South East Regional Water Balance Phase 2

The aim of Task 1 was to develop a regional groundwater flow model of the Lower Limestone Coast Prescribed Wells Area (LLC PWA), including both major aquifers, with the following primary objectives:

1. Assess and improve knowledge of the regional water balance, including recharge, groundwater extraction, groundwater inflows and outflows across the boundaries of the PWA, and outflows at the coast.
2. Quantify available surface water and groundwater volumes at a regional scale.
3. Identify critical knowledge gaps.

Longer-term objectives of the regional model were to:

• Provide boundary conditions for future local scale models of “hotspot areas” or areas where local groundwater flow processes are important, e.g. wetlands or sections of the drainage network.
• Act as a tool to investigate the impacts of climate, land use and water management scenarios on aspects of the regional water balance and on groundwater levels at a regional scale.

The regional groundwater flow model developed in Phase 2 covers the entire regional groundwater flow system that includes the LLC PWA. As such, it covers the Tertiary Gambier Basin, extending east into Victoria, as well as part of the south-western Murray Basin to the north of the LLC PWA. Task 1 also included a number of activities aimed at improving the outcomes of the regional groundwater flow model. These included:

• Development and testing of a new hydrostratigraphic model of the study area in collaboration with DEWNR.
• A comprehensive assessment of recharge and evapotranspiration (ET) processes.
• Development of methodologies for the accurate representation of recharge and evapotranspiration processes in shallow water table areas into a regional groundwater flow model.
• Development of preliminary historical land use maps to better quantify historical recharge.
• Development of a recent (metered) and historical (estimated) groundwater extraction dataset and use of this to constrain recharge estimates for irrigated areas.

In Task 2, a new MODFLOW net recharge package was developed to incorporate the results of an unsaturated zone model through a lookup-table approach. This has the benefit of being able to incorporate the effects of shallow water tables on recharge, as well as changing rainfall and land use, with much lower model run-times than a fully-coupled unsaturated-saturated zone model. As rainfall recharge is a major input to the water balance for the South East, this activity was designated as a separate task in Phase 2 of the project, with the following objectives:

1. Complete one-dimensional numerical recharge modelling, conditioned to the recharge rates estimated in Phase 1.
2. Develop look-up tables that are based on: monthly rainfall, month of year, vegetation type, soil type, and depth to watertable.
3. Develop a new MODFLOW package capable of interpreting the look-up tables, and trial the implementation of this within the regional groundwater flow model.

Future water allocation policy exercises in the LLC PWA will need to evaluate potential impacts on wetlands and other groundwater-dependent ecosystems. However, by necessity, the regional groundwater flow model was developed at a coarse spatial scale (1 km by 1 km cells) relative to the size of most wetlands in the region (a few km2 or less). Thus, the aim for Task 3 was to develop a complementary approach to the regional model to help evaluate potential impacts of future changes in climate, land-use and water allocation policy on wetland water level regimes. To achieve this, Task 3 had three objectives:

1. Development of a conceptual framework for wetland – groundwater interactions in the LLC PWA;
2. Inform the development of this framework by evaluating groundwater – surface water interactions for three wetlands in the region (Deadmans Swamp, Bool Lagoon and Lake Robe) using historical data and an environmental tracer field study;
3. Develop a MODFLOW-based wetland – groundwater modelling framework representative of deflation basins and other shallow wetlands in the region.

The three wetlands selected for the field study were hypothesised to represent a regional recharge (Deadmans Swamp), flow-through (Bool Lagoon) and discharge wetland (Lake Robe) along a regional hydrogeological system.