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.
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.
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.
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.
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.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.