The concept of the drainage basin as an open system
This topic, Hydrology, river processes and hazards, is the first of the three topics examined on Paper 1 Physical Geography at AS Level.
It starts with the idea that holds the rest together: a drainage basin is a system that water moves into, through and out of.
Everything later in the topic, from storm hydrographs to flood management, is a question about how fast water moves through that system and where it is held on the way.
A basin as a system
A drainage basin is the area of land drained by a river and its tributaries.
Its edge is the watershed, the line of high ground that separates water flowing to this river from water flowing to the next one.
Rain falling one side of the watershed ends up in one river; rain falling a few metres away on the other side ends up in another.
Geographers treat the basin as a system: a set of parts linked by flows of energy or matter.
Cambridge's wording is that the drainage basin has inputs, outputs, stores and transfers:
- Inputs bring water into the basin. The only input is precipitation.
- Outputs take water out of the basin: evaporation, evapotranspiration and channel flow leaving the basin at its mouth (condensation is covered in the next section).
- Stores hold water for a time: in vegetation, on the surface, in the soil, in the rock below the water table, in channels and lakes.
- Transfers (flows) move water from one store to another: infiltration from the surface into the soil, throughflow through the soil to the river, and so on.
Figure 1.1 sets these out as a flow diagram.
Read it from the top: water enters as precipitation, passes down and across through stores, and leaves either upwards to the atmosphere or sideways as channel flow.
Why "open"?
A system is open when matter crosses its boundary, and closed when only energy does.
The drainage basin is open because water enters it (precipitation) and leaves it (evaporation, evapotranspiration, channel flow).
The global hydrological cycle is the contrast: taken as a whole, the Earth gains and loses almost no water, so the global cycle is a closed system, while each basin within it is open.
This matters for explanation.
Because the basin is open, how much water it holds at any moment depends on the balance between what comes in and what goes out.
A basin is never "full" or "empty" for good: it fills in wet periods and drains in dry ones.
The relationship between inputs, outputs, stores and transfers
The second idea Cambridge names is the relationship between inputs, outputs, stores and transfers.
The parts do not act alone: a change in one changes the others.
Three relationships do most of the work in exam answers.
1. The water balance. Over any period, what comes in must equal what goes out plus any change in storage:
where P is precipitation, Q is channel flow (runoff) leaving the basin, E is evaporation plus evapotranspiration, and ΔS is the change in storage (soil water, groundwater, surface water).
Over a whole year in a stable basin, ΔS is close to zero, so Q ≈ P minus E. Over a single month it is not: storage rises in wet months and falls in dry ones.
No formula sheet is given, so learn this form.
2. Stores control transfers. How water moves depends on how full the stores are.
When the soil water store has spare capacity, rain infiltrates and moves slowly to the river by throughflow.
When the soil is saturated, the infiltration transfer is shut off and the same rain runs over the surface as overland flow, reaching the river in minutes or hours instead of days.
So the state of a store decides which transfer operates and how fast water reaches the channel.
3. Transfers change stores, which change outputs. Heavy rain fills the soil and raises the water table; a higher water table feeds more groundwater flow (baseflow) to the river for weeks afterwards, so channel flow stays high long after the rain stops.
In a drought, evapotranspiration draws the soil store down; the next rain first refills the soil, and the river barely responds.
These links are a chain, so explanation is about following it: input, then the state of the store, then the transfer that operates, then the output.
Figure 1.2 shows the seasonal version of the chain in a temperate basin.
Storage through the year
In Figure 1.2, precipitation exceeds evapotranspiration from September to April, and evapotranspiration exceeds precipitation from May to August.
The months when outputs to the atmosphere are larger than the input are the months when the basin draws on its stores:
- Soil moisture surplus (autumn and winter): P is greater than E, the soil store is full, and the extra water drains to the river and to groundwater. River flow is high.
- Soil moisture utilisation (late spring and summer): E is greater than P, so plants draw water from the soil store. River flow falls and is fed mainly by groundwater.
- Soil moisture deficit: by late summer the soil store holds less than it can, so the first autumn rain refills it before much reaches the river.
- Recharge (autumn): P is again greater than E, and the soil store is refilled. Only when it is full does the surplus return.
This is the same relationship as in point 2 above, stretched over a year: how full the store is decides how much of the input becomes channel flow.
Dynamic equilibrium and feedback
A basin that is not disturbed tends towards a dynamic equilibrium: inputs and outputs balance over a run of years, though they swing season by season.
A large change to one part can push the system to a new balance.
Clearing a forest removes the interception store and lowers evapotranspiration, so more of the input becomes channel flow; the river may then erode and widen its channel until it carries the larger flows.
The links work both ways, which is why the papers ask you to explain relationships, not list parts.
- Drainage basin: the area of land drained by a river and its tributaries, bounded by a watershed.
- Watershed: the boundary of a drainage basin, the line of high ground separating it from neighbouring basins.
- Open system: a system in which matter (here, water) and energy cross the boundary, as inputs and outputs.
- Store: a part of the system where water is held for a time.
- Transfer: a flow that moves water from one store to another.
- Water balance: precipitation equals runoff plus evapotranspiration, plus or minus the change in storage (P = Q + E ± ΔS).
Use Figure 1.2 to calculate the water that leaves the basin as channel flow in a year, assuming storage at the end of the year is the same as at the start.
Show your working.
1. Total precipitation: 95 + 70 + 65 + 55 + 55 + 50 + 55 + 60 + 65 + 90 + 100 + 105 = 865 mm (1).
2. Total evapotranspiration: 5 + 10 + 25 + 45 + 75 + 95 + 105 + 90 + 60 + 30 + 12 + 5 = 557 mm (1).
3. With ΔS = 0 over the year, Q = P minus E = 865 minus 557 (1).
Answer: 308 mm of channel flow (1), with the unit.
In words, about 36% of the year's precipitation (308 ÷ 865 = 0.356) leaves as channel flow and about 64% returns to the atmosphere.
A missing "mm" loses the final mark; an arithmetic slip in step 1 still earns the later marks if the method is carried through on your own total.
In July, Figure 1.2 shows P = 55 mm and E = 105 mm.
Channel flow measured at the outlet that month is equivalent to 12 mm over the basin.
Calculate the change in storage in July.
Show your working.
1. Rearrange P = Q + E ± ΔS to give ΔS = P minus Q minus E (1).
2. ΔS = 55 minus 12 minus 105 = minus 62 mm (1).
Answer: storage falls by 62 mm in July.
The river still flows although E is nearly twice P, because the water comes out of the soil and groundwater stores.
That is the relationship between stores and outputs in one number.
Explain how the amount of water in the soil store affects the transfer of water to the river channel. [4]
Top answer. When the soil has spare capacity, rain infiltrates (1) and moves slowly downslope through the soil as throughflow, so it reaches the channel over many hours or days (+1 developed).
When the soil is saturated, no more water can infiltrate (1), so further rain runs over the surface as saturation-excess overland flow, which reaches the channel much faster and raises discharge sharply (+1 developed). [4]
Weaker answer. "Wet soil means more runoff and dry soil means less runoff."
This is one undeveloped point (1): it states a link but gives no transfer, no mechanism and no effect on timing.
Two simple points without development would score 2 of 4.
When a question asks about a relationship, write the chain in order: input, state of the store, transfer, output.
"Because the soil was already saturated, infiltration stopped, so rain moved as overland flow and reached the river within hours" earns development marks that "the soil was wet" does not.
And when you calculate, write the formula first, every step with its unit, and the answer with its unit.
- Calling the drainage basin a closed system. Water crosses its boundary as precipitation, evapotranspiration and channel flow, so it is open; the global hydrological cycle as a whole is the closed one.
- Listing stores and transfers as a vocabulary list. The papers reward the relationships: how a store's state switches transfers on or off and changes the timing of outputs.
- Treating "runoff" and "discharge" as rain. Discharge is the output of the whole system, after stores and transfers have delayed and reduced the input.
- Using vague words such as "water goes into the ground". Name the transfer: infiltration (into the soil), percolation (down to groundwater).
Check your understanding
1. Identify the month in Figure 1.2 when precipitation exceeds evapotranspiration by the largest amount. [1]
2. Calculate the total amount by which evapotranspiration exceeds precipitation from May to August in Figure 1.2. Show your working. [2]
3. Describe the pattern of evapotranspiration shown in Figure 1.2. [3]
4. Explain why a drainage basin is an open system. [3]
5. Suggest why a river can keep flowing through a dry summer month in which evapotranspiration is greater than precipitation. [3]
6. Explain two reasons why the discharge of a river may respond very little to the first heavy rain after a long dry season. [4]
Answers
1. December (105 minus 5 = 100 mm). [1]
2. May 75 minus 55 = 20; June 95 minus 50 = 45; July 105 minus 55 = 50; August 90 minus 60 = 30 (1). Total = 20 + 45 + 50 + 30 = 145 mm (1, unit needed). [2]
3. Low in winter, 5 mm in January and December (1); rises steadily through spring to a peak of 105 mm in July (1, data with unit); falls through autumn, to 12 mm in November (1). One mark is reserved for data with units; listing every month in turn without the overall rise and fall earns at most 1. [3]
4. Matter (water) enters across its boundary as precipitation (1); water leaves as evaporation and evapotranspiration to the atmosphere (1) and as channel flow at the mouth (1). Credit the contrast with the closed global cycle as development. [3]
5. Water drains from stores that were filled earlier (1): the soil water store, and especially groundwater in permeable rock, which feeds baseflow to the channel (+1); so channel flow is supplied by a fall in storage (ΔS negative) while the input is small (+1). [3]
6. First reason: the soil store has a deficit, so rain infiltrates and refills it (1) before any surplus moves on to the river (+1). Second reason: dry vegetation and ground surfaces hold water and evaporate it (1), and a lowered water table means little groundwater flow reaches the channel (+1). One reason well developed scores at most 3. [4]