Evaporation is determined by measuring the amount of water evaporated from a free water surface exposed in a pan. Such evaporation depends upon a number of climatic elements mainly, temperature, humidity and wind.

Evaporation data are useful in water conservation studies and estimating a potential evapotranspiration for irrigation and plant growth studies. In Australia where surface water storage is vital over large areas, evaporation is an important element of the farm system.

Evaporation varies markedly with exposure of the instrument for example, sheltering from wind and shading pans cause local variations in measured evaporation of as much as 25%. Similarly, instruments near expanses of water such as coastal inlets, rivers, reservoirs or irrigation systems may record lower evaporation from the surrounding country due to local effects on climatic elements especially water vapour in the air.

Such reductions are about 5-10%. Annual average evaporation is shown in the diagram below:


In millimetres.

The above diagram shows that average annual evaporation ranges from more than 4 000 mm over central WA to less than 1 000 mm in alpine areas of southeast Australia and in much of Tasmania.