How does water move through the global hydrological cycle and the drainage basin, and what controls the balance between them?
The global water cycle as a closed system; the drainage basin as an open subsystem; inputs, outputs, stores and flows; the water balance; runoff variation and the storm hydrograph.
A focused answer to the AQA A-Level Geography 3.1.1 water cycle content, covering the global water cycle as a closed system, the drainage basin as an open subsystem, inputs, outputs, stores and flows, the water-balance equation, factors driving runoff variation and the storm hydrograph.
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What this dot point is asking
AQA section 3.1.1 wants you to describe the global water cycle as a closed system, treat the drainage basin as an open subsystem with inputs, outputs, stores and flows, work quantitatively with the water-balance equation, and explain the physical and human factors that change runoff and the storm hydrograph. The water cycle is taught alongside the carbon cycle because the two are physically coupled, but the water-specific detail here is about stores, fluxes and the basin response to rainfall.
The global water cycle as a closed system
At the planetary scale the water cycle is a closed system: matter (water) is conserved, so only energy (solar radiation and gravitational potential energy) crosses the boundary. The major stores, with their approximate share of all water, are the oceans (about 97 percent), the cryosphere (ice caps and glaciers, holding most freshwater), groundwater, lakes and rivers, soil moisture and the atmosphere (a tiny store but rapidly cycled, with an average residence time of only about nine days).
The fluxes that move water between stores are evaporation and transpiration (together evapotranspiration), condensation, precipitation, cryospheric exchange (melting, freezing, sublimation), surface runoff and groundwater flow. Because these transfers are driven by the Sun and by gravity, the cycle keeps moving while the total mass of water stays fixed.
The drainage basin as an open subsystem
A drainage basin is the area drained by a river and its tributaries, bounded by the watershed. Unlike the global cycle, the basin is an open system because both energy and matter cross its boundary: precipitation enters, and water leaves as evapotranspiration, river discharge to the sea and deep percolation to regional groundwater.
The inputs are precipitation (rain, snow, hail). The outputs are evapotranspiration, river discharge (channel flow leaving at the mouth) and percolation to deeper groundwater. The stores are interception (water held on vegetation), surface storage, soil moisture, groundwater and channel storage. The flows connect them: infiltration (water entering the soil), throughflow (lateral movement through soil), percolation (downward movement to groundwater), overland flow (water moving across the surface when rainfall exceeds infiltration capacity), and baseflow (slow groundwater feed that sustains rivers between storms).
Runoff variation and the storm hydrograph
The storm hydrograph links a rainfall event to the river's response. The rising limb shows discharge increasing; the peak discharge is the maximum; the lag time is the gap between peak rainfall and peak discharge; the falling (recession) limb shows the return towards baseflow.
A flashy hydrograph (short lag time, high peak) results when water reaches the channel quickly. The controlling factors are:
- Basin size and shape. Small, circular basins respond fast; large, elongated basins spread the response and flatten the hydrograph.
- Relief. Steep slopes accelerate overland flow and shorten lag time.
- Geology and soils. Impermeable rock and saturated clay soils force rapid overland flow; permeable rock and dry soils favour infiltration and slow baseflow.
- Vegetation. Dense vegetation increases interception and transpiration, delaying and reducing the peak.
- Antecedent conditions. Already-wet or frozen ground cannot store more water, raising runoff.
- Land use. Urbanisation replaces permeable ground with impermeable concrete and adds drains, sharply shortening lag time and raising the peak, which is the direct link to urban flood risk.
Try this
Q1. State the drainage-basin water-balance equation and define each term. [3 marks]
- Cue. : precipitation, evapotranspiration, runoff and change in storage.
Q2. Explain why an urbanised basin tends to have a flashier storm hydrograph. [4 marks]
- Cue. Impermeable surfaces and drains cut infiltration, force rapid overland flow, shorten lag time and raise the peak.
Q3. Distinguish between throughflow and baseflow. [3 marks]
- Cue. Throughflow is relatively rapid lateral movement through the soil layer; baseflow is the slow, sustained groundwater feed that keeps rivers flowing between storms.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 2018 (style)6 marksExplain how physical factors affect the shape of a storm hydrograph.Show worked answer →
A 6 mark "explain" question rewarding precise process links (AO1). A storm hydrograph plots river discharge against time after a rainfall event; physical factors change the lag time, peak discharge and the steepness of the rising and falling limbs.
Basin size and shape: a small, circular basin delivers water to the channel quickly, shortening lag time and raising the peak; a large, elongated basin spreads arrival times, giving a flatter hydrograph. Relief: steep slopes speed overland flow and shorten lag time. Geology and soils: impermeable rock or saturated, clay-rich soils force water into rapid overland flow, giving a flashy hydrograph, whereas permeable rock encourages infiltration and slow baseflow, lengthening lag time. Antecedent conditions: already-wet ground cannot store more water, so runoff and the peak rise.
Markers reward each factor being linked explicitly to lag time or peak discharge rather than simply listed. Top answers note that vegetation interception and drainage density also modify the response.
AQA 2020 (style)4 marksCalculate the runoff for a drainage basin where precipitation is 880 mm, evapotranspiration is 510 mm and the change in storage is minus 40 mm. Show your working.Show worked answer →
A short calculation (AO3 numeracy). Use the water-balance equation , rearranged for runoff: .
Substitute carefully with the sign: . Subtracting a negative adds it, so .
State units (mm) and, for the final mark, interpret: a negative storage change means the basin drew down its stores over the year (a utilisation or deficit phase), so runoff slightly exceeds the simple precipitation minus evapotranspiration figure. Markers reward correct rearrangement, the sign handling on , and the units.
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Sources & how we know this
- AQA A-level Geography (7037) specification — AQA (2016)