🌧 Precipitation (input)
Water falling from the atmosphere to the ground as rain, snow, sleet or hail. This is the only input to a basin.
Rivers and Drainage Basins » The Drainage Basin and the Water Cycle
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 1.1.3, 1.1.4
- How a drainage basin works as an open system with inputs, stores, flows and outputs
- What each drainage basin process is, and where it fits in the system
- Which routes are quick and which are slow
- How rock, soil, vegetation and land use change the route water takes
Street or forest floor?
Heavy rain falls on a city street. Within minutes the gutters are overflowing and water is rushing towards the nearest river. The same rain falls on a forest floor. The ground drinks it up and the stream hardly changes. Same rain, very different result. To explain why, we need to follow the water through a drainage basin.
A drainage basin is part of the water cycle, the way water moves between the sea, the air and the land. Remember, the drainage basin is the area of land drained by a river and its tributaries.
The drainage basin as an open system
A drainage basin is an open system. Water enters it from outside, and water leaves it. Geographers split the system into four parts.
Key terms:
- Open system: a system where matter (here, water) crosses the boundary, so things enter and leave.
- Input: water that enters the drainage basin from outside.
- Store: a place where water is held for a time.
- Flow (transfer): the movement of water from one place to another inside the basin.
- Output: water that leaves the drainage basin.
| Part of the system | Examples in a drainage basin |
|---|---|
| Input | Precipitation |
| Stores | Vegetation, puddles and lakes on the surface, soil, rock underground, the river channel |
| Flows (transfers) | Interception, infiltration, percolation, overland flow, throughflow, groundwater flow, channel flow |
| Outputs | Evaporation, transpiration, evapotranspiration, and the river flowing out to the sea |
The processes, one by one
Follow one raindrop through the basin. Each process below has its own name.
🌳 Interception
Precipitation caught by leaves, branches and stems before it reaches the ground. Some of it evaporates straight back into the air.
💧 Infiltration
Water soaking down from the surface into the soil.
🏔 Percolation
Water moving down from the soil into the rock below. It is deeper than infiltration.
🌊 Overland flow
Water flowing over the ground surface towards the river. Also called surface runoff.
🌱 Throughflow
Water flowing sideways through the soil towards the river.
🔮 Groundwater flow
Water flowing slowly through the rock below the ground towards the river.
- Channel flow: water moving along the river channel towards the mouth.
- Evaporation: liquid water turning into water vapour, mostly because the Sun heats it. It happens from the sea, lakes, rivers, soil and wet leaves.
- Transpiration: water lost as vapour from the leaves of plants.
- Evapotranspiration: evaporation and transpiration added together. It is the total water returned to the air from the basin.
Interception moves water from the air into the vegetation store. Evaporation, transpiration and evapotranspiration are outputs because the water leaves the basin as vapour. Out in the air it cools, forms clouds and falls again as precipitation, which completes the water cycle. The river carrying water out to the sea or a lake is an output too.
Quick routes and slow routes
Water does not reach the river at the same speed by every route. The route matters because it decides how fast the river rises after rain.
| Route | Speed | Why |
|---|---|---|
| Overland flow | Fastest | Water runs freely over the surface with nothing to slow it |
| Throughflow | Medium | Water has to move through the gaps between soil particles |
| Groundwater flow | Slowest | Water creeps through tiny gaps in the rock, and may take a very long time |
If most water takes a quick route, it arrives at the river all at once and the river rises fast. If most water takes a slow route, it arrives gradually and the river stays steadier.
What changes the route?
Four things decide how much water takes the quick routes and how much takes the slow ones.
🌋 Rock type
Permeable rock lets water pass through it, so more water percolates and the groundwater store fills. Chalk is an example. Impermeable rock does not let water through, so water stays on the surface or in the soil above. Clay and granite are examples.
🌿 Soil
Sandy soil has big gaps, so infiltration is fast. Clay soil has tiny gaps, so infiltration is slow and the soil fills with water. Once soil is full (saturated), extra rain cannot enter and runs off over the surface.
🌾 Vegetation
Plants intercept rain, so less hits the ground quickly. Roots open up the soil, helping infiltration. Plants also take up water and lose it by transpiration. More vegetation means a slower route to the river.
🏙 Land use
Tarmac, concrete and roofs are impermeable. There is no infiltration, so most rain becomes overland flow. Drains and gutters then carry it straight to the river. Cutting down forest removes interception and transpiration, so the route gets quicker.
This explains the opening picture. The city street is impermeable and has drains, so water takes the quickest route. The forest has leaves to intercept rain and roots and soil to soak it up, so most water takes the slow routes. Why this leads to floods is covered in the lesson Why Rivers Flood and What Floods Do.
Worked example
A farmer's field of clay soil is saturated after days of rain. More rain falls. Describe the route the water takes.
Answer: the soil is full, so there is little infiltration (1). The water cannot soak in, so it flows over the surface as overland flow (1). It reaches the river quickly (1).
Common mistakes
- Mixing up infiltration and percolation. Infiltration is surface into soil. Percolation is soil into rock.
- Mixing up throughflow (through soil) and groundwater flow (through rock).
- Saying plants have no effect. They intercept rain and also return water by transpiration.
- Writing "the water goes into the ground" with no process name. The exam wants the term.
Exam-style question
Fig. 1.1 shows the share of heavy rain that reached the river by each route in two drainage basins (made-up data). Basin A is forest. Basin B is a city.
| Route | Basin A (forest) % | Basin B (city) % |
|---|---|---|
| Overland flow | 5 | 85 |
| Throughflow | 55 | 5 |
| Groundwater flow | 40 | 10 |
(a) Define the term infiltration. [1 mark]
(b) State two outputs of a drainage basin. [2 marks]
(c) Compare the routes taken by water in Basin A and Basin B. Use Fig. 1.1. [3 marks]
(d) Explain how rock type, vegetation and land use can change the routes water takes through a drainage basin. [6 marks]
Model answer
(a) Water soaking from the ground surface into the soil (1).
(b) Any two from: evaporation, transpiration, evapotranspiration, river flow to the sea (1 each).
(c) In Basin B most water, 85%, is overland flow, but in Basin A only 5% is (1). In Basin A most water, 55%, is throughflow, but in B only 5% is (1). Groundwater flow is a bigger share in A, 40% against 10% (1).
(d) Top level answer (5-6 marks): Rock type matters because permeable rock such as chalk lets water percolate, which sends more water into the slow groundwater flow route (1). Impermeable rock such as clay does not, so more water becomes overland flow or throughflow (1). Vegetation intercepts rain, so less reaches the ground at once (1). Roots help infiltration and plants take up water for transpiration, so more water takes slow routes (1). Land use such as tarmac and roofs is impermeable, so there is no infiltration and most water runs off as overland flow (1). Drains carry this water to the river quickly (1). Overall, permeable rock and plenty of vegetation slow the water down, while impermeable rock and built surfaces speed it up.
Exam tip
In an "explain" answer, use the process names (infiltration, overland flow) and finish each point with its effect: quicker route or slower route.
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