Salinity - Management - Victoria

Victoria

Introduction

Water balance

As the groundwater system fills and eventually reaches a new equilibrium, the amount of water entering the landscape as recharge and the amount of water leaving as discharge is balanced. However there is a time lag between when changes in land use or improvement in water balance occurs and evidence of a response. It will take decades to reverse the water rise in most groundwater systems (see figure below).

Re-establishing the water balance requires farming systems with similar water use to that of deep-rooted native vegetation. Designing and implementing such farming systems is a major challenge.

Recharge processes are generally faster than discharge processes. If it takes 30 to 50 years for our fastest groundwater system to fill with water, then it is reasonable to expect that it might take at least 30 to 50 years for it to empty back to where it was. If the system takes 100 years or more to fill, we can again expect at least a similar amount of time to establish the original equilibrium. This is an important issue for management as the degree of recharge reduction and the time taken have important consequences on land use options during any adjustment period, and the degree of change sought. Beneficial effects of land use options may well occur before the system has returned to an equilibrium.

Salt balance

As more water moves through an aquifer, more salt is mobilised. Very long periods of time are needed for catchment salt stores to be reduced to the point where the amount entering the system equals the amount leaving the system, that is, to achieve a salt balance. The net amount of salt that exits a catchment via stream flow indicates the time it will take for the catchment to flush its store of salt, when compared with the total mass of salt stored in that catchment. In some of the more responsive groundwater flow systems, the net output of salt may take about 150 years to flush from the system. In larger catchments (e.g. the Murray groundwater basin), it may take as much as 15 000 years. This means that although management may lower the watertable and allow productive use of land, there may be ongoing salt inflow to streams via groundwater.

This makes managing stream salinity very difficult. It is very important to prevent the interception of groundwater with salt stores in regions where we still have this opportunity.

The reality

The substantial lag times for catchments to come back into water balance and change salt mobilisation mean that it is inevitable that dryland salinity will be a feature of many Australian landscapes for some time. This is true even with widespread adoption of innovative land uses that manage to turn off the recharge tap and re-establish water balance. Ultimately the decisions on the measures to be taken will be influenced by the value of the threatened assets, the capacity to manipulate the environmental processes, the economic feasibility and social acceptance of the proposed actions.

What is the scale of the groundwater systems and how can they be managed?

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.

More detailed information is available in the Victorian Dryland Salinity Assessment 2000 Report

Groundwater Flow Systems in Victoria

The groundwater flow systems for Victoria are present in the table below. Within the >300mm rainfall zone, 506,000 hectares in local, 1,186,000 hectares in intermediate and 1,378,000 hectares in regional flow systems are coincident with regions within which there are areas with a high risk of dryland salinity.

Groundwater Flow System Type	Area (ha)at risk in 2050	Percentage of total risk area (%)
Local and intermediate flow systems in deeply weathered rocks	81	0
Intermediate flow systems in sedimentary sequences in large valleys	35,400	1
Local flow systems in fractured or weathered rocks or colluvial fans	382,731	12
Intermediate flow systems in fractured rocks	862,631	28
Local flow systems in fine grained unconsolidated sediments	122,925	4
Regional flow systems in permeable alluvial sediments	359,325	12
Regional flow systems in marine sediments overlain with some local flow systems in sand dunes	341,019	11
Regional and intermediate flow systems in fractured basaltic rocks and layered sedimentary rocks	679,350	22
Intermediate and local flow systems in fractured basaltic rocks and layered sedimentary rocks	287,931	9

* Area within >300mm rainfall zone

Government responses to dryland salinity as at the year 2000

Government responses

The Victorian Salinity Program was established in 1987 with the release of Salt Action Joint Action. Under this program, dryland salinity management plans, strategies, or land and water management plans were prepared for major catchment areas of northern and south-west Victoria between the late 1980s and mid 1990s. They focus on dryland salinity but recognise the links to other natural resource issues. Government has accepted all plans and strategies and they are being implemented.

Catchment management authorities have been created and given responsibility for overseeing the plans, leading to further integration of salinity with other natural resource issues.

The State Government has recently prepared a revised salinity management framework for Victoria. Regional salinity management plans and strategies will be reviewed and second generation plans prepared by September 2001. Reviews will consider:

the progress of the plans against specified targets for works;
the progress of works towards achieving desired catchment health outcomes;
the validity of the assumptions on which the plans were based; and
new information that contributes to a better understanding of the dryland salinity problem.

It is expected that the reviews, particularly for the Murray Basin catchments, will recommend an approach to salinity management based on the protection of specific social, economic or environmental assets and the achievement of targets for catchment health outcomes.