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The Hydrosphere
Lecture 7: Runoff and the Hydrograph
Key Terms:
• Infiltration capacity

• The maximum rate at which a  particular soil under specific conditions can
absorb precipitation (mm/hr)

• Infiltration rate

• The rate at which water is actually being absorbed by the soil (mm/hr)

• Infiltration capacity is strongly affected by land-cover type: soil with more air spaces
like woodland can absorb more water

Land-cover type

Infiltration capacity (mm/hr)

Moderately grazed pasture

20

Heavily grazed pasture

5

Freshly ploughed cropland

100

Woodland

150

Bare clay

2

• Saturated soil:

• When all pore spaces in the soil are filled with water

• Unsaturated soil:

• When there are still pore spaces in the soil that are not filled with water

• Runoff:

• Water that flows over or under the land surface

• Runoff is comprised of water that flows:

1
...
Over the land surface (‘surface runoff’, or ‘overland flow’)

(a) Infiltration-excess overland flow

(b) Saturation-excess overland flow

• The hydrograph:

• Displays changes in river discharge with time
1
...
Matrix Flow:

• lateral flow of water through very fine pores in the soil

2
...
1mm in diameter, mostly
created by worms and insects, that promote quicker movement of water
than with matrix flow


3
...
g
...
e
...
Surface runoff (overland flow)
• There are two types of overland flow, dependent upon the method of generation:

(a) Infiltration-excess overland flow (‘Hortonian’ OLF)

(b) Saturation-excess overland flow

2
...
g
...
g
...
Surface runoff
(b) Saturation-excess overland flow
• It takes longer for saturation-excess overland flow to occur, as it only happens
once the soil has been saturated

• It is still quite quick though, much faster than throughflow


The Hydrograph
• Surface and sub-surface runoff can flow into river channels

• When the water enters river channels, the flow of water is called “river discharge”

• Hydrographs display variation in river discharge (m3s-1) over time:

• Comprised of:

1
...
time

2
...
‘Baseflow’ component (groundwater + throughflow)

4
...
Rise time

6
...
Subsurface runoff (also called “baseflow”)
2

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