Lecture on Water Cycle Processes

Overview

• Focus on water cycle processes in the context of Earth’s life support system for OCR A-level.
• Concentration on drainage basin level, emphasizing open systems with inputs and outputs.

Open Systems

• Drainage basins are open systems with both inputs and outputs, similar to a water butt.
• Inputs: primarily precipitation.
• Outputs: stream flow, evapotranspiration (combination of evaporation and transpiration), and changes in storage (lakes, groundwater).

Water Balance Equation

• Water Balance Equation: Precipitation = Evapotranspiration + Stream flow + Changes in storage.
• Water balance is not always equal due to the open nature of systems – periods of surplus and deficit occur.

Example of Water Balance

• Precipitation vs. Evapotranspiration illustrated over a year.
• Surplus: When precipitation exceeds evapotranspiration, leading to full rivers and groundwater.
• Deficit: Evapotranspiration exceeds precipitation, leading to decreased water stores.
• Recharge: Occurs when precipitation again exceeds evapotranspiration, refilling depleted stores.

Processes in Detail

Precipitation

• Forms: Liquid (rain) and solid (snow, hail).
• Snow and ice can delay runoff into rivers due to longer ground presence.

Intensity and Duration

• Intense precipitation can exceed ground infiltration capacity, causing surface runoff and potential flooding.
• Long-duration rain increases risk of flooding due to saturated ground.

Seasonal Variations

• Regions experience different rain patterns – monsoon season in India, wet winters in the UK.

Key Processes

Transpiration

• Water movement from plant roots to leaves, exiting via stomata.
• Influenced by temperature and wind.
• Deciduous trees shed leaves to conserve water during winter.

Condensation

• Transition of water vapor to liquid at the dew point temperature.
• Types of clouds:
  • Cumuliform Clouds: Formed by rising warm air.
  • Stratiform Clouds: Horizontal movement of air over cooler surfaces (advection).
  • Cirrus Clouds: Made of ice crystals, less impact on water cycle.
• Fog: Low-level clouds depositing water on land and vegetation.

Implications

• Type and intensity of precipitation affect flow and storage within the water cycle.
• Understanding these processes helps predict and manage water resources effectively.