Ord

Data source

Water Corporation: Operational Data Storage System database.

Data provider

Water Corporation.

Method

Storage volume is calculated at the start and end of the reporting period by using gauged water level height(s) (metres above Australian Height Datum [mAHD]) for individual reservoirs. The height measurement is converted to a volume using the storage–volume curve(s) of the storage.

The storage volume of individual reservoirs is aggregated to present the volume for the line item.

Uncertainty

Derived from measured data. Uncertainty range +/–5%.

Assumptions, approximations and caveats/limitations

• Storage–volume curves represent specifically surveyed parts of the reservoir, and may not reflect the storage–volume relationship across the entire storage.
• Reservoirs are subject to sedimentation and other physical changes over time, which in turn affects the accuracy of the storage–volume curves.

Data source

Bureau of Meteorology: monthly precipitation grids, Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class, AHGF stream network feature class, and water storages.

Data provider

Bureau of Meteorology

Method

Monthly precipitation data was produced by the Bureau. It was based on daily data from approximately 6500 rain gauge stations and interpolated to a 0.05 degrees (5 km) national grid (Jones et al. 2007).

The precipitation at each waterbody was estimated from the average of the grid-points within a 5 km radius of each water feature. The volume was then estimated by multiplying by the surface area of each waterbody. The average monthly surface area of the reservoirs was calculated from daily storage levels and capacity tables.

The AHGF surface water feature was used to estimate a static surface area for Moochalabra Dam and Lake Kununurra.

Precipitation on the connected surface water store included reservoirs and the regulated channel of the Lower Ord River between the Ord River Dam and Carltons Crossing. An estimate of 100 m was provided by the Western Australian Department of Water for the width of the Lower Ord River.

Uncertainty

Derived from modeled data. Ungraded.

Assumptions, approximations and caveats/limitations

• Monthly precipitation estimates were subject to approximations associated with interpolating the observation point data to a national grid.
• 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 use of the static AHGF waterbody feature class surface area is an approximation only. It represents the storage at total capacity and therefore likely results in an overestimation of precipitation on the storage. This data-set was only used for the small storages and therefore the impact of this approximation is likely to be small.
• 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 on the connected surface water store.

Data source

Western Australian Department of Water: Water Information (WIN) database.

Data provider

Western Australian Department of Water.

Method

Runoff into major reservoirs was calculated based on a water balance approach using measured data collected at each major reservoir. Runoff into a reservoir (inflow) is calculated using the following water balance equation:

Inflow = ΔS + O + E - Pb - T

Where:

ΔS is the change in storage
O is total outflow from the reservoir
E is evaporation from the reservoir
Pb is the volume of water pumped backed into the reservoir
T is the volume of water transferred into the reservoir from the water distribution system.

Total outflow (O) is calculated using the following equation:

O = Sp + Sc + D + R

Where:

Sp is spillage
Sc is scour
D is volume of water abstracted from the reservoir
R is volume of riparian releases from the reservoir.

The calculation of inflows to Lake Arygle is described in the following tables:

Lake Argyle

Uncertainty

Runoff to reservoirs is derived from measured and modelled data. Ungraded.

Assumptions, approximations and caveats/limitations

• Runoff into the reservoirs assumes that all inflows and outflows are adequately quantified.

Data source

Bureau of Meteorology: monthly precipitation grids, Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class, AHGF stream network feature class, and water storages.

Data provider

Bureau of Meteorology.

Method

Evaporation from the connected surface water store was estimated using monthly open water evaporation data produced by the Bureau. It is a Penman evaporation estimate based on daily gridded climate data and is available on a 0.05 degree (5 km) national grid. The Penman method estimates the evaporation that would occur from a small, open waterbody and assumes the evaporation does not modify the meteorology through evaporative cooling. It assumes aerodynamic conductance of 0.01 m/s and saturation deficit is estimated as (saturation vapour pressure at T_max) – (saturation vapour pressure at T_min).

As a potential evaporation data-set, it is an estimate of the evaporative demand of the atmosphere. The daily gridded climate data-sets are generated by the Bureau and include precipitation, downward solar irradiance, and maximum and minimum air temperatures. The methods used to generate these gridded data-sets are outlined in Jones et al. (2007).

The evaporation at each waterbody was estimated from the average of the grid-points within 5 km radius of each water feature. The volume was then estimated using the surface area of each waterbody. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables.

The AHGF surface water feature was used to estimate a static surface area for Moochalabra Dam and Lake Kununurra.

Evaporation from the connected surface water store included reservoirs and the regulated channel of the Lower Ord River between the Ord River Dam and Carltons Crossing. An estimate of 100 m was provided by the Western Australian Department of Water for the width of the Lower Ord River.

Uncertainty

Derived from modelled data. Ungraded.

Assumptions, approximations and caveats/limitations

• The Penman evaporation estimates are subject to approximations associated with interpolating the observation point data to a national grid, as detailed in Jones et al. (2007).
• The dynamic storage surface areas calculated from the levels and storage rating tables represent a monthly average and, therefore, will not capture changes that occur on a shorter temporal scale.
• The use of the static default AHGF waterbody feature class surface area was an approximation only. It represented the storage at capacity and thus likely resulted in an overestimation of evaporation from the storage. This default data-set was only used for the small storages and, therefore, the impact of this approximation is estimated to be limited.
• Defining the Ord River width as a static 100 m was an approximation only. It is likely that this width varies along the river section length and throughout the reporting period, but these variations were assumed to be minor and to have a limited influence on the total volume of evaporation from the connected surface water store.

Data source

Water Corporation: Operational Data Storage System database.

Data provider

Bureau of Meteorology.

Method

Instantaneous seepage data (L/s) was measured at Lake Argyle on a daily basis. The measurements are taken at the dam wall. These data are put into Hydstra and converted to daily volume data (ML) in order to determine the total annual seepage (in ML) at each reservoir. The total leakage is the aggregate of seepage for each monitoring site.

Uncertainty

Derived from measured data. Ungraded.

Assumptions, approximations and caveats/limitations

• Seepages are estimates only, based on measurements at seepage monitoring points below dam walls. Total leakage is based on the aggregation of seepages for each site at each dam. It is assumed that the dam seepage measuring points reflect the total seepage from the dams.
• These data were not quality coded. Significant ‘spikes’ in the data (i.e. where daily data were orders of magnitude different to daily data collected before and after the spike) were removed from the data-sets for the calculation of total seepage volume.

Data source

Western Australian Department of Water: Water Information (WIN) database.

Data provider

Western Australian Department of Water.

Method

a. Releases for hydro-electricity and subsequent downstream users

The volume of water released from Lake Argyle for hydro-electricty was estimated using the power output of the power station, measured at the terminals of the station’s generators. The volume is calculated using the difference in pressure between the inlet and outlet of the power station, and the flow rates through the power station.

b. Releases for the purpose of the environment (Spillway Creek)

The volume of water released from Lake Argyle to Spillway Creek was calculated based on the water level of Lake Argyle, and stream gauging in Spillway Creek.

c. Releases from the irrigation valves (used for recreation)

The volume of water released from Lake Argyle through the irrigation valves was calculated using hydraulic rating tables, and valve opening settings recorded by the dam operators.

d. Licenced abstraction

Direct abstraction under licence from Lake Argyleis metered.

Uncertainty

Ungraded

Assumptions, approximations and caveats/limitations

• Calculation of flow rates through the power station includes allowances for head losses in the tunnels, pipe work and turbines of the power station.
• Storage–volume curves represent specifically surveyed parts of the water storage, and may not reflect the storage–volume relationship across the entire storage.
• Water storages are subject to sedimentation and other physical changes over time, which, in turn, affects the accuracy of the storage–volume curves.

The volume disclosed in the Storage information (2,069 ML) represents the difference between the measured opening and closing balances of Lake Argyle, after physical inflows and outflows have been applied.

The unaccounted-for difference is calculated according to the table below.

The volume of unaccounted-for difference as a proportion of the total storage is less than 1%.