Global water stores and the closed system
A store holds water for a time. A flow (or transfer) moves water between stores. For the whole Earth the water cycle is a closed system: water is not added or lost, it only moves.
Rough shares of all water: oceans about 96.5%, ice sheets and glaciers about 1.7%, groundwater about 1.7%, lakes, rivers, soil and wetlands well under 0.1%, the atmosphere about 0.001%. So only around 2.5% is fresh, and most of that is locked in ice or deep rock.
Residence time is how long water stays in a store: about 9 days in the air, weeks in rivers, thousands of years in deep groundwater or the Antarctic ice sheet.
The cryosphere (ice), lithosphere (rocks), hydrosphere (oceans and rivers), atmosphere and biosphere (living things) are all stores.
Evaporation, condensation and precipitation
Evaporation: liquid water turns into vapour using the Sun's energy. It is faster when it is hot, windy, dry and when there is lots of open water. Transpiration is water lost from leaves. Together they are evapotranspiration.
Condensation: rising air cools. When it cools to its dew point, vapour turns into tiny droplets around dust or salt specks (condensation nuclei), making clouds. Air rises because of heating (convection), mountains (relief) or meeting a colder air mass (fronts).
Precipitation: droplets join until they are heavy enough to fall as rain, snow or hail. Rain can be convectional, relief (orographic) or frontal.
Cryosphere processes: snow builds up (accumulation) and melts (ablation); sublimation turns ice straight into vapour.
The drainage basin as an open system
A drainage basin is the land drained by one river and its tributaries. Its edge is the watershed, a ridge of high ground.
- Input: precipitation.
- Stores: interception (on leaves), surface storage (puddles, lakes), soil water, groundwater, channel storage.
- Flows: throughfall and stemflow (from leaves to the ground), infiltration (into soil), percolation (down into rock), throughflow (sideways in soil), groundwater flow (slowly in rock), overland flow (over the surface) and channel flow.
- Outputs: evapotranspiration and river runoff to the sea.
It is an open system because water and energy cross its boundary.
Water balance
The water balance is like a bank account: P = Q + E ± ΔS. P is precipitation, Q is river runoff (discharge), E is evapotranspiration and ΔS is the change in storage.
In a wet season P is bigger than E, so soil and groundwater fill up (water surplus). In a dry season E can be bigger than P, so stores are used up (soil water deficit). When rains return, soil is first refilled (recharge) before rivers rise much.
This is a mass balance: whatever comes in must go out or be stored.
Runoff and flood hydrographs
Discharge = cross-section area × velocity, in cubic metres per second (m³/s, also called cumecs).
A storm (flood) hydrograph plots discharge against time after a rain event, with rain shown as bars. Parts: base flow (slow groundwater supply), rising limb, peak discharge, falling (recessional) limb, and lag time (from peak rain to peak discharge).
Flashy or subdued?
A flashy hydrograph (short lag, high peak) comes from small steep basins, impermeable rock (like clay or granite), soil already saturated, very heavy rain, little vegetation and lots of towns. A subdued hydrograph (long lag, low peak) comes from large gentle basins, permeable rock (like chalk or sandstone), dry soils, forest and farmland with good soil.
A river regime is the yearly pattern of discharge, for example a monsoon river peaking in July to September, or a glacier-fed river peaking in summer melt.
Natural and human changes to the water cycle
Natural changes: storms bring short floods; seasons change rain and evaporation; droughts such as those linked to El Niño empty stores; long-term climate change shrinks glaciers and changes snowmelt.
Human changes:
- Deforestation: less interception and transpiration, more overland flow, faster floods and soil erosion.
- Urbanisation: concrete and drains make water run off fast; less infiltration.
- Farming: ploughing, irrigation and drainage ditches change infiltration and runoff.
- Water abstraction: pumping groundwater faster than it refills lowers the water table.
- Dams and reservoirs: store water, smooth floods, increase evaporation.
Skills and case studies
Number skills
1 mm of rain on 1 m² = 1 litre. 1 mm on 1 km² = 1,000 m³. 1 m³ = 1,000 litres. To get discharge, multiply mean width × mean depth (area) by mean velocity.
Presenting field data
Draw cross-sections of a river, plot hydrographs with rain bars, use scatter graphs (for example infiltration rate against vegetation cover) and describe trends with numbers.
Case study: a tropical rainforest
Rainforests (such as the Amazon or Borneo) have heavy convectional rain, very high interception and transpiration. Up to half of the rain may be recycled back to the air by the trees. When forest is cleared, runoff and soil erosion rise, local rain can fall and carbon stored in trees is released. Protection, replanting and sustainable forestry help both the water and carbon cycles.
Case study: a local river catchment
Study a river near you: where it starts, its rock and land use, how towns get drinking water from it, and when it floods. Compare a natural upstream part with an urban downstream part.
Try it at home
Pour one cup of water on bare soil and one cup on grass or a mulched pot. Time how long each takes to soak in. Which gives more 'runoff'?
Key formulas and definitions
- Water balance: P = Q + E ± ΔS (precipitation = runoff + evapotranspiration ± change in storage).
- Discharge Q = cross-section area (m²) × velocity (m/s), in m³/s (cumecs).
- Lag time = time of peak discharge − time of peak rainfall.
- 1 mm of rain on 1 m² = 1 litre; 1 m³ = 1,000 litres.
- Evapotranspiration = evaporation + transpiration.
Worked examples
1. A basin gets 1,200 mm of rain in a year. Runoff is 500 mm and evapotranspiration 650 mm. Find the change in storage.
ΔS = P − Q − E = 1,200 − 500 − 650 = +50 mm. Storage rose by 50 mm (soil and groundwater gained water).
2. A river is 8 m wide, 1.5 m deep on average and flows at 0.6 m/s. Find its discharge.
Area = 8 × 1.5 = 12 m². Q = 12 × 0.6 = 7.2 m³/s.
3. Peak rain was at 10:00 and peak discharge at 22:00. What is the lag time?
22:00 − 10:00 = 12 hours.
4. 20 mm of rain falls on a 3 km² town. How many cubic metres of water is that?
20 mm on 1 km² = 20 × 1,000 = 20,000 m³. On 3 km² = 60,000 m³ (60 million litres).
5. Explain why a basin's hydrograph became flashier after a forest was cleared for a new town.
Less interception and transpiration means more water reaches the ground. Concrete stops infiltration, so water moves as overland flow and through drains straight to the river. The peak rises and the lag time shortens.
6. In a dry season P = 20 mm and E = 80 mm, with no runoff. What happens to the stores?
ΔS = 20 − 0 − 80 = −60 mm. Soil and groundwater lose 60 mm: a soil water deficit.
Common mistakes
- Calling the drainage basin a closed system. Water enters and leaves it, so it is open; only the global cycle is closed.
- Mixing up infiltration (into the soil surface) and percolation (deeper into rock).
- Measuring lag time from the start of rain. It is from peak rain to peak discharge.
- Thinking all fresh water is in rivers and lakes. Most is ice and groundwater.