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  • Rights: The University of Waikato
    Published 15 April 2009 Referencing Hub media
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    In this video, David Hamilton explains what time and space means for the water cycle1 by taking a closer look at Lake Rotorua.

    Lake Rotorua’s water quality is the result of what happened to the water that infiltrated the land around the lake – the catchment2 area – a long time ago.

    Point of interest
    Think about the factors we need to consider in lake restoration projects.

    Nature of Science

    Scientists sometimes work with hypothetical examples, to ask ‘what if’ questions. Using models allows them to experiment without having an impact on the natural environment.

    Transcript

    PROF DAVID HAMILTON
    A good example to consider time and space in lakes is to think about water in Lake Rotorua. Lake Rotorua has an area of about 80 square kilometres. About 80% of that water or more comes from groundwater3 that supplies streams that pop up out of the ground just before they enter the lake, and that water has been often underground for a long period of time, and the largest of the Rotorua Lake inflows is the Hamurana stream. That stream flows at 3 cubic metres4 per second – that is a huge amount of water – 3,000 litres per second coming into Lake Rotorua from one source. And it’s been under the ground for 140 years. In 1991, the wastewater from Rotorua City that went into the lake was taken out. Everybody thought at the time that that would solve the problem of algal blooms5 in Lake Rotorua. 20 years on, we still had the same problem. Why did we have that problem? It was actually because the groundwater is slowly becoming enriched in nutrients6, that groundwater is feeding into Lake Rotorua, and all of the benefits of taking the wastewater and the nutrients out in 1991 had been lost because they had been replaced by the effects of that older aged groundwater that now reflects more intense land use in the catchment of the lake.

    1. water cycle: The continuous movement of water on, above and below the surface of the Earth (also known as the hydrological cycle).
    2. catchment: An area that collects all the water that drains to a particular lake, river or reservoir. Also known as a watershed or a drainage basin.
    3. groundwater: Water located beneath the Earth’s surface in soil spaces and in fractures of rocks.
    4. metre: The base unit of length in the International System of Units (SI).
    5. algal bloom: A rapid increase or accumulation in the population of algae in an aquatic system.
    6. nutrient: A substance that provides nourishment for growth or metabolism.
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      water cycle

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    2. The continuous movement of water on, above and below the surface of the Earth (also known as the hydrological cycle).

      metre

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    4. The base unit of length in the International System of Units (SI).

      catchment

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    6. An area that collects all the water that drains to a particular lake, river or reservoir. Also known as a watershed or a drainage basin.

      algal bloom

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    8. A rapid increase or accumulation in the population of algae in an aquatic system.

      groundwater

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    10. Water located beneath the Earth’s surface in soil spaces and in fractures of rocks.

      nutrient

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    12. A substance that provides nourishment for growth or metabolism.