The Water Cycle
The global water cycle involves the movement of water between major stores:
Oceans: The largest store, holding 97% of Earth's water.
Glaciers: A significant store of freshwater (60-70% of freshwater).
Groundwater (Aquifers): Water stored in saturated zones within permeable rocks
and sediments underground.
Lakes: Surface water bodies.
Soils: Water held within the soil layer.
Atmosphere: Stores a relatively small amount of water with a low residency time
(average of about 9 days for a water molecule).
Rivers: Store a small percentage of Earth's total water.
Biosphere: Water contained within living organisms.
Key processes (flows) in the water cycle include:
Evaporation: The process of liquid water turning into water vapor, driven by solar
radiation and affected by temperature and wind.
Transpiration: The release of water vapor from plants through stomata, a crucial
process for plant function and cooling. Influenced by temperature, wind, and
vegetation type.
Evapotranspiration: The combined process of evaporation and transpiration.
Condensation: The change of water vapor into liquid water, occurring when air cools
to its dew point. This process is fundamental to cloud formation and fog.
Precipitation: Water falling from the atmosphere in liquid (rain) or solid (snow, hail)
form. The type, intensity, and duration of precipitation significantly impact other
water cycle processes.
Interception: Precipitation prevented from reaching the ground by vegetation.
Interception loss occurs when intercepted water evaporates. Influenced by tree species
(evergreen vs. deciduous), vegetation type, wind, and temperature.
Throughfall: Water that falls through gaps in the vegetation canopy.
Stemflow: Water that flows down the stems and branches of plants.
Infiltration: The vertical movement of water from the surface into the soil.
Infiltration capacity refers to the maximum rate at which soil can absorb water.
Percolation: The downward movement of water through soil and permeable rock.
, Overland Flow (Surface Runoff): Water flowing over the surface when the rate of
precipitation exceeds the infiltration capacity, or when the ground is saturated
(saturated overland flow). Flat relief and impermeable surfaces like tarmac increase
runoff.
Throughflow: Horizontal movement of water through unsaturated soil.
Groundwater Flow: Slow horizontal movement of water through saturated soil and
permeable rock towards streams or other outlets.
Ablation: The loss of water from ice, snow, and glaciers through melting,
evaporation, and sublimation (direct change from solid to vapor).
Water Balance: At a local scale (drainage basin), water balance can be described by the
equation:
Precipitation = Evapotranspiration + Streamflow + Change in Storage
Water balance fluctuates seasonally, with periods of surplus (precipitation exceeds
evapotranspiration) and deficit (evapotranspiration exceeds precipitation). Recharge occurs
when precipitation exceeds evapotranspiration after a period of deficit, refilling groundwater
and other stores.
Importance of Water:
Supports Life: Essential for all known life forms.
Moderates Thermal Conditions: Oceans absorb, store, and redistribute heat. Clouds
reflect solar radiation (albedo effect), lowering surface temperature and moderating
heat.
Greenhouse Gas: Water vapor is a significant greenhouse gas, absorbing and
radiating infrared radiation, warming the atmosphere and maintaining habitable
temperatures. Excessive water vapor can contribute to the enhanced greenhouse
effect.
Plant Processes: Used in photosynthesis, nutrient transport, maintaining rigidity, and
cooling through transpiration.
Animal and Human Processes: Crucial for chemical reactions, nutrient and oxygen
transport, and cooling (e.g., sweating, panting).
Human Use: Used extensively in agriculture (irrigation), industry (manufacturing),
energy generation (hydroelectric power), domestic supply (drinking water, sewage),
and leisure activities. Overuse of groundwater for irrigation can lead to depletion if
extraction exceeds replenishment rates.
Cloud Formation
The global water cycle involves the movement of water between major stores:
Oceans: The largest store, holding 97% of Earth's water.
Glaciers: A significant store of freshwater (60-70% of freshwater).
Groundwater (Aquifers): Water stored in saturated zones within permeable rocks
and sediments underground.
Lakes: Surface water bodies.
Soils: Water held within the soil layer.
Atmosphere: Stores a relatively small amount of water with a low residency time
(average of about 9 days for a water molecule).
Rivers: Store a small percentage of Earth's total water.
Biosphere: Water contained within living organisms.
Key processes (flows) in the water cycle include:
Evaporation: The process of liquid water turning into water vapor, driven by solar
radiation and affected by temperature and wind.
Transpiration: The release of water vapor from plants through stomata, a crucial
process for plant function and cooling. Influenced by temperature, wind, and
vegetation type.
Evapotranspiration: The combined process of evaporation and transpiration.
Condensation: The change of water vapor into liquid water, occurring when air cools
to its dew point. This process is fundamental to cloud formation and fog.
Precipitation: Water falling from the atmosphere in liquid (rain) or solid (snow, hail)
form. The type, intensity, and duration of precipitation significantly impact other
water cycle processes.
Interception: Precipitation prevented from reaching the ground by vegetation.
Interception loss occurs when intercepted water evaporates. Influenced by tree species
(evergreen vs. deciduous), vegetation type, wind, and temperature.
Throughfall: Water that falls through gaps in the vegetation canopy.
Stemflow: Water that flows down the stems and branches of plants.
Infiltration: The vertical movement of water from the surface into the soil.
Infiltration capacity refers to the maximum rate at which soil can absorb water.
Percolation: The downward movement of water through soil and permeable rock.
, Overland Flow (Surface Runoff): Water flowing over the surface when the rate of
precipitation exceeds the infiltration capacity, or when the ground is saturated
(saturated overland flow). Flat relief and impermeable surfaces like tarmac increase
runoff.
Throughflow: Horizontal movement of water through unsaturated soil.
Groundwater Flow: Slow horizontal movement of water through saturated soil and
permeable rock towards streams or other outlets.
Ablation: The loss of water from ice, snow, and glaciers through melting,
evaporation, and sublimation (direct change from solid to vapor).
Water Balance: At a local scale (drainage basin), water balance can be described by the
equation:
Precipitation = Evapotranspiration + Streamflow + Change in Storage
Water balance fluctuates seasonally, with periods of surplus (precipitation exceeds
evapotranspiration) and deficit (evapotranspiration exceeds precipitation). Recharge occurs
when precipitation exceeds evapotranspiration after a period of deficit, refilling groundwater
and other stores.
Importance of Water:
Supports Life: Essential for all known life forms.
Moderates Thermal Conditions: Oceans absorb, store, and redistribute heat. Clouds
reflect solar radiation (albedo effect), lowering surface temperature and moderating
heat.
Greenhouse Gas: Water vapor is a significant greenhouse gas, absorbing and
radiating infrared radiation, warming the atmosphere and maintaining habitable
temperatures. Excessive water vapor can contribute to the enhanced greenhouse
effect.
Plant Processes: Used in photosynthesis, nutrient transport, maintaining rigidity, and
cooling through transpiration.
Animal and Human Processes: Crucial for chemical reactions, nutrient and oxygen
transport, and cooling (e.g., sweating, panting).
Human Use: Used extensively in agriculture (irrigation), industry (manufacturing),
energy generation (hydroelectric power), domestic supply (drinking water, sewage),
and leisure activities. Overuse of groundwater for irrigation can lead to depletion if
extraction exceeds replenishment rates.
Cloud Formation
