The Processes of the Water Cycle

The water cycle involves the continuous movement of water between stores in the atmosphere, lithosphere, hydrosphere, and biosphere.

Although the global cycle is a closed system, continuous processes transfer water between stores. These include evaporation, transpiration, condensation, precipitation, interception, run-off, infiltration, percolation, throughflow, groundwater flow, and cryospheric processes.

Evaporation

Evaporation is the process by which liquid water changes into water vapour due to heating by solar energy.

• Most evaporation occurs from oceans, but also from rivers, lakes, and soil surfaces.
• The rate of evaporation depends on temperature, humidity, wind speed, and the availability of water.
• As water evaporates, it absorbs latent heat from its surroundings, storing energy in the atmosphere and influencing weather and climate patterns.

Transpiration

Transpiration is the process by which plants release water vapour from small pores (stomata) in their leaves.

• This water is drawn up through the roots and stem from the soil.
• Transpiration rates depend on temperature, wind speed, humidity, and plant type.
• Together, evaporation and transpiration form evapotranspiration, a significant input of water vapour to the atmosphere.

Condensation

Condensation occurs when air cools to its dew point, and water vapour changes back into liquid water.

• Cooling happens when warm, moist air rises and expands in the atmosphere.
• As it cools, the air’s ability to hold water decreases, leading to condensation on small particles (condensation nuclei) to form clouds, mist, or dew.
• Condensation is vital to the water cycle as it forms clouds, which are needed for precipitation to occur.

Cloud Formation

Clouds are visible masses of water droplets or ice crystals suspended in the atmosphere. They form when warm, moist air rises, cools, and condenses.

1. Warming and rising of air

Conduction causes a parcel of air near the surface to warm faster than the surrounding air.
Because it is less dense, the air rises by convection.
The atmosphere is described as unstable when rising air is warmer than the surrounding air.

2. Cooling as the air rises

• As the air rises, it expands due to lower atmospheric pressure.
• Expansion causes the air to cool as the particles move further apart.
• Initially, the air cools at around 10°C per kilometre, known as the Dry Adiabatic Lapse Rate (DALR), because it is not yet saturated.

3. Reaching the dew point

• The air continues to rise and cool until it reaches its dew point, at which point water vapour begins to condense into droplets.
• This condensation forms clouds and releases latent heat, warming the rising air.

4. Cooling at the saturated adiabatic lapse rate

• Once condensation begins, the air is saturated.
• Because latent heat is released, the air cools more slowly — around 7°C per kilometre.
• This is called the Saturated Adiabatic Lapse Rate (SALR).

5. The top of the cloud

• The air mass rises until it reaches the same temperature as the surrounding air.
• Rising then stops, the atmosphere becomes stable, and this marks the top of the cloud.

Precipitation

Precipitation occurs when water droplets or ice particles in clouds grow large enough to fall under gravity.

• Types include rain, snow, sleet, hail, and drizzle.
• The form of precipitation depends on air temperature and atmospheric conditions.
• Precipitation transfers water from the atmosphere to the land or oceans.

Causes of Precipitation

Precipitation forms through three main uplift mechanisms:

• Convectional rainfall – the Sun heats the ground, warm air rises rapidly, cools, condenses, and produces heavy, short-lived showers (common in tropical regions and summer storms).
• Orographic (relief) rainfall – moist air is forced to rise over mountains, cools, and condenses to form rain on the windward side; the leeward side is often drier.
• Frontal rainfall – warm and cold air masses meet; the warmer, lighter air rises over the colder air, cools, and condenses, producing widespread rain along weather fronts.

Interception

Interception occurs when precipitation is caught and held by vegetation before reaching the ground.

• Some of this water evaporates directly from leaves or branches.
• The remainder drips to the ground (throughfall) or runs down stems and trunks (stemflow).
• Dense vegetation can delay or reduce surface run-off, influencing the speed and volume of water entering river systems.

Run-off

Run-off is the movement of water over the land surface, returning it to rivers, lakes, and eventually the oceans.

• It occurs when precipitation exceeds infiltration capacity, or the ground is saturated or impermeable.
  Run-off links the land, river, and ocean stores and forms part of the surface water transfer system.

Types of Run-off

• Overland flow – water flows across the ground surface because it cannot infiltrate quickly enough.
• Saturated overland flow – occurs when the soil is already saturated and any additional rainfall immediately flows over the surface.

Catchment Hydrology Processes

Catchment hydrology refers to the movement and storage of water within a drainage basin — the area of land drained by a river and its tributaries. It examines how precipitation becomes run-off and how water moves through the soil and underlying rock before reaching river channels. These processes influence the timing, quantity, and pathways of water flow within a catchment, which in turn affect flood risk, water quality, and river discharge patterns.

The main hydrological processes operating within a drainage basin include infiltration, percolation, throughflow, and groundwater flow.

Infiltration

Infiltration is the downward movement of water from the surface into the soil.

The infiltration rate depends on soil type, slope angle, vegetation cover, and precipitation intensity.
When infiltration capacity is exceeded, water remains on the surface, increasing run-off.

Percolation

Percolation is the movement of water from the soil into the underlying rock layers.

• It occurs through pore spaces, joints, or bedding planes in permeable rocks such as sandstone or chalk.
• Percolation replenishes groundwater stores and maintains baseflow in rivers.

Throughflow

Throughflow is the lateral movement of water through the soil towards a river channel.

• It is influenced by soil permeability and slope angle.
• Throughflow provides a delayed transfer of water to rivers after rainfall events.

Groundwater Flow

Groundwater flow is the slow movement of water through permeable rock below the water table.

• It is the deepest and slowest transfer of water within the drainage basin.
• Groundwater eventually discharges into rivers, lakes, or the sea, maintaining river flow during dry periods (baseflow).

Cryospheric Processes

Cryospheric processes include accumulation and ablation within glaciers and ice sheets.

• Accumulation adds ice through snowfall and compaction.
• Ablation removes ice through melting, calving, or sublimation.
• These processes are key to the global water balance and are influenced by temperature and climate change.