Rights: The University of Waikato Published 15 April 2009 Download

In this video, 4 New Zealand scientists – Dave Campbell, Louis Schipper, David Hamilton and Keith Hunter – talk about what it means if we say that the water cycle is dynamic and changing, highlighting variables like time and space.

The scientists point out that changes can sometimes take very long periods of time.

Keith explains that sometimes water will be stored for long periods, such as in a lake or in the groundwater. When water is stored like this, we call it a reservoir. The largest reservoir is the Earth’s oceans, which cover two-thirds of the Earth’s surface. How long water stays in a reservoir depends on factors like the size of the reservoir and movement within it, and in and out of the reservoir. How long water stays in a reservoir is called residence time.

Louis explains that sometimes not only does the water move, but it can carry nutrients with it too. Water moving through the soil will take soil particles and any chemicals with it, and these can then enter the groundwater.


The water cycle is dynamic, but this doesn't mean that all water is moving all the time. Water can be stored in so called reservoirs. This includes, for example, lakes, icebergs and the ocean.

The role the ocean plays in water storage is the dominant one, because they contain virtually all the Earth's water – 98.5%. The spatial dimensions of the ocean are very interesting. On the one hand, you know, the ocean is only about on average 3.5 kilometres [deep], and that is typically the distance from home to the next dairy, and yet it’s huge in the horizontal sense because it covers two-thirds of the Earth, and so it’s spread out.

Water can be stored in reservoirs for different lengths of time. Different variables have an effect, like, for example, the size of the reservoir, and the in and out flow. Water can even be stored in the atmosphere

How long the water stays in the atmosphere introduces the concept of a residence time really, and scientists talk about residence time as applied to the ocean, or to lakes, or to the atmosphere – in other words, how long on average does a water molecule stay within the ocean, or a lake, or the atmosphere. The atmosphere contains only a very small proportion of the water on planet Earth – about 0.001% – and therefore the residence time of water in the atmosphere tends to be quite short. If all of the water in the atmosphere rained out and it fell to the surface, we would have enough to supply precipitation for about 9 or 10 days, so the residence time in the water… of the water in the atmosphere is about 9 or 10 days on average.

The time it takes for water to move through the different pathways of the water cycle can range considerably – from very short periods of time where water moves rapidly from one system of the water cycle, to the next, to very long time spans. Water that infiltrates the ground and moves through the soil can sometimes take much longer.

One of the really important things to know is that, when water moves down through the soil profile and hits groundwater, it’s not just the water moving, it’s also nutrients being carried away from the land and eventually ending up in our lakes where they can cause pollution if too much moves. Around different parts of New Zealand, it can take decades for that water and the nutrients to move down through the soil profile, hit the groundwater and then eventually end up in the lakes.

Lake Rotorua has an area of about 80 square kilometres. About 80% of that water or more comes from groundwater that supplies streams that pop up out of the ground just before they enter the lake, and that water has been often underground for as long as 140 years in the largest of the Rotorua Lake inflows, which is the Hamurana Stream. That stream flows at 3 cubic metres 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. So in other words, the age of that water is about 1870 on average.

The time variables in the water cycle, therefore, have major impacts for the management of water resources, especially when we want to better understand and limit the effects from contamination and pollution.

Prof. Keith Hunter, Otago University
Dr David Campbell, Waikato University
Prof. Louis Schipper, Waikato University
Prof. David Hamilton, Waikato University

Iceberg image, Dr Mike Williams, NIWA
Flyover NZ/Rotorua Lakes model by Mathew Allan
Globe image, courtesy of NASA
Franz Josef Glacier image by ocwo, licensed through 123RF Ltd
Atmosphere image courtesy of Aerospaceweb.org
Cumulonimbus cloud image, Phillip Norton
Hamarana Stream image 1, Lorraine Thorne
Hamarana Stream image 2, Prabhnoor Singh
Buoy in lake and David Hamilton with buoy images, LERNZ