Water is the most precious resource on earth. About 71 percent of the earth's surface is covered with water, but most of this is salt water in the oceans and seas, which we cannot drink or use directly for agriculture. Only a small part of the earth's water is fresh water, and even less is easily available to us in rivers, lakes, and groundwater.
Water is constantly moving between the oceans, the atmosphere, and the land in a never-ending cycle called the water cycle. The oceans not only provide water for the cycle but also influence our climate, provide food, and offer routes for transport. This chapter explores the water cycle, the distribution of water on earth, the oceans and their movements, and the crucial issue of water conservation.
The sun's heat causes water in the oceans, rivers, lakes, and soil to evaporate, changing it into water vapour. The water vapour rises, cools, and condenses into clouds. When the clouds become heavy, the water falls back to the earth as rain, snow, or hail (precipitation). The water then flows back to the oceans through rivers and streams, and some of it seeps into the ground to become groundwater.
This continuous process of evaporation, condensation, and precipitation is called the water cycle or hydrological cycle. The water cycle is powered by the sun's energy, and it redistributes fresh water all over the earth. Without this cycle, fresh water would not be available on the land, and life there would be impossible.
The total amount of water on the earth is fixed, but it is very unevenly distributed:
Because so much of the world's water is either salty or frozen, fresh water is a scarce resource that must be used carefully and conserved.
The oceans are vast bodies of salt water that cover most of the earth's surface. There are five major oceans: the Pacific Ocean (the largest and deepest), the Atlantic Ocean (the second largest), the Indian Ocean (the third largest), the Southern Ocean, and the Arctic Ocean (the smallest and shallowest).
The oceans are important for many reasons. They are the largest storehouse of water for the water cycle, they moderate the climate of the coastal regions, they provide food like fish, and they are used as routes for sea transport and trade.
The ocean floor is not flat like the surface of a swimming pool. It has relief features like the continents, including continental shelves, continental slopes, abyssal plains, deep sea trenches, mid-ocean ridges, and submerged mountains called seamounts. The Mariana Trench in the Pacific Ocean is the deepest part of the world's oceans.
The water in the oceans is in constant motion, and this movement takes three main forms: waves, tides, and currents.
Waves are caused by the wind blowing over the surface of the ocean. The wind transfers energy to the water, causing it to move in up-and-down and circular motions. Waves are measured by their height, wavelength, and amplitude. Strong winds create large, powerful waves that can erode the coast.
Tides are the regular rise and fall of the sea level, caused by the gravitational pull of the moon and the sun on the earth. There are high tides, when the sea rises, and low tides, when the sea falls. Tides occur twice a day.
Tides are important for several reasons. They help in navigation, as ships can enter shallow harbours during high tides. Tides also help fishermen to catch fish and help in generating electricity. The highest tides occur during the new moon and full moon, called spring tides, while the lowest are called neap tides.
Ocean currents are streams of ocean water moving in a definite direction. They can be warm currents or cold currents. The warm currents flow from the tropical regions towards the poles, while cold currents flow from the polar regions towards the equator.
Ocean currents influence the climate of coastal areas. The warm currents make the coasts they pass warmer and rainier, while cold currents make them cooler and drier. The Labrador current is a cold current, while the Gulf Stream is a warm current. The meeting of warm and cold currents often creates dense fog and is a good fishing ground.
A tsunami is a giant, powerful wave caused by an earthquake, a volcanic eruption, or a landslide on the ocean floor. The word "tsunami" is a Japanese word meaning "harbour wave." When an earthquake occurs under the sea, it displaces a huge amount of water, creating a series of enormous waves that travel at very high speeds.
When a tsunami reaches the coast, it can rise to a great height and cause enormous destruction to life and property. The Indian Ocean tsunami of 2004, caused by an earthquake near Indonesia, devastated the coasts of many countries, including India. Tsunamis cannot be prevented, but early warning systems can help save lives.
Fresh water is a scarce and precious resource, and it is our duty to conserve it. As the population grows and industries and agriculture expand, the demand for water is increasing, while the available fresh water remains limited. In many parts of India, water is already in short supply.
We can conserve water in many simple ways: by turning off taps when not in use, by reusing water where possible, by harvesting rainwater, by not polluting rivers and lakes, and by using water wisely in agriculture and industry. Conserving water today ensures that there will be enough for everyone in the future.
| Type | Percentage |
|---|---|
| Salt water (oceans and seas) | About 97% |
| Fresh water | About 3% |
| Fresh water frozen (glaciers and ice caps) | About 70% of fresh water |
| Movement | Cause | Importance |
|---|---|---|
| Waves | Wind over the surface | Erosion of coasts |
| Tides | Gravitational pull of moon and sun | Navigation, fishing, electricity |
| Currents | Temperature and salinity differences | Influence climate, fishing grounds |
Water covers most of our planet, yet fresh water is one of the scarcest resources on earth. The sun-powered water cycle continually recycles the world's water, carrying it from the oceans to the clouds, the land, and back again. The oceans, which hold most of the earth's water, are in constant motion, with waves, tides, and currents shaping the coasts and influencing the climate. Yet the oceans can also unleash the destructive power of tsunamis. Because fresh water is so precious and limited, conserving it and protecting the oceans from pollution are among the most important responsibilities of every human being.