Water cycle

Please get in touch with your teacher or tutor in case you have a question related to this lesson

Slide 1

In this lesson, we’re going to look at the water cycle, and the effects that humans have on it.

Slide 2

Let’s start by considering why water is important. It plays a major role in our daily life. In fact, 75% of our body is made up of water. We use it for many things: drinking, washing, bathing, cooking, growing plants, watering animals and so on, and it also plays a major role in industry. Water is the most important part of our ecosystem. Without it, there would be no plants and animals.

Slide 3

Although water makes up about 70% of the Earth’s surface, only 2.5% of the Earth’s water is fresh, and most of that is inaccessible. But, where do we find water on Earth? Obviously, it’s in oceans, lakes, streams and rivers and it falls to the Earth as precipitation in the form of rain, snow and dew. About 68.7% of the Earth’s fresh water is stored in glaciers as snow and ice. We can also find water in river basins and wetlands such as swamps, bogs, marshes and lagoons. Then there’s ground water. This is the water that’s found underground in the cracks and spaces in soil, sand and rock. About 30% of the Earth’s fresh water is stored as ground water.

Slide 4

The water cycle describes how water moves around the Earth from one state to another. Water evaporates from the surface of the earth, rises into the atmosphere, cools and condenses into rain or snow in clouds and falls again to the surface as precipitation. The water falling on the land collects in rivers and lakes, soil and the porous layers of rock. Much of it flows back into the oceans, from where it will evaporate again, continuing the water cycle.

Slide 5

The water cycle is a natural cycle that is affected by a wide variety of different factors. The activities of humans can change the amount of water that is available for use in any given region. Population growth in regions that are already short of water can make water shortages worse, as can the movement of large portions of the population from the country into more densely populated towns and cities. Our demands for higher living standards and more easily available and reliable food supplies lead to increased farming in marginal areas and the increased use of irrigation, which can impact the amount of water available and its quality. Finally, pollution from factories, cities and the use of fertilisers and pesticides in agriculture can contaminate our water supply.

Slide 6

Many of the ways that humans affect the amount and quality of the water available to the water cycle are unintentional, but there are also ways that humans can deliberately affect the water cycle.

Slide 7

Some ways that we might deliberately change the water cycle include using water storage towers, drinking water treatments, sinking artesian bores, damming rivers to create artificial lakes, guiding stormwater into rivers through stormwater drains and treating wastewater in sewerage treatment plants before allowing it to enter rivers or the ocean.

Slide 8

Human beings make many deliberate changes to the water cycle, manipulating water sources to meet their own requirements. They sink artesian bores to draw water out of the ground, lowering the groundwater table. They change the flow of water by building irrigation systems to water farmland. They dam lakes and rivers to produce electricity in hydroelectric power plants to create man-made lakes, canals and ponds for transportation or for aesthetic reasons.

Slide 9

Humans can also affect the water cycle indirectly. For example, releasing green house gases into the atmosphere causes climate change, and changes in the water cycle. These include changes in sea levels: rising sea levels in some areas lead to receding coastlines, while dropping sea levels lead to expanding coastlines. In some cases, rising sea levels lead to the disappearance of islands. Because of global warming, glaciers are retreating and disappearing. This changes the amount of water in rivers. Some regions are experiencing more frequent floods or droughts. Finally, the polar ice caps are melting, leading to rising sea levels.

Slide 10

Groundwater has traditionally been known as a source of clear, fresh water, but human actions are changing that. Farmers use pesticides, fertilisers and herbicides. This and excess nitrogen runs off the soil into rivers and streams, and pollutes ground water. The runoff is further increased by deforestation. Acid rain is caused by certain emissions from industry.

Slide 11

The diagram on this slide shows the various ways in which groundwater can be contaminated. These include acid rain, and run off from waste disposal sites, which contaminate the groundwater. Artesian bore wells can cause the water table to lower. Pollution in the catchment area further pollutes the water in the rivers and in the ground water.

Slide 12

Human activities such as farming, forest-clearing, road-building and mining can also contaminate the water cycle. If improperly managed, they can lead to suspended particles and soil running off into rivers.

Slide 13

Pollution is a major cause of harm to the water cycle, and to aquatic ecosystems, contaminating our water and leading to mass fish kills. Some examples of harmful pollutants are organic matter and bacteria from waste water discharges, fertilisers, herbicides and pesticides running off agricultural lands, acid rain caused by factory emissions and the heavy metals that are released by mining and industrial activities that can be toxic to both aquatic animals and the humans who eat them or drink the water.

Slide 14

Another human activity which adversely affects the water cycle is deforestation. Irresponsible forestry practices include clear-cutting or clearing a forest or stand of trees to use the land for something else such as conversion to farming land, grazing land or urban developments. There are a number of problems with deforestation. It leads to loss of biodiversity due to habitat destruction, it increases green house gas emissions as there are fewer trees left to absorb carbon dioxide, it disrupts the water cycle and increases soil erosion. Finally, it can disrupt the livelihood of people who rely on the products of the forest for their income.

Slide 15

You’ve probably seen signs on people’s gardens saying that bore water is used for watering. This is not without its problems. Pumping from aquifers has been increasing globally, and the effects on the water cycle can be devastating. Water tables are lowered, meaning that deeper wells need to be dug, and the groundwater source can become depleted. This can lead to increased salination of the soil, reduced spring yields, diminished river flows, poorer water quality, damage to natural habitats and land, and the gradual sinking or subsidence of the land.

Slide 16

Climate change also has devastating effects on the water cycle. In areas that are already under pressure, climate change can lead to even larger water shortages. This can be seen by the fact that land and mountain glaciers are now shrinking more rapidly and disappearing, taking with them their stored fresh water. Global warming also leads to an increase in the frequency and severity of extreme weather events such as storms and floods. This causes an increase in the contamination of waterways by storm and flood debris, and soil runoff.

Slide 17

A nasty consequence of air pollution that affects the water cycle is the production of acid rain. The combustion of fossil fuels gives rise to air pollutants consisting of sulphur and nitrogen compounds. When these compounds mix with water vapour in the atmosphere, it becomes more acidic. This acidic mixture condenses to form acid precipitation, which, in turn, causes the water on the Earth’s surface to become more acidic. This is a very bad thing for many plants and animals. They die, and this has devastating effects on ecosystems. Acid rain also causes many man-made structures such as buildings and statues to deteriorate.

Slide 18

Global warming also affects the water cycle. Many human activities increase the amounts of greenhouse gases in the atmosphere. Greenhouse gases are gases such as carbon dioxide and methane that help to keep the Earth warmer. The increase in these gases is one theorised cause of global warming. An increase in air temperature leads to an increase in the evaporation of water. In addition, warm air is able to hold more water. So, global warming leads to an intensification of the water cycle and changing weather patterns.

Slide 19

So, what can we do to help prevent some of these effects? There are a number of measures we can take to avoid water shortages and reduce losses of groundwater. We can collect rainwater in tanks to replenish underground supplies and divert surface water into the ground to help reduce losses due to evaporation. We can also build dams and reservoirs to store water so that is available for irrigation and drinking. Transferring water between river basins can help to alleviate water shortages in some areas, and finally we can desalinate water to turn sea water into fresh water so that we can tap into more of the water that is present on the Earth.

Slide 20

Let’s just summarise the points that we’ve covered in this presentation. Climate change affects water availability. Human impacts on the water cycle such as pollution, water diversions and uncertainties about water availability threaten economic growth, the environment and our health. Groundwater is often overexploited and polluted, with devastating effects. To increase our fresh water supply, we now supplement traditional techniques such as rainwater collection with newer technologies such as desalination and water recycling plants.