Water and Carbon Cycles
Processes Driving Change in the Magnitude of Water Stores
Introduction
The magnitude of water stores, the amount of water held in the atmosphere, cryosphere, lithosphere, and hydrosphere, changes continually over time and space. Flows and transfers, including evaporation, condensation, cloud formation, precipitation, and cryospheric processes, drive these variations. They operate across a range of spatial scales (from hill slopes to global systems) and over timescales from minutes to millennia.
| Processes | Description | Effects on Water Stores | Typical Timescale |
|---|---|---|---|
| Evaporation | Solar energy converts liquid water into water vapour. | Decreases surface and soil water; increases atmospheric water vapour. | Minutes to days |
| Condensation | Air cools to its dew point and water vapour becomes liquid. | Reduces atmospheric vapour; increases cloud droplets or dew. | Hours to days |
| Cloud Formation | Water vapour condenses around condensation nuclei to form droplets or ice crystals. | Transfers water from vapour to suspended cloud droplets. | Hours to days |
| Precipitation | Water falls to Earth as rain, snow, hail, or sleet. | Moves water from atmosphere to surface stores. | Hours to weeks |
| Cryospheric Processes | Freezing, melting, sublimation, and calving alter cryospheric water storage. | Transfers water between ice, ocean, and atmosphere. | Seasonal to millennial |
2. Evaporation
Process
Evaporation occurs when solar radiation provides sufficient energy for water molecules to escape the surface and enter the atmosphere as vapour.
Influencing Factors
- Temperature: higher temperatures accelerate evaporation.
- Wind speed: removes saturated air and enhances evaporation.
- Humidity: low humidity increases the potential for evaporation.
- Surface area: larger areas (e.g. oceans) promote greater evaporation.
- Albedo: darker surfaces absorb more energy, increasing evaporation.
Scale and Timescale
- Hill slope: evaporation from puddles, vegetation, and soil (minutes–days).
- Drainage basin: losses from lakes, rivers, and vegetation (days–weeks).
- Global: oceanic evaporation drives the global atmospheric moisture budget (continuous).
3. Condensation
Process
When moist air cools to its dew point, it becomes saturated and condenses into liquid droplets, forming clouds, fog, or dew.
Conditions Promoting Condensation
- Air cooling: as warm air rises and expands, or contacts a colder surface.
- Atmospheric instability: uplift through convection, orography, or frontal activity.
Scale and Timescale
- Hill slope: dew and fog formation overnight (hours).
- Drainage basin: valley fog or cloud on uplands (hours–days).
- Global: widespread condensation driving latent heat release and atmospheric circulation (days–weeks).
4. Cloud Formation
Mechanisms of Air Uplift
- Orographic uplift – air rises over mountains, cools, and condenses.
- Convectional uplift – surface heating causes air to rise.
- Frontal uplift – warm air rises over cooler, denser air at a front.
- Convergence – air masses meet and are forced upwards (common near the Equator).
Cloud Types
- Cumuliform: vertically developed, linked to convection.
- Stratiform: layered clouds, often from gentle frontal uplift.
- Cirrus: high-altitude ice clouds indicating moisture movement in the upper air.
Timescale
Clouds may form in minutes (tropical convection) or persist for days (frontal systems).
5. Causes of Precipitation
Precipitation occurs when water droplets or ice crystals within clouds grow large enough that gravitational forces overcome updraughts, allowing them to fall to the Earth’s surface. Growth occurs through collision and coalescence in warm clouds, or through the Bergeron–Findeisen process in cold clouds, where ice crystals grow at the expense of water droplets.
Key Processes
- Coalescence: small droplets merge to form larger ones that fall as rain.
- Bergeron–Findeisen process: in cold clouds, ice crystals grow at the expense of water droplets and fall as snow or melt into rain.
Spatial and Temporal Variation
- Hill slope: localised showers from convection or orographic uplift.
- Drainage basin: widespread, prolonged frontal rainfall.
- Global: large-scale patterns controlled by the Inter-Tropical Convergence Zone (ITCZ) and mid-latitude storm tracks.
Timescale
- Short-term: convective storms form and release rainfall within hours.
- Seasonal: global shifts in the ITCZ cause monsoon rainfall patterns.
| Process | Description | Effect on Stores | Timescale |
|---|---|---|---|
| Accumulation | Inputs of snow and ice via snowfall and freezing. | Increases cryospheric store. | Seasonal to millennial |
| Ablation | Loss through melting, sublimation, or calving. | Transfers water to hydrosphere. | Seasonal to annual |
| Sublimation | Direct change from ice to vapour. | Reduces ice; increases atmospheric water. | Daily to seasonal |
| Freeze-Thaw | Cycles of melting and refreezing. | Controls runoff generation and glacier mass balance. | Days to decades |
| Calving | Icebergs break from glaciers or ice shelves. | Transfers ice to ocean store. | Seasonal to centennial |
Scale and Timescale
- Hill slope: snowmelt and freeze–thaw influence runoff (daily–seasonal).
- Drainage basin: glacier melt influences river discharge (seasonal–annual).
- Global: ice sheet growth and retreat alter sea level (centuries–millennia).
7. Global Climate Influence
Global climatic processes drive large-scale variation in water storage and transfers:
- Atmospheric circulation: controls patterns of evaporation, condensation, and rainfall. The ITCZ migrates seasonally, concentrating intense convection and heavy rainfall near the thermal equator.
- Glacial–interglacial cycles: during the last Ice Age (~18,000 years ago), one-third of Earth’s land was ice-covered, increasing cryospheric storage and lowering sea levels by over 100 metres.
- Contemporary climate change: global warming accelerates evaporation and ice melt, altering the magnitude of atmospheric, hydrospheric, and cryospheric stores.
8. Human Modification of Natural Processes
Although this section focuses on natural processes, human activity can alter the rate and scale of flows and transfers:
- Deforestation reduces interception and evapotranspiration, increasing surface runoff.
- Urbanisation creates impermeable surfaces, limiting infiltration and soil storage.
- Groundwater abstraction reduces subsurface stores and alters baseflow.
- Industrial emissions can increase condensation nuclei, subtly affecting local precipitation patterns.
These changes modify how quickly water moves through the system, disrupting local and regional water balances.
9. Temporal and Spatial Variability
The magnitude of water stores and the rate of flows between them vary over time (temporal variability) and across space (spatial variability). These variations result from differences in climate, relief, vegetation, and human activity, and operate over a wide range of timescales, from short-lived weather events to long-term climate cycles. Understanding these variations helps geographers explain how the water cycle remains in dynamic equilibrium despite constant change.
| Scale | Typical Processes | Timescale | Example |
|---|---|---|---|
| Hill slope | Infiltration, evaporation, interception, throughflow. | Minutes–days | Evaporation from soil after rainfall. |
| Drainage basin | Seasonal weather patterns, geology, soil permeability, vegetation type. | Days–years | Seasonal flood and drought patterns. |
| Global | Cryospheric accumulation and ablation, ocean–atmosphere transfers, glacial–interglacial variation. | Centuries–millennia | Growth and melting of polar ice sheets altering sea level. |
10. Dynamic Equilibrium
The global water cycle remains in a state of dynamic equilibrium; inputs, outputs, and stores fluctuate, but overall balance is maintained. However, both natural climate variability and human-induced change can disrupt this balance, influencing the magnitude and movement of water across all stores.
Summary
- The magnitude of water stores changes constantly due to flows and transfers such as evaporation, condensation, precipitation, and cryospheric processes.
- These processes operate across hill slope, basin, and global scales and over timescales from minutes to millennia.
- Global atmospheric circulation and glacial-interglacial cycles are key large-scale controls on water distribution.
- Human activity increasingly modifies these natural processes, altering rates of transfer and storage.
Exam Tip
When answering A Level questions on systems:
- Refer to spatial scale (hill slope → basin → global).
- Include timescale and process mechanism (e.g. latent heat, adiabatic cooling).
- Use specific examples (ITCZ rainfall, glacier melt, permafrost thaw).
- Apply system thinking – link inputs, outputs, and feedback to changes in store magnitude.