The term salinity refers to the concentration of dissolved salts in water or soil and is expressed in terms of concentration (mg/L) or electrical conductivity (EC).
Salinity management is one of the most significant environmental challenges facing the Murray–Darling Basin.
If left unmanaged, salinity has serious implications for water quality, plant growth, biodiversity, land productivity and the supply of water for critical human needs.
High salinity can:
Typically, the Murray–Darling Basin is renowned for its flat terrain, low rainfall and high evaporation rates which assist in the accumulation of increased salt in the landscape.
Salt is a natural feature of the Basin with its derivation coming from ancient ocean sediments, the weathering of rocks and deposition by rainfall over millions of years.
The amount of salt in the Basin is not increasing; however naturally occurring salt is being 'mobilised' and concentrated in certain parts of the landscape. Whilst salt mobilisation happens naturally, it is often exacerbated by human activities such as irrigation development and land clearing (removal of deep–rooted native vegetation).
Resulting removal of deep–rooted native vegetation and its replacement with shallow–rooted crops and pastures can result in rising groundwater and the accumulation of salt at the surface.
An accumulation of salt at the soil surface is known as dryland salinity. Observations of groundwater during the drought indicated that dryland salinity is cyclic in nature and is related to climate.
In parts of the Basin, saline groundwater naturally flows into rivers. When the saline groundwater systems are recharged by a succession of wet years recharged groundwater drains back to river systems. When the rivers' flow returns to base flow levels, salinity levels (concentration) in the rivers increase making the water less suitable for drinking and for irrigation.
Water flowing through the river system and out to the sea through the Murray Mouth is the only natural way that salt can leave the Basin. A non–natural way is through salt interception schemes (see below illustration).
Salinity in the landscape is managed in a number of ways. One way is through the management of river flows — for example, through timed water releases to dilute salinity. Other ways include:
Salt interception schemes are an engineering tool used to divert groundwater and drainage water away from the Murray River. In most cases, a bore and pump system extracts groundwater and pumps it to a salt management basin some distance away from the river. Without salt interception schemes in the lower part of the River Murray, approximately half a million tonnes of salt per year would reach the River, resulting in increased salinity levels (concentration). Illustrated below are the 18 salt interception schemes in operation along the Murray–Darling system.
The clearing of native vegetation and the introduction of European farming practices has affected the natural water cycle in the Basin. However, improved farming practices have reduced the amount of water entering groundwater systems.
Improved farming methods in dryland areas include:
In irrigation areas, improved farming methods include:
Salinity affected land south of Mildura, Victoria Photo by Arthur Mostead
Flow & Salinity Reports – summarise what is happening now with the flow and salinity levels of the River Murray. These reports also provide a forecast for the week ahead.
River Salinity - graphs of salinity levels at key indicator sites.