The global water cycle as a closed system of stores and flows; the drainage basin as an open sub-system with inputs, flows, stores and outputs; the water balance; and the natural and human factors that change water stores and flows across scales.

What this dot point is asking

OCR wants you to describe the global water cycle as a closed system of stores and flows, explain the drainage basin as an open sub-system with inputs, flows, stores and outputs, use the water balance to link them, and explain the natural and human factors that change water stores and flows across scales.

The answer

The global water cycle as a closed system

The cycle is driven by solar energy and gravity. The main global flows are evaporation (and transpiration) lifting water vapour into the atmosphere, condensation forming cloud, precipitation returning water to the surface, and cryospheric exchange as snow and ice accumulate and melt. The relative size of the stores matters: the oceans and ice caps are vast, slow-turnover stores, while the atmosphere is tiny but turns over in days. Because the system is closed, changes are about redistribution, for example a warming climate shrinking the cryosphere and raising ocean and atmospheric stores, rather than any net gain or loss of water.

The drainage basin as an open sub-system

Within the closed global cycle, the drainage basin is the key open sub-system. Its boundary is the watershed. The single input is precipitation. Stores include interception (on vegetation), surface storage, soil moisture storage, groundwater storage in aquifers, and channel storage. Flows or transfers move water between stores: infiltration (into soil), percolation (to groundwater), throughflow (laterally through soil), overland flow (across the surface), baseflow (groundwater feeding the channel) and channel flow. The outputs are evapotranspiration, river discharge leaving the basin, and deep groundwater loss. It is open because matter (water) crosses the boundary in and out.

The water balance

The stores and flows of a basin are tied together by the water balance, the annual accounting of inputs against outputs and storage change. It is usually written

where $P$ is precipitation, $Q$ is runoff (discharge), $E$ is evapotranspiration and $\Delta S$ is the change in storage. In a temperate basin, a winter water surplus (precipitation exceeds evapotranspiration) recharges soil and groundwater, while a summer water deficit (evapotranspiration exceeds precipitation) draws on that store, with recharge and utilisation phases in between. The balance therefore explains the seasonal rhythm of river flow and soil moisture.

Natural and human factors changing stores and flows

Natural factors dominate the broad pattern. Climate sets precipitation and evapotranspiration totals; seasonality shifts storage between snowpack, soil and channel; vegetation controls interception and transpiration; geology and soils govern infiltration and groundwater storage; and relief affects runoff speed. Human factors increasingly modify the cycle, mostly at the basin scale: deforestation cuts interception and transpiration and speeds runoff; urbanisation replaces permeable ground with impermeable surfaces and drains, raising overland flow; abstraction for water supply and irrigation depletes groundwater and reduces baseflow; and reservoirs redistribute storage in time and raise evaporative loss. Climate change overlays all of this by altering precipitation patterns and shrinking the cryosphere.

Examples in context

Example 1. A temperate drainage basin water balance (the UK). A typical lowland British basin shows a clear seasonal pattern: from autumn, precipitation exceeds evapotranspiration, so there is a water surplus that recharges soil moisture and groundwater and sustains high winter river flow. In summer, higher evapotranspiration creates a water deficit, soil moisture is utilised and rivers run lower, fed mainly by baseflow. This rhythm, captured by the water balance, explains why UK flood risk peaks in winter and low-flow stress in late summer, and why chalk-aquifer rivers (with large groundwater storage) are more buffered than impermeable upland catchments.

Example 2. Human modification by urbanisation and abstraction. Where a basin is heavily urbanised, impermeable surfaces and storm drains cut infiltration and raise overland flow, producing a flashy hydrograph and elevated flood risk, the rationale for sustainable urban drainage systems. Where groundwater is over-abstracted, as in parts of intensively irrigated regions, the water table falls, baseflow declines and rivers and wetlands can dry out, showing how human activity can dominate the stores and flows of a basin and even push an aquifer store past a sustainable threshold.

Try this

Q1. State the water balance equation and define each term. [4 marks]

• Cue. $P = Q + E \pm \Delta S$: precipitation equals runoff plus evapotranspiration plus or minus the change in storage.

Q2. Explain why urbanisation produces a flashier storm hydrograph. [4 marks]

• Cue. Impermeable surfaces and drains cut infiltration and increase overland flow, so water reaches the channel faster, raising and sharpening the discharge peak and shortening the lag time.