PYTR Class

Learning Objectives

1. Explain how solar radiation and gravity drive the movement of water in the hydrosphere.
2. Describe the global hydrological cycle as a system with stores and flows.
3. Identify and quantify the main water stores in the hydrological cycle, including oceans, glaciers, groundwater, surface freshwater, atmosphere, and organisms.
4. Explain the key flows in the hydrological cycle, such as transpiration, sublimation, evaporation, condensation, advection, precipitation, melting, freezing, surface runoff, infiltration, percolation, streamflow, and groundwater flow.

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Part 1: Hydrological Cycle

• About 70% of Earth’s surface is covered by water, collectively known as the hydrosphere.
• The hydrosphere includes oceans, rivers, lakes, glaciers, ice caps, and atmospheric water vapor.
• Water moves between different parts of the hydrosphere through processes like melting, evaporation, and precipitation.
• The movement of water is driven by two main forces: the sun and gravity.
• Solar radiation provides heat energy, causing water to evaporate from oceans, rivers, lakes, and wetlands.
• Water vapor rises, is transported by wind, and condenses to form precipitation, returning water to the surface.
• Gravity moves water from high elevations, such as mountains, to lower areas, eventually reaching the sea.
• This continuous movement of water in different phases is known as the water cycle or hydrological cycle.

Part 2: Hydrological Cycle as a System

Recall that a system would have an input, process and storage, and an output.

• The hydrological cycle is a biogeochemical cycle, functioning as a system with stores and flows.
• Stores and flows can be examined individually or collectively.

Inflows and Outflows of a Lake

Inflows:

• Rainfall or other forms of precipitation
• Overland flow of water from surrounding areas
• Sewage inputs from domestic, industrial, or agricultural sources

Outflows:

• Evaporation
• Infiltration of water into groundwater
• Abstraction of water for domestic, industrial, or agricultural use

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• Precipitation: The conversion and transfer of moisture in the atmosphere to the land, including all forms of rainfall, snow, frost, hail, and dew.
• Interception: Water that is caught and stored by vegetation.
• Water not stored by plants may be lost through:
  • Interception loss: Water retained by plant surfaces and later evaporated away.
  • Throughfall: Water that falls through gaps in the vegetation or drips from leaves, twigs, or stems.
  • Stemflow: Water that trickles along twigs and branches before flowing down the main trunk.

Part 3: Transfer & Transformation

Water Cycle Transfers

• Surface runoff:
  • Water from rainfall flows downhill over the land surface.
  • Eventually enters lakes, rivers, or other surface waterways.
  • Usually in small quantities but can be significant during floods.
• Streamflow:
  • Water moves through rivers and streams following their channels.
  • Eventually flows into lakes, wetlands, or the sea.
• Infiltration:
  • Water soaks into the soil if it stays on the surface long enough.
  • More likely on flat or gently sloping land with vegetation slowing runoff.
  • Land surfaces like pavement or tarmac reduce infiltration.
• Percolation:
  • Water moves through soil and rock particles after infiltration.
  • Eventually reaches the water table, where the ground is fully saturated.
• Groundwater flow:
  • Groundwater moves slowly through porous rocks, usually downhill.
  • Movement stops when it reaches impermeable rocks that prevent further flow.
  • Can emerge at the surface as a spring, which may develop into a stream or river.
• Advection:
  • Wind moves water vapor and clouds across long distances.

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Water Cycle Transformations

• Evaporation:
  • Heat energy breaks bonds between water molecules, converting liquid water into vapor.
  • Happens faster at higher temperatures and in windy conditions.
  • High humidity slows evaporation.
• Transpiration:
  • Plants absorb liquid water from soil and release it as vapor through their leaves.
  • Water moves from roots to leaves to replace lost moisture.
  • Evapotranspiration is the combined process of transpiration and evaporation.
• Condensation:
  • Water vapor cools and turns back into liquid water droplets.
  • Happens at low temperatures, especially at high altitudes.
  • Contributes to cloud formation.
• Sublimation:
  • Ice converts directly into water vapor, or vice versa, under specific conditions.
• Precipitation:
  • Water returns to the Earth’s surface as rain, snow, hail, sleet, dew, frost, or fog.
• Melting and Freezing:
  • Melting: Ice turns into water if the temperature rises.
  • Freezing: Water turns into ice if the temperature drops.
  • Glacial meltwater flows into rivers and eventually reaches the sea.

Part 4: Stores in the Hydrological Cycle

Major Stores in the Hydrological Cycle

Store	Best estimate (km³)	Residence time
Oceans	1,350,000,000	2500 years
Atmosphere	13,000	8 days
Rivers	1,700	16 days
Lakes	100,000	17 days
Inland seas	105,000	Data not available
Soil moisture	50,000	1 year
Groundwater	8,200,000	1400 years
Glaciers & ice caps	27,500,000	Montane glaciers: 1600 yearsPolar ice caps: 9700 years
Organisms	1,100	Hours

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Oceans (96.5% of all water)

• Composed of saltwater with an average salinity of 35 parts per thousand (%)
• High salinity makes seawater unsuitable for farming, industry, or drinking
• Desalination can convert seawater into freshwater but is energy-intensive and expensive

Glacial and Polar Ice (1.7%)

• Frozen freshwater found in high mountains (e.g., Rockies, Himalayas) and polar regions (Greenland, Antarctica, Arctic, Southern Oceans)
• Ice fluctuates seasonally but is declining due to global warming

Groundwater (1.7%)

• Water percolates into porous rocks or aquifers
• Groundwater is generally fresh but may contain dissolved minerals
• Extracted using wells

Surface Freshwater (0.02%)

• Includes rivers, lakes, and freshwater wetlands
• Naturally low salinity but prone to pollution from farming, industry, and households
• Despite being a small store, it is highly accessible

Relatively Minor Stores

Atmosphere (0.001%)

• Water evaporates from oceans and surface waters and enters the air as water vapor
• Varies over time and location
• Water vapor is an important greenhouse gas, helping regulate Earth’s temperature

Living Organisms (0.0001%)

• Water is essential for life, present in cells and tissues of all organisms

Exercises