Average annual & monthly evapotranspiration

At a glance

These evapotranspiration maps show the average annual and average monthly evapotranspiration distributed across Australia.

What do the maps show?

Evapotranspiration (ET) is a collective term for the transfer of water, as water vapour, to the atmosphere from both vegetated and un-vegetated land surfaces. It is affected by climate, availability of water and vegetation.

Areal actual ET 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. For example, this represents the evapotranspiration which would occur over a large area of land under existing (mean) rainfall conditions.

Areal potential ET 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. For example, this represents the evapotranspiration which would occur over a very large wetland or large irrigated area, with a never-ending water inflow. A "large" area is defined as an area greater than one square kilometre.

Point potential ET 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. For example, this represents the evapotranspiration which would occur from small irrigated fields with a never-ending water inflow, surrounded by unirrigated land. Point potential ET may be taken as a rough preliminary estimate of evaporation from small water bodies such as farm dams and shallow water storages.

The annual actual evapotranspiration map shows that evapotranspiration is least in areas of south-central Australia. In these areas there is very little moisture and cooler temperatures. This leads to a low rate of evapotranspiration. In coastal areas, particularly in northern Australia, greater moisture levels and warmer temperatures lead to higher rates of evapotranspiration.

How is evapotranspiration calculated?

Evapotranspiration is calculated based on the transfer and balance of energy in the environment. Temperature, vapour pressure and solar global exposure are all used as inputs in the calculation. Mortons complementary relationship was used to derive the values in these maps.

Further information

• About the maps - metadata and related information.
• Find out how climate statistics are calculated.
• Information about the range of gridded climate data we can provide.
• Information about additional climate products and other data available from the Bureau of Meteorology.
• Learn about the influences on the Australian climate.
• Explore other topics relating to climate education.
• Learn about the calculation of evapotranspiration in more detail.