UNIT 3: THE EARTH’S WATER

1.      THE HIDROSPHERE

The hydrosphere is all the water on the planet. It is made up of oceans, seas, rivers, lakes, glaciers,groundwater and water vapour.

THE WATER CYCLE

There is always the same amount of water on Earth. It sometimes changes state, and it can be in liquid, gas or solid form. It moves continually between the atmosphere, the oceans and the continents. This continual movement of water is called the water cycle. This cycle is made up of three phases:

1.      Evaporation. Water evaporates, rises and cools in the atmosphere.

2.      Condensation. It condenses in the air and forms clouds.

3.      Precipitation. When the clouds cannot absorb any more water, there is precipitation in the form of rain, snow or hail.

Some of the precipitation falls on the land, and flows into lakes and rivers. Other water filters into the ground and becomes groundwater. All this water returns to the oceans and seas and the water cycle begins again.

SALTWATER

About 97 per cent of the Earth’s water is saltwater, located in the oceans and seas.

Salinity is the amount of salt dissolved in water. Salinity is higher in hot, enclosed seas because the proportion of salt rises when the water evaporates. It is lower in cold, open oceans.

- The temperature of saltwater varies. It is warmer near the surface (between 12 and 30 degrees celsius) and colder in deep water (between 4 and -1 degrees celsius). Oceans are huge bodies of water that affect temperatures. In summer, deep cold water cools the ocean’s surface and the air above it. In winter, the water retains heat, which is slowly released. Consequently, coastal areas have mild climates.

- Saltwater is denser than fresh water. For that reason navigation is easier.

FRESH WATER

About 3 per cent of the Earth’s water is fresh water found on continents and islands.

- Most of this water is ice at the polar regions and in glaciers.

- Some of it is groundwater under the Earth’s surface.

- Only a small proportion of it is found in water vapour and in lakes or rivers.

2.      OCEANIC WATER

OCEANS AND SEAS

The Earth is sometimes called the “blue planet” because oceans and seas cover 71 per cent of its surface.

- Oceans are vast masses of saltwater that separate the continents. They cover a large proportion of the Earth’s surface, particularly in the Southern Hemisphere.

- Seas are oceanic waters that are close to continents. They are not as deep as oceans.

‣ Some seas, such as the Caribbean Sea, are part of an ocean.

‣ Other seas, such as the Mediterranean Sea, are connected to oceans through narrow straits.

‣ Some large bodies of water, such as the Caspian Sea, are separate from the oceans. They are really large, inland saltwater lakes.

WAVES, TIDES AND CURRENTS

The water in oceans and seas is in continual movement. There are three types of movement:

- Waves are the circular movements of water on the surface of oceans and seas. They are caused by winds.

- Tides are the daily rise and fall of sea levels caused by the gravitational influence of the Moon and the Sun. High tide is when sea levels rise, and low tide is when they fall.

- Ocean currents are large masses of water that move across the oceans. Some surface currents are caused by winds, but deep ocean currents are caused by differences in temperature, density and salinity.

- Cold currents begin in the polar regions and move towards tropical regions, cooling the coasts that they pass.

- Warm currents begin in warm regions and often move west until they reach a continent and change direction.

3.      CONTINENTAL WATERS: RIVERS AND GROUNDWATER

RIVERS

Rivers are continuously flowing currents of water that flow into seas, lakes and other rivers. A river that flows into another river is its tributary.

There are three stages in the course of a river:

- The upper course: as the river leaves its source, it flows rapidly through steep, mountainous areas. It erodes rocks and creates river valleys.

- The middle course: the river flows less rapidly, and transports sand and stones in the water. It creates bends called meanders.

- The lower course: the river flows slowly and deposits sediments. It reaches the river mouth.

The flow, or volume, of water in a river is due to various factors:

- The amount of rainfall, which can change during the year.

- The type of soil, because some of the water filters into the ground.

GROUNDWATER

Rainwater that infiltrates rocks and the soil can be found as groundwater under the Earth’s surface. Aquifers are stores of groundwater that accumulate underground on top of impermeable layers of rock. We can use wells to bring this water to the surface and use it for agriculture. Groundwater makes up about 20 per cent of the Earth’s fresh water.

4.      HOW TO READ AND INTERPRET A HYDROGRAPH

An annual hydrograph shows how much water a river carries in a year. The river regime is how this flow rises and falls.

- The vertical axis shows the discharge, or rate of flow. We calculate this figure by dividing the average monthly discharge by the average annual discharge.The result shows us whether the flow in a particular month is above or below average.

- The horizontal axis indicates the months of the year.

- A line connects the data to show the river regime of a particular river.

HOW TO INTERPRET THE GRAPH

We need to think about the following questions:

- Is the river regime simple or complex?

‣ A river has a simple regime when it receives all of its water from a single source, such as rain or melting ice.

‣ A river has a complex regime if it receives water from both rain and melting ice. It will reach its highest and lowest levels twice a year.

- How regular is the flow?

The river will flow regularly all year round in areas where rainfall is regular. The graph will rise and fall sharply in areas where there are rainy and dry seasons. There may be droughts during the months when rainfall is low.

5.      CONTINENTAL WATERS: GLACIERS AND LAKES

ICE CAPS AND GLACIERS

The ice caps and glaciers hold 77 per cent of the Earth’s fresh water. The mass of ice increases in winter when it is cold, but is reduced when part of the ice melts in summer. Ice is found in both hemispheres.

LAKES

Lakes are large bodies of water that accumulate in inland basins. Lakes receive water from rivers, precipitation and groundwater, and they need to receive water continually. Over a very long period of time, many lakes disappear because rivers leave sediment in them and they gradually become more shallow. Most lakes store fresh water, but a few lakes store saltwater. This is because rivers transport minerals into these lakes, and they lose water through evaporation.

Unit 3: The Earth´s water

6.      WATER AS A RESOURCE

All living things need water. But most of the Earth’s water is saltwater that we cannot drink. We can only drink fresh water, but most of it is trapped in ice at the poles or in glaciers, therefore, it is not available to us. Moreover, drinking water needs to be purified before it is ready for human consumption. Many people die after drinking polluted water.

Water is such a precious resource that we need to recycle it and use it many times so that it is not wasted.

UNEVEN WATER DISTRIBUTION

We often find considerable amounts of water in temperate and tropical regions where there is continuous precipitation. There may be floods in some tropical regions. However, water is extremely scarce in very dry regions, such as parts of Africa, which often suffer from droughts.

WATER SHORTAGE

The rapid growth in population means that water is becoming increasingly scarce. This applies not only to drinking water but also to water used for irrigation and in industry and towns. It is essential to save water and also to recycle purified water. Disputes over access to water are made worse by its increasing scarcity.

WATER POLLUTION

Many kinds of human activity affect the quality of our water:

- Factories and farms release chemicals and pesticides that enter our rivers and seas. They can filter through the soil and go into the Earth’s groundwater.

- Waste sewage is a problem in many parts of the world. Developing countries do not always have the resources to clean it up properly.

- Radioactive waste is released by nuclear power plants and raises the level of radiation in our oceans.

- Some gases are absorbed by clouds and become acid rain that contains chemicals. They then enter the water cycle.