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Spec mapping (AQA 7037): Paper 1, §3.1.1 — The drainage basin as an open system: inputs, outputs, stores, and flows; the water balance; the influence of physical and human factors on these. Synoptic links: drainage-basin processes underpin fluvial landform development (river systems), flood-hazard management, and connect to §3.1.5 Hazards where land-use change amplifies flood risk. AOs: AO1 (open-system structure, infiltration mechanisms, water-balance terms), AO2 (applying controls to specific catchments), AO3 (calculating and interpreting a water balance).
The drainage basin — also called a catchment or watershed in American usage — is the fundamental spatial unit of hydrology. It is defined as the area of land drained by a river and its tributaries, bounded by a ridge of higher ground (the watershed) that separates it from neighbouring basins. Whereas the global hydrological cycle (Lesson 2) is a closed system, the drainage basin is an unambiguously open system: it gains matter (precipitation) and energy (solar radiation) from outside, and loses matter through river discharge and evapotranspiration across its boundary. It is also a cascading system (Lesson 1), in which water is handed from one store to the next, the output of each becoming the input of the next. Mastery of drainage-basin hydrology underpins flood prediction, water-resource management, and the assessment of land-use change. Because the drainage basin is the scale at which human decisions most directly shape the water cycle, this lesson is also where the abstract systems framework becomes intensely practical — every term in the water-balance equation can be measured, budgeted, and, crucially, altered by land management, which is why it recurs throughout the flood-hazard and management content of the specification.
graph TD
P["INPUT: Precipitation (rain, snow, hail)"] --> INT["Interception store (canopy)"]
P --> TF["Throughfall and stemflow"]
INT -->|"evaporation"| ATM["ATMOSPHERE"]
TF --> INF["Infiltration into soil"]
TF --> OF["Overland flow (surface runoff)"]
INF --> SM["Soil moisture store"]
SM --> TH["Throughflow (lateral)"]
SM --> PERC["Percolation to bedrock"]
SM -->|"evapotranspiration"| ATM
PERC --> GW["Groundwater store"]
GW --> BF["Baseflow"]
TH --> CH["Channel store"]
OF --> CH
BF --> CH
CH --> Q["OUTPUT: River discharge"]
The drainage basin is the scale at which most practical hydrology, flood management, and water-resource planning is conducted, precisely because the watershed provides a natural, physically-meaningful boundary: within it, all surface water drains to a single outlet, so what happens on the slopes is hydrologically connected to the river. This makes the basin an ideal unit for applying the systems framework — its inputs, stores, flows, and outputs can all, in principle, be measured and budgeted. Whereas the global cycle is too vast and too closed to manage, the basin is the scale at which human decisions (land use, abstraction, flood defences) actually bite, which is why it is the organising unit for the rest of this topic and for §3.1.5 flood-hazard management.
| Store | Description | Typical magnitude |
|---|---|---|
| Interception store | Water held on vegetation surfaces (leaves, branches, bark) | Up to ~2–5 mm per storm for a deciduous canopy in leaf; near zero for bare ground |
| Surface (depression) store | Water in puddles, hollows, and depressions | Variable with surface roughness and microtopography |
| Soil moisture store | Water in pore spaces between soil particles | Field capacity varies with texture: sandy ~20% by volume, clay ~45% porosity |
| Groundwater store | Water in pore spaces and fractures of permeable bedrock (aquifers) | The Chalk aquifer of SE England holds water sufficient for ~70% of regional public supply |
| Channel store | Water held within the river channel at any instant | Scales with channel cross-section and length |
| Transfer | Description | Typical speed |
|---|---|---|
| Throughfall | Water dripping through gaps in the canopy | Seconds |
| Stemflow | Water running down stems and trunks to the ground | Seconds |
| Infiltration | Downward entry of water from surface into soil | Sandy soils ~25 mm hr⁻¹; clay soils ~5 mm hr⁻¹ |
| Throughflow | Lateral (downslope) movement through soil, often via macropores | Slow — metres per day |
| Percolation | Downward movement from soil into underlying rock | Very slow — may take years |
| Baseflow | Slow seepage of groundwater into the channel | Very slow — sustains rivers in droughts |
| Overland flow | Surface flow when rainfall intensity > infiltration capacity, or soil saturated | Fast — metres per minute |
| Channel flow | Flow within the channel towards the mouth | Typically 0.1–3 m s⁻¹ |
The flows can be ranked by velocity: overland flow ≫ throughflow ≫ baseflow. Because overland flow is so much faster, any change that increases the proportion of rainfall taking the overland route (urbanisation, deforestation, soil compaction) shortens the basin's response time and raises flood peaks — the central theme of the storm hydrograph (Lesson 4).
The interception store is the first link in the cascade and a frequently underrated control. When rain begins, the canopy first wets up, filling the interception store; only once its capacity is reached does throughfall (water dripping from leaves) and stemflow (water running down trunks) deliver water to the ground. Water held in the interception store is largely evaporated straight back to the atmosphere — interception loss — and never reaches the channel at all. A dense coniferous canopy can intercept and evaporate 35–45% of annual rainfall (Calder, 1990), a deciduous canopy 25–35% (much less in winter when leafless), and grassland 10–20%. Interception therefore does three things at once: it reduces the total volume reaching the soil, it delays delivery (buying time for infiltration), and it protects the soil surface from raindrop impact, preserving infiltration capacity. This is precisely why removing vegetation has such a disproportionate hydrological effect — it eliminates the first buffer in the cascade.
Once water reaches the channel it is routed to the outlet as channel flow, whose velocity depends on the channel gradient, the hydraulic radius (cross-sectional area divided by wetted perimeter — a measure of channel efficiency), and the roughness of the bed and banks (quantified by Manning's n). Smooth, deep, steep channels convey water fastest. Discharge itself, Q=A×v, therefore rises both because more water enters the channel during a storm and because the deeper flow moves faster — a compounding effect that helps explain the steep rising limb of a storm hydrograph.
Key Definition: Infiltration is the downward movement of water from the surface into the soil. The infiltration rate is the volume entering per unit time (mm hr⁻¹); the infiltration capacity is the maximum rate possible under given conditions. Infiltration capacity declines during a storm as the soil wets up.
Robert Horton (1933) established that overland flow is generated when rainfall intensity exceeds infiltration capacity — infiltration-excess or Hortonian overland flow. This dominates where soils are thin or impermeable, in semi-arid areas with crusted surfaces, and during intense convective downpours.
| Factor | Effect on infiltration |
|---|---|
| Soil texture | Sandy soils: high (large pores). Clay soils: low (fine pores, small capillaries) |
| Antecedent moisture | Already-wet soils have low remaining capacity → more runoff |
| Vegetation cover | Roots open macropores; organic matter improves structure; canopy reduces raindrop impact and surface sealing |
| Land use | Tarmac/concrete are impermeable; compacted (ploughed or grazed) land has reduced capacity |
| Slope angle | Steeper slopes give water less time to infiltrate before running off |
| Frost / frozen ground | Frozen soil is effectively impermeable → high runoff during rain-on-snow events |
| Raindrop impact | Heavy rain on bare soil forms a crust (rainsplash compaction), sealing the surface |
A second mechanism, identified by Hewlett and Hibbert (1967), is saturation-excess overland flow: where soils become fully saturated from below (typically in valley bottoms and near channels as the water table rises to the surface), any further rain — however gentle — must flow over the surface. This is the dominant runoff mechanism in humid temperate environments like the UK, where soils are frequently near saturation in winter. The variable source area concept follows from it: the area generating overland flow is not the whole basin but expands and contracts around the channel network as wetness changes.
On a real hillslope, the two overland-flow mechanisms and the subsurface flows operate side by side, and which dominates shifts through a storm and across the slope. Early in a storm onto dry soil, infiltration is high and most water enters the soil, moving downslope as throughflow — often concentrated in natural soil pipes and macropores (root channels, animal burrows, desiccation cracks) that can transmit water surprisingly fast. As rain continues, two things happen: on thin or compacted soils, infiltration capacity is exceeded and Hortonian overland flow begins on the upper slope; meanwhile, near the channel the water table rises until the soil saturates from below and saturation-excess overland flow spreads outward from the valley floor. The variable source area — the saturated, runoff-generating zone — therefore expands up the slope and along the network as the storm proceeds, then contracts again as the basin drains. Understanding this dynamic explains why the proportion of a basin contributing fast runoff is not fixed but grows with storm magnitude and antecedent wetness — and why the largest floods occur when prolonged rain onto already-wet ground maximises the contributing area.
The water balance (or water budget) quantifies the relationship between inputs and outputs over a defined period:
P=Q+E±ΔS
Where:
Over a long period (e.g. a year), ΔS approaches zero and the equation simplifies to P = Q + E. Over shorter periods, ΔS captures the seasonal "savings and withdrawals" from soil and groundwater stores. A helpful analogy is a household budget: precipitation is income, evapotranspiration and discharge are expenditure, and ΔS is the change in the savings account. Over a year the savings may net to roughly zero (you end where you started), but within the year you draw the account down in summer (deficit) and build it back up in winter (surplus). The equation must always balance — if you know any three terms you can calculate the fourth — which is precisely what makes it such a versatile analytical and exam tool.
In the UK the balance has a strong seasonal rhythm:
| Season | Precipitation | Evapotranspiration | Outcome |
|---|---|---|---|
| Winter (Oct–Mar) | Higher | Low (cool, low insolation, dormant vegetation) | Soil moisture surplus → field capacity reached → groundwater recharge → rivers at high flow |
| Summer (Apr–Sep) | Lower | High (warm, active vegetation, long days) | Soil moisture deficit → soils dry below field capacity → no recharge → rivers at low flow |
The soil moisture budget graph (PE vs precipitation through the year) is a standard AQA resource. Its key features are the recharge phase (autumn, when P first exceeds PE and refills the soil store), the surplus phase (winter, soil at field capacity, excess water becomes runoff/recharge), the utilisation phase (spring, PE exceeds P and plants draw down soil water), and the deficit phase (summer, soil water exhausted, AE < PE).
Two thresholds in the soil store matter for the budget. Field capacity is the maximum water the soil can hold against gravity after free drainage; once reached, any additional input becomes runoff or recharge (the surplus phase). The wilting point is the moisture level below which plants can no longer extract water; between field capacity and wilting point lies the available water that plants and evapotranspiration draw on during the utilisation and deficit phases. Tracking these explains why winter rainfall is so much more "effective" at generating river flow and groundwater recharge than summer rainfall of the same amount: in winter the soil store is already at field capacity and PE is low, so almost all rain becomes runoff; in summer the rain first has to refill a depleted store and much is lost to high PE before any reaches the channel.
The water-balance terms are not fixed — they respond to land-use change, which is why the equation is such a powerful analytical tool:
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