The word salinity refers to a situation when the quantity of sodium chloride (NaCl) in the landscape causes problems with soil and water quality, agriculture, the built and natural environment, and biodiversity. It could also be described as the presence of salt in the root zone of plants. Sodium chloride (NaCl) is better known to us as common table salt and causes enormous problems in the Australian agriculture industry. It is estimated the monetary cost of loss of production is in excess of $270 million each year. Due to the extent of this problem, the Government is active in identifying and studying salt affected areas of Australia.
When discussing salinity the following terms are often used:
- Recharge: where water is added to the ground water.
- Discharge: where water is lost from the ground water.
If recharge > discharge the level of the watertable will rise.
There are two types of salination caused by farming practices in Australia: these being
a) Dryland salinity and
b) Irrigation salinity.
Dryland salinity is salinity which occurs in non-irrigated rural areas. It affects approximately 2.5 million ha of Australian farmland.
Cause of dryland salinity
The major cause of dryland salinity is the clearing of native vegetation. Since the European settlement of Australia in 1788, enormous numbers of high water using native trees, shrubs and grasses have been replaced with shallow rooted crops and pastures. Many of these introduced plants take less water from the ground water. This is known as reduce discharge. As less water is taken from the system, the level of the watertable rises bringing with it dissolved salts and in turn bringing salts into the root zone.
- Reduced plant productivity because the plants have difficulty extracting water from the soil.
- Dissolved sodium, chloride and borates can be toxic to plants and may raise soil pH.
- Soil structure may be damaged leading to soil instability and erosion.
- Death of native vegetation.
- Water becomes unsuitable for livestock, humans and irrigation.
Some solutions to dryland salinity include planting salt tolerant pasture species such as marsh grass, tall wheat grass & saltbush. Tree planting programs, avoidance of overgrazing, contoured banks and drainage to control water flow and changing from shallow rooted annual pastures to deep-rooted perennial pastures such as lucerne and phalaris.
This type of salination occurs in areas where natural rainfall is supplemented with irrigation. This is known as an area with greatly increased recharge. It is estimated that 70 – 80% of irrigated land in NSW is under threat from a rising watertable.
When the volume of added water is greater than the evaporation and transpiration occurring , the excess water goes into the groundwater , causing the water table to rise. This problem is compounded by the high rate of clearing that precedes irrigation.
Clearing and irrigation results in a rising water table. This is called irrigation salinity.
These are the same as in dryland salinity. However, the irrigation water usually becomes saline too, so the effects are quickly magnified.
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