Runoff plays major role in salinity

By Rebecca Thyer

Work by NSW Department of Industry and Investment hydrologist Dr David Mitchell has found that the majority of on-farm salt movement has been caused by runoff, rather than groundwater.
The results – from the first major farm-scale study of salt stores and water at key sites across NSW – show that runoff, driven by rainfall, is a major pathway for salt transport from the land to streams, he says.
“It was previously assumed that groundwater was the major pathway for salt to enter streams. It is known that rainfall drives runoff and our research shows that runoff and subsequent wash-off of salt can contribute between 40 and 80 per cent of the salt load leaving a first-order catchment.”
So, during wetter periods there will be more runoff and therefore salt entering waterways, and in drier periods less runoff and less salt entering waterways, Dr Mitchell hypothesises, adding “we haven’t had any wet periods lately”.
His research also shows that most of the salt comes off very small areas of land. “For example, at one of our sites most of the salt was generated from five hectares out of a possible 750ha. This then leads to the ability of individual landholders to make considerable contribution to managing salinity,” he says.
“Using ground cover to minimise overland flows and strategic tree plantings to reduce saturation, which in turn minimises both overland and subsurface flows, can help manage the salt stores. Design of on-ground actions to reduce salt movement, planting perennials or earthwork for instance, should be based on the hydrological aspects of the individual site.”
Dr Mitchell says an improved understanding of how groundwater and surface flows are affected by landscape, land use and climate means clearer, more consistent advice can be given to land managers, particularly regarding the effects of dryland salinity and catchment water yield and quality.
The NSW Salinity Strategy and the National Action Plan for Salinity and Water Quality program funded eight key sites in the central west, near Wagga Wagga, and near Tamworth. All represented a range of typical landscapes and farming systems.

More information: David Mitchell, 02 6391 3852, david.mitchell@industry.nsw.gov.au

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