National Water Account 2017

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Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Burdekin: Methods

Summary of methods

Table N4 outlines the methods used to derive the item volumes for the Burdekin region. For a more detailed description of the method, click on the relevant item name in the table.

Table N4  Methods used to derive item volumes

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 as detailed in the surface water note. The uncertainty range for these volumes is +/–5%.

The assumptions made were as follows:

• 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, which in turn affects the accuracy of the storage–volume curves.

Stream monitoring data

Outflow

There are three rivers that discharge to the sea from the Burdekin region. The total river outflow was established using flow data collected at the most downstream gauging stations nearest to the outlet to the sea (Figure N1):

• Burdekin River at Clare (Station 120006B)
• Haughton River at Powerline (Station119003A)
• Barratta Creek at Northcote (Station 119101A).

Figure N1 Gauging stations used to calculate total outflow to sea

The river outflow was estimated using flow data collected at the most downstream gauging station (nearest to the outlet to the sea) along a river. These data were converted to daily volume data (ML) to determine the total annual discharge (in ML) at each station during the year.

It is assumed that the river outflow to the sea is equal to the volume of discharge measured at the most downstream station along a river, that is, there is no adjustment made for the contributing area below the gauging station used to calculate the outflow.

Quality codes are assigned to flow data in accordance with Table N5, as given in the Bureau of Meteorology's Water Data Online.

The total volume of water that discharges into the sea from Burdekin River, Haughton River, and Barratta Creek has a quality code of C (estimated) during the 2016–17 year.

Climate grid data

Precipitation and evaporation

The precipitation at each waterbody (i.e. storages and rivers) was estimated from the proportionally weighted average of grid cells that intersected each water feature. The volume was then estimated by multiplying the surface area of each waterbody by the weighted average precipitation.

For storages, the average monthly surface area was calculated from daily storage levels and capacity tables. For rivers, the daily dynamic surface area was calculated by multiplying the length of the river section between two gauging stations by the average river width. The average river width was based on the measured flow at the two stations and the stations’ flow-width relationship. The flow-width relationship was developed for each gauging station using cross-sectional data (Dutta et al. 2015). Where the cross-section data were not available, an approximate estimate of river width was made using nearby 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 waterbody (i.e. storages) 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 dynamic storage surface areas calculated from the levels and capacity tables represent a monthly average and therefore, will not capture changes that occur on a shorter temporal scale.
• The total surface area of the surface water store within the Burdekin region included only the storages and rivers, not lakes or wetlands.
• River widths are estimated at the upstream and downstream gauging stations and the average width is assumed to be representative for the entire reach.

Uncertainty estimates were not available for the modelled precipitation and evaporation estimates by the Bureau.

Runoff

Runoff to surface water in the Burdekin region was based on a combination of observed streamflow data and streamflow estimates from the AWRA-L version 5.0 model outputs.

The volume of runoff in the catchment area upstream of Lake Dalrymple (approximately 85% of the Burdekin region area) was estimated based on observed streamflow data (Figure N2). The runoff is assumed to equal the sum of flow data from gauging stations along the three primary rivers that flow into Lake Dalrymple.

• Burdekin River at Sellheim (Station 120002C)
• Cape River at Taemas (Station120302B)
• Suttor River at St Anns (Station 120303A).

Figure N2 Gauging stations used to calculate runoff upstream of Lake Dalrymple

The runoff in the remaining 15% of the region was based on AWRA-L model outputs. Using climate grid data for the Burdekin region (including precipitation, temperature, and solar radiation data), the AWRA-L model was used to estimate the runoff depth at each grid point within the 'ungauged' portion of the region. Only runoff from the landscape was considered; therefore, the surface areas of the storages were excluded from the analysis.

The average runoff from the landscape into the connected surface water store was determined as the weighted mean of the relevant grid points within the region boundary. Points were weighted based upon the area they represented within the region to remove edge effects (where the area represented is not wholly within the reporting 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 modelled region (excluding storages).

The approach was subject to the assumptions of the AWRA-L model detailed in Viney et al. 2015.

AWRA model

The AWRA-R is a node-link river network model developed with explicit representation of key hydrological processes and anthropogenic water uses. It is used to quantify various river fluxes and stores at a daily temporal resolution along the river network. The river network is conceptualised as nodes at stream gauging stations connected by river reaches. Model parameters are estimated for each river reach, optimised to an objective function combining bias with the daily Nash-Sutcliffe coefficient of efficiency using a shuffled complex evolution algorithm (Duan et al. 1992).

Unregulated and regulated river

The volume of water in the main river channels was modelled using the AWRA-R model, v5.0 (Dutta et al. 2015). In 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, however,  this shouldn't affect results due to model spin-up times being used.

The limitations associated with this approach are:

• The modelled ungauged runoff from the AWRA-L model (Viney et al. 2015) for the grid cells within the residual (or headwater) catchment of each reach was given as input of the inflows to the corresponding reach of the AWRA-R model.  Therefore, any modelling error in AWRA-L will have some impact on AWRA-R outputs. An assessment of the varying quality of the AWRA-L runoff across Australia can be found in Frost et al. 2016. Note that a correction factor to the AWRA-L runoff is applied in AWR-R as part of the calibration process.

• In the AWRA-R model, it was not possible to consider all the catchment physical and anthropogenic processes (for example some extractions in some reaches) because of the lack of available data. This might result in some  errors in the simulated current river volume as all processes interact.

Flood return and overbank flow

The volume of overbank flow from the river onto the floodplain, and the return flow from the floodplain back into the river are estimated from AWRA-R model outputs.

The 'overbank threshold' and the 'return flow rate' are calibrated parameters in the AWRA-R model. The annual volume of overbank flow in the region is estimated as the:

• excess flow above the 'overbank threshold' if the excess flow is less than 1 m³/s, or
• square root of the excess flow above the 'overbank threshold' if the excess flow is greater than 1 m³/s.

The annual volume of flood return in the region is estimated by multiplying the 'return flow rate' by the volume of water in the floodplain.

Detailed information on the model calibrations are provided by Dutta et al. (2015). The estimates of both flood return and overbank flow have high uncertainty as they were not validated due to a lack of available data.

Managed aquifer recharge

Lower Burdekin Water & DNRM manage the majority of the aquifers in the lower Burdekin delta. SunWater do not manage aquifer recharge (outside the Giru Benefited Area). The expected error associated with this reported volume is +/– 5% but only extends to surface water metering, not GW recharge.

Water resourcing licence database and annual reports/meter readings

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 (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. The allocation remaining at 30 June 2017 is calculated as shown in Table N6.

Adjustment and forfeiture

The portion of groundwater 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. Remaining surface water allocation for individual users and irrigation are carried over.

Allocations

Queensland water licences are issued with an annual abstraction amount specified and with annual compliance arrangements in place.

The maximum amount of abstraction under a water entitlement is announced by the resource operations licence holder on an annual basis. The announced allocation is made after a review of storage and aquifer levels in the region on the first day of the water year (1 July). Subsequent additional announcements may be made throughout the year if additional water becomes available.

More information on these allocations and the associated water access entitlement is given in the Water rights, entitlements, allocations and restrictions note

Allocation abstraction: individual users

The entitled abstraction of allocated water by individual users (both surface water and groundwater) during the licenced water year is derived from volumetric charging (metered data).

The expected error associated with metered data is +/– 5%. SunWater requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range. The uncertainty of the estimated data is unquantified.

Allocated diversion: urban water system

The entitled abstraction of allocated water for the urban water system during the licensed water year is based on measured data collected at the outlet of the water source using a cumulative water meter.

The expected error associated with these abstractions is +/– 5%.  SunWater requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range.

Allocated diversion: irrigation

Most of the entitled diversion of allocated surface water in the Burdekin region is for irrigation scheme water supply. The entitled diversion of allocated water for Irrigation combines total report volumes from the Bowen Broken and Burdekin Haughton water supply schemes. The expected error associated with these diversions is +/– 5%. SunWater requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range.