Ord
17.1 Evaporation from surface water

The volume recognised in the water accounting statements (1,488,640 ML) represents water that evaporated from surface water storages in the Ord region during the 2012–13 year.

The total evaporation from each storage within the Ord region during the 2012–13 year is provided in the following table.

Evaporation from surface water storages in the Ord region during the 2012–13 year

Storage	Evaporation (ML)
Arthur Creek1	18,031
Lake Argyle	1,430,393
Lake Kununurra	29,897
Lower Ord River	9,923
Moochalabra	396
Total	1,488,640

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

Daily climate grids (temperature and solar radiation), Australian Hydrological Geospatial Fabric—waterbody feature class and stream network feature class, Australian Water Resources Information System—water storages.

Bureau of Meteorology.

The potential evaporation estimate produced by the Australian Water Resources Assessment system Landscape model (AWRA-L) version 3.0 (Van Dijk 2010) was used to calculate evaporation from the surface water store. The AWRA-L model uses a modified version of the Penman-Monteith method to produce the potential evaporation. Daily AWRA-L potential evaporation grids were produced based on daily gridded climate data that were available on a 0.05 degree (approximately 5 km) national grid.

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 proportionally weighted average of grid cells that intersected 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 where data were available. Where daily storage level data were unavailable (i.e., Arthur Creek storage), a static surface area value from the Australian Hydrological Geospatial Fabric was used.

Evaporation from the connected surface water store included reservoirs and the regulated channel of the lower Ord River between Lake Kununurra and Carltons Crossing. An estimate of 100 m was provided by the Department of Water for the width of the lower Ord River.

The AWRA-L potential evaporation estimates are subject to approximations associated with interpolating the observation point input data to a national grid as described 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. 

Dynamic storage surface area data are not available for Arthur Creek. Therefore, the Australian Hydrological Geospatial Fabric surface water feature was used to estimate a static surface area for Arthur Creek. This represents the storage at total capacity and, therefore, is likely to result in an over-estimation of evaporation from the storage.

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.

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

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