9.1 Precipitation on surface water

Supporting information

The volume recognised in the water accounting statements (756,582 ML) represents the volume of rainfall that fell on surface water storages in the Ord region during the 2011–12 year.

The total volume of rainfall on each storage during the 2011–12 year within the Ord region is provided in the following table.


Total rainfall on each storage within the Ord region during the 2011–12 year


Rainfall (ML)

Lake Argyle


Lake Kununurra


Arthurs Creek Dam1


Lower Ord River


Moochalabra Dam



  1.   Precipitation calculated using a static surface area (see 'Quantification approach')

Quantification approach

Data source

Monthly precipitation grids, Australian Hydrological Geospatial Fabric—waterbody feature class and stream network feature class, Australian Water Resources Information System—water storages.

Provided by

Bureau of Meteorology.


Monthly precipitation grids were produced using 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 proportionally weighted average of grid-cells that intersected each water feature. The volume was then estimated by multiplying 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. Where daily storage level data were unavailable (i.e. Arthurs Creek Dam), a static surface area value from the Australian Hydrological Geospatial Fabric was used.

Precipitation on the surface water store included storages 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 Department of Water for the width of the Lower Ord River.

Assumptions, limitations, caveats and approximations

The precipitation estimates were 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 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 Arthurs Creek Dam. Therefore, the Australian Hydrological Geospatial Fabric surface water feature was used to estimate a static surface area for Arthurs Creek Dam. This represents the storage at total capacity and, therefore, likely results in an overestimation of precipitation on the storage.

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 surface water store.

The total surface area of the surface water store within the Ord region did not include unregulated rivers.

Uncertainty information

Derived from modelled data. The uncertainty estimate was not quantified.