Audit was released on 22 October 1999. A recent
review of the 1988 Salinity and Drainage Strategy (expanded below) indicated
that the main type of salinisation in the Basin would shift from irrigation
salinity to dryland salinity. This is because the time lag between land
use changes and salt mobilisation to rivers and the landscape in irrigation
districts is relatively rapid. In releasing the Audit, the Council observed
that major land use changes in the Basin are necessary in order to avoid
further salinisation (and adapt to higher salinity levels)(3). The Council
stated that 'Business as usual' is not an option. It has requested that
the Murray-Basin Commission prepare a draft Basin Salinity Management
Strategy by June 2000.
The Audit reported on salinity levels and it established a trend line
for salt mobilisation in the landscape to predict the rise in salinity
in river valleys if there is no change in land and water management practices.
In order to predict the degree of salt mobilisation, resultant salinity
levels and the extent to which land is threatened by rising watertables,
available data on the observed rate of groundwater rise, the current depth
of groundwater and the salinity of groundwater, was used. Estimates were
provided for 2020, 2050 and 2100.
River Salinity Predictions
The desirable salinity limit for drinking water is 800 electrical conductivity
units (EC). Within 20 years salinity levels of the Murray River at Morgan
are predicted to exceed 800 EC 40 per cent of the time. Sixty per cent
of this increase will be due to dryland sources (one quarter from outside
the Mallee region).
In Victoria, the Avoca and Loddon Rivers already record salinities above
800 EC on a flow weighted basis and these could rise significantly by
2050.
The average salinity at the end of the NSW rivers will approach or exceed
the 800 EC level by 2020, namely the Lachlan (780 EC), the Bogan (1500
EC), the Macquarie (1290 EC) and the Namoi (1050 EC).
On the basis of preliminary information, it is predicted that salinity
levels of three Queensland Rivers in the Basin will rise from present
levels of 200-300 EC to 1000-1200 EC by 2020.
Irrigation Area Salinity
The Audit refers to a prediction that all irrigation areas in the southern
Basin will have watertables within 2 metres of the surface by 2010. It
is expected that the 12 500 ha in SA which are water logged will
increase to 20 000 ha. The at-risk irrigated area in Victoria
will increase from 440 000 ha to 600 000 ha, while the
412 000 ha of NSW irrigated areas along the Murray and the Murrumbidgee
will require drainage by 2020.
Dryland Salinity
Salinity Audit predictions are only available for Victoria and SA. Presently
68 000 ha of SA (in the Basin) are affected by dryland salinity.
This will rise to 96 000 ha by 2020 and to 116 000 ha by 2050.
The area of the Basin in Victoria affected by dryland salinity is predicted
to be 843 000 ha of which 254 000 ha will be severely affected.
Bore data for NSW indicates that there are potentially 5.4 million ha
with groundwater at or near the surface. Serious salinisation could be
experienced in the order of 2 to 4 million ha of the Basin in NSW. Another
study estimate is that 7.5 million ha of the NSW has the potential to
become salinised if there is no further intervention.(4)
Dryland salinity in Queensland is not as severe as elsewhere in the Basin.
However by 2020, 633 000 ha of Queensland in the Basin could have
watertables within 2 metres of the surface and therefore potentially be
at risk of salinisation.
Impacts
The value of one EC unit change in river salinity at Morgan in SA has
been estimated at $93 000-$142 000 per year. Urban salinity
in Wagga Wagga is costing $3.2m per annum and this will total $95 m over
the next thirty years, if nothing is done.(5) An estimated 34 per cent
of State roads and 21 per cent of national highways in south-western NSW
are affected by high watertables, estimated to cost $9 million per
year.
Studies in the Loddon-Campaspe and the Upper Macquarie catchments quantified
the annual costs of rising watertables and salinity at $1 million
per 5 000 ha of visibly affected land. Assuming that up to 5m ha
will be visibly affected by 2100, then simple arithmetic would give the
Basin wide impact costs of salinity at around $1 billion per annum.
These costs were grouped into four main classes:
- additional repair and maintenance costs of infrastructure and equipment,
- the cost of undertaking protective works or actions,
- the cost of associated with shortened expected lifespans of infrastructure
and equipment, and
- revenue foregone because of reduced capacity to use, or charge for,
salinised infrastructure or services.
It is not as easy to allocate a monetary cost on the environmental damage
caused by salinity, such as impacts on wetlands and restricted fragmented
ecosystems and endemic species. However without intervention more than
half of the Chowilla wetlands, a Ramsar Convention wetland, on the Murray
River will be lost to salinity.
What should be done
To date the focus on salinity in the Basin has been on irrigation salinity
and rising river salinity. The Salinity and Drainage Strategy, implemented
over the past ten years via the construction of salt interception schemes,
has helped reduce the average salinity at Morgan by 10 per cent but this
gain is expected to be eliminated within 20 years. Measures such as groundwater
pumping, drainage, soil moisture monitoring, water reuse and more efficient
irrigation practices have also been implemented over the years with some
success in reducing groundwater mobilisation in irrigation areas. Action
addressing dryland salinity has not had this immediate success but it
is going to be just as, or more important, in the long-term to control
the increase in and hopefully reduce groundwater and river salinity levels.
The large scale clearing of deep rooted native vegetation and its replacement
with shallow rooted annual crops and pastures has lead to increasing amounts
of water 'leaking' through the root zone and into the groundwater. In
high rainfall areas (>600 mm) of the Basin, deep drainage from perennial
pastures ranged between 50-120 mm per year compared to 5-10 mm per year
for the original woodland cover. In lower rainfall areas the difference
in water use between trees and agriculture is less distinct(6).
Agricultural systems need to be modified to reduce the rate of leakage
of rainfall into groundwater to levels approaching that of 1-5 mm/year
recorded with native perennial vegetation in order to control dryland
salinity. While perennial pastures in low and medium rainfall zones can
significantly reduce leakage a high proportion of trees on pastures in
high rainfall zone (>600 mm) is the only option for salinity control.
In the southern portion of the Basin, the removal of long fallow rotations
and inclusion of lucerne in rotations has reduced leakage to 12-25 mm/year.
Agroforestry systems can halve leakage rates but not to the levels under
native vegetation. In areas with rainfall below 700 mm, however, the leakage
rate under plantations is close to zero.
The Audit indicated that while changes to farming systems in the lower
rainfall and irrigation areas of the Basin will achieve significant reductions
in leakage, the only option for salinity control in grazing zones with
rainfall of more than 600 mm/year is large scale tree planting, where
the benefits are restricted to the areas under vegetation.
Endnotes
- F. Ghassemi, A. J. Jakeman and H. A. Nix, Salinisation
of Land and Water Resources-Human causes, extent, management and case
studies, UNSW Press, Sydney, 1995.
- D. R. Williamson, 'Salinity-An Old Environmental Problem', in Year
Book Australia 1990, AGPS, Canberra, 1990, pp. 202-211.
- Communique from the Murray-Darling Basin Ministerial Council, 22 October
1999.
- PMSEIC, 'Dryland Salinity and its impact on rural industries and the
landscape', in Prime Minister's Science, Engineering and Innovation
Council Occasional Paper, no. 1, Canberra, DISR, 1999.
- 'War on Salinity gets NLP boost' Australian Landcare, Sept.
1999, pp. 26-29.
- G. Walker, M. Gilfedder and J. Williams, Effectiveness of
Current Farming Systems in the control of dryland salinity, CSIRO,
Canberra, 1999.
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