Lecture 4: Surface run off

Question 1

What is the difference between overland, through and groundwater flow? Which flows fastest?

Answer 1

Overland - on surface
Through - in shallow subsurface
Ground - in saturated zone
groundwater slower than throughflow slower than runoff

Question 2

What does a hydrograph tell us about these three pathways?

Answer 2

Which one predominates

Question 3

What is the runoff ratio?

Answer 3

Storm runoff / rainfall (the proportion of rainfall generated into runoff

Question 4

What is the area under a hydrograph equal to, and what does the dashed line represent?

Answer 4

area = total storm runoff

dashed line used to distinguish normal base flow from storm runoff

Question 5

How can throughflow vary?

Answer 5

Can be unsaturated or shallow perched saturated flow above main groundwater level.

Question 6

What was Horton’s overland flow theory?

Answer 6

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.

Question 7

What is the partitioning divide in Hortons model?

Answer 7

The infiltration capacity of the soil surface - therefore no overland flow will develop if rainfall intensity is less than capacity.

Question 8

Surface flow in hortons model may occur as thin, uniform sheet, but what determines the effective depth of this?

Answer 8

The height of topographic features / bumps as sheet flow cannot occur unless overground stores connect.

Question 9

Why does overland flow not erode soil for a certain initial distance?

Answer 9

Velocity is not great enough

Question 10

Describe Hewletts saturation - excess overland flow theory.

Answer 10

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.

Question 11

What is the difference between Horton and Hewletts models?

Answer 11

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).

Question 12

Describe Dunne and Leopold’s theory of subsurface stormflow.

Answer 12

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.

Question 13

Describe the variable source area concept of saturation.

Answer 13

Saturation occurs from river moving up the basin and topography, expanding the area of dynamic partial contributing areas

Question 14

Where on the hydrograph are the (shallow soils of) headwater basins becoming saturated?

Answer 14

Near the peak

Question 15

Asia has the highest rates of runoff, what other defining property is this linked to?

Answer 15

Highest suspended sediment discharge due to runoff and human activities (agriculture siltates dams, deposits sediment in ocean)

Question 16

What is drainage density and what does it reflect?

Answer 16

The density of river channels in a basin, reflecting balance between erosive forces and resistance.

Question 17

What is linked to a large drainage density?

Answer 17

High runoff potential (as runoff generates erosion of new channels)

Question 18

What factors affect runoff generation?

Answer 18

• Rainfall
• Cracks
• Soil profile
• Surface characteristics (i.e. vegetation)
• Slope
• Storage

Question 19

What forms of storage might contribute to runoff generation?

Answer 19

Depression storage / soil storage

Question 20

What properties of a slope might influence runoff?

Answer 20

Length, gradient, shape

Question 21

Within the soil profile, what might affect runoff?

Answer 21

The properties of the soil itself
Evaporation
Antecedent moisture redistribution

Question 22

What water pathways connect hillslope to channel?

Answer 22

Direct runoff
Infiltration excess overground flow
Saturation excess overground flow
Subsurface storm flow
Groundwater flow

Question 23

Apart from in Asia, where is runoff high?

Answer 23

North parts of South America (and along tropics in general)

Question 24

Why does infiltration capacity increase with rainfall intensity?

Answer 24

• 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.

Question 25

Why does infiltration capacity decline during later storms?

Answer 25

Colloids/clay swell, closes cracks and pores, decreased potential gradient as wetting front penetrates to greater depth and surface crust develops.

Question 26

What varies systematically along hillslopes affecting infiltration rate?

Answer 26

Soil texture and structure.

Question 27

What do global runoff rates reflect?

Answer 27

Patterns of precipitation and evaporation