19.1 Evaporation from urban water
The volume reported (3,289 ML) represents evaporation from urban water storages within the Adelaide region during the 2010–11 year. Evaporation, itemised by each urban water storage, is provided in the following table.
|Urban water storages||Volume (ML)|
|Happy Valley Reservoir1||1,893|
|Hope Valley Reservoir1||581|
|Onkaparinga Summit Reservoir2||116|
1 Dynamic surface area used
2 Static surface area used
Potential evaporation is an estimate of the evaporative demand of the environment. The Priestly and Taylor method was used to estimate potential evaporation (as calculated by the WaterDyn model (Raupach et al. 2008)) from surface water storages and weirs.
This method used monthly, open water evaporation data produced by the Bureau of Meteorology. These data are based on daily gridded climate data that are available on a 0.05 degree (5 km) national grid and included downward solar irradiance, and maximum and minimum air temperature. The methods used to generate these gridded datasets are outlined in Jones et al. (2007).
Evaporation at each waterbody was estimated from the proportionally weighted average of grid-points that intersected each storage or weir (water feature). The volume was then estimated using the monthly average surface area of each waterbody. The surface area varied dynamically with changing reservoir storage level where the relationship between storage level and surface area had been derived.
Assumptions, Limitations, Caveats and Approximations
- The Priestly and Taylor 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.
- Evaporation was only estimated for the urban water storages within the Adelaide region and did not include surface water storages, weirs and rivers.
This line item corresponds to line item 14.1 Evaporation from connected surface water reported in the 2010 Account. The comparative year volume was restated because:
- Reclassification: In the 2011 Account, the Bureau of Meteorology reclassified major storages located off-channel as part of line item 3.1 Urban water supply system to provide clearer information to users. The criteria for this reclassification required that storages were located offstream, harvested minimal runoff (due to a diversion channel around the perimeter of the storage) and were used only for urban water supply. Therefore evaporation from those storages reclassified as being part of the urban water system were reported at this line item.
- Prior period error correction: In 2010 a bias correction applied to the satellite-derived solar radiation dataset was incorrect resulting in an overestimation of incoming solar radiation. When the bias correction was updated and evaporation was re-estimated, the comparative year volume reported for evaporation decreased.
- Change in methodology: In 2010, the Penman open water evaporation method was used to estimate evaporation. In 2011 the Priestly and Taylor potential evaporation method was used to estimate evaporation, which resulted in a decrease to the comparative year volume reported for evaporation.
Restatement of the value published in the 2010 Account was made as the difference was material and increased the accuracy and comparability of the information provided to the users of the National Water Account. It was not possible to separate the change in volume due to error corrections, reclassification and changes in methodology. The changes and their respective volumes are detailed in the following table. The restated comparative year volume is 3,826 ML
|2010 Account line item name||Source||Volume at 30 June 2010 reported in 2010 Account (ML)||Value difference to volume reported due to error correction, method change and reclassification (ML)||Volume at 30 June 2010 reported in 2011 Account (ML)|
|14.1 Evaporation from connected surface water||Barossa Reservoir||820||820|
|Happy Valley Reservoir||2,191||2,191|
|Hope Valley Reservoir||676||677|
|Onkaparinga Summit Reservoir||138||138|