Environment and Resource Management

Land: Salinity

Author

Daniel M. Brough, Department of Natural Resources
and Water

Reviewer

Ian Gordon, Department of Natural Resources and Water

Key findings

Indicators and summary of status


Indicator

Status of indicator

Location, size and intensity of dryland salt-affected areas

Anecdotal evidence suggests that this indicator is stable or decreasing as a result of prolonged drought; it may increase after a period of wet years.

Location, size and intensity of irrigation salt-affected areas

Large quantities of water drain past the root zone under irrigation, bringing an increased risk of salinity. In irrigated areas location, size and intensity of salt-affected areas are difficult to estimate from current data.

Depth to groundwater (irrigated areas only)

Watertable depths under irrigation areas are generally rising, and many are already shallow

The national indicators of salinity, as a broad target-setting and reporting tool, have been used with mixed success in Queensland. The proposed national indicators of salinity are depth to groundwater; groundwater salinity; baseflow salinity; and location, size and intensity of salt-affected areas. The national indicators of salinity were trialled in a pilot study for Queensland (Bigwood 2006) that concluded that several of the indicators are difficult to assess because of data paucity. The application of all the national indicators for salinity in Queensland is difficult and concerns are held about scalability and the wide range of environmental and land management conditions present across the state. The national indicators of salinity that are most applicable to Queensland, especially to overcome the data paucity, are the location, size and intensity of land affected by salinity, and the depth to groundwater. Depth to groundwater can reliably be used as a salinity indicator only in areas where the spatial and temporal quantity of data is adequate, and in situations where there is a near-saturated regolith-that is, a direct relationship between land management and groundwater level exists.

Importance

Salinity is an important issue in terms of water quality and land degradation, and because of its effects on agriculture, aquatic ecosystems and infrastructure. Salinity is caused by shallow watertables and an increase in the amount, or concentration, of salt in the upper zone close to the land surface. Salinity becomes a matter of concern for vegetation when saline groundwater intersects the root zone, and for infrastructure when saline groundwaters enter the capillary zone (approximately two metres from the surface) because the salt can affect concrete, with potentially severe consequences.

One of the major features of salinity is that groundwaters do not need to be excessively saline to cause salinity. If slightly saline groundwater comes close to the land surface the effects of evaporation concentrate the salts in the water, thus causing a much greater impact than that of the slightly saline water alone.

When dealing with salinity it is important to keep in mind that many assets (natural or built) have varying levels of tolerance to increases in salinity; therefore salinity must be considered in the context of the particular asset at risk and the value of that asset.

Area of land affected

The National Land and Water Resources Audit (NLWRA) reported in 2000 that a total of 48 000 ha of land was estimated to be affected by salinity in Queensland (CoA 2001). The Australian Bureau of Statistics reported in 2002, however, that the current area of saline land in Queensland was 107 000 ha (ABS 2002). This represents a more than twofold increase in saline area in just two years. This increase in area is attributable mainly to landholders' greater understanding or recognition of salinity. The NLWRA has estimated that if no preventive measures are taken, the saline area will to increase to about 3.1 million ha by the year 2050.

The discrepancy can be explained partly by the different meanings attached to 'affected by salinity' and by the different methods of assessment. It also stems from the different scales at which the assessment is undertaken. Farmers are likely to assess the area affected at the paddock scale, whereas the NLWRA assessment was at a much broader scale. The ABS report may also include some areas of primary salinity; primary salinity occurs in areas that are naturally saline. Table 4.1 shows the key assets that are potentially threatened by salinity.

Value of assets affected

The value of assets affected by salinity is not well quantified, but the costs to the community of salinity affecting some 2 600 000 ha of agricultural land will potentially be significant. The community must bear the full cost of the loss of productive land, which is a finite resource, to a potentially irreversible salinity outbreak. The value of assets such as biodiversity, water quality and agriculture is often difficult to establish, while the value of infrastructure assets is comparatively simpler to estimate.

Excessive levels of salts also affect infrastructure. For example, the structural integrity of concrete is severely affected by high levels of soluble salts of waters in direct contact with the concrete. Like plants, the materials used in built structures experience varying levels of impact from salinity. The cost of salinity to community infrastructure is not well known, but a major road may cost $1 million a kilometre to construct, and substantially more where bridges and other such structures exist. The impacts of salinity on civil infrastructure are many, and probably the least well understood component of salinity risk in Queensland, where both primary and secondary salinity processes have impacts on infrastructure construction, maintenance and life span. Primary salinity has historically been poorly understood by infrastructure developers. In some instances, infrastructure may be instrumental in the development of secondary salinity.

In south-east Queensland, where many salinised lands have previously been identified, the urban fringe is expanding into areas once considered farm lands. In these rapidly developing areas the potential for salinity to have impacts on communities and infrastructure is increasing. The potential cost of salinity for urban and peri-urban communities is not well documented in Queensland, but as development pressure increases so too will the likely costs of dealing with salinity problems either during or after development.

In Australia, where water is a precious resource, the impact of salinity on water quality should not be underestimated. Declining water quality can lead to a reduction in agricultural production, impacts on infrastructure and negative effects on town water supplies. In south-east Queensland there is concern that during periods of high rainfall large quantities of salt are exported into the Brisbane water supply system. This concern is evident in the Black Snake Creek catchment (at Marburg), where the local Landcare group has invested in understanding landscape-scale processes in an effort to deal with the issue of salinity within the catchment and potentially reduce the quantity of salt exported from the catchment (Ellis et al. 2006). Such export has the potential to increase the amount of treatment required for the supply of potable water to Brisbane residents and industry. The water supply for Kingaroy, Gordon Brook Dam, is saline enough to limit some uses of the water.

Table 4.1 Key assets located in areas of dryland salinity hazard in Queensland

Asset

By the year 2050

Agricultural land (ha)

2 600 000

Remnant vegetation (ha)

92 000

Ramsar wetlands (ha)

635

Important wetlands (ha)

25 600

Streams (km)

1 800

Roads (km)

12 000

Rail line (km)

1 500

Source: CoA 2001

Pressure and condition

Land use activities contribute to the development of salinity. Dryland salinity is generally caused by rising saline groundwater after clearing of deep-rooted vegetation. It can also be caused by the inappropriate siting of infrastructure such as roads and drainage channels, which can alter the local hydrology and promote ponding and rising watertables. Irrigation salinity is caused by repeated and long-term irrigation with slightly saline water or by the infiltration of excessive irrigation water to the watertable. In the latter case, the rising watertable brings salt towards the root zone or soil surface.

Location and extent of salinity

The area of land affected by salinity is predicted to increase in Queensland over the next few decades, but the area currently affected has anecdotally decreased as a result of the current prolonged drought. Following below-average rainfall for a number of years, the quantity of water available to drive salinity processes has decreased. The state's medium-term climatic variability has a large influence on dryland salinity, particularly salinity associated with local and intermediate-sized groundwater flow systems. Most experts expect that the area affected by dryland salinity in Queensland will increase with a return to a wetter climatic regime-that is, non-drought conditions. The combination of large-scale land use change and tree clearing over the past 15 years and a marked increase in the amount of water in the regolith (zone from the soil to groundwater) may cause many new salinity outbreaks to occur.

The location and area of land affected by both dryland and irrigation salinity and the intensity of the effect in Queensland are difficult to estimate with a great deal of reliability. The current salinity sites coverage (Figure 4.1) is biased towards areas with a focused investment of resources for natural resource a ssessment and information is collected in an ad hoc manner without a strategic data capture plan. The area of land affected by irrigation salinity is easier to describe, given the reduced spatial extent of irrigation areas.

Detailed land resource mapping shows that the area of land affected by salinity in the Bundaberg and Maryborough irrigation areas is 6610 ha. The area of land affected by salinity in these two irrigation areas represents a substantial proportion when compared to the area of land under cropping, although most salt-affected areas are adjacent to cropped areas.

Depth to groundwater in irrigation areas

Groundwater levels are typically rising, or have risen to a shallow depth, in and around irrigated areas throughout Queensland, except where groundwater is used as a source of irrigation supply or is pumped specifically to control groundwater levels. Studies done since the previous reporting period continue to find high rates of deep drainage under furrow irrigated fields (Silburn and Montgomery 2004). Groundwater pumping has induced increased groundwater salinity levels in several aquifer systems, such as Lockyer and Callide. These pressures are increasing, leading to an accelerating rise in the watertable in most of Queensland's irrigated areas (Table 4.2).

Rising groundwater was found in two monitoring bores sited near irrigation fields and infrastructure in the Border Rivers west of Goondiwindi (Biggs et al. 2005). The groundwater is highly saline and often acid. Groundwater levels are not rising in nearby bores away from irrigation. Salinity induced by irrigation (horticulture) and leaky dams was observed at many sites in the Granite Belt in southern Queensland (Biggs et al. 2005). No groundwater levels data are available.

Table 4.2 Minimum watertable depth in some irrigation areas

Irrigation zone

Watertable depth
(m from the surface)

1990

1995

2000

2005

Mareeba-Dimbulah

5.0

4.5

2.5

1.14

Burdekin River

6.0

5.0

2.0

1.15

Emerald

8.5

4.0

0.5

0.89

Bundaberg

3.0

2.0

1.0

1.19

Lower Mary River

2.0

1.5

0.5

0.60

Source: DNRW


Figure 4.1 Salinity sites in Queensland
Source: DNRW

Groundwater and baseflow salinity

The depth to groundwater, baseflow salinity and salt-affected areas indicators are all late stage indicators of salinity. Loss of soil chloride in non-discharge areas is an early stage indicator of changes in hydrology that may lead to salinity. Soil chloride paired sites (native vegetation versus cropping or pasture) have been studied extensively across Queensland's cropping lands (Tolmie et al. 2003). Tolmie et al. have consistently found loss of loss of soil chloride and an increase in the rate of deep drainage after clearing, indicating a potentially large future risk of salinity.

Response

Soil salinity usually builds up over long periods. Problems may appear long after the causes started. Similarly, management responses rarely produce immediate improvements. A combination of agronomic solutions (for example, revegetation with deep-rooted plants and selection of appropriate crops, such as beetroot in the Lockyer Valley) and engineering solutions (such as surface or subsurface drainage, groundwater pumping and more efficient irrigation systems) is often required to effectively minimise and potentially reverse the expansion of salinity.

Natural resource management issues are being dealt with under the provisions of the Vegetation Management Act 1999 (Qld) and the Water Act 2000 (Qld). Numerous initiatives have been implemented with Commonwealth and state funding to combat the problem of dryland salinity in Queensland. These include projects funded through the Natural Heritage Trust, the National Action
Plan for Salinity and Water Quality, the Murray-Darling Basin Commission, the National Landcare Programme, and Land and Water Australia.

The Queensland Government has invested some $22 million in irrigation efficiency improvements through the Rural Water Use Efficiency (RWUE) program. The RWUE is a partnership between the government and industry to help growers improve water management practices and achieve water use efficiency. The aim of the program is to continue to help irrigators in each industry improve their on-farm natural resource management, particularly through efficient irrigation and nutrient management. As a result of increasing the efficiency of irrigation practices, the quantity of water that leaves the root zone of a crop (deep drainage) is reduced, thereby decreasing the potential for groundwater rise in irrigation areas.

NAPSWQ activities

Considerable salinity research has been conducted in Queensland since the NLWRA Australian Dryland Salinity Assessment (CoA 2001) and the release of salinity hazard maps for four priority Queensland catchments in 2002. This research has substantially improved our knowledge of landscapes and their associated salinity processes, groundwater systems, and the impacts of land use change.

While the NAPSWQ has invested heavily in the understanding of salinity in Queensland, little of this investment was focused on the systematic identification of areas affected by salinity. The investment focus was aimed at obtaining an improved understanding of salinity processes across the priority catchments and providing information on the management and prevention of salinity at regional scales.

The Strategic Investment Program (SIP) has focused on several key areas for improved salinity management by investing in improved data and information and a greater ability to predict where salinity is or may be an issue for the community through land management practices that are risky in terms of salinity processes. The SIP provided improved data and information by developing a network of groundwater bores for the regional monitoring of groundwater trends in priority catchments. Improved soil, regolith, land use and terrain models were produced to assist in broad-scale modelling of salinity processes for preventative and remediation activities. Salinity risk modelling at the subcatchment scale was completed for several of the priority catchments. The risk models focus on activities that increase the risk of salinity and on assets such as biodiversity, agriculture, water quality and infrastructure. The development of a groundwater monitoring network will assist in producing information about trends and will contribute to the improved reporting of salinity by increasing the ability of the state and regional Natural Resource Management groups to report on the salinity indicators.

References

ABS 2002, Salinity on Australian Farms, cat. no. 4615.0, Australian Bureau of Statistics, Canberra.

Biggs, A.J.W., Power, R.E., Silburn, D.M., Owens, J.S., Burton, D.W.G. and Hebbard, C.L. 2005, Salinity Audit-Border Rivers and Moonie Catchments, Queensland Murray-Darling Basin, QNRM05462, Department of Natural Resources and Mines, Brisbane.

Bigwood, R.C. 2006, Presentation of the Information Related to the Indicators for Land Salinity, Department of Natural Resources, Mines and Water, Brisbane.

CoA 2001, Australian Dryland Salinity Assessment 2000, National Land and Water Resources Audit, Canberra.

Ellis, M.D., Bigwood, R.C. and Moss, J.B. 2006, Landscape Salinity of the Black Snake Creek Catchment, Department of Natural Resources, Mines and Water, Brisbane.

Silburn, D.M. and Montgomery, J. 2004, Deep drainage under irrigated cotton in Australia-A review, in WATERpak a guide for irrigation management in cotton, Section 2.4, Cotton Research and Development Corporation/Australian Cotton Cooperative Research Centre, Narrabri.

Tolmie, P.E., Silburn, D.M. and Biggs, A.J.W. 2003, Estimating Deep Drainage in the Queensland Murray-Darling Basin Using Soil Chloride, QNRM03020, Department of Natural Resources and Mines, Brisbane.

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Last reviewed 16 May 2011
Last updated 4 September 2007

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