Lecture 4: Surface run off Flashcards
What is the difference between overland, through and groundwater flow? Which flows fastest?
Overland - on surface
Through - in shallow subsurface
Ground - in saturated zone
groundwater slower than throughflow slower than runoff
What does a hydrograph tell us about these three pathways?
Which one predominates
What is the runoff ratio?
Storm runoff / rainfall (the proportion of rainfall generated into runoff
What is the area under a hydrograph equal to, and what does the dashed line represent?
area = total storm runoff
dashed line used to distinguish normal base flow from storm runoff
How can throughflow vary?
Can be unsaturated or shallow perched saturated flow above main groundwater level.
What was Horton’s overland flow theory?
Infiltration excess - soil surface partitions falling rain so that one part goes rapidly as overland flow and the other initially into soil and then either through gradual groundwater flow to the stream channel or through evaporation to the atmosphere.
What is the partitioning divide in Hortons model?
The infiltration capacity of the soil surface - therefore no overland flow will develop if rainfall intensity is less than capacity.
Surface flow in hortons model may occur as thin, uniform sheet, but what determines the effective depth of this?
The height of topographic features / bumps as sheet flow cannot occur unless overground stores connect.
Why does overland flow not erode soil for a certain initial distance?
Velocity is not great enough
Describe Hewletts saturation - excess overland flow theory.
Over much of catchment area, even during intense and prolonged precipitation, all precipitation infiltrates soil surface.
Infiltration and throughflow cause rising water table to saturate the ground surface, causing these areas to have infiltration capacity of zero and subsequent precipitation to be saturated overground flow.
What is the difference between Horton and Hewletts models?
According to Hewlett, only the saturated areas of catchment act as a source of quickflow. (Source area of quickflow is therefore of variable size, growing with duration of rainfall).
Describe Dunne and Leopold’s theory of subsurface stormflow.
As rain falls, increased moisture, baseflow, subsurface stormflow and rise in water table.
The subsurface stormflow has a much slower response than steep rising and falling limb of saturation overland flow.
Describe the variable source area concept of saturation.
Saturation occurs from river moving up the basin and topography, expanding the area of dynamic partial contributing areas
Where on the hydrograph are the (shallow soils of) headwater basins becoming saturated?
Near the peak
Asia has the highest rates of runoff, what other defining property is this linked to?
Highest suspended sediment discharge due to runoff and human activities (agriculture siltates dams, deposits sediment in ocean)
What is drainage density and what does it reflect?
The density of river channels in a basin, reflecting balance between erosive forces and resistance.
What is linked to a large drainage density?
High runoff potential (as runoff generates erosion of new channels)
What factors affect runoff generation?
- Rainfall
- Cracks
- Soil profile
- Surface characteristics (i.e. vegetation)
- Slope
- Storage
What forms of storage might contribute to runoff generation?
Depression storage / soil storage
What properties of a slope might influence runoff?
Length, gradient, shape
Within the soil profile, what might affect runoff?
The properties of the soil itself
Evaporation
Antecedent moisture redistribution
What water pathways connect hillslope to channel?
Direct runoff Infiltration excess overground flow Saturation excess overground flow Subsurface storm flow Groundwater flow
Apart from in Asia, where is runoff high?
North parts of South America (and along tropics in general)
Why does infiltration capacity increase with rainfall intensity?
- Deeper runoff so larger potential pressure to drive through water surface
- Deeper runoff floods more topographic irregularities so greater surface area
- Intensity, runoff and droplet size scour surface skin, increasing permeability of crust.
Why does infiltration capacity decline during later storms?
Colloids/clay swell, closes cracks and pores, decreased potential gradient as wetting front penetrates to greater depth and surface crust develops.
What varies systematically along hillslopes affecting infiltration rate?
Soil texture and structure.
What do global runoff rates reflect?
Patterns of precipitation and evaporation
