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Salt of the Basin: 'business as usual' is not a viable option.



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Research Note 22 1999-2000

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, 1.

1995.

D. R. Williamson, 'Salinity-An Old Environmental Problem', in Year Book Australia 1990, AGPS, Canberra, 1990, pp. 202-211. 2.

Communique from the Murray-Darling Basin Ministerial Council, 22 October 1999. 3. 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.

4.

'War on Salinity gets NLP boost' Australian Landcare, Sept. 1999, pp. 26-29. 5. G. Walker, M. Gilfedder and J. Williams, Effectiveness of Current Farming Systems in the control of dryland salinity, CSIRO, Canberra, 1999. 6.

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