National Water Account 2015

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Quantification approach

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Accountability statement

Ord: Quantification approaches

Summary of quantification approaches

Table N11 outlines the quantification approaches used to derive the item volumes for the Ord region. For a more detailed description of a quantification approach, click on the relevant item name in the table.

Table N11  Quantification approaches used to derive item volumes

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. 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 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 that in turn affect the accuracy of the storage–volume curves.
• No storage data were available for Arthur Creek at 30 June 2015. Given the storage volumes in Lake Kununurra and Moochalabra changed little during the 2014–15 year, it was assumed that the storage volume in Arthur Creek remained the same as that measured on 30 June 2014. 

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 2015.

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.

Outflow

There are four rivers that flow out to 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 N10):

• 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 N10 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, that is, there is no adjustment made for the contributing area below the gauging station used to calculate the outflow.

This contributing area below the gauging stations is approximately 10% of the total area of the Ord region. Based on a drainage-area ratio equation, estimated outflow is approximately 4,230,000 ML, which is 1.1 times that reported in the Statement of Water Flows (3,806,638 ML). Given, however, that the 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 such a large amount of runoff. Instead, it is considered that the reported outflow to sea may be underestimated by up to 5–10%

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 N12 and N13, respectively.

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.

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 degrees (5 km) national grid (Jones et al. 2009).

Potential evapotranspiration across the region was estimated using the Australian Water Resources Assessment system Landscape model (AWRA-L) version 5.0 (Viney et al. 2015). The AWRA-L model uses the Penman method to produce the potential evapotranspiration. Daily AWRA-L potential evapotranspiration grids for the region were produced based on daily gridded climate data (including precipitation, solar radiation, and temperature) available on a 0.05 degree (approximately 5 km) national grid (Jones et al. 2009).

The volume of precipitation and evapotranspiration at each waterbody 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 of the storages was calculated from daily storage levels and capacity tables where data were available.

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.
• 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.

Stream monitoring data and AWRA-L model

Runoff

Runoff to surface water in the Ord 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 Argyle (approximately 60% of the Ord region area) was estimated based on observed streamflow data from gauging stations along the three primary rivers that flow into Lake Argyle (Figure N11):

• Ord River at the Old Ord Homestead (Station 809316)
• Negri River at the Mistake Creek Homestead (Station 809315)
• Wilson River at O'Donnell Range (Station 809322).

The contributing area below these three gauging stations is approximately 25% of the area upstream of Lake Argyle (Figure N11). Based on a drainage-area ratio equation, estimated flow into Lake Argyle is approximately 1.3 times the sum of the flows observed at the three gauging stations. This simple equation assumes that catchment conditions in the gauged and ungauged areas are relatively similar.

Figure N11 Gauging stations used to calculate runoff upstream of Lake Argyle

The runoff in the remaining 40% of the region was based on AWRA-L model outputs. Using gridded climate data for the Ord region (including precipitation, temperature, and solar radiation data), AWRA-L model was used to estimate the runoff depth at each grid cell 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 depth from the landscape into the connected surface water store was determined as the weighted mean of the relevant grid cells 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 version 5.0 model detailed in Viney et al. (2015).

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 2014–15 year is the unused component of the annual allocation for the licence. The allocation remaining at 30 June 2015 is calculated as shown in Table N14.

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 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.

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. 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 2014–15 year, the volume of abstraction is estimated to be the full licensed allocation.
• For licences that expired during the 2014–15 year (and were not renewed), the volume of abstraction is estimated to be the allocation remaining at 1 July 2014 (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 2014–15 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 2015 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.

Allocated diversion: irrigation

The allocated diversion of surface water for the Ord River Irrigation Area during the licensed water year is derived from a combination of metered data and estimates. 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 volume of water diverted to the Ivanhoe and Packsaddle plains is based on metered data taken at the M1 supply channel and the Packsaddle pump station respectively. Metered data were available for the entire 2014–15 year.

The volume of water diverted to Water Corporation's irrigation customers along the M1 supply channel, as well as for channel maintenance, was measured between 1 July and 31 December 2014; however, data for the remaining 6 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 year's water diversion.

The licence entitlement to supply water to the Goomig Farmlands is a new licence created in mid-2015; no metered data associated with this licence were available at the time this report was prepared. Therefore, the volume of diversion is estimated to be the full allocation, multiplied by the ratio of the number of days from the licence issue date until 30 June 2015 to the number of days in the year.

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.

Operational Data Storage System: metered data 

Discharge: wastewater

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%.

Allocated abstraction: urban system

The allocated abstraction of water for the urban 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 2014–15 year. For the previous 2 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).

Leakage: landscape

Instantaneous seepage data are measured at storages within the Ord region on an approximately daily basis. These data were converted to daily volume data (ML) to determine the total annual seepage (in ML) at each storage.

Seepage data are collected at Lake Argyle, Lake Kununurra and Lake Moochalabra within the Ord region. No data are collected at the Arthur Creek storage, so the total reported leakage volume is likely to be an underestimate. It is assumed that the dam seepage measuring points reflect the total seepage from the storages.

Estimated data

Recharge: landscape

The volume of groundwater recharge is estimated to be equal to the extraction from groundwater in the region during the 2014–15 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).