Ord
17.1 Evaporation from surface water

Supporting Information

The volume recognised in the water accounting statements (2,080,765 ML) represents water that evaporated from surface water storages in the Ord region during the 2010–11 year.

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

Evaporation from various sources in Ord region during the 2010–11 year

Storage

Evaporation (ML)

201011

Lake Argyle

2,010,268

Lake Kununurra

34,545

Arthur Creek Dam

22,948

Lower Ord River

12,797

Moochalabra Dam

207

Total

2,080,765

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.

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 Tmax) – (saturation vapour pressure at Tmin).

As a potential evaporation dataset, it is an estimate of the evaporative demand of the atmosphere. The daily gridded climate datasets are generated by the Bureau and include precipitation, downward solar irradiance, and maximum and minimum air temperatures. The methods used to generate these gridded datasets 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, Lake Kununurra and Arthur Creek Dam.

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.

Assumptions, Limitations, Caveats and Approximations

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 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 17.1

2011 Account line item

2010 Account line item

Value reported in the 2010 Account (ML)

Value reported in the 2011 Account (ML)

Additional information

17.1 Evaporation from surface water

14.1 Evaporation from connected surface water

2,505,866

2,179,465

Change in method and extra storage considered

The volume of evaporation from Arthur Creek Dam in the 2009–10 year was estimated as 25,696 ML.