The rate of evaporation is in excess of 2 metres per year over most of Australia’s area and mean rainfall in Australia is less than 500mm per year and falling.  On such a hot dry continent, it has been estimated that up to 95% of the rain which falls in Australia is re-evaporated and does not contribute to run-off.

Water, when harvested is commonly stored in small storages and dams, but it is estimated that up to half of this may be lost due to evaporation.  This represents a huge waste of our resource.  The price and value of water increasing dramatically and the scarcity of water is the main limiting factor working against agricultural production in Australia.

Water loss from storage dams can firstly be managed by increasing their depth.  Wind breaks can also be used in certain circumstances, but their overall effect in reducing evaporation is likely to be small, as solar radiation rather than wind is the key driver of evaporation.  With increasing value of water, a realistic management option is to invest in a cover over the dam to reduce evaporation.

The definitions of evaporation below are based on those given by Morton (1983) - Taken from the Bureau of Meteorology website:-

Areal actual ET* - This is the ET that actually takes place, under the condition of existing water supply, from an area so large that the effects of any upwind boundary transitions are negligible and local variations are integrated  to an areal average.

Areal potential ET* - This is the ET that would take place, under the condition of unlimited water supply, from an area so large that the effects of any upwind boundary transitions are negligible and local variations are integrated to an areal average.

Point potential ET* - This is the ET that would take place, under the condition of unlimited water supply, from an area so small that the local ET effects do not alter local air mass properties.  It is assumed that latent and sensible heat transfers within the height of measurement are through convection only.

Further information on evaporation and dam covers available in Loss of Water Through Evaporation with Particular Reference to Arid and Semi-Arid Zone Pastoralism in Australia  Review compiled by Ian P Craig USQ
(information on this page extracted from review)

Link to Bureau of Meteorology Climate and evaporation information:
http://www.bom.gov.au/climate/averages/#skip

(ET*) Evapotranspiration: the water lost to the atmosphere by two processes-evaporation and transpiration. Evaporation is the loss from open bodies of water, such as lakes and reservoirs, wetlands, bare soil, and snow cover; transpiration is the loss from living-plant surfaces.

 Evaporation control devices trialled in South Australia and Western Queensland:

E -Vap Cap from project partner Darling Downs Tarpaulins

This patented system of  evaporation control is based around a floating cover made from light-impervious polyethylene sheeting that is environmentally benign.It will minimise evaporation from large water storages.According to independent university testing E-VapCap ® effectively stops 90%+ of the water vapour loss to the atmosphere ( up to 100%).

Project Partner AgFloat

AgFloat is a new product, made from recycled waste products (truck and car tyres), that significantly reduces evaporation from dams and other agricultural waterways.