Drainage Basin Hydrology Advanced

A drainage basin is an area of land drained by a river and its tributaries. Unlike the global hydrological cycle (a closed system), the drainage basin is an open system with clearly defined inputs, outputs, stores, and transfers. Mastering the quantitative aspects of drainage basin hydrology — infiltration, soil moisture dynamics, flow separation, and the water balance equation — is essential for high-mark responses at A-Level.

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The Drainage Basin as an Open System

Inputs

• Precipitation — the primary input, arriving as rain, snow, sleet, or hail.
• Solar energy — drives evapotranspiration and snowmelt.

Outputs

• River discharge (Q) — measured at the basin outlet (gauging station) in cumecs (m³/s).
• Evapotranspiration (E) — water returned to the atmosphere from soil, vegetation, and water surfaces.
• Deep groundwater seepage — water that percolates below the basin's bedrock boundary (often negligible in short-term analysis).

Stores

Store	Description
Interception storage	Water held on vegetation surfaces; typically 1–3 mm per storm in deciduous woodland
Surface storage	Water in puddles, lakes, and wetlands
Soil moisture	Water held in pore spaces between soil particles
Groundwater	Water in saturated rock below the water table
Channel storage	Water held within the river channel and floodplain
Snow/ice storage	Seasonal or permanent frozen water within the basin

Transfers

• Infiltration — movement of water from the surface into the soil.
• Percolation — downward movement through soil and rock to the water table.
• Throughflow — lateral movement of water through the soil, often along horizons of differing permeability.
• Groundwater flow (baseflow) — slow lateral movement through saturated rock towards the river.
• Overland flow (surface runoff) — water flowing across the surface when infiltration capacity is exceeded or the soil is saturated.
• Stemflow and throughfall — routes by which intercepted water reaches the ground.
• Channel flow — water moving within the river channel.

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Infiltration and Infiltration Capacity

Infiltration is the process by which water enters the soil surface. The rate at which soil can absorb water is called the infiltration capacity (or infiltration rate), measured in mm/hr.

Horton's Infiltration Model

Robert Horton (1933) proposed that infiltration capacity is highest at the start of a rainfall event and decreases exponentially over time as soil pores become saturated:

• Initial infiltration capacity (f₀): The maximum rate at the start of rainfall, before surface sealing occurs. Values range from >200 mm/hr for well-structured sandy soils to <5 mm/hr for compacted clay.
• Final (equilibrium) infiltration capacity (fc): The constant rate reached after prolonged rainfall, essentially equal to the saturated hydraulic conductivity of the soil.

Factors Affecting Infiltration Capacity

Factor	Effect
Soil texture	Sandy soils have large pores and high infiltration rates; clay soils have small pores and low rates
Soil structure	Well-aggregated soils with macropores (worm burrows, root channels) allow rapid infiltration
Antecedent moisture	Wet soils have less pore space available; infiltration capacity is reduced
Vegetation cover	Root networks maintain soil structure; leaf litter protects the surface from raindrop impact
Land use	Urbanisation (impermeable surfaces) and agriculture (compaction, ploughing) reduce infiltration
Surface crusting	Raindrop impact on bare soil can create a thin, impermeable crust
Slope angle	Steeper slopes allow less time for infiltration; water moves downslope more quickly

Key Distinction: Infiltration-excess (Hortonian) overland flow occurs when rainfall intensity exceeds infiltration capacity. Saturation-excess overland flow occurs when the soil is fully saturated from below, regardless of rainfall intensity, and is common in valley bottoms and areas with a high water table.

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Soil Moisture Dynamics

Key Concepts

• Soil moisture content: The volume of water held in soil pore spaces, expressed as a percentage or in mm.
• Gravitational water: Water that drains freely under gravity through macropores.
• Field capacity: The moisture content of soil after gravitational water has drained away (typically within 1–2 days of rainfall). At field capacity, the soil is moist but not waterlogged.
• Wilting point: The moisture content below which plants can no longer extract water from soil. Remaining water is held too tightly by surface tension in micropores.
• Available water capacity: The difference between field capacity and wilting point — this is the water available for plant uptake.

Soil Moisture Budget

The soil moisture budget describes seasonal variations in soil moisture: