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.

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.

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.

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.

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.