Runoff

Question 1

Why is runoff important?

Answer 1

Flood forecasting

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

Question 2

Precipitation into runoff

Answer 2

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

Question 3

Infiltration capacity

Answer 3

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

Question 4

flow through soil depend on:

Answer 4

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

Question 5

Slow delayed flow

Answer 5

flow rates of up to 2m/hour

Question 6

Seldom fully developed

Answer 6

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

Question 7

Contributing areas/source

Answer 7

• 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

Question 8

Variable source Area theory

Answer 8

• 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

Question 9

Riparian area

Answer 9

• 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

Question 10

Dilution gauging

Answer 10

• 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

Question 11

Weirs

Answer 11

• 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