Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7
Water quality and sources of pollution (continued)
Salinity of surface waters (continued)
The major impact of dryland salinity is from saline groundwaters reaching the land surface and causing soil salinisation and waterlogging. The major cause of dryland salinity is the clearing of deep-rooted perennial woody vegetation and its replacement with shallow-rooted annual crops (see Figure 8). In uncleared areas, deep-rooted vegetation uses more of the water (e.g. rainfall), rather than allowing it to 'leak' into the groundwater. In areas that are cleared, water can 'leak' into saline groundwater tables, raising their levels until they reach the surface. As well as causing soil salinisation, raised saline groundwater tables may discharge saline water directly into rivers, streams and lakes.
Figure 8: Causes of dryland salinity
Source: Jolly et al. 2000, reproduced with the permission of CSIRO Australia.
In many areas, salt leached from the surface accumulates in an unsaturated zone between the soil surface and the groundwater table (see Figure 9). With increased 'leakage' of water from the surface and rising groundwater tables, salt in this zone may leach into the groundwater causing groundwater salinity to increase.
Figure 9: Process of groundwater salinisation
Source: Jolly et al. 2000, reproduced by permission of CSIRO Australia CSIRO Australia 2001.
A full discussion of the impacts and extent of dryland salinity is contained in the Land Theme Report and is summarised below. A recent study concluded that 5.7 million hectares of land are currently affected by dryland salinity with another 7.5 million hectares at serious risk due to shallow groundwater tables (NLWRRA 2001b). Up to 17 million hectares could be affected by the year 2050. Seventy-six per cent of the land already affected is in Western Australia, with Victoria and South Australia having the next largest areas, respectively. In Queensland, large increases in the area of land affected by salinity are predicted. In the Murray-Darling Basin, 300 000 ha of land are currently affected by dryland salinity (LWRRDC 1998a), although potentially up to 5 million hectares are at risk (MDBMC 1999). Land affected by dryland salinity is difficult to return to productive use.
- increased inflow of rising saline groundwaters into rivers, streams, wetlands and lakes
- higher salt loads in catchment run-off from salinised land
- degradation of riparian vegetation due to increased soil and water salinisation.
The impacts of increased saline groundwater inflow and higher salt loads in catchment run-off are not just confined to the sections of the river systems where they are occurring. Dryland salinity can affect areas some distance downstream from the cleared areas, with severe impacts on biodiversity remnants. In Western Australia for example, many low-lying remnant vegetation communities will almost certainly become extinct due to salinity (DEP 1998). Many wetlands are threatened by salinisation in the eastern states as well as Western Australia.
The impact of dryland salinity on the salinity of rivers, streams and lakes can be significant. For example, a comparative study of two similar streams in the Western Australian wheatbelt found that salt loads in a stream with an uncleared catchment were less than one tonne per year, whereas in a stream with a cleared catchment, salt loads ranged from 500 to 4000 tonnes per year (Salama & Bartle 1998). Saline groundwater inflow was the primary reason for the high salt load in the stream with the cleared catchment. Although groundwater in the uncleared catchment was highly saline, the woody perennial vegetation maintained groundwater levels seven metres below the surface.
Areas that have been identified as having a high risk of being affected by dryland salinity by 2050 due to shallow groundwater tables are presented in Figure 10. These are areas where the groundwater tables are within two metres of the surface or are within five metres of the surface and are rising. The Northern Territory and northern Western Australia were not analysed due to their high annual rainfalls. South-west Western Australia, western Victoria, southern South Australia and substantial areas of the Murray-Darling Basin in New South Wales are at risk from increasing dryland salinity. In Queensland, the areas with a high risk of dryland salinity are widespread throughout the state.
Figure 10: Areas with a high risk of dryland salinity by 2050 due to shallow groundwater tables
Source: National Land and Water Resources Audit 2001.
Irrigation-induced salinity is caused by increased 'leakage' of water from the surface into groundwaters due to over-irrigation or the irrigation of inappropriate soils. It has similar impacts to dryland salinity; however, it is easier to manage. A reduction in irrigation-induced salinity impacts can be achieved by improving irrigation practices (e.g. ensuring over-watering does not occur). In areas where the soils or hydrogeological conditions are unsuitable for irrigation, more comprehensive measures such as extensive tree planting, diversion schemes, structural readjustments or maximising plant water need to be considered.
In the late 1980s, irrigation-induced salinity was considered to be the major cause of the increasing salinity of the Murray River (MDBMC 1987), with 71% of irrigated land in Australia in the Murray-Darling Basin. However, over the next 50 years irrigation-induced salinity is predicted to contribute to only 13% of the increase in salinity of the Murray River at Morgan, with dryland salinity contributing to the remainder (MDBMC 1999).
The clearing of deep-rooted woody perennial vegetation is the major cause of dryland salinity. Despite the clear link between land clearing and dryland salinity (and many other problems such as increased soil erosion and loss of biodiversity), land clearing in some areas of Australia continues at an unsustainable rate. For a full discussion of land clearing and existing catchment vegetation cover see the Land Theme Report.
- Dryland salinity may result in the increased inflow of saline groundwaters into surface waters and salinisation of groundwater resources, and affect the health of riparian and catchment vegetation. Currently 5.7 million hectares of land are affected by dryland salinity with another 7.5 million hectares at serious risk of being affected. Most of the land currently affected is in south-west Western Australia. However, many areas of the Murray-Darling Basin are also at risk from dryland salinity.
- Dryland salinity is caused by the clearing of deep-rooted woody vegetation and its replacement with shallow-rooted annual species. High rates of land clearing have occurred in Queensland over the last 10 years.
- Irrigation-induced salinity may be a problem in some areas. However, its overall impact is not significant and can be managed and reduced.