Water (Basic)

Water Cycle

The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. It is a fundamental process that sustains life and shapes our planet's climate and geography.

Key Processes in the Water Cycle:

1. Evaporation: The process by which liquid water changes into water vapour (a gas). This primarily occurs from the surface of oceans, seas, lakes, rivers, and even moist soil. Solar energy provides the heat needed for evaporation. The rate of evaporation is influenced by temperature, humidity, wind speed, and the surface area exposed.

   Formula for Evaporation Rate (Simplified concept): While complex, the concept is related to the difference in water vapour pressure between the water surface and the air, and wind speed. A simplified representation of potential evaporation might consider factors like:

   $$ E_p \propto (e_s - e_a) \times f(u) $$ Where:
   • $E_p$ is potential evaporation.
   • $e_s$ is the saturation vapour pressure at the water surface temperature.
   • $e_a$ is the actual vapour pressure of the air.
   • $f(u)$ is a function of wind speed ($u$).
2. Transpiration: The process by which plants release water vapour into the atmosphere from their leaves through small pores called stomata. This is essentially evaporation from plant surfaces. The combined process of evaporation and transpiration is called evapotranspiration.
3. Evapotranspiration: The total amount of water transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.
4. Sublimation: The process by which solid ice or snow changes directly into water vapour, without first melting into liquid water. This occurs primarily in cold, dry, and windy conditions.
5. Condensation: The process by which water vapour in the atmosphere changes into liquid water droplets or ice crystals. This occurs when air cools to its dew point (saturation) and water vapour condenses onto tiny particles called condensation nuclei (e.g., dust, salt, pollen). Condensation forms clouds, fog, and dew.
6. Precipitation: Any form of water that falls from clouds and reaches the Earth's surface. This includes rain, snow, sleet, hail, and drizzle. Precipitation occurs when cloud droplets or ice crystals grow large enough to overcome updrafts and fall due to gravity.
7. Runoff: The flow of water over the land surface, typically occurring after precipitation when the ground is saturated or the rainfall rate exceeds the infiltration capacity of the soil. Runoff collects in streams, rivers, lakes, and eventually flows back to the oceans.
8. Infiltration: The process by which water on the ground surface enters the soil. This water can then percolate deeper to recharge groundwater aquifers.
9. Percolation: The downward movement of water through soil and rock layers, driven by gravity. This process replenishes groundwater.
10. Groundwater Flow: The movement of water underground through aquifers. This water eventually emerges at springs, seeps into rivers and lakes, or flows into the oceans.
11. Collection: The accumulation of water in various bodies like oceans, lakes, rivers, glaciers, and groundwater reservoirs. Oceans are the largest reservoir, holding about 97% of Earth's water.

Importance of the Water Cycle:

• Sustains Life: Provides fresh water for drinking, agriculture, and ecosystems.
• Shapes Landscapes: Through processes like erosion and deposition by rivers and glaciers.
• Regulates Climate: Water vapour is a greenhouse gas, and processes like evaporation and condensation are vital for heat transfer and weather patterns.
• Cleanses Water: Evaporation naturally purifies water, leaving behind salts and impurities.

Distribution Of Water Bodies

Water covers approximately 71% of the Earth's surface. However, the distribution of this water among different bodies is highly uneven. The vast majority is saline, and only a small fraction is freshwater.

Distribution of Water on Earth (Approximate Percentages):

Major Water Bodies and Their Characteristics:

• Oceans and Seas:
  • Salinity: Contain dissolved salts, primarily sodium chloride. Average salinity is about 35 parts per thousand (ppt).
  • Volume: Hold the vast majority of Earth's water (97.5%).
  • Influence: Play a critical role in regulating global climate, driving weather patterns, and supporting marine ecosystems.
  • Examples: Pacific Ocean, Atlantic Ocean, Indian Ocean, Arctic Ocean, Southern Ocean, Mediterranean Sea, Arabian Sea.
• Ice Caps and Glaciers:
  • State: Frozen freshwater, primarily located in polar regions (Antarctica, Greenland) and mountain ranges.
  • Volume: Contain the largest portion of Earth's freshwater (about 68.7%).
  • Significance: Act as massive freshwater reservoirs and significantly influence global sea levels and climate.
• Groundwater:
  • Location: Water held underground in the soil or in pores and fractures of rock formations (aquifers).
  • Volume: The second largest reservoir of freshwater (about 30.1%).
  • Source: Replenished by precipitation that infiltrates the ground.
  • Importance: A vital source of drinking water and irrigation, especially in arid and semi-arid regions. However, it can be depleted if extraction rates exceed recharge rates.
• Surface Water:
  • Lakes: Inland bodies of standing water, can be freshwater or saline (e.g., Great Lakes, Caspian Sea).
  • Rivers and Streams: Flowing bodies of freshwater, a crucial part of the water cycle and a major source of freshwater for human use.
  • Swamps and Wetlands: Areas saturated with water, either permanently or seasonally, supporting unique ecosystems.
  • Volume: Relatively small percentage of total freshwater (about 1.2%).
• Atmospheric Water:
  • State: Water vapour, clouds (liquid droplets or ice crystals).
  • Volume: Very small percentage of total water, but crucial for weather and climate.
• Soil Moisture:
  • State: Water held in the soil pores.
  • Importance: Essential for plant growth.

Freshwater Availability: While 71% of the Earth is covered in water, only about 2.5% is freshwater. Of this freshwater, most is locked up in ice caps and glaciers, making readily accessible liquid freshwater (primarily groundwater and surface water) a very scarce resource, highlighting the importance of water conservation.