the processes and human activities causing salinity as a form of land cover change, and the impacts of and responses to salinity

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.

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.

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.