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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.
| 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 |
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.
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:
| 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.
The soil moisture budget describes seasonal variations in soil moisture:
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