South East Regional Water Balance Phase 2

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Project Partners:

Flinders University and CSIRO

Research Theme:

N/A

Status:

Project Overview

The overall purpose of the South East Regional Water Balance Project was to address a number of gaps in the conceptual model of the overall water balance of the Lower Limestone Coast Prescribed Wells Area (LLC PWA) and bring together all existing and new knowledge into a prototype model that, with future work, will enable assessments of risks to groundwater resources and wetlands from changing climate, land use and water management practices.

Phase 1, completed and reported on in November 2013, comprised a series of preliminary investigations into various components of the water balance of the LLC PWA, and development of the framework for a regional scale numerical groundwater flow model. 

Phase 2 consisted of three main tasks:

  1. Development of a regional water balance model
  2. Recharge modelling
  3. Wetland connectivity modelling

Progress Update and Key Findings

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.

Project Impacts

The South East Regional Water Balance Project has delivered the following outcomes to support water resource management for the South East:

A spatially continuous net recharge dataset for the period 2001 to 2010. For the 10 year period 2001-2010 the areal average net recharge was found to be 40 mm/yr.

An assessment of factors controlling net and gross recharge.

A regional scale numerical groundwater flow model that:

  • Provides a platform for exploring the water balance of the LLC PWA based on current knowledge and data.
  • Incorporates all available relevant data and knowledge on the hydrological system in the model domain.
  • Uses gross recharge inputs from a LEACHM unsaturated zone model that was developed as part of the project and which provides monthly outputs for the period 1965 to 2013.
  • Uses sampled land surface variability to parameterise evapotranspiration and water table depth relationships, to address concerns about upscaling groundwater evapotranspiration within regional-scale models.
  • Has undergone preliminary calibration using available data, including chemistry and isotope derived fluxes and groundwater flow information, and “soft knowledge”.
  • Conforms to the Australian Groundwater Modelling Guidelines and DEWNR’s Groundwater Model Warehouse Guidelines.
  • Can effectively support the development of local scale numerical models to address local scale policy questions, including the models being developed through the Wetland – groundwater connectivity project.

Estimates of the regional water balance for the LLC PWA that are based upon the best available data and knowledge, and a new understanding of how this water balance changes over time.

Recommendations for the areas of the conceptual model that require further investigation (critical knowledge gaps) and for the direction of future modelling activities.

A better understanding of the dynamics of rainfall recharge in the South East, and a range of new tools that enable the representation of the depth to water table dependence of this process in groundwater flow models.

A new lookup-table based approach that uses a newly developed MODFLOW package (the NetR package) to incorporate the effects of depth to watertable on recharge and evapotranspiration into MODFLOW regional groundwater flow models. A lookup table has been developed for the study area in the South East, and the methodology has been tested with the regional groundwater flow model, however, further calibration is required. Once fully validated this approach will be applicable to any area with shallow water tables.

A conceptual model for the geological factors determining groundwater-surface water interactions for South-East wetlands.

An evaluation of regional and local factors controlling groundwater – surface water interactions at three high value wetlands (Lake Robe, Bool Lagoon and Deadmans Swamp.

A new methodology for assessing the risks to wetland water leve regimes from changes in regional groundwater levels.

Research Outputs

South-East Regional Groundwater Flow Model - LEACHM Model

South-East Regional Groundwater Flow Model

South-East Wetlands-Groundwater Interaction Model

Goyder South East Project - A new approach for modelling groundwater recharge in the South East of South Australia using MODFLOW

South East Regional Balance Project - Phase 2 Project Summary Report

South East Regional Water Balance Project - Phase 2 Development of a Regional Groundwater Flow Model

Development of a Groundwater Extraction Dataset for the South East of South Australia: 1970-2013

Development of Preliminary 1969 and 1983 Land Use Maps for the South East of SA

A Hydrostratigraphic Model for the Shallow Aquifer Systems of the Gambier Basin and South Western Murray Basin

A MODFLOW-based approach to simulating wetland-groundwater interactions in the Lower Limestone Coast Prescribed Wells Area

Groundwater-surface water interactions at Bool Lagoon, Lake Robe and Deadman Swamp (Limestone Coast, SA): Data review

Evaluation of groundwater-surface water interactions at Bool Lagoon and Lake Robe using environmental tracers

Project News

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