South East Queensland

Data source

Bureau of Meteorology: National Climate Centre daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric waterbody feature class and Australian Water Resources Information System water storages.

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 degrees (5km) national grid. The Penman method estimates the evaporation that would occur from a small open waterbody and assumes that the evaporation does not modify the meteorology through evaporative cooling. It assumes aerodynamic conductance of 0.01m/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. It is based on daily gridded climate data-sets, including precipitation, downward solar irradiance, and maximum and minimum air temperature. 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 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.

The surface area of most of the storages in the South East Queensland (SEQ) region was calculated using this dynamic method. Storage-rating table and regular-level data were not available for all of the storages, so static surface areas were used. Cedar Grove and Mount Crosby weirs were not included in the evaporation calculation as surface area data was not available. For Lake Cressbrook, Lake Perseverance and Splityard Creek Dam, the waterbody feature class was used to estimate a static surface area. In the SEQ region, the evaporation from the connected surface water store included evaporation from storages only.

Uncertainty

Ungraded.

Approximations, assumptions, caveats and limitations

• The Penman evaporation estimates are subject to approximations associated with interpolating the observation point data to a national grid.
• 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 waterbody feature class surface area is an approximation only. It represents the storage at capacity and therefore probably results in an overestimation of evaporation from the storage. This default data-set was only used for the small storages; therefore, the impact of this approximation is estimated to be small.