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Supporting Environmental Decision-Making on the Lowbidgee

Sundown over Tala Lakes by James MaguireSundown over Tala Lakes by James MaguireThe timing, quantity and duration of environmental flows can mean the difference between a healthy wetland ecosystem pulsing with life and one in decline.

Ensuring every release achieves maximum affect requires thorough understanding of a wetland’s characteristics and the way its geomorphology, water and ecology interact.

Without such knowledge, effectively applied, the health of our wetlands is threatened, and precious water intended to improve the environment is wasted.

That’s why a well-conceived and executed ecosystem response model for river flows can be such a powerful weapon in the armoury of natural resource managers, letting them test a range of scenarios as they juggle competing needs for water.

Now the NSW Office of Environment and Heritage (OEH) believes it has just that, after engaging SKM and eWater CRC to develop an ecological response model within a decision support system (DSS) for the Lowbidgee wetlands.

Currently undergoing thorough testing, the DSS is letting OEH investigate the response of ecological communities to different watering regimes. The agency hopes the DSS will help it to determine how best to use available environmental entitlements and underpin future revisions to the Murrumbidgee Regulated River Water Sharing Plan.

Planned water

Environmental water in NSW is currently managed through the relevant water sharing plans (WSPs), which control ‘planned water’, and the Annual Environmental Management Watering Plans prepared by OEH and approved by the Office of Water. Implementation of environmental flows through the WSPs has been an important first step in arresting the decline of wetlands. With more water now being made available for environmental use, ecological modelling tools like eWater’s Eco Modeller can play a part in decisions on how to optimise the effect of this environmental water on rehabilitating the wetlands.

This is the case in the Lowbidgee, a nationally important wetland spanning 217 000 ha which is both a drought refuge and significant breeding habitat for at least 41 species of waterbirds. Significant amounts of water are vital to those birds well being: the Nimmie-Caira area alone needs fully 58,000 ML of water in the bird breeding season to provide stable water in rookeries, and must be flooded for at least five months of the year.

Great egret by Mark FanthorpeGreat egret by Mark FanthorpeThe wetlands provide important habitat for fish, frogs and macroinvertebrates.

It is also the home of the second largest river red gum forest in Australia and contains significant black box, lignum and reed-bed communities, as well as 15,000 ha of important reeds and grasses.

Before human settlement the Lowbidgee wetlands were regularly inundated by floodwaters from the Murrumbidgee River, driven by reliable winter and spring rainfall and snow melt. The entire system was ephemeral, with the channel, riparian zone and floodplain each linked in a wetting and drying regime that supported a diverse 'boom and bust' ecology typical of inland river systems in Australia. That is just the way the area’s wildlife and vegetation liked it – the science has shown that inland wetlands are most productive when flooding follows a period of complete drying.

These days the wetlands are in serious trouble. The extended drought of recent years, the effects of river regulation, the impact of land use practices such as land clearing, burning, and cropping; declining water quality, and the impact of exotic flora and fauna (especially common carp) have all taken a toll, placing severe environmental stress on water dependent ecosystems.

Sedimentation and erosion patterns have suffered, water dependent ecosystems have received less floodwater, distribution of aquatic and riparian plants has changed, and populations of native birds and fish have all declined precipitously. River red gum communities in particular are showing significant signs of stress.

OEH hopes manipulating delivery of water to these wetlands might re-introduce a more natural flow and water level variability, resulting in the regeneration of riparian vegetation.

It also hopes it will reconnect the river channel with the adjacent floodplain and wetland habitats and allow for a drying phase between flows which will consolidate sediment and enhance nutrient cycles.

Scenario comparisons

The key purpose of the DSS is to compare scenarios relating to water delivery (volume, timing and duration) to ecological outcomes in order to provide a transparent and rigorous decision-making process to optimise the use of environmental water. Cormorant chicks by James MaguireCormorant chicks by James Maguire

It will help OEH plan and manage environmental flows at the valley and wetland level over different time scales, while allowing water managers to compare scenarios relating the volume and timing of water delivery to ecological outcomes.

Based on Eco Modeller, and providing links to hydrological (IQQM) and hydraulic models, the DSS will allow natural resources managers to accurately calculate how the Lowbidgee will respond to management practices. Eco Modeller is a tool for building and storing quantitative models of ecological responses to physical and biological factors (such as river flow) to explore various solutions to problems. Eco Modeller will be a major component of the future alternative hydrological modelling platform to IQQM: eWater Source, making it reasonably “future-proof”.

The resulting models can simulate the response of ecological systems to both changes in factors that define habitat, such as water flow, water quality and temperature, and physical attributes like substrate or presence or absence of woody debris. ‘Plug-in’ functionality lets users build unique models incorporating complex relationships.

Underpinning the DSS is a conceptual model of the Lowbidgee wetlands describing system drivers and ecological interactions. This provides a foundation and framework for specific responses by ecological assets in the Ecosystem Response Models in the DSS. Those assets were selected according to their rarity and status, distribution, degree of reliance on water and floodplain habitats and other qualities. Ecological response models were developed for the endangered southern bell frog Litoria raniformis, common carp  Cyprinus carpio, Murray cod Maccullochella peelii peelii, un-specked hardyhead Craterocephalus stercusmuscarum, colonial breeding waterbirds (ibis, cormorants, herons and egrets), yabby Cherax destructor, river red gum  Eucalyptus camaldulensis, black box Eucalyptus largiflorens, lignum Muehlenbeckia florulenta and tall spike-rush Eleocharis sphacelata.

The DSS is easy to use and readily adaptable.

“It is a useful tool allowing the Department to examine the benefits of different watering regimes, providing a level of sophistication that has never been available before,” says SKM Senior Water Resource Engineer Ian Varley.

“We know that in the future there will be more water available for environmental purposes than what there has been in the past, so it’s a tool to allow them to explore how best to use that water. The tool will be thoroughly tested over the next year or so.”

It allows OEH to examine ecological behaviour over a long time period using approximately 100 years of historical climate and flow data in order to understand long-term behaviour and trends under different management settings.

It will also be used over shorter time frames to assist in managing specific watering events. In this mode the DSS will provide informatiSouthern bell frog by Kathy DueSouthern bell frog by Kathy Dueon such as the likelihood of a flood event being sufficient to induce and sustain a bird breeding event, or whether additional water (and what volume) would be required to sustain the event. eWater Ecological Response Product Developer Nick Marsh says the DSS is unique in its ability to address both planning and operational timescales.

“In this particular case the problem that we’re trying to solve is what happens to these big wetland areas under the different water delivery scenarios. The State’s water planning authority, the NSW Office of Water, runs 100 year simulations to compare different release rules and possible configurations to understand water supply and environmental requirements under changing climate conditions.

“On the other hand the managers of these major wetlands want to make decisions on a year by year basis about how they should deliver water. The DSS can deal with both short and long-range temporal scales.”

Marsh says the DSS is also extremely flexible on the spatial scale, allowing the system to be used on the scale of the wetland and in sub-wetlands.