Salinity as land cover change: VCE Geography
“The processes and human activities causing salinity as a form of land cover change, and the impacts of and responses to salinity”
Salinity is the build-up of salt in soil and water that degrades land cover, turning productive vegetation and farmland into bare, salt-scalded ground. It is driven mainly by clearing native vegetation (dryland salinity) and over-watering crops (irrigation salinity), which raise saline water tables. Impacts include lost agriculture, dead vegetation and damaged infrastructure. Responses include replanting deep-rooted vegetation, better irrigation, and engineering such as drains and pumps.
What this dot point is asking
VCAA treats salinity as a form of land cover change: you must explain the processes and human activities that cause it, evaluate its impacts on land and people, and assess the responses, using located examples with data. Salinity is a strongly Australian case study, which examiners reward.
What salinity is
Salinity is the concentration of dissolved salts in soil and water. Australian landscapes naturally store large amounts of salt deep in the soil, deposited by ancient seas and rainfall over millions of years. The problem arises when that salt is mobilised toward the surface, where it kills vegetation, poisons soil and changes the land cover from productive farmland or native bush to bare, white salt scalds and salt-tolerant scrub.
The two main processes
- Dryland salinity. Native deep-rooted vegetation such as eucalypts and mallee used most of the rainfall before it could sink deep. When this is cleared for shallow-rooted crops and pasture, more water passes through the soil, raising the water table and carrying stored salt to the surface. This is the dominant process across southern Australia.
- Irrigation salinity. Applying more water than crops can use, often on poorly drained soils, raises the water table from below. As the saline groundwater nears the surface it evaporates, leaving salt behind. This affects irrigated districts along major rivers.
Human activities that cause salinity
- Broad-scale clearing of native vegetation for agriculture.
- Over-irrigation and inefficient flood irrigation.
- Poor drainage that lets saline groundwater accumulate.
- Removal of perennial pasture in favour of annual crops with shallow roots.
Case study: the Murray-Darling Basin and the WA wheatbelt
The Murray-Darling Basin in south-eastern Australia suffers both irrigation and dryland salinity. Decades of clearing and irrigation raised water tables, mobilising salt into soils and into the Murray River itself, which threatened drinking water and irrigation supplies downstream in South Australia. In the Western Australian wheatbelt, extensive clearing of mallee and woodland for wheat and sheep has produced severe dryland salinity, with large areas of farmland degraded into salt scalds.
Around 70 percent of pre-European native vegetation in the Western Australian wheatbelt has been cleared, which is the underlying cause of its severe dryland salinity. Salinity has been estimated to threaten millions of hectares of agricultural land across southern Australia, making it one of the nation's most significant forms of land degradation.
Impacts on the environment
Rising salt kills native vegetation that cannot tolerate it, removing habitat and reducing biodiversity. Salt scalds leave bare, eroding ground where little will grow. Saline water enters wetlands and rivers, harming freshwater ecosystems and the species that depend on them. The land cover shifts from forest, woodland or pasture toward bare ground and salt-tolerant scrub.
Impacts on people
Salinity reduces crop yields and can render farmland unusable, cutting farm incomes and rural employment. Saline groundwater corrodes roads, building foundations and pipes, raising maintenance costs for towns. Rising salt in rivers threatens town and irrigation water supplies, with costs borne far downstream of where clearing occurred.
Question: Explain how the clearing of native vegetation causes dryland salinity at a located example.
Step 1: State the starting condition
Before clearing, deep-rooted native vegetation (eucalypts and mallee) in the Western Australian wheatbelt draws water from deep in the soil and transpires it, keeping the water table low and the naturally stored salt locked well below the surface.
Step 2: Identify the human activity and its immediate effect
Broad-scale clearing for wheat and sheep replaces this deep-rooted cover with shallow-rooted annual crops and pasture. These plants use far less of the rainfall, so more water drains below the root zone (deep drainage increases).
Step 3: Explain the mechanism that mobilises salt
The extra water recharges the groundwater, so the water table rises. As it rises through naturally salt-rich subsoils, it dissolves the stored salt and carries it upward toward the surface.
Step 4: Link to the change in land cover
Near the surface the saline water evaporates, concentrating salt in the topsoil. Salt-intolerant plants die and the land cover changes from productive pasture or native bush into bare, white salt scalds, as seen across the Western Australian wheatbelt where around 70 percent of native vegetation has been cleared.
Responses to salinity
- Revegetation with deep-rooted perennials and trees lowers water tables by using more water before it reaches the salt store.
- Improved irrigation, such as drip systems and better scheduling, reduces excess water that raises water tables.
- Engineering works, including groundwater pumps, drains and salt interception schemes along the Murray, divert saline water away from rivers and lower local water tables.
- Land use change, such as switching to salt-tolerant crops and perennial pasture, adapts farming to affected land.
- Government programs and catchment management coordinate revegetation and monitoring across whole catchments rather than single farms.
A common error is treating salinity as a purely natural process. The salt is natural, but examiners want you to explain the human trigger, especially vegetation clearing and over-irrigation, that mobilises it. Another trap is confusing dryland and irrigation salinity: name which process applies in your case study and explain the water-table mechanism, rather than just saying salt builds up.
Exam-style questions
Questions in the style of VCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice6 marksExplain how two interconnected human activities cause salinity at a selected location you have studied this year.Show worked answer →
For 6 marks, name a precise location, explain two human activities, and show explicitly how they reinforce one another rather than just listing them (roughly 2 to 3 marks per activity plus a mark for the interconnection).
Using the Western Australian wheatbelt: "The two leading human causes are broad-scale clearing of deep-rooted native vegetation (mallee and woodland) for wheat and sheep, and the replacement of perennial native cover with shallow-rooted annual crops. Clearing removes the deep roots that once used most of the rainfall, so more water drains below the root zone and the water table rises, dissolving naturally stored salt and carrying it toward the surface."
Interconnection: "These activities are interconnected because clearing and the switch to shallow-rooted annuals both reduce how much rainfall the vegetation uses, so together they raise the water table faster than either alone, mobilising salt to the surface where evaporation concentrates it into salt scalds."
Markers reward a located example, two genuine human activities, the rising-water-table mechanism, and an explicit causal link between the two activities.
Practice8 marksName the location of your selected example of salinity. a. Outline one impact of salinity on the environment and one impact on people at this location (4 marks). b. Evaluate the likely effectiveness of one response to salinity at this location (4 marks).Show worked answer →
Part a (4 marks): name a precise location, then outline one environmental and one social impact, about 2 marks each. Using the Murray-Darling Basin: "Environmental: rising saline groundwater kills salt-intolerant native vegetation and degrades wetlands and river ecosystems. Social: salt entering the Murray River threatens town and irrigation water supplies downstream in South Australia, and saline soils cut crop yields and farm incomes."
Part b (4 marks): name a response, then evaluate strengths and limitations with a judgement. "A response is the salt interception scheme: bores and drains along the Murray pump saline groundwater away from the river into evaporation basins. Strength: it has measurably lowered salinity in the river, protecting downstream water supplies. Limitation: it treats the symptom rather than the cause (continued clearing and irrigation upstream), is costly to run, and the disposal basins create local salt-management problems, so it is only partially effective unless paired with revegetation and improved irrigation."
Markers reward a located example, distinct environmental and social impacts, a named response, balanced evaluation and a clear judgement.