Environmental Flow Assessments

​​​​​​​​​​​​​Environmental Flows in Tasmania

Throughout Australia, environmental flow assessments are used to estimate the quantity and timing of flows required to sustain identified aquatic values in riverine environments. Typically, water management planning and other natural resource planning processes use environmental flow assessments to make informed decisions about water management.

There is no single standardised method for estimating environmental flows in Australia or in other countries; instead several  methods are available, and their application is often tailored to meet the specific requirements of each assessment. Environmental flow assessments have been undertaken for a number of Tasmanian rivers and streams, using the most appropriate methods available at the time. Previously, these assessments have focused largely on minimum flow requirements for instream fauna (such invertebrates and fish) by employing the instream Flow Incremental Methodology (IFIM) or other desktop approaches. However, new holistic methodologies that incorporate natural variability in stream flow, and the high flow water requirements of entire riverine ecosystems, are now being used by NTE Tas to develop environmental flows for catchments.

  The Tasmanian Environmental Flows Framework Technical Report   (452Kb)

  The Tasmanian Environmental Flows Framework Appendices   (13Mb)

Tasmanian Environmental Flows Framework (TEFF)

Tasmanian catchments that require environmental flow assessments are prioritised using information on current water use, river and estuarine health indicators, and water management planning activities. For each assessment, NRE Tas identifies water demand at different times of the year and uses the most appropriate environmental flow assessment method on a case by case basis. Generally, a desktop approach is used to determine environmental flows in smaller systems where water use is minimal, and more detailed field-based methods are employed in catchments where there is a high demand for water.

The new methodology developed by NRE Tas (TEFF) is employed in catchments with high water demand and is a flexible approach, whereby assessments can be tailored to suit the ecological characteristics of different catchments. Broadly, the TEFF aims to link the biological components and physical heterogeneity present within a river to specific flow events, and to link these flow events to specific environmental flow objectives. The process involves four main steps:

1. Identify freshwater ecosystem values in each catchment in order to define the objectives of the environmental flow assessment. Develop conceptual models that identify the ecosystem processes that support these values.
2. Identify representative river reaches, conduct assessments using hydraulic and hydrological models to characterise physical habitat and biological diversity of the system, and identify specific flow events that relate to these attributes.
3. Conduct hydrological analyses of flow data to define the pattern of occurrence of important flow events and the availability of important habitats for fauna.
4. Recommend a flow regime that meets the objectives of the environmental flow assessment, including rules for water abstraction.

Step 1. Identification of Ecosystem Values

Ecosystem values in each catchment are derived from an assessment of the freshwater-dependent ecosystems defined within the Conservation of Freshwater Ecosystem Values (CFEV) database. All major freshwater-dependent ecosystems are included in the CFEV database: rivers, waterbodies, wetlands, saltmarshes, karst and estuaries.

The CFEV database provides conservation management priorities for individual freshwater-dependent ecosystems based on their current condition, representativeness and distinctiveness in comparison to other similar ecosystems in Tasmania. Because of this robust, landscape-scale approach, CFEV objectively identifies areas of conservation significance and high freshwater-dependent "value" within a catchment. Conceptual models are then developed to illustrate the dependence of the identified ecosystem values on the movement of water through the landscape. These models help define the elements of the ecosystem and the flow regime which should be targeted in the environmental flow assessment.

Link to CFEV site

Step 2. Field Assessments, Hydrological and Hydraulic Modelling

Once the objectives of the environmental flows have been identified, the hydrology of the catchment is characterised using hydrological models. This is done in order to assess the degree to which the current flow regime has been altered from that of the "natural" (modelled pre-abstraction) flow regime, and allow risk to the various ecosystem values to be identified. Field sites are then selected for assessments to determine the flow events required to meet the environmental flow objectives.

Sites used in the assessment are representative of river reaches in the catchment (e.g. headwaters, lowland reaches, etc.) and, thus, incorporate the range of habitats (such as pools and riffles) and hydraulic conditions found throughout the river. In catchments where water use is high, sites may also occur in locations that are relevant to water management. Site assessments involve surveying the physical structure and heterogeneity of reaches, and constructing hydraulic models to determine the specific flow magnitudes associated with a range of physical features.

Step 3. Hydrological Analysis

Site assessments enable the magnitude of flow events that influence the physical heterogeneity of the river to be determined (such as flows at which over-bank flooding occurs). Once the magnitudes of particular flow events have been determined, the frequency, duration, timing and rate of change of these events are determined. Additionally, the flow conditions that provide important habitats for fauna are also identified. Based on these analyses, monthly minimum flows and high flow thresholds that maintain healthy physical habitats and suitable instream habitat conditions for faunal assemblages are derived.

Step 4. Recommending Environmental Flows

The final step in the TEFF is to combine the important flow events that were identified in the previous steps into a flow regime to meet the objectives of the assessment. Environmental flow regimes typically comprise:

• Large-scale flow events, which in many systems will be essentially natural
• Small- and intermediate-scale flow events which will be explicitly linked to the habitat and life history requirements of the system's biota, the physical heterogeneity of the system, and retain characteristics of the natural flow regime as much as possible
• Minimum flows (depending on season) to maintain hydrological connectivity during periods of high water abstraction

These events are defined by their hydrological parameters (i.e. magnitude, frequency, duration, timing and rate of change) and are combined to describe the recommended environmental flow regime.

Water Planning and Environmental Flows

Environmental flow assessments essentially describe the importance of different flows to the aquatic values of riverine systems. It is critical when interpreting environmental flow assessments that they are considered in the context of the community's environmental, economic and social objectives for the river(s) being examined. Thus, assessments provide an essential input to NRE Tas water management planning process; however, all water users within a catchment are considered by NRE Tas during the development of water management plans. During this process, environmental water requirements are negotiated and implemented as environmental water provisions, which aim to balance economic, social and environmental values within catchments.

The Tasmanian Environmental Flows Project

The TEFlows Project was conducted between 2007 and 2010 by the Water Assessment Branch (Primary Industries and Water Division, NRE Tas) for NRM South and NRM North, and was funded under the National Action Plan for Salinity and Water Quality.

The Project aimed to test and refine the Tasmanian Environmental Flows Framework (TEFF) by investigating the influence of flow variability on the physical and biological structure of freshwater dependent ecosystems, including wetlands and estuaries. It built on known flow requirements of freshwater-dependent biota and processes, by investigating links between flow variability and river ecology.

Project Catchments

The TEFlows Project was conducted in six catchments in eastern Tasmania. These catchments where selected to represent the two extremes of the four hydrological "types" which characterise Tasmanian river systems.

• The Ringarooma River, the Great Forester River, and Dans Rivulet (a tributary of the upper South Esk) are all in the north-east of the state and have perennial and predictable hydrology with low flow variability. They tend to display low flows in summer, high flows in winter, and floods generally occur in winter and spring.

• The Little Swanport River, the upper Macquarie River, and Brushy Plains Rivulet (a tributary of the Prosser River) in eastern Tasmanian lie at the other extreme, having intermittent and very unpredictable hydrology and high flow variability, with floods occurring any time of the year and cease-to-flow periods are a natural occurrence.

The third and fourth hydrological regions include rivers in the northwest and southwest, and have not been included in this project.

The results of this project are now being used to guide environmental flow assessments in catchments of corresponding hydrology.

  Tasmanian Environmental Flows Project Technical Report   (14Mb)

Environmental Flow Assessment Reports

NRE Tas has undertaken several environmental flow assessments. Reports detailing these assessments can be found here:
Environmental Flow Assessment Reports

Further Information

Conservation of Fresh Water Ecosystem Values Program (CFEV)
Water Management Plans