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This lesson covers the global hydrological cycle as a closed system, the major stores and fluxes of water, residence times and the global water budget. It addresses Edexcel A-Level Geography (9GE0) Paper 1, Topic 5: The Water Cycle and Water Insecurity, Enquiry Question 1: What are the processes operating within the hydrological cycle from global to local scale?
In geography, a system is a set of interrelated components that work together. Systems have inputs, outputs, stores (sometimes called components or stocks) and flows (also called transfers or fluxes).
The global hydrological cycle is a closed system — this means that no water enters or leaves the system. The total amount of water on Earth is fixed at approximately 1.386 billion km³. Water is neither created nor destroyed; it is simply moved between stores and changed between states (solid, liquid, gas).
graph TD
A["Atmosphere<br/>Store: 12,900 km³"] -->|"Precipitation<br/>~505,000 km³/yr"| B["Land Surface<br/>(Ice, Lakes, Rivers, Soil)"]
B -->|"Evapotranspiration<br/>~72,000 km³/yr"| A
B -->|"Runoff<br/>~45,500 km³/yr"| C["Oceans<br/>Store: 1,335,040,000 km³"]
C -->|"Evaporation<br/>~434,000 km³/yr"| A
A -->|"Precipitation over oceans<br/>~398,000 km³/yr"| C
B -->|"Groundwater flow<br/>~2,200 km³/yr"| C
Exam Tip: The closed-system concept is fundamental. In your answers, always state that the global hydrological cycle is a closed system because no water is added from or lost to external sources. Contrast this with a drainage basin, which is an open system (it receives precipitation inputs and loses water through evapotranspiration and river discharge outputs).
Water on Earth is held in a variety of stores (also called reservoirs or stocks). The distribution of water between these stores is extremely uneven. The vast majority is held in the oceans.
| Store | Volume (km³) | % of Total Water | % of Fresh Water |
|---|---|---|---|
| Oceans | 1,335,040,000 | 96.54% | — |
| Ice caps and glaciers | 26,350,000 | 1.74% | 68.7% |
| Groundwater (total) | 10,530,000 | 0.76% | — |
| — Fresh groundwater | 10,530,000 | 0.76% | 30.1% |
| Permafrost (ground ice) | 300,000 | 0.022% | 0.86% |
| Freshwater lakes | 91,000 | 0.007% | 0.26% |
| Saline (salt) lakes | 85,400 | 0.006% | — |
| Soil moisture | 16,500 | 0.001% | 0.05% |
| Atmosphere (water vapour) | 12,900 | 0.001% | 0.04% |
| Rivers | 2,120 | 0.0002% | 0.006% |
| Biota (living organisms) | 1,120 | 0.0001% | 0.003% |
Oceans dominate, holding approximately 96.5% of all water. This is saline water (average salinity ~35 parts per thousand) and is not directly usable for drinking or agriculture.
Of the 2.5% that is fresh water, the vast majority (~68.7%) is locked in ice caps and glaciers, primarily the Antarctic ice sheet (~26.5 million km³), the Greenland ice sheet (~2.6 million km³) and mountain glaciers. This water is essentially unavailable for human use unless it melts.
Groundwater is the second-largest freshwater store (~30.1% of all fresh water). It is held in pores, cracks and fissures in permeable rocks called aquifers. Some groundwater is thousands of years old (fossil water).
Surface freshwater — rivers, lakes and soil moisture — constitutes less than 1% of all fresh water and less than 0.01% of total global water. Yet this is the water that ecosystems and human societies depend on most directly.
The atmosphere holds only about 12,900 km³ of water at any given time — a tiny fraction of the total. However, it is the most dynamic store, with an average residence time of only about 9 days.
Exam Tip: You may be asked to explain why water scarcity exists even though Earth has 1.386 billion km³ of water. The answer lies in accessibility: 96.5% is saline, ~1.7% is frozen, and much groundwater is too deep to extract economically. Only about 0.3% of all freshwater is readily accessible surface water in rivers and lakes.
A flux is the movement of water between stores. The main fluxes in the global hydrological cycle are:
| Flux | Direction | Volume (km³/yr) | Description |
|---|---|---|---|
| Evaporation from oceans | Ocean → Atmosphere | ~434,000 | Solar energy converts liquid water to water vapour |
| Precipitation over oceans | Atmosphere → Ocean | ~398,000 | Water vapour condenses and falls directly back to oceans |
| Precipitation over land | Atmosphere → Land | ~107,000 | Rain, snow, sleet, hail falling on land surfaces |
| Evapotranspiration from land | Land → Atmosphere | ~72,000 | Combined evaporation from surfaces + transpiration from vegetation |
| Runoff (surface + groundwater) | Land → Ocean | ~45,500 | Water returning to oceans via rivers and groundwater flow |
| Infiltration | Surface → Soil/Groundwater | Variable | Water seeping downwards into soil and rock |
| Percolation | Soil → Groundwater | Variable | Deeper downward movement of water through rock |
At the global scale, the hydrological cycle is in dynamic equilibrium — the total amount of water in the system remains constant, but water moves continuously between stores. For the global system:
Total evaporation = Total precipitation
At a more detailed level:
Evaporation is the process by which liquid water is converted to water vapour. It occurs when water molecules at the surface gain enough kinetic energy (from solar radiation) to overcome the attractive forces holding them in the liquid phase and escape into the atmosphere.
Factors affecting the rate of evaporation include:
When moist air rises (through convective uplift, orographic uplift or frontal uplift), it cools adiabatically. When it reaches the dew point temperature, water vapour condenses onto condensation nuclei (tiny particles such as dust, pollen, sea salt or pollution) to form cloud droplets. These droplets may coalesce to form precipitation.
Precipitation occurs in several forms:
Global mean precipitation is approximately 1,000 mm/yr over the land surface and 1,100 mm/yr over the oceans.
Transpiration is the loss of water vapour from the stomata of plant leaves. It accounts for a significant proportion of water returned to the atmosphere from land — estimates suggest that 60-80% of evapotranspiration over vegetated land surfaces is transpiration rather than direct evaporation.
Evapotranspiration is the combined total of evaporation from surfaces and transpiration from vegetation. Potential evapotranspiration (PET) is the amount that would occur if unlimited water were available; actual evapotranspiration (AET) is the amount that actually occurs given available water supply.
Exam Tip: When discussing evapotranspiration, distinguish between PET and AET. In arid regions, PET far exceeds AET because water supply limits actual evapotranspiration. This concept is crucial for understanding water budgets and water balance graphs.
Residence time is the average length of time a water molecule spends in a particular store before being transferred to another store. It is calculated as:
Residence time = Volume of store ÷ Rate of transfer (flux)
| Store | Approximate Residence Time |
|---|---|
| Atmosphere | ~9 days |
| Rivers | ~2–6 weeks |
| Soil moisture | ~1–2 months |
| Seasonal snow cover | ~2–6 months |
| Freshwater lakes | ~10 years |
| Oceans | ~3,100 years |
| Glaciers | ~20–100 years (mountain); ~10,000+ years (ice sheets) |
| Deep groundwater | ~10,000 years |
| Antarctic ice sheet | ~900,000 years |
Exam Tip: Residence times are frequently examined. Remember that ocean water has a residence time of ~3,100 years, atmospheric water vapour ~9 days, and deep groundwater can be >10,000 years. Link these to real-world issues — e.g., the rapid cycling of atmospheric water means that changes in evaporation or precipitation can have quick effects on weather patterns.
The distribution of water between stores is not permanently fixed — it varies over different timescales.
| Human Activity | Effect on Water Stores |
|---|---|
| Dam construction | Increases surface water storage; ~10,800 km³ held in reservoirs globally |
| Groundwater abstraction | Depletes groundwater stores; some aquifers falling by >1 m/yr |
| Deforestation | Reduces interception and transpiration; increases runoff |
| Urbanisation | Reduces infiltration; increases surface runoff |
| Irrigation | Transfers water from groundwater/rivers to soil and atmosphere |
| Climate change (fossil fuel burning) | Accelerates ice melt; alters precipitation patterns; increases atmospheric moisture (~7% per °C warming, per Clausius-Clapeyron relation) |
The cryosphere refers to the parts of the Earth system where water is in its solid (frozen) form. This includes:
The cryosphere stores approximately 26.35 million km³ of water (1.74% of total). It acts as a long-term regulator of the hydrological cycle and climate. When the cryosphere shrinks (as is happening under current warming), it:
graph LR
A["Global Temperature Rises"] --> B["Ice/Snow Melts"]
B --> C["Albedo Decreases"]
C --> D["More Solar Energy Absorbed"]
D --> A
style A fill:#ff9999
style B fill:#99ccff
style C fill:#ffcc99
style D fill:#ff9999
Exam Tip: The ice-albedo feedback is a classic example of a positive feedback loop in the water cycle. Make sure you can explain how reduced ice cover leads to reduced albedo, which leads to further warming and further ice loss — a self-reinforcing cycle.
| Key Concept | Detail |
|---|---|
| System type | Global hydrological cycle is a closed system |
| Total water | ~1.386 billion km³ (fixed) |
| Largest store | Oceans (96.54%) |
| Largest freshwater store | Ice caps and glaciers (68.7% of fresh water) |
| Most dynamic store | Atmosphere (residence time ~9 days) |
| Most important flux | Evaporation from oceans (~434,000 km³/yr) |
| Water balance | Global evaporation = global precipitation |
| Key trend | Cryosphere shrinking; ocean store increasing; atmospheric moisture increasing |
| Human impacts | Dams, abstraction, deforestation, urbanisation, climate change |
Understanding the global hydrological cycle as a system — with quantifiable stores, fluxes and residence times — is the foundation for everything else in Topic 5. In the next lesson, we scale down from the global to the drainage basin level and explore how the cycle operates as an open system.