Class 7 Science Notes Chapter 16 (Water: a Precious Resource) – Science Book

Okay, let's prepare detailed notes for Chapter 16: "Water: A Precious Resource" from the NCERT Class 7 Science textbook, focusing on aspects relevant for government exam preparation.

NCERT Class 7 Science - Chapter 16: Water: A Precious Resource

Objective: To understand the importance of water, its limited availability, the water cycle, groundwater resources, causes of water depletion, and methods for water conservation.

1. Introduction: Importance of Water

• Essential for Life: Water is fundamental for the existence of all living organisms (plants, animals, humans).
• Daily Needs: Required for drinking, cooking, bathing, washing, sanitation, agriculture, industries, and transportation.
• World Water Day: Celebrated on March 22nd every year to raise awareness about the importance of freshwater conservation.
• UN Recommendation: The United Nations recommends a minimum of 50 litres of water per person per day for basic needs (drinking, washing, cooking, sanitation).

2. Availability of Water

• Earth's Surface: About 71% of the Earth's surface is covered with water.
• Saline Water: Most of this water (approx. 97%) is in oceans and seas and is saline (salty), making it unfit for direct human consumption, agriculture, or most industrial uses without desalination.
• Freshwater: Only about 3% of the Earth's water is freshwater.
  • Frozen State: A large portion of freshwater is locked up in glaciers and ice caps (approx. 68-69% of freshwater).
  • Groundwater: A significant amount is stored as groundwater (approx. 30% of freshwater).
  • Surface Water: A very small fraction (less than 1%) is available as surface water in rivers, lakes, and swamps, readily accessible for human use.
• Conclusion: The amount of water readily available and fit for human use is extremely limited (estimated around 0.006% of all water on Earth).

3. Forms of Water

Water exists on Earth in three forms, which are inter-convertible:

• Solid: Ice, snow, glaciers, ice caps. Found at poles, high mountains.
• Liquid: Water in oceans, rivers, lakes, groundwater, rain.
• Gaseous: Water vapour in the atmosphere. Invisible.

4. The Water Cycle (Hydrological Cycle)

• Definition: The continuous circulation of water on, above, and below the surface of the Earth. It ensures the availability of freshwater.
• Key Processes:
  • Evaporation: The process where liquid water changes into water vapour (gas) due to heat, primarily from oceans, rivers, and lakes.
  • Transpiration: The process where plants release water vapour into the atmosphere through pores (stomata) in their leaves.
  • Condensation: The process where water vapour in the air cools down and changes back into tiny liquid water droplets or ice crystals, forming clouds.
  • Precipitation: Water falling back to the Earth from clouds in the form of rain, snow, sleet, or hail.
  • Infiltration: The process by which water on the ground surface seeps into the soil.
  • Percolation: The downward movement of infiltrated water through soil and rock layers.
  • Surface Runoff: Water flowing over the land surface into rivers, lakes, and eventually oceans, when the ground is saturated or the rate of precipitation exceeds infiltration capacity.
  • Collection: Water gathering in large bodies like oceans, lakes, and rivers, or as groundwater.

5. Groundwater: An Important Source of Water

• Definition: Water held underground in the soil or in pores and crevices in rock.
• Infiltration: Rainwater and water from other sources (rivers, lakes) seep through the soil and fill the empty spaces and cracks deep below the ground. This process recharges groundwater.
• Water Table: The upper level of the zone where the ground is saturated with water.
  • It can vary from place to place.
  • It can fluctuate depending on rainfall (recharge) and extraction (discharge).
  • It can be found at different depths – less than a meter to several meters below the ground.
• Aquifer: A layer of permeable rock, sand, or gravel below the water table that holds groundwater. Water can be extracted from aquifers using wells, tube wells, or hand pumps.

6. Depletion of Water Table

The water table is decreasing in many areas due to excessive extraction and insufficient recharge. Major causes include:

• Increased Population: Higher demand for drinking water, sanitation, and domestic use.
• Industrialization: Industries require large quantities of water for various processes.
• Agricultural Activities:
  • Irrigation is a major consumer of water, especially groundwater.
  • Irregular rainfall increases dependency on groundwater for irrigation.
• Deforestation: Cutting down trees reduces the seepage of rainwater into the ground (less infiltration) and increases surface runoff. Trees help bind soil and allow water to percolate.
• Decreased Seepage Area: Construction of concrete roads, pavements, and buildings reduces the open area available for rainwater to seep into the ground.
• Scanty Rainfall: Prolonged periods of low rainfall lead to less recharge of groundwater.
• Uneven Distribution of Water: Some regions experience floods, while others face droughts, leading to regional water stress.

7. Water Management and Conservation

Managing water resources effectively and conserving water are crucial.

• Rainwater Harvesting (RWH):
  • Collecting rainwater from rooftops and other surfaces and storing it for later use or directing it to recharge groundwater aquifers.
  • Traditional methods like Bawris (step wells) were used historically in India for water storage and recharge.
• Efficient Irrigation Techniques:
  • Drip Irrigation: A technique where water is delivered directly to the base of the plants through narrow tubes, drop by drop. This minimizes water loss through evaporation and runoff. Highly efficient.
  • Sprinkler Irrigation: Another method that can be more efficient than traditional flood irrigation in certain conditions.
• Conservation at Individual Level:
  • Turn off taps while brushing or shaving.
  • Fix leaking taps and pipes immediately.
  • Use buckets for bathing and washing cars instead of showers or hoses.
  • Reuse water (e.g., water used for washing vegetables can be used to water plants).
  • Practice the 3 R's: Reduce, Reuse, Recycle water whenever possible.
• Revival of Traditional Methods: Restoring old structures like bawris, johads, check dams.
• Afforestation: Planting more trees helps increase groundwater recharge and maintains the water cycle.

8. Effect of Water Scarcity on Plants

• Plants need water for photosynthesis (to make food) and to transport nutrients.
• Lack of water causes plants to wilt.
• Prolonged water scarcity can lead to plants drying up and dying.
• This impacts agriculture, food security, and the entire ecosystem (loss of green cover, impact on oxygen levels).

Key Takeaways for Exams:

• Remember World Water Day (March 22nd) and the UN recommended minimum (50 litres/person/day).
• Understand the distribution of water (saline vs. freshwater, frozen vs. liquid/groundwater). Know that usable freshwater is very scarce.
• Be clear about the processes in the water cycle (evaporation, transpiration, condensation, precipitation, infiltration, runoff).
• Define groundwater, water table, aquifer, and infiltration.
• Know the main causes of water table depletion (population, industry, agriculture, deforestation, reduced seepage).
• Understand water management techniques, especially Rainwater Harvesting (including traditional methods like Bawris) and Drip Irrigation.
• Recognize the importance of individual actions and afforestation in water conservation.

This detailed breakdown covers the essential points of the chapter relevant for competitive exams, focusing on definitions, key facts, processes, causes, and solutions related to water resources. Remember to correlate these points with diagrams from the textbook (like the water cycle and groundwater structure) for better understanding.