Water moves through the hydrologic cycle via evaporation, transpiration, and condensation. These processes transform water between liquid and vapor states, driven by the sun's energy and temperature differences. They play a crucial role in distributing water and heat globally.

Evaporation occurs at water surfaces, while transpiration moves water through plants. Condensation forms clouds and precipitation. These processes cool or warm their surroundings, affecting weather patterns and climate systems. Understanding them is key to grasping the water cycle's dynamics.

Evaporation, Transpiration, and Condensation

Water State Changes in the Hydrologic Cycle

• Evaporation transforms liquid water into water vapor at the surface of water bodies, moist ground, or vegetation
• Transpiration moves water through plants and evaporates it from aerial parts (leaves, stems, flowers)
• Condensation changes water vapor in the air into liquid water when warm air encounters cooler surfaces or air
• Water cycle (hydrologic cycle) interconnects these processes to distribute water globally
• Sun's energy and temperature/humidity differences drive these physical state changes of water
• Evaporation and transpiration absorb heat from surroundings (cooling effect)
• Condensation releases heat to the environment (warming effect)

Process Mechanisms and Importance

• Evaporation occurs at the interface between liquid water and air
  • Molecules with sufficient kinetic energy escape liquid surface
  • Requires breaking of hydrogen bonds between water molecules
• Transpiration involves water movement through plant vascular system
  • Occurs primarily through stomata (small pores) on leaf surfaces
  • Driven by water potential gradient from roots to leaves
• Condensation happens when air becomes saturated with water vapor
  • Typically requires the presence of condensation nuclei (dust, salt particles)
  • Forms clouds, fog, and dew
• These processes regulate global heat distribution
  • Transport energy from Earth's surface to atmosphere
  • Influence weather patterns and climate systems
• Water vapor acts as a greenhouse gas, affecting Earth's radiation balance
• Evapotranspiration (combined evaporation and transpiration) key in water balance
  • Significant component of terrestrial water cycle
  • Influences soil moisture, groundwater recharge, and streamflow

Factors Affecting Evaporation and Transpiration

Environmental Factors

• Solar radiation provides primary energy for evaporation and transpiration
  • Higher intensity increases rates (e.g., midday vs. early morning)
  • Seasonal variations affect process rates (summer vs. winter)
• Air temperature directly influences both processes
  • Higher temperatures generally increase evaporation and transpiration
  • Example: Increased evaporation from lakes during heat waves
• Relative humidity impacts vapor pressure gradient
  • Lower humidity promotes higher evaporation and transpiration rates
  • Example: Faster drying of clothes in dry climates
• Wind speed removes water vapor from air above evaporating surface
  • Maintains vapor pressure gradient
  • Example: Increased evaporation on windy days at the beach
• Atmospheric pressure affects molecular motion
  • Lower pressure generally leads to higher evaporation rates
  • Example: Faster boiling of water at high altitudes

Biological and Surface Factors

• Available water supply crucial for both processes
  • Soil moisture content particularly important for plant transpiration
  • Example: Reduced transpiration during drought conditions
• Vegetation characteristics significantly influence transpiration rates
  • Leaf area index affects total transpiring surface
  • Stomatal conductance regulates water vapor loss
  • Root depth determines access to soil water
  • Example: Deep-rooted trees maintain transpiration during dry periods
• Surface albedo impacts energy absorption for evaporation/transpiration
  • Higher albedo (e.g., snow) reduces available energy
  • Lower albedo (e.g., dark soil) increases available energy
• Surface roughness affects wind patterns near the surface
  • Rougher surfaces (forests) increase turbulence and vapor removal
  • Smoother surfaces (lakes) have more laminar flow
• Soil properties influence water retention and availability
  • Texture (sand, silt, clay) affects water-holding capacity
  • Structure impacts water movement in soil
  • Organic matter content enhances water retention
• Urban surfaces alter evaporation patterns
  • Impermeable surfaces (asphalt, concrete) reduce evaporation
  • Heat island effect increases potential evaporation in cities

Latent Heat in Water Phase Changes

Concept and Measurement

• Latent heat defined as energy absorbed or released during phase changes
  • No temperature change occurs during the process
• Latent heat of vaporization for water approximately 2,260 kJ/kg at 100°C
  • Represents energy required to change liquid water to vapor
• Latent heat of condensation equal in magnitude but opposite in sign
  • Energy released when water vapor condenses to liquid
• Measurement units typically in joules per kilogram (J/kg) or calories per gram (cal/g)
• Latent heat values vary slightly with temperature and pressure
  • Example: Latent heat of vaporization at 0°C ≈ 2,501 kJ/kg

Atmospheric and Climate Impacts

• Evaporation and transpiration absorb latent heat from surroundings
  • Causes cooling effect on surface or plant
  • Example: Evaporative cooling in swamp coolers
• Condensation releases latent heat back into environment
  • Warms surrounding air or surface
  • Example: Fog formation warming the air near the ground
• Latent heat exchange crucial in atmospheric energy transfer
  • Redistributes energy globally through water vapor transport
  • Influences formation and intensity of storms
• Latent heat flux significant component of surface energy budget
  • Affects local and global climate patterns
  • Example: Monsoon systems driven by land-sea temperature differences
• Cloud formation and precipitation processes involve latent heat exchange
  • Cumulus cloud development releases latent heat, fueling further convection
  • Hurricanes derive energy from latent heat release in eyewall
• Latent heat transport helps balance global energy distribution
  • Moves energy from tropics to higher latitudes
  • Moderates temperature extremes in coastal areas

Surface Characteristics and Evaporation/Transpiration

Land Cover and Vegetation Effects

• Forests generally have higher transpiration rates than grasslands
  • Greater leaf area and deeper roots access more water
  • Example: Amazon rainforest significant source of atmospheric moisture
• Crop type and growth stage influence evapotranspiration rates
  • Mature corn field transpires more than newly planted field
  • Rice paddies have high evaporation rates due to standing water
• Vegetation canopy structure affects energy interception and wind patterns
  • Dense canopies reduce evaporation from soil surface
  • Sparse vegetation allows more direct soil evaporation
• Plant adaptations to water stress impact transpiration
  • Succulent plants (cacti) have reduced transpiration in arid environments
  • Deciduous trees shed leaves to reduce water loss in dry seasons

Topography and Water Body Influences

• Topography affects evaporation and transpiration through local climate conditions
  • Slope aspect influences received solar radiation (north vs. south-facing slopes)
  • Elevation impacts temperature and atmospheric pressure
  • Valley bottoms may have higher humidity, reducing evaporation rates
• Water body characteristics significantly influence evaporation rates
  • Depth affects water temperature and heat storage
    • Shallow ponds warm faster, potentially increasing evaporation
  • Surface area determines total evaporative surface
    • Large lakes have greater total evaporation than small ponds
  • Water temperature relative to air affects evaporation rate
    • Warm water in cool air increases evaporation (fall season on lakes)
• Coastal areas experience unique evaporation patterns
  • Sea breezes influence humidity and wind patterns
  • Salt spray can affect vegetation transpiration rates
• Wetlands and marshes have high evapotranspiration rates
  • Abundant water supply and aquatic vegetation
  • Important in local and regional water cycles