Storage in the water cycle
Scientists generally agree that the amount of water on Earth is finite, and that the water cycle constantly recycles water through the Earth’s systems. But how long does this take? How long does water stay in any one part of the cycle? These are questions that many scientists investigate, and the answers might just surprise you!
Lake Wakatipu
Lake Wakatipu is New Zealand’s third largest lake, making it an important water reservoir. It is a glacial lake, collecting water from the Southern Alps.
The water cycle is dynamic and always active, but that doesn’t mean every molecule of water is constantly moving through the system. In fact, water is stored in various parts of the cycle, often referred to as reservoirs. These might be as large as water in the oceans, or, on a smaller scale, water can be ‘trapped’ in an iceberg or a lake. Much more water is in storage than actively moving through the water cycle.
Franz Josef Glacier
Glaciers store large volumes of water. Franz Josef Glacier, in the South Island, is both beautiful and unique because it is one of only three glaciers that end less than 300 metres above sea level.
How long water spends in each reservoir is called ‘residence time’. These are some estimated average residence times, but it’s important to remember that some water will spend much longer or shorter time than this.
Reservoir
Estimated residence time
Atmosphere
7–9 days
Soil moisture
14–28 days
Snow
30–150 days
Rivers
weeks–months
Groundwater
months–10,000 years
Glaciers
40–400 years
Lake
100 years
Ocean
3,000–3,500 years
Why is residence time important?
These different residence times are important to remember when we are trying to minimise human impacts on the environment, for example, knowing that water might spend months to 10,000 years in the groundwater before seeping out of the ground.
Age of water
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.
Imagine an old garbage dump has been found to be leaking toxins into the groundwater but testing of a nearby lake has shown no traces of these toxins. If efforts are made to fix the pollution so that no more toxins can enter the groundwater system, how long should the lake be monitored?
It might be that there is never a problem, or contamination may occur 1,000 years later. This has major impacts when we are trying to manage our past contamination problems and trying to remedy things for the future.
Activity ideas
In Constructing an aquifer model students build an aquifer model and examine how water gets into an aquifer system.
In Groundwater contamination students build an aquifer model to look at point source and non-point source pollution.
