National Water Account 2018

Region report

Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Focus area

Border Rivers

Murray–Darling Basin: Methods

Summary of methods

There were five key methods for establishing item volumes in the 2018 Account. Click the down arrow in the table below to view the list of items derived by each method type. For detailed information about each method scroll down this page or click on the links in the table.

Detail of methods

AWRA-R model

AWRA-R is a river network model that represents key hydrological processes and diversions at a daily time step (Dutta et al. 2017; 2015). The model was used in the National Water Account to quantify river fluxes and stores along the river network.

The river system is conceptualised in AWRA-R as a node-link network comprising nodes connected by river reaches. Gauged streamflow data are used where available. For ungauged portions of catchment, the landscape runoff from the AWRA-L model is used (Viney et al. 2015). River processes represented in the AWRA-R model are shown in Figure N1.

Figure N1 Conceptual diagram of AWRA-R reach showing model components (from Dutta et al. 2015)

Rivers

The volume of water in the river channels at 30 June was estimated by using the daily water balance approach within the AWRA-R model. The water balance includes inflow at the upstream nodes and outflow at the downstream nodes; contributing catchment runoff, reservoir contribution, irrigation diversion and return; overbank flooding and floodplain return; loss to groundwater; anabranch flow; rainfall; and evaporation.

Overbank flow and flood return

The AWRA-R floodplain module was used to model the volume of overbank flow from the river onto the floodplain, and the return flow from the floodplain back into the river. The module applies a simple storage-based floodplain model to each river reach. The floodplain modelling method is detailed in Dutta et al. (2013).

The volume of water harvested from the landscape as a result of heavy rainfall events and overbank flood spilling is recognised as non-allocated water use. This information is available for several jurisdictions in s.71 data received from MDBA. This volume is estimated based on a combination of user returns and local knowledge with the uncertainty estimate of +/– 40%. In the National Water Account, this volume is treated as a consumptive water use.

The overbank flow volume presented in the account is equal to the estimated AWRA-R overbank flow volume minus the estimated consumptive water use.

Runoff

Runoff to surface water was estimated using the modelled runoff from the AWRA-R model. Runoff within AWRA-R is in turn derived from landscape runoff modelled in the AWRA-L model, with a scaling factor applied within AWRA-R during the calibration process.

The AWRA-L model is a daily grid-based water balance model that is conceptualised as a small unimpaired catchment (Viney et al. 2015). It simulates the flow of water through the landscape from rainfall entering the grid cell through the vegetation and soil, and then out of the grid cell through evapotranspiration, runoff or deep drainage to the groundwater. Its inputs include gridded climate, soil, vegetation and topographic data. For more information see the Bureau of Meteorology's Australian Landscape Water Balance webpage.

Precipitation and evaporation

Rainfall and evaporation into/from storages and rivers were calculated using climate data from the Bureau of Meteorology interpolated to 0.05 degree (5 km) national grids (Jones et al. 2009). Calculations were done on a daily time step, with the annual totals summed from the daily values.

Climate data for each water body at each time step were estimated from the proportionally weighted average of grid-cells that intersected the water body. Evaporation was estimated using Morton's shallow lake formulation (Morton 1983). Rainfall and evaporation volumes were then estimated by multiplying the surface area of each waterbody by the weighted average rainfall and evaporation respectively. The average daily surface area of rivers was estimated using the AWRA-R model and the average daily surface area of storages was calculated from daily storage levels and capacity tables.

Outflow

To estimate the total surface water outflow from the Murray–Darling Basin region, the AWRA-R and AWRA-L models were used to simulate daily flows at the end of the River Murray. These flows included:

Water storage product data and Murray-Darling Basin Authority's HYDRO database

Storages

Storage volume at the start and end of the year was calculated using water level data (metres above Australian Height Datum) collected at each storage. Capacity tables established for each storage were used to convert the height measurement to a volume.

The volume of individual storages was aggregated to present the total volume for the item as detailed in the Surface water assets section in the 'Statement details' note. The uncertainty range for these volumes is +/–5%.

The assumptions made and limitations in calculations were as follows:

• Storage volumes provided in the 2018 Account include dead storage volumes.
• Storages with the capacity equal or above 1,000 ML were considered in the calculations.
• Storage–volume curves represent specifically surveyed parts of the storage and may not reflect the storage–volume relationship across the entire storage.
• Storages are subject to sedimentation and other physical changes over time that in turn affect the accuracy of the storage–volume curves.
• Storages within the operational area of Snowy Hydro Limited were not included in the calculations (the volume of water received from these storages have been treated as inter-region transfer to the Murray–Darling Basin region).

Lakes and wetlands

The volume of water in lakes and wetlands is based on both measured and estimated data. The volume of water in Lake Burley Griffin at the start and end of the year was calculated using water level data (metres above Australian Height Datum) collected at the lake. Rating tables established for the lake were used to convert the height measurement to a volume.

The volumes of water in Lake Ginninderra and Lake Tuggeranong were estimated based on the known capacities of the lakes, that is, the lakes were assumed to be full at 30 June 2018.

The assumptions made were as follows:

• Water levels in Lake Ginninderra and Lake Tuggeranong are generally managed within 200 mm of full supply level throughout the year. Therefore, the estimated storage volumes of these lakes are considered to be only slightly overestimated.
• The capacity of Lake Burley Griffin is based on survey data collected at the time of construction and fill in 1964.

Weirs and locks

River levels were directly measured and converted into volumes using capacity tables for the individual weirs and locks, including locks 6–10 and 15 and Mildura Weir.

The assumptions and limitations of this approach were:

• The capacity of the lock is taken to be the volume contained in the lock at target storage/full supply level.
• The dead storage associated with the locks is taken to be the volume in storage at the lower level of the operating range.
• This approach allows for comparative measure across years and is preferred to estimating the total volume behind the lock wall, which cannot be accurately measured.
• The accuracy of the capacity tables employed was not evaluated.
• Euston Weir volume excludes the Euston Lakes.

Reporting partner databases and estimated data

Claims: inter-region

Snowy Hydro, located outside the region boundary, must release the required annual release (RAR) each Snowy Hydro water management year to the Tumut and Murray rivers. The RAR is set at the commencement of the Snowy Hydro water year which runs from 1 May–30 April. The inter-region claim at the end of the 2017–18 year is calculated as shown in Table N3.

Snowy Hydro RAR is based on the calculations in the Snowy Water Licence. The Snowy Hydro RAR remaining at 30 June adjusts the RAR at 1 May by the estimated progressive releases (ML) from 1 May to 30 June. Inputs to the calculation of the RAR include:

• 1,062 GL for the River Murray and 1,026 GL for the Tumut River—the precorporatisation RAR
• the dry inflow sequence volume
• environmental water savings transferred from the Murrumbidgee River and the River Murray to the Snowy River or the montane streams
• recognition of when Snowy Hydro has made a water release in advance of the Snowy Hydro water year
• transfers between the Tumut River and the River Murray
• recognition of water deals between Snowy Hydro and downstream irrigators (both the borrowings and the subsequent paybacks)
• any other RAR adjustment.

Claim increase: inter-region

Inter-region claim increases for the region include:

• increases in the RAR that Snowy Hydro was required to deliver to the Tumut and the Murray rivers
• a claim by Grampians Wimmera–Mallee Water in the Glenelg catchment.

The total inter-region claim increase during the 2017–18 year was calculated as shown in Table N4.

The limitations associated with this approach are:

• Volumes associated with the Snowy Hydro claim were estimated at the commencement of the water year and subject to revision and confirmation at the end of the year.
• Inflows to Lake Hume for the River Murray are subject to a formula for calculating the respective New South Wales and Victorian shares, which is split 50/50 between New South Wales and Victoria with some adjustments.

Delivery: inter-region agreement

The volumes of water delivered under the above inter-region agreements were based on metered data.

Claim decrease: inter-region

Inter-region claim decreases for the region associated with the Snowy Hydro's RAR was calculated as the sum of the following components:

• repayment of water deals
• reduction in RAR for Snowy River deals (Mowamba)
• Snowy Water Licence reduction agreed between parties for the 2017–18 year
• relation volume reduction (due to irrigators' entitlements reaching full allocations and fullness of downstream storages)
• reserved as directed by the NSW Office of Water under clause 13.2 of the Snowy Water licence to facilitate a potential inter-valley transfer
• allocated for the drought account
• call-out of relation volume that occurred in the previous water year.

Inter-region inflow (surface water)

This is the discretionary flow made by Snowy Hydro to the Murray and Tumut rivers.

Recharge/discharge: landscape

The groundwater recharge and discharge volume to landscape was calculated in selected SDL units across the region. The SDL areas calculated by NSW Department of Industry Lands and Water using MODFLOW and water table fluctuation methods are shown in Figure N2. Recharge / discharge estimates are available and presented only for these areas.

Figure N2 Sustainable diversion limit areas for modelled groundwater discharge to landscape and surface water

Recharge and discharge volumes were calculated for selected SDL resource units applying New South Wales groundwater models based on MODFLOW (United States Geological Survey 2013) and water table fluctuation modelling process. In MODFLOW process, discharge volumes were calculated where evapotranspiration routines were activated to represent groundwater discharge.

Groundwater recharge is both an input to and an output from a groundwater model. There is no single method for estimating recharge used in the New South Wales groundwater models; however, several models estimate recharge as a percentage of rainfall. The magnitude of recharge (as a percentage of rainfall) can be adjusted during the calibration of a groundwater model so that the observed groundwater levels are reproduced in model outputs as accurately as possible, typically for a period of around 20 years if data are available.

The assumptions and limitations of this approach were:

• Groundwater models make many assumptions and approximations to represent a water balance (United States Geological Survey 2013
• Several of the New South Wales groundwater models assume estimation of recharge volume as a percentage of rainfall.

Uncertainty of recharge/discharge estimates were not evaluated for the New South Wales groundwater models.  

Recharge/discharge: surface water

Groundwater interactions with surface water (discharge to and recharge from) can be represented in MODFLOW and water table fluctuation models in several ways. Figure N2 shows the modelled areas by Department of Industry - Lands and Water for which volume is calculated in the region. Recharge / discharge estimates are available and presented only for these areas.

Options that have been used in the New South Wales groundwater models are the MODFLOW river package and the MODFLOW drain package (United States Geological Survey 2013), and water table fluctuation models.

Groundwater flow into the river is modelled when groundwater levels are higher than river water levels and water flow is out of the river when river water levels are higher than groundwater levels. MODFLOW also has a subroutine to represent drains. When this is activated and groundwater levels are above the base of the drain, water flow to the drain is estimated and this water volume is removed from the cell of the groundwater model.

For more details about MODFLOW calculations, see documentation at the MODFLOW website (United States Geological Survey 2013

The assumptions and limitations of this approach were:

• Groundwater models make numerous assumptions and approximations to represent water balance (refer to the MODFLOW website for more details).
• Estimates of water level in rivers that are input to groundwater models are usually taken to be monthly average levels, and the levels would usually have a high level of uncertainty unless a river gauge is located within the groundwater model cell.

The uncertainty estimate was not quantified. It is currently not feasible to estimate the uncertainty of modelled groundwater recharge from surface water from outputs of a MODFLOW groundwater model.

Discharge: wastewater system

'Treated wastewater discharged to surface water' was metered at wastewater treatment plant outflow points. The estimated uncertainty is +/– 10%.

s.71 (Section 71 of the Water Act 2007) data and water sharing plans

Other surface water assets

The volume of surface water stored in Rocky Valley Reservoir, owned by a hydro-electric operator, was provided to the Murray–Darling Basin Authority as part of the s.71 Water Resource Report Victorian input; however, the calculations used to derive this volume were not available.

The volume of water in Rocky Valley Reservoir is not included in the region's storage volume (see Storages) because no orders can be placed on this storage for delivery of water to the entitlement system. It is only an asset when the hydro-operator physically makes a release.

Other groundwater assets

The extractable volume of groundwater (groundwater asset) was estimated as the sum of sustainable diversion limits (SDL) volumes based on information provided by the Murray–Darling Basin Authority. 

Allocation remaining

The water management year commences on the date the licence is issued. In most cases, particularly for individual users, the licence anniversary falls outside the standard water year of 1 July–30 June. As a result, the water allocation remaining at the end of the 2016–17 year is the unused component of the annual allocation for the licence. In general, the total volume of allocation remaining is not available for the entitlement holders in the following year. As such, volumes provided as carryover for the following year in the account are indicative. The allocation remaining at end of the year is calculated as shown in Table N5.

Adjustment and forfeiture

For most licences in the region, the portion of water allocation that has not been abstracted at the end of the licence water year is forfeited (i.e. there is no carryover of entitlements). Therefore, forfeiture is calculated as the total annual allocation for each licence minus the allocation abstraction during the licence water year. Individual user entitlements that are terminated during the year are also considered to be forfeitures.

In some cases, such as the Angas Bremer and Marne Saunders groundwater wells in South Australia, carryover is permitted. That is, a portion of the unused annual allocation can be carried over into the next water year. In these cases, forfeiture is calculated as the total annual allocation minus the allocation abstraction and the carryover.

It is likely that forfeiture volumes are underestimated for NSW and Victoria as return flows are added to net diversions to report gross diversions. The return flow portion is not covered under announced allocations.

Allocations

Individual user licences are generally issued for periods of between one and ten years, with an annual abstraction amount specified and with annual compliance arrangements in place.

The maximum amount of abstraction for each year is announced annually and is usually a percentage of the licence entitlement. It is also usually based on a review of storage, river, and aquifer levels in the region at the start of the water management year.

More information on these allocations and the associated water access entitlement is given in the Water access and use note.

Allocated abstraction

The volume of allocated diversions and extractions of surface water and groundwater are for individual users, environmental purposes and the urban water system. They are based on the licensed water year and derived from metered data.

Non-allocated diversion

The non-allocated diversion of surface water for individual users, environmental purposes and the urban water system are based on a combination of metered and estimates. Where metered data are available, the diversion is calculated as the actual diversion during the year. Where metered data are not available, an estimate is made based on historical usage data or modelled data.

Overland flow take

The volume of water harvested from the landscape as a result of heavy rainfall events and overbank flood spilling is recognised as overland flow take which is a non-allocated water use. This volume is estimated based on a combination of user returns and local knowledge with the uncertainty estimate of +/– 40% . In the National Water Account, this volume is treated as a consumptive water use. Currently, information is available only for Queensland and New South Wales areas within the region.

Diversion: statutory rights

The volume of diversion under statutory rights was received the s.71 Water resource report. The reported volume is likely to be an underestimate as only limited data were available.

Extraction: statutory rights

Groundwater extraction data were obtained from the relevant clauses in the jurisdictional groundwater plans and the s.71 Water resource report. The reported volume is likely to be an underestimate as only limited data were available.

Conveyance losses for allocated diversions

Conveyance losses for allocated diversions in SDL resource units in Victoria were separately accounted as there are no provisions for these losses in specified allocations. This information was included in s.71 data received from MDBA. The losses are based on the licensed water year, and derived from metered data for source and destination supplies. Conveyance losses for the SDL resource units in the other jurisdictions are included in accounted allocated diversions.

Point return: irrigation

The volumes of point return from irrigation schemes within the Murrumbidgee and Victorian Murray river systems were derived from metered flow data. Point return data were not available for the other irrigation areas across the region.

The uncertainty estimate for the Broken Creek irrigation scheme is +/– 40%. The uncertainty for all other schemes is +/– 5%.

Return flow: environmental purposes

The volume of environmental return flow was estimated for SDL resource units in Victoria. This information was included in s.71 data received from MDBA. Details of the estimation method are not available.

River and floodplain losses

A water balance approach was adopted in calculating river and floodplain leakage, evaporation and errors. The calculations were based on hydrological boundaries of river catchments within the region. Total inflows less total outflows and changes in surface water storage for surface water in a reporting unit (a water resource plan area) was considered as the river loss (river and floodplain leakage, evaporation, and errors) for the same unit.

Other groundwater increases/decreases

Other groundwater increases and decreases do not represent any physical groundwater flow. The volumes provided represent the changes to and inclusion of volumes for sustainable diversion limits for extraction between the reporting year and the previous year. These volumes were calculated by the Bureau of Meteorology and based on s.71 Water resource report jurisdictional data.

Groundwater modelling

Inter-region flow (groundwater)

Inter-region inflow into the region' was estimated for the unconfined aquifer (Murray Group Limestone and Parilla Sands) and confined aquifer (Renmark Group) that underlie this boundary.

'Inter-region outflow from the region' refers to the lateral outflow from the aquifers into the Murray Limestone and Renmark Group aquifers near the Murray mouth.

The uncertainty in the field-measured data (e.g. groundwater levels, hydraulic conductivity) was not specified and is unknown, and hence the impacts of such uncertainty on the calculated groundwater flow were not estimated.

Inter-region coastal flow

Inter-region coastal inflow into the Murray-Darling Basin region boundary was only considered to be significant in the area near the mouth of the River Murray in South Australia.

Inter-region coastal outflow from the region refers to the volume of outflow from the aquifers to the Southern Ocean near the River Murray mouth.

Groundwater flow was calculated using a simple geographic information system (GIS) approach based on Darcy's Law. Groundwater levels were interpolated for seasons using the ArcGIS Topo-to-Raster tool from reduced groundwater levels measured at monitoring bores.

Geofabric was used to estimate aquifer thickness. The hydraulic conductivity values were sourced from the Mallee Prescribed Wells Area–Murrayville Water Supply Protection area groundwater model (Barnett and Osei-bonsu 2006). The transmissivity values were calculated by multiplying the aquifer thickness with the relevant hydraulic conductivity.

Seasonal groundwater flow grids were derived from groundwater level grids, aquifer thickness and hydraulic conductivity using a modification of the ArcGIS Darcy Velocity tool. Groundwater flow across selected flow boundaries was then calculated using a simple GIS analysis and seasonal values were aggregated for the 2017–18 year.

The assumptions and limitations were as follows:

• Regional flow estimations were provided for the Murray Group Limestone Aquifer, which was chosen to represent the unconfined aquifer and the Renmark Group Aquifer. These were considered to be the main aquifer systems that cross the boundary of the Murray–Darling Basin region.
• Inflows and outflows for the Murray–Darling Basin region were assumed to occur at or near the coastline only; all the other boundaries were assumed no-flow boundaries mostly representing a groundwater divide.
• Groundwater levels in the unconfined aquifer were assumed to be 0 mAHD (metres above Australian height datum) along the coastline.
• Groundwater flow from the Great Artesian Basin (GAB) to the Murray–Darling Basin and groundwater abstraction from the GAB were not evaluated for the 2018 Account due to lack of data (although this vertical leakage is recognised to be important in some SDL resource units).

Uncertainty information

• The uncertainty estimate was not quantified.
• The uncertainty in the field-measured data (e.g. groundwater levels, hydraulic conductivity) was not specified and is unknown, and hence the impacts of such uncertainty on the calculated groundwater flow were not estimated.
• The regional flow estimations were based on the interpolated groundwater level grids produced using a simple GIS analysis. Use of different interpolation methods may impact on the values of the groundwater level grids and hence the estimated regional flow; however, a comparison of this methodology was carried out using a simple groundwater flow model developed on MODFLOW model (United States Geological Survey 2013). The results from the two methodologies indicated a 6–7% difference.
• Groundwater flow was estimated for a simplified boundary constructed from a series of line segments. Groundwater flow across this boundary was calculated using the method described above. The uncertainty surrounding this simplification was not analysed.