Catchment and factors affecting runoff

Catchment

A catchment (drainage basin, watershed) is the area of land which drains the surface runoff from precipitation to a common outlet (a river or a point on it) through a connected system of streams. Its boundary is the water divide (ridge line).

Factors affecting runoff from a catchment

1. Climatic factors

• Type of precipitation (rain, snow, hail).
• Rainfall intensity: an intensity above the infiltration capacity gives more runoff.
• Duration and areal distribution of the storm; direction of storm movement relative to the drainage network.
• Antecedent precipitation (soil moisture): wet soil gives higher runoff.
• Evaporation, temperature, wind and snow-melt.

2. Physiographic (catchment) factors

• Size: a larger area gives larger total runoff but a lower specific peak, and a longer lag time.
• Shape: fan-shaped or circular basins produce high, sharp peaks; elongated basins produce lower, flatter peaks.
• Slope: steep slopes give quick runoff, higher peaks and less infiltration.
• Land use and vegetation: forest and crops intercept and infiltrate water; urbanisation and bare land increase runoff.
• Soil type and geology: the permeability of the soil and rock governs infiltration; sandy soil gives low runoff, clay gives high runoff.
• Drainage density and stream pattern: a high density gives quick drainage.
• Topography, elevation and orientation: these affect rainfall, snow and temperature.
• Storage: lakes, ponds, depressions, wetlands and reservoirs reduce and delay peaks.

3. Channel factors

• Cross-section, roughness and slope of the channels; presence of check dams and bank storage.

4. Human factors

• Dams, diversions, irrigation, drainage works and land-use change.