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  • Rights: The University of Waikato
    Published 15 April 2009 Referencing Hub media

    In this video, four New Zealand scientists – Dave Campbell, Louis Schipper, David Hamilton and Keith Hunter – talk about how only a small percentage of the Earth’s water is freshwater, and of that small percentage, only a fraction is available for human use. Most of the freshwater we can readily use is contained in lakes. This water is available for us to use again and again because, as water moves through the four Earth subsystems, it is being recycled.

    A specific focus in this video is the process of evaporation. Dave and Keith talk about evaporation on land and in the ocean and about the drivers of evaporation.

    Louis talks about what happens when water disappears into the ground – scientists describe this as infiltration.

    David talks about how, even in a lake that seems still and tranquil, dynamic processes occur that help water move along its way in the water cycle

    Points of interest
    When you watch this video, think about where most of the evaporation occurs. Where does most of the rainfall reach the Earth? Think about the reason for this.

    How much of the solar radiation that reaches the planet is used for evaporation?


    The Earth has a finite amount of water, and only a small amount is available to be readily used by living things.

    Most of the freshwater on the Earth is locked up in glaciers. There is actually only a very, very small percentage that is freshwater as we know it, and of that freshwater, there is only actually only a tiny amount that is in streams. So the majority of that freshwater – the freshwater that we essentially need – is contained in lakes, and so lakes act as a very, very important reservoir for freshwater.

    The reason why freshwater is not used up is that water is constantly recycled through the Earth's system by a process called the water cycle. The water cycle includes a number of processes that circulate water through the Earth's subsystems.

    Well the main way the ocean circulates water is through evaporation and then condensation, which takes place in the atmosphere, and most of the evaporation of ocean water occurs near the equator, because that is where you get the most intense solar radiation, and you've also got an enormous amount of water vapour coming up from the ocean. And then the atmospheric winds move that water vapour towards higher latitude, and of course the air then gets colder, and so as the air cools down, it can't hold as much water vapour any more and so it falls out, and you get rain, or even at really low temperature, you get snow.

    When water is circulated, it often goes through physical and chemical changes. Each change requires energy. Evaporation is one of these processes where water undergoes change.

    Evaporation is the physical process by which water changes state, or changes phase. And it’s an energy intensive process, and approximately half of the solar radiation that reaches the surface of the planet is used in that one process. Wherever there is liquid water – whether it’s in the oceans, or whether it’s in the soil as soil water, or in a river, or a lake, or within plants – that liquid water can be turned to water vapour, and those water vapour molecules can move into the atmosphere.

    These hydrospheric processes occur continually over time, but often at different rates, and in different places. An example of another water cycle process is infiltration.

    Infiltration is the rate at which the water can move into the soil through those pores at the surface. Some of those pores will be connected all the way through, some might be blocked off with little soil particles which will slow water flow down through them.

    Prof. David Hamilton, Waikato University
    Prof. Keith Hunter, Otago University
    Dr David Campbell, Waikato University
    Prof. Louis Schipper, Waikato University
    Flyover NZ/Rotorua Lakes model by Mathew Allan
    Franz Josef Glacier image by ocwo, licensed through 123RF Ltd
    Globe image and animation, courtesy of NASA
    Snow by road image, Pun Yan Lit
    Evaporating lake image by Michael Shake, licensed through 123RF Ltd
    Grass and soil cross-section image by Richard Thomas, licensed through 123RF Ltd

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