National Water Account 2016

Murray–Darling Basin: Methods

Murray River at Renmark, South Australia © Michael Bell, MDBA

Summary of methods

Table N4 outlines the quantification approaches used to derive the item volumes for the Murray–Darling Basin region. For a more detailed description of the quantification approach, click on the relevant item name in the table.

 

Table N4 Methods used to derive item volumes

Assets
Approach or data usedItemSource
Water storage product data
  • Bureau of Meteorology
  • National Capital Authority
  • Environment and Planning Directorate
AWRA-R model
  • Bureau of Meteorology
HYDRO database and height-volume relationship
  • Murray–Darling Basin Authority (MDBA)
s.71 (Section 71 of the Water Act 2007) Water resource report
  • MDBA
s.71 Water resource report and water sharing plans
  • MDBA
Not quantified
  • Water table aquifer
  • Underlying aquifers
 

 

Liabilities and claims
Approach or data usedItemSource
s.71 Water resource report
  • MDBA
Snowy Hydro database and s.71 Water resource report
  • MDBA
  • Snowy Hydro

 

Inflows and outflows
Approach or data usedItemSource
Gridded climate data, AWRA-L model
  • Bureau of Meteorology
Streamflow data
  • MDBA
s.71 Water resource report
  • MDBA
  • Bureau of Meteorology
s.71 Water resource report and water sharing plans
  • Bureau of Meteorology
  • MDBA
Metered and estimated data provided by water authorities
  •  Icon Water and Queanbeyan City Council
Snowy Hydro database and s.71 Water resource report
  • Snowy Hydro
Groundwater models
  • Bureau of Meteorology
  • DPI Water

 

Abstractions and deliveries
Approach or data usedItemSource
s.71 Water resource report
  • MDBA
s.71 Water resource report and water sharing plans
  • MDBA
Snowy Hydro database and s.71 Water resource report
  • MDBA
  • Snowy Hydro
Jurisdictional water sharing plans
  • MDBA

s.71 = Section 71 of the Water Act 2007

MDBA = Murray–Darling Basin Authority

 

Detail of methods

Water storage product data

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 2016 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 2016.

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.

AWRA-R model

Regulated and unregulated rivers

The volume of water in the main river channels was modelled using the AWRA-R model, v5.0 (Dutta et al. 2015). In the AWRA-R, a water balance approach is used to calculate the volume of water in a river reach. To the initial volume of water in a reach at the start of a time step, inflow at the upstream nodes, contributing catchment runoff, reservoir contribution, irrigation return, rainfall on the river reach and the return from the floodplain are added and the irrigation diversion, urban diversion (if any), evaporation from the river reach, anabranch flow, overbank flooding, loss to groundwater and the outflow at the downstream node are subtracted to obtain the volume of water in the river reach at the end of the time step. The volume of water in a reach is assumed to be zero at the beginning of the calculation.

The limitations associated with this approach are:

  • The modelled ungauged runoff from the AWRA-L was given as input to the AWRA-R model. Therefore, any modelling error in AWRA-L will have some impact on AWRA-R outputs.
  • In the AWRA-R model, it was not possible to consider all the catchment physical processes because of the lack of available data. This might result in some modelling errors.

HYDRO database and height-volume relationship

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.

 

Section 71 Water resource report

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.

 

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 2015–16 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.

 

Table N5 Calculation of water allocation remaining
 Account
 Opening balance at the beginning of the year
addAllocation during the year
lessAllocated abstraction during the year
lessAdjustment and forfeiture during the year
equalsClosing balance at end of the year

 

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.

 

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 rights, entitlements, allocations and restrictions section of 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.

The volume reported under non-allocated environmental diversions and other environmental decreases includes environmental volumes transferred to the Snowy River from relevant NSW and Victorian SDL resource units.

 

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%.

 

Overflow: landscape

Overbank flood spilling is classified as an unregulated event. The volume is applicable to several jurisdictions within the region; however, volumes were only available for Queensland. The volume of water that overflows from river channels onto the landscape is estimated based on a combination of user returns and local knowledge.

A limitation of this approach is that a portion of the reported volume may also include the volume of water harvested from the landscape. It is not possible to distinguish between the volume of water harvested from the landscape and the volume of water that overflows from the river.

The uncertainty estimate for the volume reported is +/– 40%.

 

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.

 

Section 71 Water resource report and water sharing plans

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.

 

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.

 

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.

 

Section 71 Water resource report and Snowy hydro database

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 2015–16 year is calculated as shown in Table N6.

 

Table N6 Calculation of inter-region claim remaining
 Account
 Fixed annual calculated yield for development
adddry inflow sequence volume (DISV) at 1 March from previous Snowy Hydro water year (1 May 2015–30 April 2016)
lessreleases made during the previous Snowy Hydro water year in excess of the previous year's RAR adjusted by the DISV at 1 March
lesswater allocation from prior Snowy Hydro water year for environmental releases
addother RAR adjustment
equalsRAR at the commencement of the Snowy Hydro water year (1 May)
lessaccountable release to 30 June
lessDISV increase at 30 June
equalsRAR remaining at 30 June

 

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 2015–16 year was calculated as shown in Table N7.

 

Table N7 Calculation of inter-region claim increase
 Account
 Fixed annual calculated yield for development
lessSnowy Hydro water allocation
lessdry inflow sequence volume (DISV) increase over the year
addother RAR adjustment
equalsincrease of claims against Snowy Hydro
addGlenelg River
equalsTotal increase of inter-region claim

 

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 2015–16 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.

 

Gridded climate data and AWRA-L model

Precipitation and evaporation

Monthly precipitation grids for the region were produced using daily data from approximately 6,500 rain gauge stations across Australia and interpolated to a 0.05 degree (5 km) national grid (Jones et al. 2009).

For the water body (storage, lake and weir), the volume of precipitation and transpiration at each water body was estimated by multiplying the proportionally weighted average of grid cells that intersected each water feature by the surface area of each waterbody. The average monthly surface area was calculated from daily storage levels and capacity tables where data were available. Where dynamic storage surface area data were not available, the Australian Hydrological Geospatial Fabric (AGHF) surface water feature was used to estimate a static surface area.

For the river sections, the volume of precipitation and transpiration at each river section was estimated by multiplying the average of the grid cells that intersected the contributing catchment for the river section. The daily dynamic surface area was calculated by averaging river width at the inlet and outlet gauging stations, and multiplying the average by the length of the river section. The river width at a gauging station was obtained based on the flow at the station and the flow-width relationship. The relationship was developed for each gauging station using the cross-section data (Dutta et al. 2015). Where the cross-section data were not available, an approximate estimate of river width was made by examining the neighbouring stations.

Evaporation from water bodies was calculated on a daily basis using the Morton's shallow lake formulation (Morton 1983a, 1983b, 1986). For annual evaporation estimate, there is no difference between shallow and deep lake evaporation (Sacks et al. 1994). The climate data required for the Morton's method are maximum temperature, minimum temperature, vapour pressure and solar radiation. The climate data for each water body was estimated from the proportionally weighted average of grid cells that intersected each water feature and input to the Morton's program to obtain the evaporation values. The volume was then estimated by multiplying the surface area of each waterbody by the evaporation values.

The limitations associated with this approach are:

  • The precipitation and AWRA-L potential evapotranspiration estimates were subject to approximations associated with interpolating the observation point data to a national grid as detailed in Jones et al. (2009).
  • The dynamic storage surface areas calculated from the levels and storage rating tables represent a monthly average and therefore will not capture changes that occur on a shorter temporal scale.
  • The use of the static default AHGF surface area is an approximation only. It represents the water features at capacity and therefore is likely to result in an overestimation of precipitation on the water features.
  • The accuracy of the river width estimated using the flow-width relationship depends on the quality of the relationship and the type of the function used (Dutta et al. 2015). In AWRA-R, a power law function was used. In addition, the reaches with no cross-section data will have further errors due to the use of an approximate estimate of the river width.

 

Runoff

'Runoff to surface water' was based on streamflow estimates from the AWRA-L version 5.0 model (Viney et al. 2015) outputs. Using gridded climate data for the region (including precipitation, temperature, and solar radiation data), AWRA-L was used to estimate the runoff depth at each grid cell within the region. Only runoff from the landscape was considered; therefore, the surface areas of the major storages were excluded from the analysis.

Runoff from the landscape is divided into two components: runoff into the surface water store (surface water storages, weirs, rivers and drains) and runoff into off-channel water storages. Only runoff into the surface water store was considered here.

The average runoff depth from the landscape into surface water was determined as the weighted mean of the relevant grid-points within the Murray–Darling Basin region. Points were weighted based upon the area they represented within the reporting region to remove edge effects (where the area represented is not wholly within the region) and the effect of changing area represented with changing latitude. Runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding surface water storages, weirs and off-channel storages) and was used as an input to the water balance algorithm.

The assumptions and limitations of this approach were:

  • Runoff estimates were subject to the assumptions of the AWRA-L model (Viney et al. 2015).   
  • The estimated runoff corresponds to the runoff expected from an unimpaired catchment. The impairment on runoff from local catchment storages is estimated using a local catchment storage balance model. Where this is applied, the runoff estimates inherit the approximations, assumptions and caveats of the local catchment storage water balance model and the parameters used.

 

Streamflow data

Outflow

The annual volume of water that flows out to sea from the River Murray is estimated based on data collected at several barrages near the mouth of the river. These data include the flow rates across the barrage and the length of time the barrage is open.

 

 Metered and estimated data provided by water authorities

Discharge: wastewater system

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

 

Groundwater model

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 2015–16 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 MurrayDarling 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 2016 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.

 

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 DPI Water using MODFLOW and water table fluctuation methods are shown in Figure N1.

 

Figure N1 Sustainable diversion limit areas for modelled groundwater discharge to landscape and surface water
Figure N1 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 N1 shows the DPI Water SDL modelled areas for which volume is calculated in the region. 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.

 

Jurisdictional water sharing plans

Diversion: statutory rights

The volume of diversion under statutory rights was estimated as equal to the total volume of basic landholder rights detailed in the water sharing plan at the commencement of the plan. No information was available for SDLs outside New South Wales.