Water — Class 7 Social Science

The water cycle (also called the hydrological cycle) is the continuous movement of water on, above, and below the surface of the Earth. Water keeps changing its form — liquid, gas (vapour), and solid (ice) — in this never-ending cycle.

Stages of the Water Cycle:

• Evaporation: The Sun heats water in oceans, rivers, lakes, and other water bodies. The water turns into water vapour (gas) and rises into the atmosphere
• Transpiration: Plants also release water vapour into the atmosphere through their leaves
• Condensation: As the water vapour rises, it cools down and condenses into tiny water droplets around dust particles, forming clouds
• Precipitation: When the water droplets in clouds become too heavy, they fall back to Earth as rain, snow, sleet, or hail
• Collection: The water that falls collects in rivers, lakes, and oceans, or seeps into the ground as groundwater
• The cycle then begins again as the Sun continues to heat water bodies

Water is one of the most essential resources for life, yet most of the water on Earth is not usable by humans.

How Water is Distributed:

• About 71% of the Earth's surface is covered with water
• 97.3% of all water is in the oceans — this is saltwater and not directly usable
• Only 2.7% is freshwater
• Of this freshwater: about 2% is locked in ice caps and glaciers
• Only about 0.7% is available as usable freshwater in rivers, lakes, and groundwater
• Less than 1% of all water on Earth is available for human use

Sources of Freshwater:

• Rivers and streams: Flowing bodies of freshwater
• Lakes and ponds: Standing bodies of freshwater
• Groundwater: Water stored underground in soil and rock layers (aquifers)
• Glaciers and ice caps: Frozen freshwater (not easily accessible)
• Rainwater: Fresh precipitation that feeds all other sources

Ocean water is different from freshwater in important ways — it is salty and its temperature varies with location and depth.

Salinity:

• Salinity is the amount of salt dissolved in 1,000 grams of ocean water (measured in parts per thousand — ppt or ‰)
• The average salinity of ocean water is 35 parts per thousand (35‰) — meaning 35 grams of salt in every 1,000 grams of water
• Salinity varies from ocean to ocean and even within the same ocean
• Higher salinity: In enclosed seas and areas with high evaporation (e.g., Dead Sea has very high salinity)
• Lower salinity: Near river mouths where freshwater mixes with ocean water, and in polar regions where ice melts

Temperature of Ocean Water:

• The temperature of ocean water varies based on location and depth
• Surface water is warmer because it receives direct sunlight
• Temperature decreases with depth — deep ocean water is very cold
• Oceans near the equator are warmer; oceans near the poles are colder
• The average temperature of surface ocean water is about 27°C (in tropical regions)

Ocean water is constantly in motion. The three main movements of ocean water are waves, tides, and ocean currents.

Waves:

• Waves are the rise and fall of water on the surface of the ocean
• They are mainly caused by wind blowing over the surface of the water
• During a wave, water does not move forward — it moves in a circular motion. Only the energy moves forward
• The highest point of a wave is called the crest
• The lowest point of a wave is called the trough
• The stronger the wind, the bigger the waves

Tsunami:

• A tsunami is a huge wave caused by an earthquake, volcanic eruption, or landslide on the ocean floor
• Tsunamis are not caused by wind — they are caused by sudden disturbances under the sea
• They can travel at very high speeds across the ocean
• When they reach the shore, they can be extremely destructive, causing massive flooding and loss of life
• The word tsunami comes from Japanese and means "harbour wave"
• Example: The 2004 Indian Ocean tsunami caused massive destruction in several countries

Tides are the rhythmic rise and fall of ocean water caused by the gravitational pull of the Moon and the Sun on the Earth's water.

How Tides Work:

• The Moon's gravitational pull is the main cause of tides (though the Sun also contributes)
• When the water rises to its highest level, it is called high tide
• When the water falls to its lowest level, it is called low tide
• Most places experience two high tides and two low tides every day
• The time between a high tide and the next high tide is approximately 12 hours and 25 minutes

Types of Tides:

• Spring Tides: Occur during full moon and new moon when the Sun, Moon, and Earth are in a straight line. The gravitational pull of the Sun and Moon combine, producing very high tides and very low tides
• Neap Tides: Occur during first quarter and last quarter of the Moon when the Sun and Moon are at right angles to each other. Their gravitational pulls partially cancel each other, producing moderate tides (neither very high nor very low)

Importance of Tides:

• Navigation: High tides help ships enter and leave harbours safely
• Fishing: Fishermen use tides to plan their fishing activities
• Energy: Tidal energy can be used to generate electricity
• Cleaning: Tides help clean the shoreline by carrying away waste

Ocean currents are large-scale movements of water in a definite direction through the ocean. They are like rivers flowing within the ocean.

Causes of Ocean Currents:

• Wind: Permanent winds push surface water in specific directions
• Temperature differences: Warm water rises and cold water sinks, creating circulation
• Salinity differences: Water with different salt content moves differently
• Earth's rotation: The Coriolis effect deflects currents

Types of Ocean Currents:

• Warm Currents: Flow from equatorial (warm) regions towards the poles. They carry warm water and raise the temperature of the areas they pass through
• Cold Currents: Flow from polar (cold) regions towards the equator. They carry cold water and lower the temperature of the areas they pass through

Examples:

• Gulf Stream: A warm current in the Atlantic Ocean that flows from the Gulf of Mexico towards Europe, keeping Western Europe warmer than expected for its latitude
• Labrador Current: A cold current that flows from the Arctic along the east coast of Canada
• Where the warm Gulf Stream and cold Labrador Current meet (near Newfoundland), it creates ideal conditions for fishing and is one of the best fishing grounds in the world

Effects of Ocean Currents on Climate:

• Warm currents bring warm, moist air and cause rainfall on nearby coasts
• Cold currents bring cool, dry air and can cause deserts on nearby coasts
• Example: The Namib Desert on the west coast of Africa is dry partly because of the cold Benguela Current
• Western Europe has a mild climate partly because of the warm Gulf Stream

Since only about 1% of all water on Earth is usable freshwater, it is essential to conserve water. Water scarcity is a growing problem worldwide.

Why Water Conservation is Important:

• Freshwater is a limited resource — only about 1% of Earth's water is usable
• The world's population is growing, increasing demand for water
• Pollution of rivers and lakes makes water unsafe for use
• Many regions face water scarcity due to uneven distribution of rainfall
• Groundwater levels are falling due to excessive pumping
• Without conservation, future generations may not have enough water

Ways to Conserve Water:

• Rainwater harvesting: Collecting and storing rainwater for later use
• Fixing leaks: Repairing leaking taps and pipes to prevent wastage
• Drip irrigation: Using efficient irrigation methods in agriculture
• Reducing usage: Turning off taps when not in use, taking shorter showers
• Recycling water: Treating and reusing wastewater
• Protecting water bodies: Preventing pollution of rivers, lakes, and groundwater
• Planting trees: Trees help maintain the water cycle and prevent soil erosion