The variable

The rate of evaporation is controlled by the relative humidity and temperature of the air, the amount of net radiation, the wind speed at the surface, the amount of water available, the nature of the surface (for example its roughness) and the type of vegetation. Open water presents another situation, as do ice and snow. The net incoming energy is apportioned to the three fluxes – sensible, latent and soil heat – according to the infinite variety and combination of circumstances.

It is much more difficult to sense the rate of loss of water from the surface through evaporation – the latent heat flux – than it is to measure the gain of water through precipitation. Nevertheless, the rate of evaporation is expressed in the same units as precipitation: it is the equivalent depth of liquid water lost into the atmosphere as water vapour, expressed in millimetres lost over an hour or a day.

Measuring and estimating evaporation

Evaporation can be measured either as the loss of liquid water from the surface or as the gain of water vapour by the atmosphere, but few of the methods involve a direct measurement, most inferring the amount by indirect measurement.