Water Cycle Processes

This lesson examines the key physical processes that drive the water cycle in detail. While earlier lessons introduced these processes in the context of the global water cycle and drainage basin hydrology, here we explore the underlying science, spatial variations, and the factors that modify each process. A thorough understanding of these processes — and the terminology associated with them — is essential for answering AQA A-Level questions on physical geography.

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Evaporation

Key Definition: Evaporation is the process by which liquid water is converted to water vapour (a gas) at a temperature below its boiling point. It occurs at the surface of water bodies, soil, and wet surfaces.

The Physics of Evaporation

Evaporation requires energy to break the hydrogen bonds between water molecules. This energy is known as the latent heat of vaporisation — approximately 2,260 kJ per kg of water evaporated. When water evaporates, it absorbs this energy from the surrounding environment, producing a cooling effect. This is why evaporation is an important mechanism for heat transfer in the climate system.

Factors Controlling Evaporation Rate

Factor	Effect	Explanation
Temperature	Higher → faster evaporation	More molecules have sufficient kinetic energy to escape the liquid surface
Humidity	Lower → faster evaporation	The vapour pressure gradient between the surface and the air is steeper
Wind speed	Higher → faster evaporation	Wind removes saturated air from above the surface, maintaining the vapour pressure gradient
Surface area	Greater → more evaporation	More area for molecules to escape from
Salinity	Higher → slower evaporation	Dissolved salts reduce the vapour pressure of water (Raoult's Law) — ocean water evaporates ~5% slower than pure water
Solar radiation	More → faster evaporation	Provides the energy required for phase change

Global Patterns of Evaporation

Global evaporation is estimated at approximately 577,000 km³/year (Trenberth et al., 2007). Oceanic evaporation (~505,000 km³/year) dominates because:

• Oceans cover 71% of Earth's surface
• Ocean water is continuously available (unlike land surfaces, which may dry out)
• Warm ocean currents (e.g., Gulf Stream, Kuroshio) produce particularly high evaporation rates

The subtropical oceans (around 15°–30° latitude) have the highest evaporation rates — high solar input combined with relatively low humidity and persistent trade winds. The equatorial zone has slightly lower evaporation because humidity is very high, reducing the vapour pressure gradient.

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Transpiration

Key Definition: Transpiration is the loss of water vapour from plant surfaces, primarily through stomata (microscopic pores on the underside of leaves).

The Mechanism

Transpiration is driven by the soil-plant-atmosphere continuum (SPAC), a concept developed by Philip (1966):

1. Water is absorbed by plant roots from the soil through osmosis.
2. It moves upwards through the xylem vessels, driven by a tension gradient (cohesion-tension theory, Dixon & Joly, 1895).
3. Water evaporates from cell surfaces within the leaf into substomatal air spaces.
4. Water vapour diffuses out through open stomata into the atmosphere.

Factors Controlling Transpiration

Factor	Effect
Temperature	Higher temperatures increase the rate of evaporation from leaf cell surfaces
Humidity	Low humidity increases the diffusion gradient from leaf interior to atmosphere
Wind	Increases removal of humid air from the leaf boundary layer
Light intensity	Stomata open in response to light (for photosynthesis), allowing transpiration
Soil moisture availability	Low soil moisture → stomata close to conserve water → transpiration decreases
Leaf area index (LAI)	Higher LAI → more leaf surface area → greater total transpiration

The Significance of Transpiration

• A mature oak tree can transpire up to 600 litres per day in summer (Roberts, 1983).
• The Amazon rainforest returns approximately 50% of its precipitation back to the atmosphere through transpiration, contributing to local and regional rainfall recycling (Salati et al., 1979). This "flying rivers" phenomenon means that deforestation in the Amazon can reduce rainfall thousands of kilometres downwind.
• Globally, transpiration accounts for approximately 60–80% of total terrestrial evapotranspiration (Jasechko et al., 2013).

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Evapotranspiration and Its Measurement

Key Definition: Evapotranspiration (ET) is the combined water loss from a surface through evaporation and transpiration. Potential evapotranspiration (PE) is the maximum ET that would occur if water supply were unlimited. Actual evapotranspiration (AE) is the real-world ET limited by available moisture.

The Penman Equation

Howard Penman (1948) developed an equation to estimate PE from meteorological data:

PE is calculated using:

• Net radiation (energy available for evaporation)
• Air temperature
• Wind speed
• Humidity (vapour pressure deficit)

The Penman-Monteith equation (Monteith, 1965) extended this to include vegetation resistance to water loss (stomatal conductance), making it applicable to vegetated surfaces. It is the standard method recommended by the FAO for estimating crop water requirements.

Seasonal Variation in the UK

Month	Typical PE (mm)	Typical Rainfall (mm)	Water Balance
January	10	80	Surplus (+70 mm)
April	45	55	Surplus (+10 mm)
July	95	50	Deficit (−45 mm)
October	30	75	Surplus (+45 mm)

Approximate values for SE England

From April to September, PE typically exceeds rainfall, creating a soil moisture deficit. From October to March, rainfall exceeds PE, allowing soil moisture recharge and groundwater recharge.

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Condensation

Key Definition: Condensation is the process by which water vapour changes to liquid water. It releases latent heat — the same 2,260 kJ/kg absorbed during evaporation — warming the surrounding air.

Requirements for Condensation