Lecture Notes: The Hydrologic Cycle

Overview of the Hydrologic Cycle

• The hydrologic cycle represents the storage locations and transfer mechanisms of water within the Earth's atmospheric system.
• Major Storage Locations:
  • Oceans: Majority of Earth's water is in ocean basins, not suitable for human consumption.
  • Cryosphere: Largest source of freshwater, in solid form (glaciers, sea ice, snow), yet largely inaccessible.
  • Surface Freshwater: A very small percentage found in lakes and streams, far less than half a percent.
  • Groundwater: More water is found below the ground than in lakes and rivers.

Transfer Processes in the Hydrologic Cycle

• Evaporation: The process of water turning into vapor and rising into the atmosphere.
• Precipitation: Water falling to the earth as rain, snow, sleet, or hail.
• Transpiration: Loss of water vapor from plants.
• Evapotranspiration: Combined loss of water from plants and land surfaces.

Interception

• Definition: Interruption of precipitation's direct contact with the soil, primarily by vegetation.
• Functions of Interception:
  • Allows gradual soil infiltration.
  • Prevents soil particle displacement and soil plugging.
  • Reduces sheet erosion, which is topsoil loss due to runoff.

Soil and Water Zones

• Zone of Aeration: Upper soil profile with air pockets, where water is called soil moisture.
• Zone of Saturation: Soil saturated with water, known as groundwater.
• Water Table: Boundary between aeration and saturation zones, fluctuates with precipitation and groundwater usage.

Water Movement Mechanisms

• Stem Flow: Water traveling down plant branches and trunks.
• Canopy Drip: Water dripping off the canopy.
• Through Fall: Water passing through gaps in the canopy.

Infiltration

• Definition: Initial movement of precipitation into the soil.
• Factors Affecting Infiltration:
  • Soil type composition (clay, silt, sand ratio).
  • Urban and desert surfaces have low infiltration rates, prone to flooding.

Percolation

• Definition: Downward movement of water through soil after infiltration.
• Controlled by:
  • Porosity: Availability of pore spaces, influenced by existing saturation.
  • Permeability: Rate of water movement through soil, dependent on soil type.