The extent of dryland salinity

A large area of Victoria is affected by dryland salinity. Approximately 120 000 hectares (ha) of dryland salinity induced by land management practices of the last 200 years is known to occur, and of this 85 000 ha has been accurately mapped and is on the DSE Geographic Information System.

Salinity mapping is on-going. Additional salinity sites are known to exist but are not yet recorded in the Glenelg, western Wimmera, western North Central and the North East Catchment Management Regions.

Each dot represents the centre of a salinity site. The size of the dot used on the map under represents some sites and over represents others. The actual size of the sites varies from less than one hectare to more than 2000 hectares. Fifty percent of the sites are smaller than two hectares while just 3% are larger than 100 hectares.

The high density of dots in the south-west represents a large number of relatively small discharge sites (average size 8 ha). The obvious straight lines in this region represent the extent of currently available mapping. Further field work will reveal the full extent of salinity in this area.

Accurately Mapped Dryland Salinity (ha) by Catchment Management Region

CMA Region	Area of Induced Dryland Salinity (ha)
Corangamite	15250
East Gippsland	1140
Glenelg	29410
Goulburn-Broken	4900
North Central	27635
North East	2367
Port Phillip	1790
West Gippsland	15872
Wimmera	16348
Total for State	85292

Geology and Dryland Salinity

Plains

The largest sites of dryland salinity occur on the plains of northern Victoria. In the plains areas (eg the Wimmera Plains and the Northern Riverine Plains) regional groundwater aquifers discharge in low points in the landscape (river channels and wetlands). These aquifers may be recharged hundreds of kilometres away in the headwaters of the ancient alluvial channel systems called Deep Leads as well as more generally across the entire plains area. Establishment and astute management of higher water using vegetation such as trees and deep-rooted crops and pastures will help to control the problem.

Around Lake Wellington in West Gippsland Region large salinity sites are found which are caused by recharge in both dryland and irrigation areas of the catchment.

The basalt plains run from Melbourne, west to the South Australian border. Many salinity sites are associated with this geology.

Hills and Mountains

The second most common landscapes in Victoria experiencing salinity are the bedrock (mostly sedimentary bedrock) hills of the Central and Western Highlands. Although annual rainfall is typically less than 800 mm/year, significant recharge occurs through thin, stoney soils under areas supporting shallow-rooted annual vegetation. The salinity from this recharge generally occurs locally.

Much of the bedrock in these highland areas is deeply weathered with high salt storage and the resulting rising watertables have brought much of this salt to the surface in low points and breaks of slope in the landscape.

Unlike the plains, salinity occurrence is usually associated with local scale groundwater systems. However, although the salinity sites tend to be small in comparison to those of the plains, they are numerous and pose a serious problem throughout the Central and Western Highlands.
Management of the problem is focused on re-establishing deep-rooted perennial vegetation, including trees, to make greater productive use of water from rainfall and reduce recharge and runoff.

The Eastern Highlands, Otway Ranges and Strzelecki Ranges are relatively unaffected by salinity. This is probably due to
(i) the high percentage of area still under forest cover
(ii) the generally high rainfall and good drainage in these areas. Groundwater salinities tend to be low and clearing of land in these areas more commonly leads to waterlogging than secondary salinity.

Dundas Tablelands

Salinity discharge in the Tablelands, north of Hamilton in the Glenelg Region, typically occurs along valley floors and has been associated with local groundwater systems existing within the deeply weathered material forming these tablelands.

Mallee

The Mallee region is highly salt-affected. However, approximately 85% of salinity in the region was present before European agriculture altered the water balance. This is termed natural or primary salinity. Of the 15 000 ha of salinity identified as induced (or secondary), 12 000 ha is discharging from a regional groundwater system.

General

Dryland salinity is more prevalent in the western half of the state for the following reasons:

Clearing of land and replacement with lower water using vegetation has been more widespread
Lower relief and generally poorer drainage
Generally high natural salt storage in the area.