4.3 Induced soil salinity and sodicity

Salinity continues to be a major land degradation issue in NSW, though new understandings may lead to reassessment of the potential future extent of dryland salinity. Measures to limit and reverse water table rises in irrigation areas are starting to take effect.

A new survey covering part of NSW has shown that the extent of dryland salinity has not increased since 2003. Predicted rises in water tables are not likely to occur unless significantly wetter conditions are experienced.

While climate appears to be the major driver of dryland salinity, investment and activities to combat the landscape-wide causes of this problem in NSW appear to be working. Salinity remediation efforts have been most successful in irrigation areas. In dryland areas there have been some successes but government programs and community efforts have been struggling against the wide extent of the problem and the slow pace of the remediation process.

Sodicity remains a major problem in the State. Further change to existing agricultural and urban development practices is required to prevent problems due to land salinity and sodicity worsening and there is need for extensive remediation of past damage.

NSW indicators

Introduction

Land surface salinity occurs naturally across much of NSW, with high levels of soluble salts stored in the soil and groundwater as a result of landscape processes over thousands of years. However, in some areas there is 'induced' salinity as a result of human activities altering the balance of the water cycle in the landscape and bringing this salt to the surface, killing vegetation and altering the soil structure. This section concerns this induced salinity.

Of the major types of induced salinity, dryland salinity affects the largest area of the State, but salinity also occurs in irrigation districts and urban areas where it can be exacerbated by the development of urban infrastructure. Dryland salinity was first noticed in NSW in the south-east of the State between 1880 and 1900 with new scalds (bare patches of surface soil, occasionally showing salt crystals) appearing following periods of high rainfall. The extent of the problem increased significantly in the late 1950s and up to the early 1970s (Wagner 2005).

The process of salinisation involves changes in landscape hydrology mobilising salts that are stored in the soil. The combination of land clearing and inappropriate land-management practices can decrease evapo-transpiration of soil moisture and increase the proportion of rainfall becoming groundwater recharge. Rising water tables can bring dissolved salts to within two metres of the soil surface, which then allows capillary action to carry the salts to the soil surface, usually on lower slopes. There, they are concentrated by evaporation, killing vegetation cover and forming scalds, and may be discharged by runoff into surface waters where they affect water quality. Erosion of affected areas causes further problems and salt mobilisation, sometimes in areas also affected by soil sodicity. Salt export to downstream waterways may place water resources, aquatic ecosystems, farm dams, riparian vegetation and wetland areas at risk (see Water 5.3).

While it is broadly agreed that the dryland salinity problem stems from initial clearing of the land for farming, there is less consensus about more recent drivers and whether the observed fluctuations in groundwater level follow a localised or regional model.

The processes of irrigation salinity are similar to those for dryland salinity, except that the increased recharge of groundwater results from excess irrigation water applied to pastures and crops, and also from 'leaky' earthen irrigation channels, resulting in rising water tables. Irrigation methods include flood irrigation (as in rice and some older forms of pasture and crop management), overhead sprays and drip systems. In urban areas, leaky infrastructure for stormwater, potable water and sewage can contribute to the recharge (and pollution) of groundwater, while earthworks and soil compaction can also affect groundwater hydrology and lead to urban salinity.

Salinity, whatever its origin or cause, has serious and wide-ranging environmental and socio-economic implications, nationally, in urban and rural catchments and on farms. The effects include decreased agricultural production, damage to infrastructure such as roads and bridges, damage to housing, landscape degradation and a decline in ecosystem health. In 2001–02, a direct annual cost from salinity in excess of $151 million was incurred by local governments, households, businesses, State Government agencies, utilities and agricultural producers (Wilson 2001a; Wilson 2001b; Wilson 2001c; Wilson 2002a; Wilson 2002b; Wilson 2002c; Ivey ATP 2002a; Ivey ATP 2002b). The loss of agricultural production from dryland salinity was estimated to be $6 million annually (Lockwood et al. 2003). These figures have not been updated.

Salinity is often linked to another soil degradation process known as soil sodicity because it is fairly common for both saline and sodic conditions to occur together, though they may also occur independently. Sodic soils occur naturally in areas of former marine sediments and are characterised by high concentrations of sodium ions, expressed as an exchangeable sodium percentage (ESP) of 6 (more than 6% of total exchangeable cations in the soil). At this concentration and above, the sodium ions separate from the chloride ions and attach to clay particles, which causes the soil to disperse when wet, yet set hard when dry. This leads to problems such as poor water infiltration, slow internal drainage, erosion and surface crusting, which in turn reduces vegetation growth by affecting seed germination and preventing root penetration of the soil.

Current status and trends

Dryland salinity

The extent of dryland salinity is quantified in three ways:

• aerial photography to identify areas of actual saline discharge or salt scald
• measurements of depth to groundwater
• modelled risk mapping, which attempts to predict future extents of saline areas.

The formal assessment and mapping of dryland salinity outbreaks in NSW began in 1990 using aerial photography taken in 1980. This approach identified approximately 14,000 hectares, compared to only 4000 hectares identified by ground observation in 1982. By 1996 the area identified as being affected had risen to 25,000 hectares (Wagner 2005).

The National Land and Water Resources Audit (NLWRA 2002) found that approximately 180,000 hectares of land in NSW was affected by dryland salinity or had water tables that were predicted to be within two metres of the surface. More than 90% of this area occurred in five catchments – the Murray, Murrumbidgee, Lachlan, Macquarie and Hunter rivers – and it was estimated that by 2020 some 577,000 hectares could potentially be at risk, based on average rises in catchment water tables. This prediction was similar to the 1999 Salinity Audit for the Murray-Darling Basin Commission (MDBC 1999).

Recent studies and reviews of historical observations have confirmed the actual extent of outbreaks, but have also cast serious doubt on trends predicted using rising groundwater extrapolations (DNR 2006; Summerell et al. 2005; Keogh 2005; Wagner 2005). Since 2001 improved understanding of the extent of salinity, and its relationship with the hydrological cycle, climate variability and groundwater behaviour in different contexts, has highlighted inconsistencies with the general regional rising water tables theory (Ferdowsian et al. 2003; Wagner 2005; Summerell et al. 2005). As a result, the 1999 Salinity Audit is being updated as part of the NSW Salinity Strategy (2000) in the Murray–Darling Basin (DNR 2006).

The revised data and knowledge are being applied to models to reassess the risk estimates for dryland salinity and their contributions to instream salinity. It is expected that the extent of saline area predicted by 2020 and 2050 will not be as widespread as previously thought. The new work has found that:

• climatic patterns can so dominate salinity trends that it is difficult to discern the impacts of land-use or management interventions.
• response times between recharge and discharge, especially in the local-scale fractured rock aquifer systems that dominate in the tablelands and slopes of eastern NSW, are much shorter than previously thought
• the impacts of clearing on groundwater levels have already been incurred, so no continuing effect can be attributed to this cause
• many catchments, but not all, are in a state of 'dynamic equilibrium' in which groundwater levels are fluctuating about a new average value in response to climate variability (that is, to spells of wet weather and drought). In the Murray, Murrumbidgee and Lachlan basins, 8 of the 26 southern sub-catchments have river salinity trends that appear stable and in equilibrium, and 10 sub-catchments have major rising trends. In the Gwydir, Namoi and Border Rivers, 8 of the 36 northern sub-catchments are displaying trends towards lower salinity values, while 16 have trend slopes that are statistically insignificant.

While risk assessments based on the assumption of rising regional water tables (MDBC 1999; NLWRA 2001) will need to be recalculated in the light of this new understanding, the research has confirmed the current status of salinity for the Murray–Darling Basin portion of NSW. This means that work undertaken by CMAs to prioritise salinity actions and investment between locations is soundly based. It has also reinforced that there is an existing salinity problem with serious impacts on infrastructure and the environment.

Consistent with the new work, the actual extent of dryland salinity outbreaks appears to be relatively stable. Map 4.3 shows the geographic distribution of actual dryland salinity outbreaks as at 2005 in both urban and rural environments in eastern and central NSW. The map shows a total area of about 177,800 hectares affected by salt. Nearly 77,000 hectares occurs in marine (coastal) areas with 4800 hectares occurring in irrigation areas. About 96,000 hectares is affected by dryland salinity of which 71,000 hectares is in early stages of salinisation and 25,000 hectares is associated with sheet and gully erosion. Approximately 64,000 hectares occurs within the Murray–Darling Basin in NSW. However, these figures are likely to underestimate the true extent of salinity for the whole of the State as there are known outbreaks outside of the area where mapping has been completed (shown in yellow on Map 4.3). Previous mapping has identified areas of salinity to the west of the mapped coverage, particularly along the Murray River, and significant areas of the Hawkesbury–Nepean catchment are known to be affected by dryland and urban salinity (Dias & Thomas 1997; Nicholson 2003; DNR data 2002). Although calculated by different methods to those illustrated in Map 4.3, a total of around 240,000 hectares is at high or moderate risk from salinity (DNR data 2002). A further 90,000 hectares is considered to be at low risk.

Map 4.3: Known extent of salinity outbreaks

Source: DNR data 2005

In the Hunter Valley, surface water salinity presents threats to the wine industry, power generation and town water supplies, and Map 4.3 shows extensive areas of salinity outbreaks in this catchment. Data from 1975 to1999 shows evidence of a background rising trend in groundwater pressures across geologies and the catchment as a whole (Beale et al. 2000). Although the number of bores analysed was small in proportion to the total catchment area, these trends were confirmed by further fieldwork in the course of the 2000 Hunter Salinity Audit. It was considered that rising trends found in the upper Hunter River at Muswellbrook could support a link with rising water tables, while falling trends at Liddell and Greta could be the result of several factors, including:

• falling groundwater trends in alluvial aquifers in the lower catchment
• changes to river regulation following the commissioning of the Glennies Creek dam
• the introduction of the Hunter River Salinity Trading Scheme.

Irrigation salinity

Irrigation salinity is most evident in southern regions (EPA 2000). Mapping of NSW is incomplete (Map 4.3); therefore a direct assessment of the current extent of salinity outbreaks and trends in irrigation areas is not possible. Measurements of depth to water table of less than two metres indicate the potential for salt accumulation on the surface, but these figures are not available for all irrigated lands. However, Murray Irrigation Limited (MIL) and Murrumbidgee Irrigation (MBI) report annually on environmental performance, including the depth to water table. In the Murray Irrigation Area (MIA), water tables have generally fallen over a 10-year period but about 0.5% of the area continues to have water tables within two metres of the surface (MIL 2005). No new outbreaks were reported by the MIA in 2005. In the Murrumbidgee Irrigation Area, 17.7% of the area has water tables within two metres of the surface, and these levels fell on average by 0.44 metres in 2003–04 (MBI 2004).

Monitoring associated with land and water management plans across the State is still at an early stage of development, but it is expected that more data will become available for a wider area in future reporting periods.

Sodic soils

Unlike salinity, sodicity is virtually permanent. Treatment with gypsum is an option, but this is often neither practical nor cost-effective. Reduced tillage and encouraging a build-up of organic matter can reduce the degree of dispersion in a sodic soil, but these will not affect the soil's ESP (DPI 2005a).

Map 4.4 shows the areas of the State where soils are sodic and locations where soil profiles have been tested as sodic. However, no new data is available to indicate recent trends.

Map 4.4: Current knowledge of distribution of sodic soils and sodic soil profiles in NSW

Source: DLWC data 2003

Research into the effects of sodic soils suggest that as ESP increases from 0 to 6%, expected wheat yields can decline by 5–10% (Rengasamy 2002). Adopting a combination of zero tillage/stubble retention and applying lime and gypsum to a sodic soil significantly elevated wheat yields in the Central West of NSW (Valzano et al. 2001). Long-term research on a grey clay sodic soil near Coonamble has shown that a 3–4 year pasture rotation on land cropped continuously for 40 years significantly increased subsequent wheat yields (Mitchell 2006). When surface water allocations are low, irrigators extract more water from lower-quality groundwater sources. Of particular concern are those sources that are both saline and sodic. A number of studies have documented the effects of using such water for irrigation (Slavich et al. 2002; Hulugalle & Finlay 2003; Mitchell et al. 2004) and reinforced that irrigation may increase the risk of sodicification on some soils (Mitchell et al. 2004).

Response to the issue

The scale of the challenges from dryland, irrigation, urban and instream salinity has required strong intergovernmental cooperation and the development of an integrated approach to the problem. The NSW Government's response to induced soil salinity over the past decade is embodied in the NSW Salinity Strategy 2000, which is now integrated with the CMA's NRM service delivery. This strategy has close links to the National Action Plan for Salinity and Water Quality 2000 (NAP), the National Dryland Salinity Program 1993 and the Basin Salinity Management Strategy 2001 coordinated by the MDBC. A major review of the 1999 Salinity Audit of the Murray–Darling Basin is underway. Preliminary findings from Stage 1, covering instream salinity, groundwater levels and saline discharge areas, have been communicated to CMAs, and Stage 2 commenced in May 2006.

There has also been considerable investment in the prevention and remediation of actual salinity outbreaks at national, State, catchment and farm levels. Nearly $24 million had been approved under the NAP/NHT to June 2004 for projects in NSW, such as vegetation management, changed land use, development of property management plans and salinity monitoring (Australian Government 2005).

As part of the wider changes to NRM in NSW (see Appendix 2), funding from the NAP and the NSW Sustainability Fund is now directed through the CAPs and regional investment strategies coordinated by the 13 CMAs whose investment priorities and resource condition targets are guided by national and statewide NRM targets. CMAs will address issues according to agreed regional priorities, with DNR taking a more advisory and strategic role.

Increasing attention is being given to monitoring and evaluating outcomes in NRM issues, including salinity. All CMAs have developed salinity targets (management action targets and resource condition targets) that are linked to end-of-valley targets, as required by the MDBC's Basin Salinity Management Strategy.

In addressing irrigation salinity, land and water management plans (also administered by CMAs) for irrigation areas have been completed and are being implemented. Many of these plans are being revised and updated, guided by experience with implementing the plans over the previous five years (MIL 2005). The Independent Salinity Audit Group recognises that in NSW these plans have been successful in achieving better environmental outcomes, including improvements in instream salinity (MDBMC 2005). Engineering works such as drainage and salt interception are also contributing to the improvements (MIL 2005; MBI 2005).

The Western Sydney Regional Organisation of Councils, in partnership with the Macarthur Regional Organisation of Councils, the construction industry and the NSW Government, has developed a code of practice for urban and dryland salinity management in Western Sydney which includes planning and development guidelines (Nicholson 2003).

The Government continues to increase awareness among agencies and landholders of the spread and significance of sodic soils in NSW. Sodicity is best managed through the use of ground cover, conservation farming techniques and an understanding of water balances in the landscape. Special precautions are taken by the forestry industry when logging on sodic soils. The Government provides a range of SOILpak brochures that advise on management strategies. Sodicity is being managed in cropping systems by combining measures such as zero tillage/stubble retention with the addition of ameliorants such as lime or gypsum. In pasture systems a combination of ameliorants allows pasture establishment and suitable grazing management leading to productive and sustainable farming systems. There is evidence that widespread adoption of conservation farming techniques (zero tillage/stubble retention) in the north-western cropping zone has allowed better management of soil sodicity (Mitchell 2006).

The statewide natural resource management targets relevant to soil salinity and sodicity are 'By 2015 there is an improvement in soil condition' and 'By 2015 there is an increase in the area of land that is managed within its capability'.

Future directions

Current review work will lead to a better understanding of the status of individual catchments with respect to instream salinity contributions, actual outbreaks and water table trends, allowing CMAs to more accurately target prevention and remediation efforts. Use of standard assessment techniques in future updates of this work would greatly assist in allowing trends to be more accurately reported.

Revised modelling of the risk of future dryland salinity outbreaks based on a new understanding of the problem may lead to some reassessment of strategy options. However, the current problem will remain as an important environmental and economic issue for governments, landholders and local councils.

The effectiveness, need and cost of proposed solutions to salinity, such as broadscale reforestation, have been questioned. It is likely that a combination of tools and approaches will continue to be needed as salinity impacts and processes are found to differ in different landscapes and catchments (NLWRA 2002; NDSP 2004). However, sodicity will remain a significant environmental and production problem. As with related problems such as salinity, erosion and soil acidity, the adoption of best management practices, particularly with regard to water balances, will help to limit the extent of sodicity in NSW.