Ord

The volume recognised in the water accounting statements (2,505,866 ML) represents the volume of water that evaporated from surface water storages in the Ord region during 2009–10.

The breakdown of evaporation from surface water storages is provided in the table below.

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.