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 type of vegetation. ET is a large component of the water balance across Australia, because almost 90 per cent of precipitation (rainfall or in some cases hail or snow) is returned through ET to the atmosphere. Despite its importance, ET has until recently been very difficult (if not impossible) to measure directly and hence there is little available information available to quantify ET currently. Recent development of new technology using satellite imagery now allows evapotranspiration to be comprehensively measured in a cost-effective manner.
Evaporation rates vary depending on a number of factors including temperature, cloudiness, humidity and wind speed. The first map below shows the long term annual average evaporation rates across Australia, the second shows the long term annual average evapotranspiration (ET) rates. By comparison with annual average rainfall, in general the annual average potential evaporation rate across Australia far exceeds the rainfall (i.e. what could occur if water supply was unlimited).
Annual average potential evaporation rates across Australia (Source: Bureau of Meteorology)
Annual average areal actual evapotranspiration across Australia (Source: Bureau of Meteorology)
The map showing the annual average areal actual ET across Australia, is the actual amount of ET that occurs on average in the current landscape (e.g. vegetation cover and irrigation etc) and it is much lower than the potential ET. In general terms the difference between rainfall and actual ET is what becomes runoff to streams and rivers or recharge to groundwater.
The Bureau of Meteorology website provides further information on