Humans and the water cycle
We are becoming increasingly aware of our impacts on nature. There is considerable evidence that humans are responsible for disruptions and changes to local and global water cycles.
Earth’s water distribution
A number of human activities can impact on the water cycle: damming rivers for hydroelectricity, using water for farming, deforestation and the burning of fossil fuels. When people use water for irrigation, they are taking water from streams or from the ground and, as a result, the water table drops. It can take a long time for groundwater to be recharged. The flow-on effect is a reduction in the amount of water in rivers or lakes, which then impacts on the environment – animals living in the water may find themselves affected.
Water that seeps through the soil carries particles of nutrients, and these particles will eventually reach the groundwater. Too many nutrients can pollute freshwater storages. Also, the removal of too many nutrients means farmers have to add more chemicals to their soils to keep the plants growing.
However, not all water that falls on the ground will seep into the soil. Some of the water will run off overland and transport soil particles with the water into streams and eventually into lakes. This can affect the stability of the land.
Deforestation or removal of trees changes water take-up and can lead, in extreme cases, to an increase in over-land run-off and erosion of land.
Points of interest
Students could consider what farmers could do to reduce over-land run-off. Think about what David Hamilton means when he talks about how we take water for human consumption without considering the water needs for our lakes, streams and groundwater.
Different countries use different amounts of water, but we all tend to use them in the same ways, and some of these actions can impact on the water cycle – generating hydroelectricity, irrigation, deforestation and the greenhouse effect, as well as motor vehicle use and animal farming.
Hydroelectricity
Most of New Zealand’s electricity is generated using hydro dams. This involves changing the stored gravitational energy of water held behind the dam into electrical energy that can be used. While this is a non-polluting renewable way to generate electricity, it does have environmental impacts – especially when mismanaged.
Clyde hydro dam
The Clyde dam in the South Island uses water to generate electricity. It is New Zealand’s third largest hydro dam.
Rivers must be dammed, which can affect the function of the river both upstream and downstream – lakes are usually formed from the water accumulating above the dam and a build-up of silt can occur, while the amount of water is reduced further downstream. This can be problematic for any plants and animals that may find themselves with too much or too little water, and migrating fish cannot get through the dams.
Seriously mismanaged dams can result in droughts downstream, with smaller streams completely drying up, leaving areas of unwatered land. People then have to look at ways of getting more water into these dry areas.
Find out more about hydro power.
Irrigation
As the human population has increased, so have our demands on the land. We need more food, and to make food, we need water. Irrigation is the artificial watering of land that does not get enough water through rainfall. Irrigation is used substantially by most countries, some more than others. Arid (dry) lands require far more water, as do countries that have large intensive farming communities.
Spray irrigator
Irrigation is a method used to water farm land. Unfortunately it removes water from rivers and can cause surface run-off and leaching.
The problem with irrigation is that it removes water from its natural source and often causes leaching and run-off where it is used. This removal of nutrients results in farmers using more fertilisers to keep their pastures productive while the waterways become polluted. Another problem is that salt is brought up from lower levels (salination).
Deforestation
The removal of trees (deforestation) is having a major impact on the water cycle, as local and global climates change.
Normally, trees release water vapour when they transpire, producing a localised humidity. This water vapour then evaporates into the atmosphere where it accumulates before precipitating back to the Earth as rain, sleet or snow. Deforestation in one area can therefore affect the weather in another area because if trees are cut down, there is less water to be evaporated into the atmosphere and subsequently less rain.
At a local level, the land becomes drier and less stable. When it rains, instead of the water being soaked up, there is increased run-off and leaching. Areas can become more prone to both droughts and flooding, impacting on plants and animals, and also humans living near deforested areas.
Dynamic and complex: the global water cycle
Water in the Earth system is influencing all aspects of life on Earth. Pathways, storage, transfers and transformations have an effect on the global climate and human welfare. Within this interactive 4 scientists talk about some of the complex aspects of the water cycle.
To use this interactive, move your mouse or finger over any of the labelled boxes and click to obtain more information.
Climate change
The greenhouse effect is a natural phenomenon of Earth’s atmosphere trapping a range of gases, which in turn capture infrared radiation to keep our Earth at a moderate temperature range compared to the other planets in our solar system.
Human activity such as the burning of fossil fuels has an effect on the overall increase of the Earth’s temperature. Raising the Earth’s temperature means that there is an increase of evaporation, melting of land and sea ice, and impacts on other processes of the water cycle that adversely affect the climate on Earth. Find out why climate change matters.
Nature of science
As societies change, so do scientific priorities. Water was once simply a commodity for human use and manipulation. Now, science and society have a greater awareness of how our actions impact nature.
Activity ideas
The Hub has a number of activities that model aspects of the water cycle.
Building a water cycle models evaporation and precipitation.
Water run-off models how rain can affect soil erosion and ground stability.
Constructing an aquifer model explores the link between precipitation, surface and ground water.
Groundwater contamination models how contaminants enter aquifer systems.
Use this interactive to learn about the water cycle, including human interactions.
Other teaching resources include:
Precipitation and cloud formation is a slide show explaining cloud and precipitation processes.
What is the Earth system? is a teacher resource that explains the concept of systems.
Tie it all together with a unit plan featuring resources a our water range of resources.
