9.1 Precipitation on surface water

Supporting Information

The volume recognised in the water accounting statements (1,485,020 ML) represents the volume of rainfall that fell on surface water storages in the Ord region during the 2010–11 year.

Rainfall in the 2009–10 year was below average. Rainfall in the 2010–11 year was above average. More information is provided in the contextual statement. 

The breakdown of rainfall on surface water storages is provided in the table below.


Rainfall on storages in the Ord region for the 2010–11 year


Rainfall (ML)


Rainfall (ML)


Lake Argyle



Lake Kununurra



Arthurs Creek Dam



Lower Ord River



Moochalabra Dam









Quantification Approach

Data Source

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

Provided by

Bureau of Meteorology.


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 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, Arthurs Creek Dam and Lake Kununurra.

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 Western Australian Department of Water for the width of the Lower Ord River.

Assumptions, Limitations, Caveats and Approximations

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 surface water store.

Uncertainty Information

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

Comparative year

The following table compares the value for the 2009–10 year reported in this account with the corresponding value reported in the 2010 Account.

Comparative information from the 2010 Account for line item 9.1

2011 Account line item

2010 Account line item

Value reported in the 2010 Account (ML)

Value reported in the 2011 Account (ML)

Additional information

9.1 Precipitation on surface water

13.1 Precipitation on connected surface water



Change in line item name and number. Change in scope

The precipitation on surface water during the 2009–10 year was restated as Arthur Creek Dam was included and because of improvements in the methodology of calculating precipitation on surface water.