Ord

The volume recognised in the water accounting statements (635,742ML) represents the volume of rainfall that fell on surface water storages in the Ord region during 2009–10.

The breakdown of rainfall on 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

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 modelled 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.