ABSTRACT
The evaporation process involves the transfer of water from a liquid state into a gaseous form in the atmosphere. For this to happen requires an available energy source, a water supply and the ability of the atmosphere to receive it. We introduce the fundamental processes involved in evaporation. We show that evaporation is difficult to measure directly, and we discuss various estimation techniques. These range from water budget techniques, such as evaporation pans and lysimeters, to modelling techniques, such as the Penman-Monteith equation. We note that as a process, evaporation suffers from the same problems with measurement and estimation as does precipitation (i.e. extreme variability in space and time). This variability leads to difficulties in moving from point measurements to areal estimates such as are required for a catchment study. These can be overcome by using spatial averaging techniques or using evaporation estimations that assume a wide area over which it is calculated (e.g. Priestly-Taylor). Vegetation can play an important part in determining the evaporation rate at a site which then brings in the added complication that plants are living organisms that change in structure through their life cycle and are able to respond in different ways to biophysical stresses placed on them. Therefore the age, stage and type of vegetation at a particular site will influence the amount of evaporation occurring there.
