17.1 Evaporation from surface water
The volume presented in the water accounting statements (42,190 ML) represents evaporation from the surface of water storages.
The following table presents the breakdown of evaporation from water storages in the Melbourne region during the 2011–12 year.
|Storage operator||Storage||Volume (ML)|
|Melbourne Water||Cardinia Reservoir||10,245|
|Upper Yarra Reservoir||5,535|
|Yan Yean Reservoir||5,237|
|Southern Rural Water||Melton Reservoir||1,935|
|Pykes Creek Reservoir||1,864|
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 available on a 0.05 degree (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 radiation, 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 proportionally weighted average of grid-points cells 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 reservoirs where the relationship between storage level and surface area had been derived. In the Melbourne region, the surface area of all water storages was calculated dynamically.
Assumptions, limitations, caveats and approximations
- 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 total surface area of the surface water store within the Melbourne region included only the water storages not river channels.
- Melbourne Water measures evaporation using the factored pan method. Melbourne Water's calculations of evaporation from water storages during the 2011–12 year were approximately 17% lower than modelled estimates by the Bureau. Melbourne Water evaporation estimates from water storages they managed were shown in the following table:
|Storage||Evaporation volume (ML)
|Evaporation volume (ML)
|Upper Yarra Reservoir||5,535||5,129|
|Yan Yean Reservoir||5,237||4,170|
In addition, data available for the evaporation calculation have improved since the 2011 Account publication, due to a change in modelling methods. The AWRA-L potential evaporation was used instead of the Priestly and Taylor potential evaporation method.
The error and updated modelling approach has been applied retrospectively to the 2010–11 year volume. Consequently, the volume has been restated from 35,364 ML (as published in the 2011 Account) to 36,087 ML, as shown in the following table.
|Storage operator||Storage||2012 Account volume for
the 2010–11 year (ML)
|2011 Account volume for
the 2010–11 year (ML)
|Melbourne Water||Cardinia Reservoir||7,898||7,532|
|Upper Yarra Reservoir||5,051||6,754|
|Yan Yean Reservoir||4,638||4,731|
|Southern Rural Water||Melton Reservoir||1,857||666|
|Pykes Creek Reservoir||1,570||519|