What is salinity?
Salinity refers to the movement and concentration of salt, in landscapes. Both soil and natural waters can become saline. Hence, salinity can be described as either soil salinity or water salinity.
Also read salinity explained.
The effects of salinity are broad
- reduction in the productive capacity of affected land (eg. crop yields)
- degradation of the environment and wildlife habitats
- loss of water quality for stock and domestic water supplies
- production losses causing economic hardship
- damage to roads
- damage to water-using household equipment.
Sources of salt
- Retreating seas - Many areas, such as the Mallee, were once covered by an inland sea. When the sea retreated about 10 million years ago, the sediments it left behind contain large quantities of salt. Many soils in the Mallee region have been derived from these materials.
- Rain - Salt from the sea is carried inland by strong winds, and falls in rain. As you would expect, salts in rainfall are higher near the coast, decreasing inland. Deposition of windborne salts can range from 30g/m2/y close to the coast down to 5g/m2/y further inland (CSIRO, 1980).
- Rocks - Salts are present in rocks and are released by weathering. Many rock types including marine sediments, granites and rhyolites contain high levels of sodium and potassium, which may be mobilised after weathering.
The water balance
During and after rainfall, water moves into the soil. This water is stored in the soil profile. Most is used by plants, however a proportion seeps down past the root zone into the groundwater system. In many cases this will cause groundwater levels to rise, mobilising salts. The upper surface of the groundwater is called the watertable. When a saline water table rises to within one or two metres of the ground's surface, water moves to the surface by capillary action, bringing salt with it. The water evaporates leaving the salt in the surface layers of the soil. Over time the soil becomes saline, limiting the growth of vegetation.
As salinity levels increase salt sensitive plant species die out leaving the soil prone to erosion. This is further exacerbated by declining soil structure caused by the high levels of salt. Irrigation is just like increasing rainfall. In irrigation areas, where saline ground water is present, sub-surface drainage and salt disposal are needed to prevent the otherwise inevitable rise in saline water table and accumulation of surface salt. If the saline water table rises to within two metre of ground level, waterlogging and salinisation occur causing substantial productivity losses.
The extent of the problem
Problems with salinity occur predominantly throughout northern and western Victoria and are considered to be increasing in severity (The most concentrated problems occur in the Murray-Goulburn Irrigation District, where a shallow water table threatens about 385,000 hectares of productive land. This salt-prone area includes 72% of the Kerang Region and 22% of the Shepparton Region.
Secondary salting occurs throughout Victoria, with 120,000 hectares identified to date.
Managing the problem - A catchment approach
Rarely can one land manager solve a salinity problem. While sometimes causes and effects can be seen within a single farm; mostly the cause and effects cross property boundaries. Everyone in a catchment is affected by the agricultural activities of others in it. Therefore effective salinity management requires a total catchment approach.
The long-term solution to salinity, is to restore the balance of inputs and outputs of the soil-water system, by controlling the process of groundwater recharge. Treatment of salt affected land is an important component of salinity management.
In dryland areas, large scale establishment of deep rooted plants such as trees and lucerne will be required to restore the water balance. In high rainfall areas plantation forestry may be one means of achieving the scale of tree planting required. Strategic tree planting as a part of a whole farm plan or catchment plan will also provide some benefit to recharge management. Deep-rooted perennial pastures are more likely to be effective in lower rainfall than in higher rainfall areas.
In irrigation regions control techniques may include more efficient irrigation, drainage and reuse systems, groundwater pumping and re-layout of land to facilitate surface drainage and water application.
For salt affected areas appropriate management can reduce further degradation of the site and enhance their productivity. Utilisation of salt tolerant pasture species and grazing management may substantially increase productivity.
Control of salinity in the Murray-Darling basin depends on joint action by all States in the Basin. Victoria is already committed to a major program of salinity control, coordinating the efforts of landholders and community groups with salinity control activities across catchments large and small. The success of the program depends as much on community support as on government resources. We all, therefore, need to be alerted to the salinity issue and to our opportunity for input and involvement.