Drainage Basin Systems (AQA A-Level Geography): Revision Notes

Drainage Basin Systems

Introduction to drainage basins

A drainage basin (sometimes called a catchment area) represents the land area that channels water to a specific river and its tributaries. Think of it as nature's water collection system - everything from surface water to groundwater within the basin eventually makes its way to the main river channel.

The boundary separating one drainage basin from another is marked by high ground called a watershed. Below ground, the system extends to include water stored in soils and rocks beneath the surface.

System components

Inputs, stores, transfers and outputs

Like all natural systems, drainage basins can be broken down into four main components that help us understand how water moves through the landscape:

Inputs represent water entering the system. The primary input is precipitation falling onto the land surface. The characteristics of this precipitation - its intensity, duration and form (rain, snow, hail) - significantly influence what happens next in the system.

Stores are locations where water is held temporarily within the drainage basin. These include:

• Lakes and surface water bodies
• River channels themselves
• Interception storage on vegetation
• Soil water held in pore spaces
• Groundwater stored in rocks below the surface

Transfers describe the various pathways water takes as it moves through the system. These processes include overland flow across the surface, throughflow moving laterally through soil, and percolation descending through rocks.

Outputs represent water leaving the drainage basin. The main outputs are evaporation from water surfaces, transpiration from vegetation (often combined as evapotranspiration), and run-off where water exits via the river channel.

How water moves through the drainage basin

The journey from precipitation to river

When precipitation falls onto a hillside, several processes determine its path to the river channel. The nature and speed of this journey depend on factors including rainfall intensity, vegetation cover, soil type and slope gradient.

Interception - the first barrier

Not all vegetation intercepts water equally effectively. Research shows that forests composed of needle-leaf trees capture approximately 22% of rainfall, whilst broad-leaf deciduous forests intercept around 19%. This difference likely relates to the overall density of the vegetation canopy rather than individual leaf structure. Some tropical rainforests can intercept as much as 58% of total rainfall.

Water held in the interception store has three possible fates:

• Direct evaporation back to the atmosphere
• Absorption into the plant structure
• Transfer to the ground through throughfall or stemflow

Infiltration - entering the soil

Once water reaches the ground surface (whether directly or via throughfall and stemflow), it begins to soak into the soil through a process called infiltration.

The infiltration rate controls how much water enters the soil versus flowing across the surface. This rate depends on several factors, with soil porosity being most critical. A soil's porosity is determined by its texture, structure and organic content:

• Coarse-textured soils (sandy soils) have larger pores and fissures, allowing water to pass through quickly
• Fine-grained soils (clay soils) have smaller pores that restrict water movement

How infiltration rates change during rainfall:

The infiltration rate typically declines rapidly during the early stages of a rainstorm, then stabilises after several hours. Three main processes cause this decline:

1. Small pores at the soil surface fill with water, reducing capillary forces that normally draw water into the soil
2. Clay particles absorb water and expand, decreasing the size of existing pores
3. Raindrop impact breaks apart larger soil clumps, creating smaller particles that clog surface pores and restrict water movement

When infiltration capacity is exceeded

If rainfall intensity exceeds the infiltration capacity, water cannot enter the soil quickly enough and begins to accumulate on the surface as surface storage. This commonly occurs after prolonged rainfall when soils become saturated. The soil is considered saturated when all available pore spaces are filled with water.

On natural surfaces, this surface water typically disappears into the ground relatively quickly. However, on impermeable surfaces (concrete pavements, tarmac roads, bare rock, frozen ground), water has nowhere to infiltrate and must flow across the surface.

This represents very rapid water movement with few obstacles slowing the flow. Urban areas experience particularly fast overland flow because surfaces are often designed with strategic slopes and smooth finishes to move water quickly. In contrast, water moves through soil much more slowly.

Lateral movement through soil - throughflow

Even whilst water is infiltrating, lateral movement may begin.

This lateral water movement eventually reaches the nearest channel, though it travels considerably more slowly than overland flow. The rate of throughflow increases in vegetated areas because plants constantly extract water from the soil for transpiration. This process, known as vegetation storage, involves plants removing water from the soil and temporarily storing it in their structure before releasing it to the atmosphere through stomata on their leaves.

Downward movement - percolation and groundwater

Following infiltration, water continues moving vertically downward through the soil and underlying rock.

Water first travels through unsaturated rock, where pore spaces may be held in the rocks as groundwater. From here, it moves very slowly both vertically and laterally.

Some rocks can store substantial amounts of water, particularly if they are porous. These water-bearing rocks are called aquifers.

Water reaching the river channel

All these various flows and transfers eventually deliver water to the nearest river or lake (or to deep groundwater storage). The river then transports water through channel flow, which represents the amount of water exiting the drainage basin.

The main sources of water feeding into a stream or river include:

• Direct precipitation falling onto the water surface
• Tributary streams joining the main channel
• Overland flow from surrounding land
• Throughflow emerging at the channel edge
• Groundwater discharge where the water table intersects the river bed

Water balance in drainage basins

The concept of water balance helps us understand the relationship between water entering and leaving a drainage basin over time.

This balance can be expressed mathematically as:

$P = Q + E \pm \Delta S$

Where:

• $P$ = Precipitation (input)
• $Q$ = Discharge (river flow/run-off)
• $E$ = Evapotranspiration (output to atmosphere)
• $\Delta S$ = Change in storage (soil moisture and groundwater)

The two most critical components of this relationship are precipitation and potential evapotranspiration.

Potential evapotranspiration closely relates to prevailing temperature. Higher temperatures increase evapotranspiration rates because the warmer atmosphere can hold more water vapour. Sometimes the atmosphere's capacity to hold water vapour exceeds the actual water available for evaporation, creating a deficit.

Seasonal variations in water balance

Rivers typically exhibit an annual pattern of discharge levels called the river's regime. However, discharge also fluctuates in the short term following heavy rainfall events.

The soil moisture budget graph illustrates how precipitation and potential evapotranspiration interact throughout the year. Consider an area in eastern England with 750mm annual rainfall:

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