The drainage basin as an open system
A drainage basin is the land drained by a river and its tributaries.
Always fix the outlet first: a gauge part-way down the river drains a smaller area than the river mouth.
The watershed, or drainage divide, is the high ground that separates one basin from the next.
Rain that lands on either side runs downhill towards a different channel.
A basin is an open system because water and energy cross its boundary.
Rain and snow bring water in.
Sunshine drives evaporation and transpiration back to the air, and the river carries water and sediment out through the outlet.
Where you draw the boundary decides what counts as an internal transfer and what counts as an output.
Inputs: amount, intensity, type and location
Precipitation includes rain, snow and hail.
Amount is the depth that falls over a period, usually in millimetres.
Intensity is the rate, usually in millimetres per hour (mm h⁻¹).
Spread over fifteen hours, 30 mm may all soak in.
Falling in thirty minutes, it can arrive faster than the soil can take it.
The type of precipitation changes the timing.
Snow goes into a frozen store first and only becomes runoff when it melts.
Rain on an old snowpack adds its own water and can speed up the melt.
Where the rain falls also matters.
A storm over one small tributary can cause a local flood while the rest of a large basin stays calm.
Rain near the outlet has a short journey to the gauge.
A storm that moves downstream can keep pace with the runoff, so water from many places arrives together.
A basin-average rainfall figure hides all of this.
Stores and how long water stays in them
A store is water held inside the system.
Water stays in each store for a typical length of time, its residence time.
- Interception store: water held on leaves and stems. Some evaporates. The rest drips to the ground as throughfall or runs down trunks as stemflow. The water on the leaves is a store; the water that evaporates from them (interception loss) is an output.
- Surface store: puddles, lakes and wetlands. This water can later evaporate, soak in or join a channel. A wetland with spare room can hold back a flood pulse; a wetland that is already full cannot.
- Soil moisture store: water in the spaces between soil grains, above the water table. When soil is saturated, every pore is full. Saturated soil still lets water through, but it has no room to store more at that moment.
- Groundwater store: water in saturated pores and cracks below the water table. An aquifer is rock that both stores groundwater and lets it move. Porosity is how much empty space a rock has; permeability is how well those spaces connect so water can flow. Clay has many tiny pores but lets water through slowly, while cracks can make a dense rock permeable. The groundwater divide does not always sit under the surface watershed.
- Cryosphere store: snow, glacier ice and frozen ground. Snow delays runoff by months; glacier ice holds water for much longer. Frozen ground (permafrost) blocks infiltration while it stays frozen. When it thaws, it can create new room for water below ground, so thaw does not always raise runoff at once.
Transfers link the stores
Infiltration is water soaking into the ground surface.
Infiltration capacity is the fastest rate at which the ground can take water at that time.
Rain at 18 mm h⁻¹ on soil that can take only 6 mm h⁻¹ produces infiltration-excess overland flow, even on dry ground.
A crust on the surface, trampled or compacted soil, frozen ground and paving all slow infiltration.
Percolation carries water downwards through soil and rock.
Throughflow carries it sideways through the soil, often above a less permeable layer.
Old root channels and animal burrows (macropores) can move water quickly, so underground flow is not always slow.
Overland flow is water running across the land surface to a channel.
It starts in two ways:
- the rain falls faster than the ground can soak it up (infiltration-excess);
- the soil is already full (saturation-excess).
In long storms, saturation-excess flow starts on wet valley floors and spreads up the slopes.
Base flow is the steady part of river flow, fed mostly by groundwater.
In a gaining stream, groundwater seeps into the river.
In a losing stream, river water leaks down into the ground.
Pumping groundwater can cut a river's base flow even if rainfall stays the same.
The cross-section shows these stores and transfers on one hillslope.
Trace a raindrop to the river along each route.
Outputs and the water balance
Evaporation turns liquid water on surfaces into vapour.
Transpiration releases water that plants have drawn up from the soil.
Together they make evapotranspiration (ET).
Actual ET depends on two things: how much the air can take (hot, dry, windy air takes a lot) and how much water is available.
In a heatwave with dry soil, demand is high but actual ET stays low.
River flow out through the outlet is the other main output.
The systems diagram puts every store inside the basin boundary.
The input comes in across the boundary, and the outputs leave across it.
For a simple basin with no piped water and no groundwater crossing the boundary, the water balance is :
- is precipitation;
- is runoff, written as a depth of water spread over the whole basin, not as a discharge in m³ s⁻¹;
- is the change in storage: positive when stores fill, negative when they are drawn down.
Use the same time period and the same boundary for every term.
To turn a depth into a volume, multiply the depth in metres by the area in square metres.
One millimetre over one square kilometre is 1,000 m³.
Measuring errors feed into the storage term, and the balance cannot tell you which store changed.
Part of the system
Examples
What it does to runoff
Input
Rain, snow, hail
Amount, intensity and location set how much water arrives and where
Store
Interception, surface, soil moisture, groundwater, snow and ice
Holds water back for hours (leaves) to years (aquifers, glaciers)
Transfer
Infiltration, percolation, throughflow, overland flow, base flow
Moves water between stores; fast routes give quick runoff
Output
Evapotranspiration, river discharge, leakage to deep groundwater
Removes water from the basin
Two real basins
The Test and Itchen catchment in Hampshire, England, shows how geology changes the system inside one basin.
The Environment Agency describes groundwater-dependent chalk in the north and contrasting clay geology further south.
On the chalk, much of the rain soaks in and recharges the aquifer.
USGS Circular 1139 (1998) explains what happens next: groundwater moves slowly towards the river along the slope of the water table, so the aquifer feeds base flow long after the rain has stopped.
In a long drought, recharge falls and the same aquifer can supply less base flow.
The clay in the south lets less water soak in, so more of it travels as surface flow.
This does not mean every chalk river flows at a constant rate, or that groundwater can never flood.
The Okavango Delta in north-west Botswana shows storage working at a much larger scale.
UNESCO describes permanent swamps of about 600,000 ha and up to 1.2 million ha of seasonally flooded grassland.
UNESCO also states that the sources of its water lie upstream in Angola and Namibia, and that the annual flood arrives during Botswana's dry winter season.
You can reason from those two facts that local rain at that time cannot be causing the flood: the water must have fallen earlier and far upstream, and been delayed by the long journey and by storage along the way.
UNESCO does not give the travel time.
The delta has no outlet to the sea: its water drains into the sands of the Kalahari Basin, and evapotranspiration returns water to the air.
Here river discharge to the sea is not an output at all.
A good system explanation follows the links: less infiltration means less recharge, and less groundwater can mean weaker dry-season flow.
Name the route, the timing and the place that receives the water before you make a claim about a whole basin.
How the IB examines this
- Paper 1, resource reading: state the value, store or transfer that the resource shows, with its unit. An explanation cannot replace a reading you were asked for.
- Paper 1, structured explanation [2] and [3]: link a cause to a process and then to its result, for example "compacted soil, so lower infiltration capacity, so more overland flow". Develop the link with the resource or a located example; a list of factors is not a mechanism.
- Paper 1, extended response [10]: answer the whole question with one connected argument, use located evidence, and reach a judgement. The shorter tasks below each train one skill.
- Drainage basin: the area that supplies water to a river and its tributaries above a given outlet.
- Open system: a system that exchanges matter and energy with its surroundings.
- Infiltration capacity: the fastest rate at which water can soak into the ground at a given time.
- Throughflow: sideways movement of water through the soil towards a channel.
- Base flow: steady river flow fed mainly by groundwater.
- Residence time: the average time water stays in a store.
The basins in the worked examples and check questions are fictional; all data are invented.
A basin covers 160 km².
In one year precipitation is 920 mm, actual ET is 510 mm and runoff is 350 mm.
Calculate the storage change as a depth and as a volume. [3]
1. Use the same year and the same outlet: . Correct balance and substitution [1].
2. mm, with its sign [1].
3. 60 mm is 0.060 m and 160 km² is 160,000,000 m²; m³ [1]. The plus sign shows water building up in the basin.
Answer: +60 mm, or +9.6 million m³.
Recharge, wetter soil or a rise in lake level could explain it.
The budget alone cannot tell them apart, so accept any store that could rise.
Explain why equal rainfall totals may produce different amounts of overland flow on two hillslopes. [4]
1. On compacted ground, fewer surface pores are connected, so infiltration capacity is low [1]. Intense rain then arrives faster than the soil can take it, and the extra water runs off quickly [1].
2. On a hillslope that was already wet, the soil store is full [1]. Even moderate rain then runs off as saturation-excess flow [1].
Answer: compaction limits how fast water can enter; earlier wet weather limits how much more the soil can hold.
Both raise overland flow, by different routes.
OR develop two other valid pairs, such as frozen ground against cracked, permeable rock.
Cap a list of controls with no development at 2 marks.
The Cedar basin covers 40 km².
The table shows one month's budget.
All values are depths spread over the basin.
Component
Depth (mm)
Precipitation
110
Actual ET
35
Outlet runoff
85
(a) Calculate the storage change. [2]
(b) Calculate the runoff volume. [2]
(c) Explain one way runoff can be greater than precipitation minus ET. [2]
(d) Suggest one measurement that would show which store is being drawn down. [2]
1. (a) mm: method [1], negative sign [1].
2. (b) m³: conversion [1], volume [1]. Carry forward a consistent depth after an earlier error.
3. (c) Groundwater stored in earlier months feeds base flow [1], so this month's runoff includes water that fell before the month began [1]. OR snowmelt releases water that fell earlier as snow.
4. (d) Compare groundwater levels at the start and end of the month [1]; a fall would point to the aquifer [1]. OR measure snow water, soil moisture or lake level over the same month and explain the expected change.
Answer: storage falls by 10 mm; runoff is 3.4 million m³.
The stores carry water from one month to the next.
Total: 2 + 2 + 2 + 2 = 8 marks.
Discuss the view that storage is the most important influence on a drainage basin's response to precipitation. [10]
1. Define the response: how much runoff, how fast, and by which routes.
2. Compare storage with rainfall intensity and with how well each route moves water.
3. Use evidence to set out when each matters, then judge.
Answer: Storage strongly shapes a basin's response, because rain does not pass straight to the outlet.
How much it matters depends on how full the stores are and on the storm itself.
An aquifer holds water that has soaked in and releases it slowly towards the river.
This spreads the response over weeks or longer.
The Test and Itchen catchment in Hampshire is a clear example.
The Environment Agency describes groundwater-dependent chalk rivers in the north, so subsurface storage shapes their flow.
The clay further south warns against treating the whole basin as one type.
How full the stores are before a storm can reverse a simple pattern.
Soil with plenty of empty pore space soaks up rain and delays runoff.
After a long wet spell, the same soil is saturated.
Extra rain then runs off the surface, and the area producing runoff grows.
So the room left before the storm matters more than how big the soil store is.
The rain itself can control a fast response.
When rain falls faster than the infiltration capacity, overland flow starts even on dry ground.
Here storage explains little: the limit is the rate of entry.
Storage matters most at seasonal and larger scales.
Snow holds water in a frozen store and melt releases it later, although warm rain on snow can release it suddenly.
The Okavango shows storage at the scale of a whole river system: UNESCO notes that its annual flood arrives in Botswana's dry winter, and that its water comes from upstream in Angola and Namibia.
A flood in the dry season points to upstream water delayed by a long journey and storage, not to local rain.
Overall, storage is the main influence on delayed and seasonal response, especially in groundwater-fed or snow-fed basins.
It is often not the strongest influence on fast surface runoff.
The best explanation combines the room left in the stores, infiltration capacity and the intensity of the rain.
Best-fit markbands: 1–2: names stores or precipitation with little link to how the basin responds.
3–4: describes interception, soil or groundwater stores but mostly asserts that storage controls runoff, without routes or other influences.
5–6: explains how spare storage delays or reduces runoff, with a relevant example; rainfall or routes are considered but their relative importance is partly asserted.
7–8: uses evidence to compare storage with rainfall intensity, permeability and routing, and explains why the same store gives different responses after wet and dry spells.
9–10: tests the "most important" claim throughout, showing how capacity, how full the stores are, storm length and connected routes decide storage effects; accurate evidence supports a judgement about when storage dominates and when rainfall or transfers override it.
Apply the bands to the whole answer; other examples and supported conclusions are equally valid.
Before you use a water balance, write down the outlet, the time period and the units.
A negative storage change means stored water has fed an output; it never means negative rainfall.
- Calling groundwater an output. It is a store inside the boundary; only water crossing the boundary is an output.
- Mixing up infiltration and percolation. Infiltration crosses the ground surface; percolation carries water deeper.
- Assuming all underground flow is slow. Cracks and macropores can move water quickly.
- Naming which store changed from the balance alone. You need separate storage measurements.
Check your understanding
1. Define an open system. [1]
2. Using the cross-section of a hillslope, identify the transfer that moves water sideways through the soil towards the river. [1]
3. State the unit usually used for rainfall intensity. [1]
4. Distinguish between a store and a transfer, using one water example of each. [2]
5. Outline why a porous rock may have low permeability. [2]
6. Describe how a gaining stream is linked to an aquifer. [2]
7. Explain how interception can reduce immediate runoff. [3]
8. Compare infiltration-excess and saturation-excess overland flow. [3]
9. Suggest why actual evapotranspiration may stay low during hot, dry weather. [2]
10. A 75 km² basin has the equivalent annual depths in the table. The import is piped in from a neighbouring basin. Recharge is water moving from this basin's soil into its own aquifer. Assume there are no other transfers across the boundary.
Component
Depth (mm)
Precipitation
760
Inter-basin import
20
Actual ET
430
Outlet runoff
360
Internal aquifer recharge
120
(a) Calculate the whole-basin storage change as a depth. [2]
(b) Calculate the volume of that change. [2]
(c) Explain why subtracting the 120 mm of recharge as well would give the wrong whole-basin budget. [2]
Total: [6]
11. Resource: the Environment Agency's Test and Itchen catchment description, consulted October 2026, identifies groundwater-dependent chalk in the north and contrasting clay geology in the south. It describes the catchment in words; it gives no paired storm measurements. Evaluate how far it supports the claim that geology decides the runoff response throughout a drainage basin. [6]
12. Evaluate the importance of antecedent conditions in explaining drainage-basin runoff. [10]