Drainage Basin Hydrology

The drainage basin (also called a catchment or watershed) is the fundamental unit of study in hydrology. It is defined as the area of land drained by a river and its tributaries, bounded by a ridge of high land called the watershed. Unlike the global hydrological cycle, the drainage basin is an open system — it receives inputs from the atmosphere and loses outputs through river discharge and evapotranspiration. Understanding drainage basin hydrology is essential for predicting flood risk, managing water resources, and evaluating the impact of land-use change.

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The Drainage Basin as a System

graph TD
    P["INPUTS
Precipitation
(rain, snow, hail)"] --> INT["Interception
(by vegetation canopy)"]
    P --> TF["Throughfall &
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 movement in soil)"]
    SM --> PERC["Percolation
(to groundwater)"]
    SM -->|"Evapotranspiration"| ATM
    PERC --> GW["Groundwater Store"]
    GW --> BF["Baseflow
(to river channel)"]
    TH --> CH["River Channel"]
    OF --> CH
    BF --> CH
    CH --> Q["OUTPUT
River Discharge"]

Inputs

• Precipitation — the primary input. Includes rain, snow, sleet, and hail. In the UK, average annual precipitation ranges from ~550 mm in East Anglia to over 3,000 mm in the western Highlands of Scotland. The form, duration, intensity, and seasonal distribution of precipitation all influence the drainage basin response.

Stores

Store	Description	Typical Capacity
Interception store	Water held on vegetation surfaces (leaves, branches, bark)	Up to 2–5 mm per storm event for a deciduous forest canopy
Surface store	Water held in puddles, depressions, and on the ground surface	Variable — depends on surface roughness and topography
Soil moisture store	Water held in pore spaces between soil particles	Varies with soil type: sandy soils ~20%, clay soils ~45% porosity
Groundwater store	Water held in pore spaces and fractures of permeable rock (aquifers)	Chalk aquifers in SE England hold billions of litres
Channel store	Water held within the river channel at any time	Varies with channel dimensions

Transfers (Flows)

Transfer	Description	Typical Speed
Throughfall	Water dripping through gaps in the vegetation canopy	Rapid — seconds
Stemflow	Water running down plant stems and trunks to the ground	Rapid — seconds
Infiltration	Downward movement of water from the surface into the soil	Varies: sandy soils ~25 mm/hr; clay soils ~5 mm/hr
Throughflow	Lateral (downhill) movement of water through the soil	Slow — metres per day
Percolation	Downward movement of water from the soil into underlying rock	Very slow — may take years
Baseflow	Slow seepage of groundwater into the river channel	Very slow — sustains river flow during dry periods
Overland flow	Water flowing across the surface (when rainfall intensity > infiltration capacity)	Fast — metres per minute
Channel flow	Water flowing within the river channel towards the mouth	Variable — typically 0.5–3 m/s

Outputs

• River discharge (Q) — the volume of water passing a given point per unit time, measured in cumecs (m³/s).
• Evapotranspiration — the combined loss of water to the atmosphere through evaporation and transpiration.

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Infiltration and Its Controls

Key Definition: Infiltration is the downward movement of water from the ground surface into the soil. The infiltration rate is the volume of water passing into the soil per unit time (mm/hr). The infiltration capacity is the maximum rate at which water can infiltrate under given conditions.

Robert Horton (1933) established that overland flow occurs when rainfall intensity exceeds the infiltration capacity of the soil — this is known as Hortonian overland flow (or infiltration-excess overland flow).

Factors affecting infiltration rate: