Rights: The University of Waikato Published 15 April 2009 Download

Dave Campbell explains that evaporation occurs when water changes from a liquid state to a gaseous state. It can happen anywhere there is water – in the soil, lakes, oceans and plants. When it occurs in plants, water is lost through microscopic pores in the plant’s leaves (stomata). This process is called transpiration. Transpiration differs from evaporation not only because it occurs in plants, but also because the plants have some control over how much water they lose. Plants can actively open and close their stomata, limiting how much water the plant will lose.

Points of interest: Think about what affects the rate of transpiration.


Evaporation is defined as the physical process by which water changes state from a liquid to a gas, so wherever there is liquid water – whether it’s in the oceans, or in a river, or a lake, or whether it's in the soil, or within plants – that liquid water can be turned to water vapour, and those water vapour molecules can move into the atmosphere.

Transpiration is just a subset of the evaporation process, and it’s the water that is inside plants, and then when that evaporates and moves into the atmosphere, that process is called transpiration. And it’s very much part of the living processes of the plant. It’s all occurring within a cycle – and I guess when it comes to plants, we talk about a daily cycle – and that cycle is driven by the sun. Obviously at night time, there is no solar radiation, so there is a lot less energy for driving evaporation or transpiration. And during the middle part of the day, around noon, we've got the maximum amount of solar radiation, so there is more energy, so you tend to get highest evaporation rates during the middle part of the day. Plants are a little bit different. They are active, actively involved – the microscopic pores on the surface of plant leaves, called stomata, are very active in restricting or regulating water loss, so that is another factor that comes in.

Dr David Campbell, Waikato University

Evaporating lake image by Michael Shake, licensed through 123RF Ltd
Geothermal steam still image by Kavram, licensed through 123RF Ltd
Steam on road, Dance Floor Lizard
Atmosphere image courtesy of Aerospaceweb.org