42.1 Evaporation from storages
|Storage||Volume 2010–11 (ML)||Volume 2009–10 (ML)|
|Arthurs Creek Dam||22,949||25,696|
Evaporation from storages was estimated using monthly open water evaporation data produced by the Bureau. It is a Penman evaporation estimate based on daily gridded climate data and is available on a 0.05 degree (5 km) national grid. The Penman method estimates the evaporation that would occur from a small, open waterbody and assumes the evaporation does not modify the meteorology through evaporative cooling. It assumes aerodynamic conductance of 0.01 m/s and saturation deficit is estimated as (saturation vapour pressure at Tmax) – (saturation vapour pressure at Tmin).
As a potential evaporation dataset, it is an estimate of the evaporative demand of the atmosphere. The daily gridded climate data-sets are generated by the Bureau and include precipitation, downward solar irradiance, and maximum and minimum air temperatures. The methods used to generate these gridded datasets are outlined in Jones et al. (2007).
The evaporation at each waterbody was estimated from the average of the grid points within 5 km radius of each water feature. The volume was then estimated using the surface area of each waterbody. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables.
The AHGF surface water feature was used to estimate a static surface area for Moochalabra Dam and Lake Kununurra.
Evaporation from the connected surface water store included reservoirs and the regulated channel of the Lower Ord River between the Ord River Dam and Carltons Crossing. An estimate of 100 m was provided by the Western Australian Department of Water for the width of the Lower Ord River.
Assumptions, Limitations, Caveats and Approximations
- The Penman evaporation estimates are subject to approximations associated with interpolating the observation point data to a national grid, as detailed 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 temporal scale.
- The use of the static default AHGF waterbody feature class surface area was an approximation only. It represented the storage at capacity and thus likely resulted in an overestimation of evaporation from the storage. This default dataset was only used for the small storages and, therefore, the impact of this approximation is estimated to be limited.
Derived from modelled data. The uncertainty estimate was not quantified.