The big idea: A drainage basin is the area of land drained by a river and its tributaries — the catchment. The edge of a basin is the watershed (the high ground separating one basin from the next).
Geographers study a basin as an open system: water enters as inputs, is held in stores, moves through the basin along flows (transfers), and leaves as outputs. It is open because both water (and energy) cross the boundary in and out.
The master equation is the water balance: precipitation = evapotranspiration + run-off ± change in storage.
The four parts of the drainage-basin system
- Inputs — water entering the basin: precipitation (rain, snow, hail) is the only major input.
- Stores — where water is held: interception, surface storage, soil water, groundwater and channel storage.
- Flows (transfers) — how water moves between stores: infiltration, throughflow, overland flow, percolation and base flow.
- Outputs — water leaving the basin: evapotranspiration and river discharge (channel flow to the sea).
- Watershed — the boundary of the basin; drainage basin = the catchment inside it.
Open, not closed: The basin is an open system because energy AND matter cross its boundary: rain comes in, and water leaves both as river discharge at the mouth and as evapotranspiration to the air. A closed system would let only energy cross — a drainage basin is not closed.
How this is tested: Paper 1 Option A opens with a data-response on the drainage-basin system — often a stores-and-flows graph that tracks each store and flow through a rainfall event. You Estimate or State a value (e.g. the % of rain held as surface storage at the start) or read a duration (e.g. how many hours overland flow lasts). Other data items use a basin map or satellite image — read a compass direction, a distance off the scale, or estimate % vegetation cover. Always quote the units.
| Part of the system | Example | What it does |
|---|---|---|
| INPUT — precipitation | Rain, snow, hail | The water entering the basin |
| STORE — interception | Rain caught on leaves and branches | Holds water above the ground; some evaporates |
| STORE — surface storage | Puddles, ponds, lakes, wetlands | Holds water on the surface before it moves |
| STORE — soil water | Water in the soil layer | Held in the soil; feeds throughflow and plants |
| STORE — groundwater | Water in the rock (aquifer) | The long-term store; releases slow base flow |
| STORE — channel storage | Water held in the river itself | The water currently in the channel |
| FLOW — infiltration | Water soaking into the soil | Moves surface water down into soil storage |
| FLOW — throughflow | Water moving sideways through soil | Carries soil water slowly towards the channel |
| FLOW — overland flow | Water running across the surface | The fastest flow; high when ground is impermeable |
| FLOW — percolation / base flow | Water seeping into / out of rock | Recharges then slowly releases groundwater |
| OUTPUT — evapotranspiration | Evaporation + transpiration | Water leaving the basin to the atmosphere |
| OUTPUT — river discharge | Channel flow at the mouth | Water leaving the basin to the sea |
Fast flows vs slow flows
- Overland flow is the fastest route to the river — it dominates when the ground is impermeable, saturated or frozen.
- Throughflow is slower — water moves sideways through the soil to the channel.
- Base flow is the slowest — groundwater seeps into the river, keeping it flowing between storms.
Reading a stores-and-flows graph: Each line is one store or flow (% of the rainfall). For a value, read straight up from the time axis to the right line. For a duration, find when a flow starts and when it ends and subtract — overland flow, for example, only appears once the soil is saturated, then fades.
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Changing the land surface rearranges the stores and flows. Anything that seals the ground or strips vegetation cuts storage and speeds run-off; anything that adds storage or cover slows water down. This is exactly what examiners want you to explain — not just name a change, but trace it through the system to a store or flow.
How building on a basin alters stores and flows
- Impermeable surfaces — concrete and tarmac stop infiltration, so soil and groundwater storage fall and overland flow rises.
- Drains and gutters — carry surface water straight to the river, replacing slow throughflow with fast channel flow.
- Removed vegetation — less interception storage, so more rain reaches the ground and runs off quickly.
- Compacted, bare soil on building sites stores less water, again raising overland flow.
Trace it through the system: Don't stop at naming the change — push it into a store or flow. Concrete -> no infiltration -> less soil/groundwater storage -> more overland flow. That chain is what earns the second mark each time.
Real basins change too: On the lower Los Angeles River the channel and floodplain were lined with concrete, so almost all storm rain becomes overland flow and reaches the channel in minutes. By contrast, restoring wetland storage along parts of Australia's Murray-Darling basin lets water sit and infiltrate, rebuilding soil and groundwater stores and slowing the system down.
The systems approach as a tool: Treating a basin as a system lets geographers see it as linked parts — inputs, stores, flows and outputs that affect one another — rather than a list of separate features. It shows interrelationships (change one store and the flows downstream change) and makes the basin's response to rain or to land-use change predictable. But it is a simplification: real basins are messy, data are patchy, and boundaries (the watershed) are hard to fix exactly.
How this is tested — the [10] Examine essay: Paper 1 Option A ends with a 10-mark Examine essay, marked on markbands. The headline version here asks why geographers treat drainage basins as open systems and how useful that approach is.
Top band needs: accurate use of inputs/stores/flows/outputs, developed strengths (shows interrelationships, predicts discharge and the effect of land use) set against limitations (a simplification, hard boundaries, data gaps), and a clear judgement on how useful the approach is.