Ord
Quantification approaches
Summary of quantification approaches
Table 1 outlines the quantification approaches used to derive the line item volumes for the Ord region. For a more detailed description of the quantification approach, click on the relevant line item name in the table.
Department of Water = Western Australian Department of Water
Department of Land Resource Management = Northern Territory Department of Land Resource Management
Water Corporation = Water Corporation of Western Australia
Detail of quantification approaches
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. Rating 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 line item as detailed in the supporting information table. 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.
- No storage data were available for Arthur Creek at 30 June 2014. Given the storage volumes in Lake Kununurra and Moochalabra changed little during the 2013–14 year, it was assumed that the storage volume in Arthur Creek remained the same as that measured on 30 June 2013.
Stream monitoring data
Regulated river
A series of approximately 40 cross-sections detailing channel geometry of the lower Ord River was surveyed between 1997 and 2002. The survey information was used to create a hydraulic model of the Ord River between Lake Kununurra and the limit of tidal incursion, 76 km below the Kununurra Diversion Dam (Braimbridge and Malseed, 2007). The river storage volume is calculated using the model based on the flow rate measured at the Tarrara Bar (Station 809339) on 30 June.
The limitations associated with this approach are as follows:
- The cross-sections along the lower Ord River were surveyed for the purposes of making environmental assessments of various parts of the river downstream of Lake Kununurra, not for calculating an accurate volume (i.e., the depths of the river pools were not surveyed).
- The cross-sectional profile of the river varies substantially downstream of Lake Kununurra. Approximately 40 cross-section profiles were surveyed and the calculated hydraulic model volume is considered only to provide an order of magnitude estimate of the true river volume. It is expected that since the cross-sections were surveyed between 1997 and 2002, there will have been changes within the channel. The extent and rate of these changes is unknown.
- The method of estimation also assumes that a constant flow rate is occurring along the entire (76 km) length of the lower Ord River for which the calculation was made.
River outflow from the region
The total volume of water that flows out to sea is the sum of the available volume of outflow from the four rivers that flow into the sea:
- Ord River
- King River
- Keep River
- Sandy Creek.
The river outflow was estimated using instantaneous discharge data (L/s) 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.
The Western Australian Department of Water and the Northern Territory Department of Land Resource Management assign quality codes to flow data in accordance with tables 2 and 3, respectively.
| Quality code | Description |
| 1 | uncertainty +/– 10% |
| 2 | uncertainty +/– 15% |
| 3 | uncertainty +/– 20% |
| 4 | uncertainty > +/– 20% |
| 5 | outside measured range |
| 6 | not reviewed/quality not known |
| Quality code | Description |
| 41 | good rating |
| 91 | satisfactory rating |
| 141 | poor rating |
| 145 | poor rating—extrapolated |
| 148 | below measuring device |
| 150 | rating under review—not quality-coded |
| 192 | rating table exceeded |
| 255 | no data |
The total volume of water that outflows into the sea from the Ord River catchment (Ord and King rivers) has a quality code of 4 (uncertainty > +/– 20%). The total volume of water that outflows into the sea from the Keep River catchment (Keep River and Sandy Creek) has a quality code of 192 (rating table exceeded). These quality codes indicate the lowest quality of data recorded for all the station data for the respective catchments.
Climate grid data
Precipitation and evaporation on/from surface water
Monthly precipitation grids for the region were produced using daily data from approximately 6,500 rain gauge stations and interpolated to a 0.05 degrees (5 km) national grid (Jones et al. 2007).
Potential evaporation across the region was estimated using the Australian Water Resources Assessment system Landscape model (AWRA-L) version 3.0 (Van Dijk 2010). The AWRA-L model uses a modified version of the Penman-Monteith method to produce the potential evaporation. Daily AWRA-L potential evaporation grids for the region were produced based on daily gridded climate data (including precipitation, solar radiance and temperature) that were available on a 0.05 degree (approximately 5 km) national grid (Jones et al. 2007).
The precipitation and evaporation at each waterbody was estimated from the proportionally weighted average of grid-cells that intersected each water feature. The volume was then estimated by multiplying by the surface area of each waterbody. The average monthly surface area of the storages was calculated from daily storage levels and capacity tables where data were available. Where daily storage level data were unavailable (i.e., Arthur Creek storage), a static surface area value from the Australian Hydrological Geospatial Fabric was used.
The water features within the region included storages and the regulated channel of the lower Ord River between Lake Kununurra and Carltons Crossing. An estimate of 100 m was provided by the Department of Water for the width of the lower Ord River.
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.
- Dynamic storage surface area data are not available for Arthur Creek. Therefore, the Australian Hydrological Geospatial Fabric surface water feature was used to estimate a static surface area for Arthur Creek. This represents the storage at total capacity and, therefore, likely results in an over-estimation of precipitation and evaporation at the storage.
- Defining the Ord River width as a static 100 m is an approximation only. It is likely that this width varies along the river section length and throughout the reporting period, but these variations are assumed to be minor and to have a limited influence on the total volume of precipitation and evaporation at the surface water store.
- The total surface area of the surface water store within the Ord region did not include unregulated rivers.
Runoff to surface water
Runoff to surface water in the Ord region was based on streamflow estimates from the AWRA-L model outputs. Using climate grid data for the Ord region (including precipitation, temperature and solar radiation data), AWRA-L was used to estimate the runoff depth at each grid point within the region. Only runoff from the landscape was considered; therefore, the surface areas of the major storages were excluded from the analysis.
The average runoff depth 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 reporting 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 region (excluding storages).
The estimated runoff was compared against historical flows at unimpaired catchments within the Ord region for the 2013–14 water year and provided a suitable representation of the runoff for this year.
The approach was subject to the assumptions of the AWRA-L model detailed in Van Dijk 2010.
Water resourcing licence database and annual reports/meter readings
Water 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 2013–14 year is the unused component of the annual allocation for the licence. The allocation remaining at 30 June 2014 is calculated as shown in Table 4.
| Account | |
| Opening balance (at 1 July 2013) | |
| add | Allocation announcement |
| less | Entitled abstraction of allocated water |
| less | Adjustment and forfeiture |
| equals | Closing balance (at 30 June 2014) |
Adjustment and forfeitures of water allocation
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 carry-over 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.
Water allocation announcements
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 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 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 rights, entitlements, allocations and restrictions note.
Entitled abstraction of allocated water to individual users
The entitled abstraction of allocated water by individual users (both surface water and groundwater) during the licenced 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. Where metered data are not available, the following methods were used to estimate the volume of abstraction:
- For licences that expired and were renewed during the 2013–14 year, the volume of abstraction is estimated to be the full licensed allocation.
- For licences that expired during the 2013–14 year (and were not renewed), the volume of abstraction is estimated to be the allocation remaining at 1 July 2013 (i.e., the allocation remaining at the start of the year is assumed to be abstracted before the licence expired).
- For new licences that were created during the 2013–14 year, the volume of abstraction is estimated to be the full allocation, multiplied by the ratio of the number of days from the licence issue date until 30 June 2014 to the number of days in the year.
There is not sufficient information relating to actual abstraction to provide more accurate estimates of abstraction for all licences, particularly individual users. The pro-rata estimates of abstraction assume that the full annual entitlement is abstracted each year and that the rate of abstraction is uniform throughout the year. It is unlikely on both counts that this will be the case for all licences.
The expected error associated with metered data is +/– 5%. The Department of Water 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). For estimated data the uncertainty is unquantified.
Entitled diversion of allocated surface water to irrigation scheme
The entitled diversion of allocated water for the Ord River Irrigation Area during the licenced water year is equal to the total volume of water diverted at the main Ord River channel. The volumes diverted are metered at the M1 supply channel offtake and the Packsaddle pump station continuously and reported on a monthly basis.
Water diversion to the Ord River Irrigation Area was based on both measured and estimated data. The volume of surface water diverted to the Ivanhoe and Packsaddle irrigation districts and the M1 supply channel was measured between 1 July and 31 December 2013, but data for the remaining six months of the reporting period were unavailable at the time this report was prepared. The volume of water diverted during this period of missing data was estimated based on the previous years' water abstraction.
The expected error associated with these diversions is +/– 5%. The Department of Water 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).
Operational Data Storage System: metered data
Discharge from urban water system
The volume of treated wastewater discharged from the urban water system to the river is based on measured discharge data collected by flow meters installed at the treatment plants.
Uncertainty range for flow meters installed at wastewater treatment plants is estimated to be +/– 10%.
Entitled abstraction of allocated water to urban water system
The entitled abstraction of allocated water for the urban water system (both surface water and groundwater) during the licensed water year is based on measured data collected at the outlet of the water source using a cumulative water meter.
The volume of surface water diverted from the main Ord River channel for supply to Lake Argyle Village was not measured during the 2013–14 year. For the previous two years, the volume of water diverted to Lake Argyle Village has been approximately 20 ML; therefore, it is likely that the reported volume of surface water diverted to the urban water supply system is slightly underestimated.
The expected error associated with these abstractions is +/– 5%. The Department of Water 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).
Operational Data Storage System: seepage data
Surface water leakage to landscape
Instantaneous seepage data (L/s) are measured at storages within the Ord region on an approximately daily basis. These data were inputted to Hydstra and converted to daily volume data (ML) to determine the total annual seepage (in ML) at each storage.
Seepage data are only collected at Lake Argyle and Lake Moochalabra within the Ord region; consequently, the reported leakage volume of 19 ML is likely to be an underestimate.
Calculated based on other line items
Recharge from landscape
The volume of groundwater recharge is estimated to be equal to the extraction from groundwater in the region during the 2013–14 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.