Here is the notes for Class 7 NCERT Geography – Chapter 5: Water

Water cycle

  • The process by which water continually changes its form and circulates between oceans, atmosphere and land is known as the water cycle.
  • Our earth is like a terrarium.
    • Terrarium: It is an artificial enclosure for keeping small house plants.
  • The same water that existed centuries ago still exists today.

Distribution of water(%)

      • Oceans: 97.3
      • Ice-caps: 2
      • Ground water:0.68
      • Fresh water lakes: 0.009
      • Inland seas and salt lakes: 0.009
      • Atmosphere:  0.0019
      • Rivers: 0.0001

March 22 is celebrated as World Water Day when the need to conserve water is reinforced in different ways.

Salinity

  • Salinity is the amount of salt in grams present in 1000 grams of water. The average salinity of the oceans is 35 parts per thousand.
  • Dead sea in Israel has salinity of 340 grams per litre of water.
    • Swimmers can float in it because the increased salt content makes it dense.

OCEAN CIRCULATION

The movements that occur in oceans can be broadly categorised as

 waves

  • When the water on the surface of the ocean rises and falls alternately, they are called waves.
  • During a storm, the winds blowing at very high speed form huge waves.
  • These may cause tremendous destruction.
  • An earthquake, a volcanic eruption or underwater landslides can shift large amounts of ocean water. As a result a huge tidal wave called tsunami is formed.

Tides

  • The rhythmic rise and fall of ocean water twice in a day is called a tide.
  • The strong gravitational pull exerted by the sun and the moon on the earth’s surface causes the tides.
  • Two types of tide
  • High Tides
    • The water of the earth closer to the moon gets pulled under the influence of the moon’s gravitational force and causes high tide.
    • During the full moon and new moon days, the sun, the moon and the earth are in the same line and the tides are highest.
    • These tides are called spring tides.
    • High tides helps the ships to arrive at the harbour more easily.
    • The high tides also help in fishing.
  • Low Tides
    • When the moon is in its first and last quarter, the ocean waters get drawn in diagonally opposite directions by the gravitational pull of sun and moon resulting in low tides.
    • These tides are called neap tides

                           The rise and fall of water due to tides is being used to generate tidal electricity.

Currents

  • Ocean currents are streams of water flowing constantly on the ocean surface in definite directions.
  • The ocean currents may be warm or cold.
  • The ocean current influence the temperature conditions of the area.
  • Two types of current
    • Warm current
      • The warm ocean currents originate near the equator and move towards the poles.
      • Example- Gulf current
    • Cold current
      • The cold currents carry water from polar or higher latitudes to tropical or lower latitudes.
      • Example- Labrador current
Warm + Cold current
    • The areas where the warm and cold currents meet provide the best fishing grounds of the world.
    • Example – Seas around Japan and the eastern coast of North America
    • The areas where a warm and cold current meet also experience foggy weather making it difficult for navigation
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