Melbourne

The volume recognised in the water accounting statements (44,289 ML) represents evaporation from the surfaces of the reservoirs listed in the following table.

See ‘Approximations’ section, below, for further information. Note that Melbourne Water uses a different methodology to calculate evaporation resulting in different estimates to those provided above.

Data source

Bureau of Meteorology, National Climate Centre (NCC): version 3 daily rainfall grids; daily maximum temperature grids; daily minimum temperature grids; daily satellite observed solar radiation grids; Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; Australian Water Resources Information System (AWRIS) water storage.

Data provider

Bureau of Meteorology.

Method

Evaporation from the connected surface water store was estimated using monthly open water evaporation data produced by the Bureau of Meteorology. 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 data set, it is an estimate of the evaporative demand of the atmosphere and is based on daily gridded climate data. The daily gridded climate data-sets are generated by the Bureau of Meteorology and include precipitation, downward solar irradiance, and maximum and minimum air temperature. The methods used to generate these gridded data sets are outlined in Jones et al. (2007a).

Evaporation from waterbodies was estimated using the 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. In Melbourne region, evaporation from connected surface water store includes only that from these major storages.

Uncertainty

Ungraded, but see comparison with figures derived using an alternative method below.

Approximations, assumptions, caveats/limitations

• The Penman evaporation estimates are subject to approximations associated with interpolating the observation point data to a national grid 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.
• Melbourne Water adopts different evaporation information for their storages. The following table describes the evaporation values adopted by Melbourne Water at each site. The methodology used by Melbourne Water is based on measured pan evaporation that is multiplied by monthly pan coefficient factors. The average level of each storage is accommodated for each month. Note that Melbourne Water values are significantly less than the Bureau of Meteorology values. The pan evaporation approach is a ‘point’ method, rather than an ‘areal’ method. These figures are provided below in the interests of full disclosure and to help decision making.