Salinity - Risk and Hazard - Victoria

Victoria

Where is there a dryland salinity risk or hazard and how was this determined?

Current salinity risk areas are based on mapping of land affected by dryland salinity at 1:25 000 scale and analysis of groundwater levels at a scale of approximately 1:250 000. Mapping was based on the nine-second digital elevation model for Victoria and work undertaken for the Murray-Darling Salinity Audit, for much of northern Victoria. Large contiguous areas of forest or woodland (mostly public land) were excluded from the analysis due to the lack of groundwater data and generally low threat of salinisation. Future salinity risk is based on current risk areas and analysis of groundwater trend information. Worst- and best-case trend values were calculated based on relatively wet and dry climatic sequences, respectively. Information on salinity risk and potential impact presented generally represents the worst-case trend scenario.

Groundwater trends

Groundwater trend values were assigned to physiographic units in each major catchment or region. These units were largely based on elevation and slope and, in part, reflected regional geology. For the Wimmera and the non-Murray Basin regions, trend values were calculated for two periods:

the 10 years to 1998 - considered to be the best case trend scenario with prevailing downwards trends in water table level and included a prolonged sequence of relatively dry years;
the five years 1989-1993 - considered to be the worst case trend scenario with prevailing upwards trends in water table level and was a period of relatively high rainfall.

Groundwater trend values for physiographic units in the remainder of the state were determined as part of the MDB Salinity Audit. Only a single value was calculated - which roughly corresponds to the worst case trend scenario.

The sharpest rates of rise in groundwater level were recorded in the Ovens and Kiewa catchments of north-east Victoria, with water tables rising at between 10 and 30 cm/y in most physiographic units. Rates of rise were less than 10 cm/y in most other parts of the state. In the best case trend scenario for southern Victoria and the Wimmera, water tables were falling at up to 10 cm/y.

Salinity risk

Salinity risk ratings were developed that combined current estimated depth to water table and groundwater trend for both best and worst case trend scenarios. In the worst case, the only substantial areas of the state with a low salinity risk rating were the Mallee, West Gippsland and parts of central Victoria. Almost 9 Mha of the state was included in high and moderate risk categories, with 2.5 Mha in the high risk category. The area of land in the high and moderate risk category in southern Victoria and the Wimmera under the best case trend scenario was 1.9 Mha, compared with 5.4 Mha for those regions under the worst case scenario.

How can dryland salinity risk change over time?

Under the worst case scenario (with upper limit of groundwater level trends) there is predicted to be a very large expansion in the area of land with shallow water tables. This is predicted to be concentrated in several regions, including:

the Dundas tablelands and basalt and alluvial plains of south-west Victoria
the southern slopes of the Great Dividing Range in west central Victoria
the lower Avoca riverine plain
the Goulburn riverine plain to the foothills of the Great Dividing range.

Lower limit of groundwater level trends across much of southern Victoria were negative (i.e. water tables predicted to fall) and so the area of land with water tables shallower than 5 m is predicted to contract over the coming 50 years.

The predicted area and percentage of each CMA region with shallow water tables in 1998, 2020 and 2050 are given in the table below. Under the worst case trend scenario, the largest changes in the area of land with shallow water tables were predicted for the Corangamite, Glenelg-Hopkins and Goulburn-Broken CMA regions, with an up to tenfold increase. In the worst case, the area predicted to contain land with shallow water tables will increase from 4.9% of the dryland non-forested land to 22.8%. The proportion of the CMA region predicted to contain shallow water tables in this scenario exceeds 40% for the Corangamite, Glenelg-Hopkins and Goulburn-Broken regions and exceeds 20% for the North Central region. This scale of land with potential for shallow water tables and salinity is similar to predicted for agricultural regions in Western Australia (Agriculture Western Australia et al. 1996)

Table: Areas* (ha) of land predicted to have shallow watertables for catchment management authority regions in 1998, 2020 and 2050 in Victoria.
CMA Region	1998	2020 worst-case	2050 worst-case
Corangamite	51 200	213 300	499 100
East Gippsland	1 800	1 800	19 100
Glenelg-Hopkins	144 500	429 600	947 200
Goulburn-Broken	123 600	193 500	739 800
Mallee	60 700	63 500	74 400
North Central	124 300	176 500	401 400
North East	40 400	48 000	68 100
Port Phillip	8 500	43 200	134 100
West Gippsland	14 100	14 000	70 600
Wimmera	96 400	122 500	160 800
Total	665 500	1 305 900	3 114 600