North-west flows in early 2020

Rainfall created substantial flows

From late January rain fell across parts of north-west New South Wales and southern Queensland. Over the weekend of 8th and 9th of February severe storms caused localised flooding in the Namoi and Gwydir valleys, particularly on the floodplain. Rainfall continued in various parts of the Northern Basin in February, March, April and May. This rainfall created substantial flows in the Border Rivers, Peel, Namoi, Gwydir and Macquarie valleys and along the Barwon-Darling River.

Source: ©Commonwealth of Australia 2020, Bureau of Meteorology. Reproduced in accordance with the Creative Commons Attribution Australia Licence. The map has not been modified.

Before these rains created flows, some major river systems had ceased to flow for extended periods. These river systems include the:

• Macintyre River below Boggabilla
• Mehi and Carole Rivers in the Gwydir
• Namoi River below Keepit Dam
• Peel River below Dungowan
• Macquarie River below Warren
• Barwon-Darling system, and
• Lower Darling River below Menindee Lakes

Rain created flows that recommenced and connected the northern tributaries with the Barwon-Darling River. By the 20th of February, the initial target flows of 60-70 GL were forecast to reach the Menindee Lakes. With further substantial rain and inflows in Queensland, the Barwon Darling River, and the Macquarie River, the volume forecast to reach the Menindee Lakes became 205-250 GL. From 10th March, the Barwon-Darling River was flowing along its full length from Mungindi on the Queensland border, into the Menindee Lakes. Thousands of kilometres of rivers flowed for the first time in many months. Substantial additional rain fell in late March and April, creating flows of 480 GL at Brewarrina. This was followed by May rainfall of 40-50 mm across central NSW. By the end of June, the Menindee Lakes had received more than 583 GL of total inflows.

The map above shows the end of system and key gauge flows for the period from 1 February 2020 to 28 April 2020.

Releases from the Menindee Lakes to re-start flows in the Lower Darling River commenced on the 26th of March and reached the Murray River in mid-April 2020. Water was initially released from the Menindee Lakes at a high rate to manage any potential water quality impacts of restarting the river, and to help the movement of fish populations previously stranded in refuge pools. Lower, base flow releases are now following the first pulse flow, and these will continue, maintaining supply to Lower Darling landholders for the next 12 to 18 months.

Replenishment of town water supplies

Many towns benefited from these flows. Water supplies were secured, and all town weir pools filled for Goondiwindi (population 5,500), Boggabilla (550), Mungindi (600), Collarenebri (650), Walgett (1,530) Brewarrina (930), Bourke (2,100), Wilcannia (610), Menindee and Sunset Strip (450) and Pooncarie (80).

The rainfall created substantial flows in the Border Rivers, Peel, Namoi, Gwydir and Macquarie valleys. The rain fell, in the most part, below the major storages. Pumping access had to be actively managed to protect the flows as they travelled downstream.

Under normal conditions, water sharing plans set the rules for water extraction. However, during extreme events, such as drought, additional restrictions can be applied. Department of Planning and Environment–Water temporarily restricted most commercial pumping access to river flows and overland flows in these areas. This allowed the replenishment of town supplies and fish refuge pools downstream. It also allowed the reconnection of the rivers of the Northern Basin, including the Barwon-Darling and the Lower Darling River. For more information on what was achieved for people and the environment see Replenishment of Town Water Supplies, Replenishment of Groundwater, Water Quality, Fish, Wetlands and Riparian Habitats.

The temporary water restrictions were managed using a set of principles and targets (PDF. 363KB) developed by Department of Planning and Environment–Water, the Department of Primary Industries (Fisheries), Water NSW, and Department of Planning and Environment–Environment and Heritage. For more information on how the temporary water restrictions were applied, see our series of fact sheets.

An independent panel is reviewing the management of the temporary water restrictions. For information on this work, view the Northern Basin First Flush Assessment webpage.

Water balance

We have prepared a simplified water balance for the period 1 February to 28 April 2020 as shown in the table below. We used measured flow data and estimated the unmeasured inflows for each regulated water source (downstream of the major dams) and in the unregulated Barwon-Darling water source.

This water balance below shows estimated inflows, licenced extractions, system replenishments and flows out from each river system to the downstream water sources. System replenishment includes evapotranspiration, groundwater seepage, instream and wetland replenishment, as well as any measurement error. We also corrected measurements of the end of system outflows if they were affected by water backing up into the tributaries from the high flows in the Barwon-Darling River.

^{1 Estimated total inflow to the regulated river systems downstream of major storages, 1st February to April 28th.}

^{2 Provides consumptive extraction volumes from the rivers including supplementary water take. Held environmental water usage was 3.7 GL in Gwydir and 4.6 GL in the Macquarie and is included in the system replenishments. The water balance excludes floodplain harvesting activities. That is, any water that was captured on-farm or on the floodplain and did not flow into a river.}

^{3 Queensland extractions for the period 14th to 24th February 2020.}

^{4 Inflows and direct outflows to the Barwon Darling are for the regulated river downstream of Burrendong Dam. The system replenishments presented for the regulated balance are inclusive of flows in Gunningbar Creek (8 GL) and Duck Creek (6 GL) which connect to the unregulated Bogan River system.}

^{5 Marthaguy Creek cumulative flows recorded at Carinda 421011.}

^{6 Bogan River cumulative flows recorded at Gongolgon 421023.}

^{7 Barwon-Darling River inflow estimate includes the outflows from the regulated Border Rivers, Gwydir, Namoi, Macquarie Rivers together with an estimate of the contribution from all other unregulated inflow sources such as the Culgoa, Warrego, Castlereagh, Paroo and Bogan Rivers. The estimate will be refined when further information on extraction volumes is available.}

^{8 The final extraction volumes for the Barwon-Darling were not available at time of publication, due to A class extractions continuing, and the timing of meter reads being scheduled for the end of the water year. The preliminary extraction volume is 230 GL. This table will be updated and republished when the final information is available.}

^{9 Barwon Darling River water balance outflows at Wilcannia Main Channel 425008. This volume will be refined as the Wilcannia and upstream gauges are checked in the field.}

Supplementary water

Supplementary water licence holders in the Border Rivers, Gwydir and Lower Namoi valleys had limited periods during February when access was allowed. For more information on each supplementary flow announcements visit the WaterNSW website. The table below shows the impact of the pumping restrictions applied in February, comparing the actual supplementary event extractions against an estimate of what could have been made available for supplementary extractions under standard water sharing plan rule operation.

^{1 Includes 3,448 ML held environmental water}

^{2 Includes 1,375 ML held environmental water}

Report

A satellite imagery derived assessment of take and water protected in the Northern Basin first flush flows of February 2020.

Download the report (PDF. 1,496KB)

Water quality

Flows that fill remnant river pools and break down pool stratification effects, including low oxygen, provide immediate and long-term water quality benefits. These flows move salt (salinity) and debris (particulate matter and dissolved organic carbon) through the system as well as breaking up algal blooms that may have developed in stagnant water bodies. Flows also provide food (nutrients) for aquatic animal and plant life cycles.

Rain may cause short term water quality problems following drought or bushfire. Sediment, ash and debris washed into waterways can cause rapid drops in oxygen levels and smother habitats. Continuing flows are needed to flush the system, transporting and diluting material, which leads to improved longer-term water quality.

Our Lower Darling releases fact sheets and water quality updates provided regular information as the flows progressed down the Lower Darling River. Before the flows arrived, salinity in the Lower Darling River was high. The flows flushed and diluted poor quality water from isolated remnant pools and significantly reduced salinity levels. Water quality was monitored prior to and during the release to avoid fish deaths in the Lower Darling River. Our key water quality findings for the Lower Darling River release discusses the resumption of flow and water quality in more detail.

Similarly, catchments in the Northern Basin have shown improvements in water quality. Initial data shows that salinity levels along the Barwon-Darling River have improved. For example, salinity dropped from 650 mS/cm at Bourke in early February to 150 mS/cm in late February. The flows also disrupted stratification and resulted in increased dissolved oxygen levels throughout the water column. Flows also dispersed a number of algal blooms that were present in the system.

Northern basin on-farm storage during the flows

During the event we monitored water captured in eligible farm storages across the Northern Basin. We used remote sensing technology (satellite imagery) to observe the presence of water, and in combination with storage curves, to estimate storage volumes.

There are approximately 1,400 large on-farm storages in the Northern Basin floodplains of the Border Rivers, Gwydir, Namoi, Macquarie and Barwon-Darling valleys. If full, these storages could hold a volume of 1,300 GL. At the start of February these storages held an estimated 23 GL, by mid-February 51 GL and by the end of April 292 GL. We therefore estimate an increase in stored water of approximately 270 GL between February to April 2020. This includes an estimated 30 GL by mid-February, during which there were several days when restrictions on floodplain harvesting were lifted in specified areas.

It is important to note that:

• the stored volumes are derived from remote sensing information, rather than directly measured. They are estimates only. We have used assumptions and there is scientific uncertainty in the results. Future telemetry and measurement of floodplain harvesting will improve how we quantify floodplain water take.
• the analysis captures the change in volume of water held in storages and does not identify the individual sources contributing to this stored water. This stored water might be from direct rainfall, on-farm runoff/tailwater capture, harvesting from floodplains or pumping from rivers and aquifers.

To estimate the stored volumes, we used the method below. This process was repeated three times to represent pre-, mid- and end-of-event stored water volumes.

Volume estimation method:

1. Observe storage inundation by measuring the water surface area by satellite imagery.
2. Validate the observations with a second set of high-resolution satellite imagery from a different satellite source.
3. Estimate the volume of water held in each storage using a storage capacity curve derived from aerial survey (LiDAR) data. The curve matches observed water surface height (AHD) to surface area (m2) and volume (ML). In this way, a measured wet surface area is turned into a stored volume of water for each storage.

Report

A satellite imagery derived assessment of take and water protected in the Northern Basin first flush flows of February 2020.

Download the report (PDF. 1,496KB)

Response of groundwater resources to rainfall

Monitoring helps us to find out what impact the recent rain and surface water flows have had on groundwater levels. The data has been reviewed for 20 groundwater sources so far. Early results confirm that shallow systems, such as the Peel Alluvium, show a clear but relatively minor response to the rains and flows. Dry soil profiles from the prolonged drought have soaked up much of the recent rainfall resulting in only a small amount of this reaching the groundwater systems.

Watch the video

This video demonstrates just how reactive some groundwater systems can be to rainfall. In it, a pink ribbon flutters as the groundwater level in the bore rises rapidly in response to significant rainfall and pushes the air out of the bore.

Watch the video

Larger, deeper systems show some recovery in water levels following the recent rain. However, this response is more than likely due to a reduction in groundwater pumping as surface water is available and used preferentially. It takes a much longer time for rainfall to reach the deep groundwater systems than the small shallow systems.

More rain and flood events are needed for significant groundwater recharge to occur. The department and WaterNSW will continue to monitor groundwater levels to see how the different groundwater systems respond to significant rainfall and flood events.

Fish

Flows improve the connection of the river system so that deep pools are linked together. The Barwon-Darling River is again connected with its tributaries. Fish and other aquatic animals can move up and down significant lengths of the rivers. Iconic species like Murray Cod, Golden Perch and Silver Perch breed and disperse during these connecting flows. Flows like this help maintain healthy fish populations throughout the Northern Basin. View more information about fish and flows.

Researchers from the Department of Primary Industries (Fisheries) monitored fish populations and fish movement during the flow event. Monitoring will continue over the next year. Preliminary results indicate that Golden Perch had numerous breeding events because of these flows. Larvae and juveniles were found in the flowing waters from the Moonie River down the Barwon-Darling River and in Lake Wetherell (Figure 1). Finding Golden Perch juveniles and larvae at the Menindee Lakes is of great significance because the lakes are an important nursery for Golden Perch. It is known that fish that grow up in this nursery move throughout the Northern and Southern Basin in later years.

During the drought, water mixers were placed in ten large weir and refuge pools in the river at Menindee and in the Lower Darling River. These mixers protected some fish so that the population could recover when the river began to flow again. The mixers are no longer required as the water quality has improved because of these flows. View more information about the native fish drought response.

Golden Perch larvae and early juveniles collected at the northern end of Lake Wetherell in late March 2020 (left) and a drift net sample of numerous other fish species and aquatic animals (right). Image courtesy of NSW DPI Fisheries 2020.

Fact sheet

To find out more about how native fish benefited from the flows read Native fish outcomes from the 2020 North-west flows.

Download the fact sheet (PDF. 434KB)

Wetlands and riparian habitats

Rainfall and flow events improve the health of the environment. Critical wetlands and riparian habitat wet or fill with water. During times of drought animals and vegetation can be sustained. Such events also help to start the recovery from drought.

Satellite imagery has been used to monitor flows. Inundation maps of the Gwydir Wetlands and Macquarie Marshes are being made for the rainfall events and indicate that parts of the Macquarie Marshes received flows from mid-February. By the end of April, more than 2600 ha of the critical Northern Reedbed was inundated.

Fact sheet

This fact sheet provides background information on drought recovery in the Macquarie Marshes. On April 5 2020, the department allowed supplementary access for water users in sections of the Lower Macquarie River system for a limited period of time.

Download the fact sheet (PDF. 919KB)

In the Gwydir Wetlands, flows began to reach the core reeds by the end of January. More of the wetlands were inundated during February and March. In the Central Gingham Water Management Area, more than 1700 ha of semi-permanent wetlands (Common Reed, Cumbungi, Water Couch) were inundated. In the Lower Gwydir, more than 1800 ha of semi-permanent wetland was inundated.

More information

• Vegetation outcomes from the north-west flow event 2020

Following recent floods in QLD and NSW, significant flows have entered the Barwon Darling river system. We’re keeping a close watch on the flows as they proceed down the system.

We are pleased that the resumption of flow rule we put in place earlier this year has wet the system enough to maximise the benefits of the current event.

Given the amount of water on the floodplain, providing an accurate forecast into Menindee is difficult at this stage. For example, much of the flooding we’ve seen in the Gwydir valley will enter the Gwydir wetlands and not necessarily flow into Barwon Darling.

However, based on current forecast inflow volumes, there will potentially be between 100 and 300 GL of additional water available for use in Menindee or the Lower Darling.

We are watching the weather and flow forecasts closely and given the time that it takes for the water to move down the system we are taking this time to make well informed decisions.

Based on current flows seen upstream, it is possible that Lakes Pamamaroo and Wetherell will fill. However, the decision on how any water above that which is required to fill these upper lakes will depend on a range of factors including the total volume likely to reach the Lakes.

WaterNSW insights portal provides real time data that is readily accessible.

1. The normal water sharing plan rules, including access to supplementary water, are operating.
2. Temporary water restrictions can be imposed if it is in the public interest to do so. Examples of this are to cope with water shortages, where there are threats to public health and safety or to protect flows for environmental purposes.
3. There are currently no town water shortages nor critical environmental needs that require the protection of water for environmental purposes. Menindee Lakes is holding at least 12 months supply.
4. Our rivers systems received good flows in the first few months of 2020 across the northern basin. These flows were protected to re-start the rivers and replenish aquatic habitats. Some 670 gigalitres (GL) entered Menindee Lakes by June 2020.
5. Rain in March 2021 is again providing good flows across the north-west and Barwon-Darling. Further inflows are expected into Menindee Lakes which will extend the current supply.
6. Information on the temporary water restrictions and the reasons they were imposed in early 2020 is provided in the fact sheets below.

• By 1 July 2021 – publish draft new triggers for critical environmental needs to be considered when imposing temporary restrictions.
• By 1 July 2022 – publish draft new triggers for critical human needs to be considered when imposing temporary restrictions.
• By 1 September 2021 – review the existing targets and develop a draft implementation program for the targets under the North-west Unregulated Flow Management Plan.
• By 1 January 2022 – consult on the North-west Unregulated Flow Management Plan targets and their implementation.

The draft Western Regional Water Strategy, which is due to be released towards the end of 2021, will include:

• A proposed definition of connectivity.
• Hydrological modelling to assess the extent to which the actions already undertaken to improve connectivity have been effective.
• Additional connectivity options for discussion, such as proposed flow targets for the Menindee Lakes and/or at the confluence with the Murray River.
• Initial results of modelling incorporating new climate data.