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UNIT 2

Saltland Basics

 

2.1  History of salinity

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Causes of dryland salinity?

Inland Australia is a salty place. All rain brings in small amounts of salt – typically 20 to 50 kilograms per hectare per year across the agricultural regions. Because of our relatively low rainfall and relatively flat landscape, it can take a long time (sometimes tens of thousands of years) for that salt to get carried back to the ocean via our rivers. When plants use the rainwater, they leave the salt behind which then builds up in the landscape – sometimes in massive amounts. As an example, drilling at Merredin in the WA wheatbelt showed that the soil profile contained about 650 tonnes of salt per hectare or 33,000 times the annual amount deposited in rain. This salt was in the landscape when European settlers arrived with their annual crops and pastures that would disturb the hydrological balance and start to mobilise the stored salt.

The first clear evidence of the link between clearing of native vegetation and the appearance of dryland salinity was gathered by a West Australian railway engineer, Walter Ernest Wood. In 1924, he published a paper titled Increase in salt in soil and streams following the destruction of native vegetation in the Journal of the Royal Society of Western Australia. Wood noted that the native forests and grasslands that originally occurred across southern Australia used nearly all the rainfall. However, when these were cleared by farmers to grow crops and pastures, some rainwater percolated into the soil profile, and the groundwater rose towards the soil surface bringing with it the salt stored in the soil profile. When the watertable reaches about 2 m from the soil surface, salt begins to move into the plant root zone, and plant growth and survival became affected.

However, the long time lag between clearing and the evidence of salinity, and the vast spatial separation often experienced between cause (the areas of the landscape where water infiltrates) and effect (the areas of the landscape where salinity occurs) proved to be a barrier to a full appreciation of the consequences of widespread land clearance. More recently, additional causes of dryland salinity (especially transient salinity) have been associated with overgrazing and a decline in soil health.

There is little value in speculating whether the expansion of agriculture in southern Australia could have proceeded without dryland salinity occurring. With the benefit of hindsight, the huge salt stores in the landscape, the widespread adoption of annual crops and pastures, and the mainly Mediterranean rainfall pattern with excess water over winter and spring, combined to make dryland salinity in many lower parts of the landscape inevitable. For more information see The causes and risk of dryland Salinity

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Different types of salinity

There is no widely accepted definition for ‘saltland soils’, but this website defines these as soils where there is sufficient salt and waterlogging to depress the growth of crops and pastures. There are several types of saltland in Australia, characterised by their various causes.

Primary salinity
There are many areas in Australia that were already saline at the time of European settlement – perhaps as much as 30 million hectares either along the coastline or in the rangelands and the arid interior. In other words, a high proportion of the saline areas in Australia are ‘natural’ and are not the result of agricultural activities.

Secondary salinity
Secondary dryland salinity differs from primary dryland salinity in that it can be directly attributed to human impact – principally the clearing of native vegetation for the growth of annual crops and pastures, and the subsequent soil degradation. Some of the classic signs of secondary salinity include dead remnants of vegetation that grew on the site before it became saline, colonisation by salt-loving species, and increases in waterlogging and inundation, with fences disappearing into saline lakes.

It can often be difficult to determine whether a particular saline site represents primary or secondary salinity, and in many situations, primary salinity has expanded as a result of agricultural activity, further confusing the issue. Secondary dryland salinity is caused by 3 main processes: 

  1. Rising watertables
    The lower water use of annual crops and pastures compared to the native vegetation they replace often leads to water draining below the root zone, where it becomes part of the groundwater mixing with the salt stored in the soil profile. The watertable then rises, bringing stored salt to the soil surface. When this salt reaches the root zone and inhibits plant growth and survival, we say that the site is affected by dryland (or seepage) salinity. This has been the most widely accepted cause of dryland salinity in southern Australia, and was the subject of most of the research undertaken through the National Dryland Salinity Program and formed the basis for the Salinity Audit in 2000.
     
  2. Transient salinity and Dry saline land
    There are increasing areas of ‘dryland salinity’ being identified that are not the result of rising saline groundwater bringing salts into the root zone. Transient salinity was first identified in the 1940s and is sometimes called ‘magnesia’ patches. It is the result of the seasonal movement of salt into and out of the soil profile. Evaporation from the soil surface concentrates the salts in the root zone, from where they are subsequent leached out by rainfall. This type of salinity may occur when the upper layers of soil are sodic, severely restricting the downward movement of water and leading to the formation of a perched watertable. When transient salinity, concentrated by evaporation, occurs within the root zone of crops it can be detrimental to their growth.
    For more information on transient salinity, see CSIRO report on dry saline land in South Australia or Dryland salinity in south east Australia.
     
  3. Irrigation salinity
    Excessive irrigation can lead to locally elevated watertables which in time can result in soil salinity if the irrigation water is slightly salty. This is particularly the case if annual rainfall is insufficient or if the subsoil is so impermeable that the salt cannot leach deeper into the soil profile.
    Irrigation salinity is a problem for agriculture and horticulture, and also affects parks, gardens and sporting fields in urban areas. Modern irrigation practices have helped reduce the incidence of irrigation salinity, but there are many situations where irrigators are forced to adopt more salt-tolerant crops, pastures or turfs. The growth of salt and waterlogging turfgrasses is being actively pursued for urban areas where irrigation salinity occurs.
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Groundwater flow systems

Catchment characterisation based on groundwater flow systems has proved a particularly important tool for regional planning of responses to salinity, especially in relation to understanding the time frame and distances over which a catchment might respond. However, the system is not as effective at the sub-catchment or property scale because of the complex nature of the recharge-discharge process and the lack of local groundwater data that might shed light on the local drivers and processes. A good example of this local difficulty has been explored in the Gumble area of central NSW.

Hydrologists categorise catchments on the basis of the distances over which groundwater moves to cause salinity problems.

  • Local groundwater flow systems typically have recharge and discharge areas within a few kilometres of one another. They tend to occur within individual catchments in areas of higher relief such as foothills to ranges. These systems generally respond rapidly to increased groundwater recharge and show dryland salinity within a decade of clearing. These systems can also respond relatively rapidly to salinity management practices, and afford opportunities to mitigate salinity at a farm scale.
     
  • Intermediate groundwater flow systems are intermediate between local and regional systems, generally occuring within individual catchments but also sometimes flowing between smaller subcatchments. They tend to occur in valleys, and typically occur over a horizontal extent of five to ten kilometres, have a greater storage capacity and higher permeability than local systems. They take longer to 'fill' following increased recharge. Increased discharge typically occurs within 50 to 100 years of clearing of native vegetation for agriculture. The extent and responsiveness of these groundwater systems present much greater challenges for dryland salinity management than local groundwater flow systems.
     
  • Regional groundwater flow systems generally occur in areas of low relief such as alluvial plains. They may have aquifers thicker than 300 metres, and the distances between recharge and discharge areas may be separated by distances of 50 or more kilometers. They have a high storage capacity and permeability. They take much longer to develop increased groundwater discharge than local or intermediate flow systems – probably more than 100 years after clearing the native vegetation. The full extent of change may take thousands of years. The scale of regional systems is such that farm-based catchment management options are ineffective in re-establishing an acceptable water balance. These systems will require widespread community action and major land use change to secure improvements to water balance.
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Salinity statistics

The first comprehensive assessment of the national extent of dryland salinity in Australia was undertaken as part of the National Land & Water Resources Audit in the late 1990s and published in 2000. Table 2.1 indicates the distribution of this risk across agricultural land.

Table 2.1 Salinity risk by States for 2000 and 2050 from the National Land and Water Resources Audit.

State 

At High Risk 
Year 2000
(ha)
 

At High Risk
Year 2050
(ha)
 

WA

3,552,700

4,181,700

VIC

555,000

1,170,000

NSW

161,000

526,570

SA

326,000

421,000

QLD

65,000

Not determined

TAS

53,000

69,500

TOTAL

4,712,700

6,369,000

 

These figures relate only to the hazard of salinity as defined by the groundwater recharge/discharge model – that is, salinity that is caused by rising saline groundwater within the landscape, as a result of increased recharge following land clearing and subsequent discharge down slope.

The Audit results are available at www.nlwra.gov.au.

The NLRWA used the best science available at the time and represented a real breakthrough in raising awareness of the extent of salinity, the salinity hazard, and the possible risks. However science moves on, and this information, which underpinned much of the initial planning for salinity management, continues to be reviewed and updated using new and better modelling based on the latest data and longer trends.

It is now generally agreed that the total area at risk from salinity in Australia is probably less than that estimated by the Audit – partly due to the limitations of the original models used, and partly due to the fact that rainfall across southern Australia has been consistently below average since the estimates were made. However, it is important to note that the Audit did not include transient salinity which is now accepted as an additional and widespread form of secondary salinity. State by state updates were prepared for the National Dryland Salinity Program in 2006.

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Recent trends

  1. Salinity threat downgraded
    Though official figures to replace those published by the National Land & Water Resources Audit are not yet available, the overall threat that dryland salinity poses to the nation has been downgraded. Dryland salinity has a significant cyclical component linked to climatic conditions, so it can be expected to spread during wetter periods, and contract during dry periods, even if the underlying trend is for expansion.
     
  2. New salinity models
    It is clear that not all dryland salinity is the result of clearing native vegetation and the subsequent rise in watertables. Transient salinity or Dry saline land is now widely recognised as a form of dryland salinity, but one that is not associated with watertables rising from below the root zone.
     
  3. The balance between preventing recharge and managing discharge has changed
    Preventing recharge is challenging because it can occur over large areas that can be hard to identify definitively. Recharge abatement often requires the replacement of large areas of profitable agriculture with less profitable options in order to reclaim relatively small areas of salt-affected land. On the other hand discharge areas tend to be smaller, very easy to identify and therefore more amenable to focussed management. This applies to discharge areas that occur in towns as well as on farms.

    Recent research in NSW has confirmed the trend towards direct saltland management as a less ‘risky’ option than attempting to prevent recharge. Not only is there often some uncertainty around the likely outcome from attempts to reduce recharge, there is also the high probability in some catchments saltland revegetation will reduce surface water run-off to streams and water storages. For more information, see Integrated Gumble Site Report.
     
  4. Well tested management options now available
    Since the 1950s there has been a small but consistent input into understanding saltland and developing management options, primarily across the southern states.

    This R&D was initially conducted in state agencies by researchers like Clive Malcolm (WA), Dennis West (VIC) and their colleagues. This research was focussed on the needs and priorities of individual states resulting in local rather than national changes. Information exchange between states was limited.

    This situation changed in the 1990s with the development of the Productive Use and Rehabilitation of Saline Land (PUR$L) committee - a national network of researchers and farmers who were actively involved in salinity management and wanted to see information that had been confined within State boundaries distributed more widely. The PUR$L group coordinated 9 conferences and workshops over the 15 years from 1990 to 2003 - Tatura VIC 1990, Adelaide SA 1992, Echuca VIC 1994, Albany WA 1996, Tamworth NSW 1999, Naracoorte SA 1999, Launceston TAS 2001, Fremantle WA 2002, and Yepoon QLD 2003.

    The National Dryland Salinity Program (NDSP 1993–2003) was the first national attempt to better understand the causes, impacts, costs and management options for preventing and/or overcoming dryland salinity. The final year of the program was dedicated to harvesting the knowledge and making it available to the diverse range of stakeholders through the Managing Dryland Salinity in Australia resource kit. An update was published by Land & Water Australia in 2006.

    More recently, the Sustainable Grazing on Saline Land (SGSL) initiative and the CRC Salinity brought significant focus and funding to the issue. There are now a range of proven options for managing saltland which are widely relevant across Australia. This work has provided much of the information for this website, including the 7 units of general saltland information and the 11 saltland solutions.
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