South East Queensland
17.1 Evaporation from surface water

The volumetric value for the line item at the end of the 2012–13 year was 279,163 ML.

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

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 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 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 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 South East Queensland (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 Mount Crosby weirs was provided by Seqwater. For Lake Cressbrook, Lake Perseverance, 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 prior period error in the line item volume reported in the 2012 Account.  The prior period error was a result of using solar radiation data with bias correction errors. Solar radiation is an input for line item volume calculation.  The line item volume for the 2011-12 year has been recalculated accommodating the prior period error correction. As a result, the volume for the 2011-12 year has been changed from 252,957 ML reported in the 2012 Account to  245,731 ML in the 2013 Account.