National Water Account 2017

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Key findings

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Supporting information

Region description

Reference information

Ord: Methods

Summary of methods

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

Table N4  Methods used to derive item volumes

Department of Water and Environmental Regulation = Western Australian Department of Water and Environmental Regulation

Water Corporation = Water Corporation of Western Australia

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 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 2017. It is assumed that the storage volume in Arthur Creek changes little from year to year and, similar to other smaller storages within the region (e.g., Lake Kununurra), is usually at approximately 95% capacity on 30 June each year. 

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 2017.

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 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 N1):

• 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 N1 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 18,800,000 ML, which is 1.1 times that reported in the Statement of Water Flows (16,921,169 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%

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 outflows into the sea from the Ord region has a quality code of E, which indicates the lowest quality of data recorded for all the station data.

Gridded climate data

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). The precipitation at each waterbody (e.g., 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, which 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 (e.g. storages and rivers) 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 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.
• 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.
• River widths are estimated at the upstream and downstream gauging stations and the average width is assumed to be representative for the entire reach.

Australian Landscape Water Balance model

Runoff

Runoff to surface water in the Ord region was based on streamflow estimates from the AWRA-L version 5.0 model outputs (see the Australian Landscape Water Balance model website).

Using gridded climate data for the Ord region (including precipitation, temperature, and solar radiation data), the AWRA-L model 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 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).

AWRA model

The AWRA-R model 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).

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.

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 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. 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 2016–17 year, the volume of abstraction is estimated to be the full licensed allocation.
• For licences that expired during the 2016–17 year (and were not renewed), the volume of abstraction is estimated to be the allocation remaining at 1 July 2016 (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 2016–17 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 2017 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 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). For estimated data the uncertainty is unquantified.

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).

Operational Data Storage System: metered 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 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%. 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

Recharge: landscape

The volume of groundwater recharge is estimated to be equal to the extraction from groundwater in the region during the 2016–17 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 2016–17 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 2016–17 year was equivalent to the average annual leakage between 2010 and 2015.