South East Queensland
17.1 Evaporation from surface water

The volumetric value for the line item at the end of the 2011–12 year was 259,859 ML.

The evaporation from each of the storages listed in line item 1.1 Storages can be seen in the following table.

Notes:

¹ Evaporation calculated using variable surface area

² Evaporation not calculated as surface area of the storage was not available

³ Evaporation calculated using static surface area.

– = data not available

National Climate Centre (NCC) daily climate grids (rainfall, temperature and solar radiation); Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class. Australian Water Resources Information System (AWRIS) water storage.

Bureau of Meteorology (the Bureau).

The potential evaporation estimate produced by the Australian Water Resources Assessment system Landscape model (AWRA-L) version 2.0.0 (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 avialable on a 0.05⁰ (approximately 5 km) national grid.

Potential evaporation is an estimate of the evaporative demand of the environment. The daily gridded climate datasets used to produce this estimate are generated by the Bureau of Meteorology and include 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 from all three of these larger storages was calculated using the dynamic surface area ie. the relationship between level and surface area. Evaporation was estimated from the proportionally weighted average of grid points that intersected each water feature. The volume was then estimated using the monthly average surface area of each waterbody. The surface area varied dynamically with changing water storage level for water storages where the relationship between storage level and surface area had been derived.

The surface area of most of the storages in the SEQ region was calculated using this dynamic method. Storage rating table data and regularly updated storage level data were not available for some storages, so static surface areas were used. The surface area for Cedar Grove and MountCrosby weirs was provided by Seqwater. For LakeCressbrook, LakePerseverance and Splityard Creek Dam, the AHGF waterbody feature class was used to estimate a static surface area. Surface areas were not available for Bromelton Off-Stream Storage or Wyaralong Dam. For the SEQ region, the evaporation from the surface water store included evaporation from storages only.

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 timescale.

• The use of the static default AHGF surface area is an approximation only. It represents the lakes at capacity and therefore likely results in an overestimation of precipitation on the lakes.

The uncertainty estimate was not quantified.

There was a change in the modelling methods for the 2012 Account. As a result, the 2010–11 year volume was recalculated using the new modelling method.

Modelling method change was due to selecting AWRA-L method in the 2012 Account instead of Priestly and Taylor potential evaporation method used for the 2011 Account. The change was made for better estimate of evaporation. Resulting volume change due to the method change for the 2010–11 year was decrease of the volume by 21,167 ML which represents a change of 7% of the originally published volume.