Water cycles: budgets, residence time and feedback
Analyse catchment stores and transfers, compare turnover and evaluate changes across scales.
A-level study: check your qualification and chosen options below.
Part 1 of 4
Close a catchment water budget
A basin is an open system: precipitation and transfers can supply water; evapotranspiration, discharge and transfers remove it. For a stated period, change in storage equals total inputs minus total outputs.
A residual may include measurement error and omitted flows. The budget period matters because a wet month can replenish stores that decline over a year.
Worked example
For a fictional basin, precipitation is 900 mm, evapotranspiration 450 mm and discharge 350 mm over the same year. With no other flows, storage change is +100 mm.
Common mistake: A positive residual is not automatically measured groundwater recharge.
Part 2 of 4
Compare stores and residence time
Residence time can be approximated as a store divided by its throughflow under steady-state assumptions. Match units and identify which flow exchanges the store.
A large store can turn over slowly; an intense flow can turn over a smaller store rapidly. Mixing, changing storage and multiple pathways limit a simple average.
Worked example
A fictional 600 million m³ store with a 30 million m³/year throughflow has a 20-year turnover estimate. Doubling the assumed flow halves this estimate if storage remains fixed.
Common mistake: A mean residence time does not mean every water molecule remains for that duration.
Part 3 of 4
Evaluate a land-cover feedback
Vegetation, soils and human drainage influence interception, infiltration, evapotranspiration and channel arrival. A change in one pathway can alter the balance and timing of others.
Explain feedback direction and a limiting condition. Reduced infiltration may promote runoff and erosion, further reducing soil storage, but rainfall and management can interrupt this sequence.
Worked example
A fictional model allocates 60 of 100 units to rapid runoff after paving versus 30 before. The change describes that model’s allocation; measured peak discharge also depends on routing.
Common mistake: A budget fraction alone cannot predict the hydrograph peak.
Part 4 of 4
Aquifer depletion and delayed recovery
A pumping well changes the hydraulic gradient around it. Water can come from reduced storage, extra recharge or reduced discharge to rivers and wetlands. Comparing abstraction only with rainfall misses these connections: rainfall is not all recharge.
A fall in water level can increase pumping costs and reduce shallow-well yields. Some sediments compact as pressure falls, making subsidence partly irreversible. Management should distinguish recovery of water level from recovery of storage capacity. Seasonal recovery at one well does not establish a basin-wide sustainable balance.
Worked example
Fictional annual recharge is 24 million m³, natural discharge 9 million m³ and abstraction 22 million m³. Storage change = 24 − 9 − 22 = −7 million m³. Cutting abstraction to 15 million m³ balances this simplified budget, but does not refill the previous deficit. Recharge and river discharge may also change.
Common mistake: A stable water level during one wet season is not evidence that long-term depletion has ended.
Try a different resource
River discharge
Use the explicitly rectangular cross section. The supplied mean velocity represents the whole cross section; it is not an uncorrected surface-float velocity.
| Measurement | Value |
|---|---|
| Width (m) | 6 |
| Uniform depth (m) | 0.3 |
| Mean cross-sectional velocity (m/s) | 0.7 |
Resource description
Idealised fictional rectangular channel
Write an explanation
Check your answer
- Use a budget and at least one process chain.
- Distinguish volume, rate and timing.
- Examine scale and initial conditions.
- Evaluate alternative causes and the strength of case evidence.
Practise and review a written answer
Try the model
Case studies
Slowing the flow at Pickering · The shrinking Aral Sea · Cape Town’s water-security crisis
These pages provide selected evidence. Check the whole case requirement with your teacher.
Read further
- water.usgs.gov — source or course reference
- www.aqa.org.uk — source or course reference
- www.aqa.org.uk — source or course reference
- qualifications.pearson.com — source or course reference
- www.sqa.org.uk — source or course reference
- www.usgs.gov — source or course reference
- www.ocr.org.uk — source or course reference
- www.eduqas.co.uk — source or course reference
Drainage basins as systems · Water security and competing uses · Association and geographical explanations
Course links
AQA A-level Geography · Pearson Edexcel A-level Geography · Higher Geography · OCR A-level Geography · Eduqas A-level Geography
Next topics
Carbon cycles: stocks, fluxes and changing feedbacks · Resource security: stocks, conflicts and alternative futures
Selected teaching sequence with fictional worked data. The linked case records supply bounded evidence; they do not satisfy every course case requirement.
Scottish links identify reusable parts of the topic. Scottish landforms, soil, weather and disease requirements need additional course-specific teaching.