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Flashcards in Runoff Deck (11):

Why is runoff important?

Flood forecasting
- operational real time forecasting
- strategic catchment response to environmental change
- formal theories of catchment hydrology, research tools to develop understanding and knowledge


Precipitation into runoff

- direct precipitation
- overland flow
- through-flow
- groundwater discharge


Infiltration capacity

spatial variation in initial and final infiltration capacity varies
- soil properties, structure and texture
- soil faunal activity, burrowing, plant root canals
- vegetation cover and land management
- topography


flow through soil depend on:

- moisture content of soil
- hydraulic conductivity of material
- pressure gradient, water table
- presence of macropores/preferential flow


Slow delayed flow

flow rates of up to 2m/hour


Seldom fully developed

- hard pans
- fingered flow
- percolines, swales etc.


Contributing areas/source

• As rainfall infiltrates, the soil becomes wetter and the proportion of the catchment that is saturated increases.
• This part of the catchment is termed the contributing or source area – because it is where most of the quickflow response to rainfall originates.
• Saturation-excess overland flow is generated from this.
• Saturated throughflow is much quicker than throughflow in unsaturated soils
• Macropore flow may be triggered – flow only occurs in macropores once soil moisture reaches a certain value i.e. there is an initiation threshold
• Small streams can be supported in saturated contributing areas


Variable source Area theory

•Saturated area is dynamic; it grows and contracts during an event
• Expansion into riparian area
• Expanding and contracting channel area
• Catchment responds in highly non-linear way to rainfall


Riparian area

- storage and water table higher nearer valley bottom
- near steam area = riparian area
- 1-3% of total catchment area
- storage of water due to: downslope flow, convergence in hill-slope hollows, return of flows from deeper layers. soils deeper in valley bottoms


Dilution gauging

• Useful for turbulent mountain streams where a gauging
structure would be impossible
• Also useful for low velocity locations
• Tracer should dissolve readily, be stable and non-toxic
• Gulp or continuous injection
• Sample downstream of injection



• Velocity and cross-sectional area are fixed → stage-discharge relationship
• Allows continuous measurement
• Sensitive to small changes in flow → low-flow hydrology of small channels
• Raises water level upstream above natural level
• Slows flow down → sediment build-up upstream
• Overtop under flood conditions