Ord: Methods
Summary of methods
There were five key methods for establishing item volumes in the 2019 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.
Methods approach
AWRA-R model
Water storage product data
Stream monitoring data
Water resourcing licence database and meter readings
Estimated data
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, diversions for consumptive use; overbank flooding and floodplain return; rainfall; evaporation; and losses.
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.
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's Australian Landscape Water Balance webpage.
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).
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 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 that in turn affect the accuracy of the storage–volume curves.
- No storage data were available for Arthur Creek at 30 June 2019. Given the storage capacity in Arthur Creek is less than 1% of the region's total storage capacity, not including this storage in the account made no material difference to the region's total storage volume.
Stream monitoring data
Outflow
There are four rivers that discharge to the sea from the Ord region. The total river outflow was estimated using flow data collected at the most downstream gauging stations nearest to the outlet to the sea (Figure N2):
- Ord River at Tarrara Bar (Station 809339)
- King River at Cockburn North (Station 809314)
- Keep River at Legune Road Crossing (Station G8100225)
- Sandy Creek at Legune Road Crossing (Station G8100210).
These data were used to determine the total annual discharge (in ML) at each station during the year.
Figure N2 Gauging stations used to calculate total outflow to sea
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.
The contributing area below the gauging stations is approximately 10% of the total area of the Ord region; however, given that this ungauged component of the Ord region lies mainly on the lowlands, which is an area of relatively high rainfall-recharge, it is unlikely that this area will generate a large amount of runoff. Therefore, no adjustment to outflow is made for the contributing area below that station.
Quality codes are assigned to flow data in accordance with Table N5, as given in the Bureau of Meteorology's Water Data Online.
| Quality code | Description |
| A | The record set is the best available given the technologies, techniques and monitoring objectives at the time of classification |
| B | The record set is compromised in its ability to truly represent the parameter |
| C | The record set is an estimate |
| E | The record set's ability to truly represent the monitored parameter is not known |
| F | The record set is not of release quality or contains missing data |
The total volume of water that outflows into the sea from the Ord region has a quality code of E, which indicates the lowest quality of data recorded at the stations.
Water resourcing licence database and 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 year is the unused component of the annual allocation for the licence. The allocation remaining at 30 June is calculated as shown in the table below.
| Account | |
| Opening balance at 1 July 2018 | |
| add | Allocation |
| less | Allocated abstraction |
| less | Adjustment and forfeiture |
| equals | Closing balance at 30 June 2019 |
Adjustment and forfeiture
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.
Allocations
Individual user licences are generally issued for periods of between 1 and 10 years, with an annual abstraction amount specified and with annual compliance arrangements in place.
The maximum amount of abstraction for each year for urban water and irrigation scheme supply is announced by the Western Australian Minister for Water on an annual basis. The announced allocation is made after a review by the Department of Water and Environmental Regulation of storage and aquifer levels in the region in April of the reporting year.
More information on these allocations and the associated water access entitlement is given in the Water access and use note
Allocated abstraction: individual users
The allocated abstraction of water by individual users (both surface water and groundwater) during the licensed water year is derived from a combination of metered data and estimates.
Where metered data are available, the abstraction is calculated as the actual abstraction during the year. Metered data are supplied by users to the Department of Water and Environmental Regulation and the expected error associated with metered data is +/– 5%. The department requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009).
Where metered data are not available the volume of abstraction is assumed to be the full annual entitlement. There is not sufficient information relating to actual abstraction to provide more accurate estimates of abstraction for all licences.
Allocated diversion: irrigation
The allocated diversion of surface water for the Ord River Irrigation Area is derived from metered data. Allocated diversions to the irrigation area are associated with three licence entitlements:
- Ord Irrigation Cooperative: Packsaddle and Ivanhoe plains
- Water Corporation: M1 channel
- Kimberley Agricultural Investment: Goomig Farmlands
The Ord Irrigation Cooperative monitor the volume of water diverted from Lake Kununurra to the Ord River Irrigation Area through flow meters located in the M1 channel and the Packsaddle pump station. Water diverted through the M1 channel supplies water to the Ivanhoe plains, Water Corporation's irrigation customers along the channel, and the Goomig Farmlands. Water diverted through the Packsaddle pump supplies water to the Packsaddle plains.
The expected error associated with these diversions is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009).
Estimated data
Allocated abstraction: urban system
The allocated abstraction of water for the urban system (both surface water and groundwater) during the licensed water year is estimated based on eight years of annual metered data collected between 2010 and 2017. Town supply is measured at the outlet of the water source using a cumulative water meter.
During this 8-year period, the total annual volume of water abstracted for the urban system changed little from year to year, so it was assumed that town water supply during the 2018–19 year was equivalent to the average annual supply between 2010 and 2017.
Recharge: landscape
The volume of groundwater recharge is estimated to be equal to the extraction from groundwater in the region during the year; however, this approach is likely to be an under-estimation of actual recharge. An expansion of the monitoring bore network and improved understanding of aquifer extent, aquifer properties, and groundwater processes are required to adequately quantify the flow of groundwater in the region.
Point return: irrigation
The volume of return flow from the Ord River Irrigation Area to the Ord River is an estimated volume based on the assumption that approximately 2 m3/s of surplus irrigation supply water and return flows contribute to the lower Ord River during the dry season (Department of Water 2012).
Discharge: wastewater
The volume of treated wastewater discharged from the urban water system to the river is estimated based on six years of annual metered data collected between 2010 and 2015. Discharge data are collected by flow meters installed at the treatment plants within the region.
During this 6-year period, total annual wastewater discharge at these treatment plants changed little from year to year, so it was assumed that the wastewater discharge during the 2018–19 year was equivalent to the average annual discharge between 2010 and 2015.
Uncertainty range for flow meters installed at wastewater treatment plants is estimated to be +/– 10%.
Leakage: landscape
The volume of leakage to the landscape from surface water storages is estimated based on six years of annual metered data collected between 2010 and 2015. Leakage data are collected at Lake Argyle, Lake Kununurra and Lake Moochalabra within the Ord region.
During this 6-year period, total annual leakage at these storages changed little from year to year, so it was assumed that leakage to the landscape during the 2018–19 year was equivalent to the average annual leakage between 2010 and 2015.