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Salinity - an introduction

Salinity

Definitions, processes and extent

Dryland salinity (on non-irrigated land) occurs when the concentration of soluble salts near the soil surface is sufficient to reduce plant growth. In Western Australia, this is basically a water management problem: Increased recharge raises the watertable, bringing naturally stored salts from depth to the surface.

Problems that develop from surface salinity include loss of agricultural productivity, loss of natural biodiversity, damage to buildings, roads and other structures, and degradation of water supplies.

Primary Salinity

Natural, or 'primary salinity' occurs throughout the world in arid climates, including about 29 million hectares in Australia: 14 million hectares as salt marshes, salt lakes and salt flats, and another 15 million hectares with naturally saline subsoil but no groundwater or perched water to take it to the surface. Moist and wet primary saline areas have very high natural diversity in Western Australia, but are at risk from increased flooding.

Secondary salinity - Man made

Salinity which has developed by changing land use and management is called secondary salinity. It is caused by a change in the the water balance, leading to more water in the soil and a rising watertable. This mobilises stored salts which rise with the watertable towards the surface. Clearing for agriculture has been the major cause of secondary salinity in Australia.

Salinity is usually noticed first when plants grow poorly, and yields of farm plants (crops and pastures) are reduced by more than 25 to 30 %. In severe cases, bare or 'weedy' patches develop with salt obvious on the surface. These patches are known as 'salt scalds'.

Natural diversity on secondary salinity sites is usually lower than surrounding land.

About two and a half million hectares of secondary salinity was estimated in Australia in 1996.

Chapters

• Classes of salt affected land
• Where does the salt come from?
  • Stored salt levels
• Where does the water come from and go to?
  • Recharge
  • Clearing and recharge
• Groundwater movements
  • Aquifer types
  • Discharge
• Further Reading

Classes of salt-affected land (from Martin and Metcalf 1998)

Class	Description
Not at risk	Land not susceptible to salinity, regardless of land use or management
Stable	Land susceptible to salinity but unlikely to become saline under the current land use or management
At risk	Land susceptible to salinity and likely to become saline under the current land use or management
Slightly affected <10% decrease in productivity	Land with reduced productivity from non-salt tolerant plants, some salt tolerant plants present, seasonally or permanently shallow watertable and some small bare areas
Moderately affected 10-50% decrease in productivity	Land showing a significant loss in non-salt tolerant plants, salt tolerant plants common, seasonally or permanently shallow watertable, bare areas up to 5m2 in size, some erosion present
Severely affected >50% decrease in productivity	Land showing an absence of non-salt tolerant plants, permanently shallow watertable, large bare areas which are often badly eroded

There is no single definition of whether a piece of ground is or is not 'salt affected', but the question is best answered by soil and water testing.

Where does the salt come from?

Soil salt can come from three main sources:

1. From the breakdown of parent rock: A very slow process.
2. From geological inundation by the oceans: Only on discrete parts of Australia.
3. From wind blown salt, usually in rain water from the ocean.

Salt in rainfall can range from about 20 kg/ha/per annum (usually inland with low rainfall) to more than 200 kg/ha/per annum (usually coastal with high rainfall). In most of Australia, this is the source of stored salts.

Stored salt levels

Salt becomes stored in the landscape through the balance of salt input (through rainfall) and loss through leaching or drainage from the catchment. In areas where potential evaporation is high and rainfall is low (semi-arid and arid zones), salt falls on the landscape but is not flushed out. It therefore accumulates, usually below the root zone of original native vegetation.

Accumulation is very noticeable in poorly dissected country (flat) where there is no clear escape for groundwater or surface water. It also accumulates in landscape 'sumps'; where most of the drainage is internal (that is, drainage cannot escape the catchment, and moves to a low point such as a saltlake).

Where the depth of 'soil' above bedrock is great, storage can rise to thousands of tonnes per surface hectare. With an input of 50 kg/ha/p.a. of salt and no flushing, 1000 tonnes per hectare could be accumulated in 20,000 years. This is a short time geologically.

Where does the water come from and go to?

Rainfall provides most water input to dryland catchments. In some cases, rivers and streams flowing through the catchment (in which case it is actually a sub-catchment) will be important recharge sources. Irrigated catchments can have a large proportion of water from irrigation and effluent, which is also the case in heavily populated areas.

Water coming into the catchment can either:

• be evaporated from plant foliage or the soil surface
• run off the surface and out of the catchment
• enter the soil and then be transpired by plants
• flow out of the catchment as groundwater (groundwater flow)
• add to surface water storage (lakes, dams)
• add to unsaturated soil moisture, or
• add to the watertable (recharge - seen as a rising watertable)

Recharge

'Recharge' is the water that gets through to the watertable (groundwater). It is usually measured in millimetres (mm). Average recharge rates, over catchments, of about 10% of average rainfall are not uncommon in dryland agriculture, although this varies widely.

Recharge within a catchment varies a lot: So-called 'preferential recharge zones' allow rapid water entry at the surface and flow through to the groundwater. Examples are sands and gravels, on the interface of rock outcrops, and through fractured rock. Where groundwater aquifers are confined, recharge occurs in very specific areas on higher sections of the groundwater system.

Recharge in a given year is strongly affected by rainfall distribution. Wetter than average years have a higher percentage of that rainfall going to recharge, and dryer years have a lower percentage. Heavy and 'out-of-season' rainfall events will also dramatically increase the percentage of rain going to recharge. This means that recharge tends to be strongly influenced by the season, and will often give 'step' rises in watertables over a run of years.

Clearing and recharge

The fundamental cause of salinity is the replacement of perennial, deep rooted native vegetation with the annual crops and pastures used in agriculture.

Bush has many perennial plants with deep roots and tolerance to the highly variable climate in southern Australia. Most agricultural plants in the Mediterranean climate of southern Australia are short season annuals. These annuals generally have shallow root systems and use less water.

Unused rainfall either runs-off or infiltrates beyond the root zone, and accumulates as groundwater.

The link between clearing and salinity development is well documented (see references below)

Groundwater movement

Groundwater moves by slowly percolating through gaps (or 'pores') in soils and rocks. The groundwater system is often called an 'aquifer'. It is very rarely present as underground stream.

When the groundwater is also saline, or moves through saline soils, salt transport becomes a major problem (see 'Discharge' below).

The rate of groundwater movement depends on:

• the relative ability to carry/transmit water (defined as 'hydraulic conductivity). In well sorted sands and gravels this is high (water moves tens of metres a day) and in clay it is generally very low (less than 1 mm per day). This acts like sponge in a pipe.
• hydraulic pressure. Water tries to find a level. That is, it usually flows downhill, and will flow from regions of high pressure to regions of low pressure. Water in a saturated aquifer (groundwater) acts like a pipe - water added to the groundwater at the top of a catchment (recharge) will 'push' water out the bottom of the catchment almost immediately. This acts like having different heights of water in a vertical pipe. Actual water movement is usually very slow though, limited by the hydraulic conductivity and the 'size' of the aquifer.
• the area available for flow. The catchment aquifers narrow down towards the catchment outflow. This acts like having a small tap on a large pipe, and results in either a build-up of pressure (in confined systems) or groundwater

Aquifer types

On the basis of size, aquifers can be:

• Local flow systems: Where recharge and discharge are close to each other, and discharge rapidly balances recharge.
• Regional flow systems: Where recharge and discharge may be widely separated. Groundwater movement is relatively independent of local topography (sub-catchments), and may have very long flow paths. Watertable levels outside the recharge area are slow to respond, and equilibrium may take hundreds of years.

Depending on how confined they are, aquifers can be:

• Confined aquifers; where groundwater is confined to one layer by impermeable material above and below. In a confined system, recharge can often be managed at a particular area, but plant access to the groundwater is limited elsewhere. Pressure can be transmitted over long distances.
• Unconfined aquifers; Effects are more local, and plant access to the system is over the full aquifer.
• Semi-confined aquifers; 'leaky' aquifers, which are very common in Australian discharge areas.
• Perched aquifers; which are water tables that sit on an impermeable or less permeable layer, with unsaturated soil underneath. Perched systems are often fresh. There is often a deeper watertable above bedrock. Deep bores will often go through several perched aquifers. Swamps and lakes can be perched systems over unsaturated soil.

Discharge

Discharge is the loss of groundwater from a catchment.

There are three main ways for this to happen:

• Subsurface flow (underground water movement) out of the catchment. This does not cause salinity in the catchment, but may down-slope.
• Surface loss. This could be into streams, rivers, seeps, lakes, and will generally cause increasing salinity unless it is flushed downstream.
• Loss through transpiration by plants may concentrate salt within the root zone, but this depends on many factors. Enhancing discharge with revegetation is one tool for managing salinity.

As many as 15 different categories of discharge to the surface have been proposed (Martin and Metcalf 1998 p18-19), to characterise Australian catchments affected by dryland salinity.

It is important to know that, in most case, aquifers and discharge mechanisms are complex, three dimensional structures.

An understanding of the recharge and discharge systems in a catchment will help to design effective solutions and management of salinity. In some cases, this knowledge will be the difference between success and failure. However, collecting information can be expensive: Try to match costs to expected benefits of collecting information. Use existing knowledge and experience wherever possible.

Extent of secondary salinity

The extent of secondary salinity has not been fully documented (or a final definition accepted), but it is clear that salinity has been expanding in all semi-arid agricultural areas of Australia.

Long-term observations of watertables indicate that levels have risen by tens of metres since clearing began. It is estimated that in some places, groundwater has risen by 30 metres since the 1880s in south-eastern Australia, and by about 20 metres in south-western Western Australia.

Western Australia was estimated to have nearly 2 million hectares of secondary salinity in the South-West zone (9.4% of cleared land) in 1994. It is projected to be 3.3 million ha by 2020 (17.1%) and a potential area at equilibrium of 6.1 million ha (31.8%). Most of this is expected to be in broad valley floors, where it will seriously reduce current agricultural production and affect a large proportion of existing remnant vegetation.

Area of land reported to be affected by secondary dryland salinity in Australia (Government of Western Australia 1996)

Table 2

State	Area salt affected in 1982 (ha)	Area salt affected in 1996 (ha)	Potential area at equilibrium
Western Australia	264,000	1,804,000	6,109,000
South Australia	55,000	402,000	600,000
Victoria	90,000	120,000	unknown
New South Wales	unknown**	120,000	500,000,000
Tasmania	unknown**	20,000	unknown
Queensland	unknown**	10,000***	74,000
Northern Territory	unknown**	minor	unknown
Total	na	2,476,000	11,783,000

Further Reading