Humans and the water cycle
We are becoming increasingly aware of our impacts on nature, but unfortunately many of the things we do have become so ingrained in our way of life that it is hard to change.
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.
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.
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).
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.
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.
Some scientists believe that 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 may mean that there is an increase of evaporation, melting of ice or other processes of the water cycle that adversely affect the climate on Earth.
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.