Adelaide
17.1 Evaporation from surface water

The volume reported (17,433 ML) represents evaporation from surface water storages and weirs within the Adelaide region during the 2011–12 year. Evaporation, itemised by each surface water storage and weir, is provided in the following table.

^1–11 Evaporation calculated using dynamic surface area.
^12–15 Evaporation calculated using static surface area.

Bureau of Meteorology: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 storages.

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 0.05⁰ (approximately 5 km) national grid.The daily gridded climate datasets used to produce this estimate were 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).

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 storage level where the relationship between storage level and surface area had been derived. In the Adelaide region, the surface area of the storages was calculated dynamically and the surface area of the weirs were a static value produced from the AHGF.

• 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 evaporation from the lakes.
• Evaporation was only estimated for the surface water storages and weirs (for which data were available) within the Adelaide region and did not include river channels.

In the 2011 Account, storages that were used for urban water supply only  (located off-stream and harvested only minimal catchment runoff) were reported at line item 3.1 'Urban water system'. The storages were Barossa Reservoir, Happy Valley Reservoir, Hope Valley Reservoir and Onkaparinga Summit Reservoir.

In the 2012 Account, these storages were reported at line item 1.1 Storages in order to be consistent with the classification of storages applied to the other regions of the National Water Account.

The quantification method was improved in the 2012 Account compared to the 2011 Account as the AWRA-L version 2.0.0 was used instead of the WaterDyn Priestly-Taylor method. When applied to the 2010–11 year, the difference in volume produced by both methods (4%) was not, however,  material.

Consequently, the 2010–11 volume of line item 17.1 'Evaporation from surface water' was only restated in order to include precipitation on the storages that were previously considered part of the urban water system, as shown in the following table.