How does the drainage basin operate as an open system, and how do human actions disturb it?
The drainage basin as an open system; the water balance; storm hydrographs; and the impact of land-use change, abstraction and climate change on the basin.
An Eduqas A-Level Geography answer to the drainage basin system in Component 2, covering the basin as an open system, the water balance equation, inputs, stores, flows and outputs, storm hydrographs and their controls, and the impact of land-use change, abstraction and climate change, with examples and KaTeX statistics.
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What this dot point is asking
Eduqas wants you to explain the drainage basin as an open system, use the water balance equation, interpret storm hydrographs and their controls, and explain how land-use change, abstraction and climate change disturb the basin system.
The answer
The drainage basin as an open system
The drainage basin is the local, open-system expression of the water cycle. Its single input is precipitation; its stores are interception (on vegetation), surface storage, soil moisture and groundwater; its flows (transfers) are infiltration, percolation, throughflow, overland flow (surface runoff), groundwater flow and channel flow; and its outputs are evapotranspiration and channel discharge at the basin mouth. The way precipitation is partitioned between these stores and flows determines how the river responds to rain.
The water balance and storm hydrograph
The water balance summarises the basin over time as precipitation equals evapotranspiration plus runoff plus or minus the change in storage , written . The hydrograph shape depends on basin and storm controls: large, steep, impermeable, urbanised or deforested basins with intense rainfall produce flashy hydrographs (short lag, high peak), while small, gentle, permeable, vegetated basins with gentle rainfall produce subdued ones. Reading and explaining hydrographs is a core Eduqas AO3 skill.
Human and climatic disturbance
The basin system is sensitive to change. Urbanisation seals surfaces with impermeable concrete and tarmac and adds drains, cutting infiltration and storage and increasing rapid runoff, so the hydrograph becomes flashier and flood risk rises. Deforestation reduces interception and transpiration and increases overland flow, raising flood and erosion risk; afforestation does the reverse. Abstraction from rivers and aquifers reduces the storage and discharge terms, lowering flows and depleting groundwater. Climate change alters precipitation totals and intensity and raises evapotranspiration, shifting the whole balance, so droughts and floods both become more likely in many basins.
Examples in context
Example 1. Urbanisation and flooding in a UK catchment. A catchment that has been progressively urbanised, for example parts of the Thames or a fast-growing town's basin, shows the hydrograph effect clearly. Replacing fields and woodland with roofs, roads and car parks seals the surface, so infiltration falls and rapid runoff rises; storm drains route water to the channel quickly, shortening lag time and raising peak discharge. The result is a flashier hydrograph and greater flash-flood risk, which is why sustainable urban drainage systems (permeable paving, green roofs, retention ponds) are used to restore storage and slow runoff. It is the standard Eduqas case for human disturbance of the basin.
Example 2. Deforestation and runoff in the Amazon or a tropical basin. Large-scale deforestation in a tropical basin reduces interception and transpiration and exposes soil, so a greater share of intense rainfall becomes overland flow. Runoff and peak discharge rise, soil erosion and sediment loads increase, and the dry-season baseflow can fall because less water infiltrates to recharge groundwater. The change also feeds back into the regional climate by cutting the moisture the forest recycles to the atmosphere, linking the drainage basin to the coupled water and carbon cycles, a strong synoptic example.
Try this
Q1. Write the water balance equation and define each term. [3 marks]
- Cue. : precipitation equals evapotranspiration plus runoff plus or minus the change in storage.
Q2. Explain why urbanisation produces a flashier storm hydrograph. [3 marks]
- Cue. Impermeable surfaces and drains reduce infiltration and storage and increase rapid surface runoff, delivering water to the channel faster, so lag time falls and peak discharge rises.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 2019 (style)5 marksUsing Figure 2 (two storm hydrographs for the same basin before and after urbanisation), describe and explain the differences.Show worked answer →
An AO3 resource question: describe the change in the hydrograph and explain it.
Describe: after urbanisation the hydrograph has a higher, earlier peak discharge, a shorter lag time, a steeper rising limb and a higher peak, with a steeper falling limb.
Explain: impermeable surfaces and drains reduce infiltration and increase rapid surface runoff, delivering water to the channel faster, so lag time falls and peak discharge rises, raising flood risk.
A strong answer quotes lag times and peak values from the resource.
Markers reward accurate reading of both hydrographs and a process explanation linking urbanisation to a flashier response.
Eduqas 2022 (style)8 marksExplain how land-use change can alter the water balance of a drainage basin.Show worked answer →
Define the water balance, then explain how specific land-use changes shift its terms.
The water balance is precipitation equals evapotranspiration plus runoff plus or minus changes in storage. Land-use change alters how precipitation is partitioned between these.
Deforestation reduces interception and transpiration and increases overland flow and runoff, raising flood and erosion risk; urbanisation seals surfaces, cutting infiltration and storage and raising rapid runoff; afforestation does the reverse, raising interception and evapotranspiration and reducing runoff.
A strong answer links a named change to the shifted balance term and the downstream consequence.
Markers reward the water balance framework and process links to runoff and storage.
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Sources & how we know this
- Eduqas A-level Geography specification (from 2016) — Eduqas (2016)