Variable
source area
concept
John Hewlett
from Hewlett:
Simultaneous recognition on 3
continents of a Variable Source Area
à la
Hewlett-san
and
Hibbert-san
(1967)
Runoff Pathways
R
Hortonian OF
a i n
f a l l
Infiltration
Capacity
Percolation
Saturation OF
Saturation
Regolith
Regolith Subsurface Flow
Percolation
Bedrock
Aquifer
Aquifer Subsurface Flow
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
Hewlett on the hillslope scale
from Hewlett
Runoff Pathways
R
Hortonian OF
a i n
f a l l
Infiltration
Capacity
Percolation
Saturation OF
Saturation
Regolith
Regolith Subsurface Flow
Percolation
Bedrock
Aquifer
Aquifer Subsurface Flow
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
Southern Sweden—much like NE US
(Grip and Rodhe, 1994)
A different form of overland flow
Overland flow (infiltration excess+ saturation excess) emerging from a sugar
cane paddock over Kasnozem (Oxisol) soils (originating from Basalt), South
Johnstone near Innisfail during a monsoon event, March 1985.
Photo courtesy of Brian Prove
Experimental Design of Dunne and
Black (1970)
Seasonal Variations in VSA
Dunne, 1969;
78
The
link
to
flow
From Dunne and Leopold, 1978
Direct Precipitation onto Saturated Areas and
Return Flow
• Expands and contracts
during events
• Expands and contracts
seasonally
• Key zone for partitioning
fast and slow runoff
• Key non-point source hot
spot!
Brooks et al., Fig
4.11
From the original diagram by Hewlett, 1982
Saturation Overland Flow
Dunne and
Black, 1970
Where Saturation Occurs
Ward, 1970
Relation to live streams
Saturated areas:
We can sometimes estimate based on topography
Dave Tarboton, Utah State U.
Generalised
dependence
of Runoff
Coefficient
and Style of
Overland
Flow on
Arid-Humid
scale and on
Storm
Rainfall
Intensities
HOF vs SOF
100
More
Humid
Mainly
Saturation
Overland Flow
75%
ETACT
ETPOT
50%
%
More
Arid
Mainly
Hortonian
Overland Flow
Seasonal or
storm period
fluctuations
25%
Total Runoff
0%
0
0 More Intense
% Rain Days
Less Intense
100
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
Runoff Pathways
R
Hortonian OF
a i n
f a l l
Infiltration
Capacity
Percolation
Saturation OF
Saturation
Regolith
Regolith Subsurface Flow
Percolation
Bedrock
Aquifer
Aquifer Subsurface Flow
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
The British Invasion
Benchmark papers
by Burt, 1970s and
early 1980s
and Weyman,
Anderson, Kirkby,
Chorley……….
From Kirkby, 1978
Topographic Convergence
Anderson and
Burt, 1978
Hornberger et al text
Topographic Controls on Saturation Development
Ruhe and Walker,
Subsurface Stormflow
• At the start of an event,
percolation occurs vertically
• Soil moisture increases & some
water bypasses to depth
• Where percolation reaches a
less permeable layer that will
not accept the wetting front,
saturation will develop
• Saturation development
controlled by permeability &
available storage
• The saturated “wedge” or
perched water table contributes
Weyman 1973
Whipkey’s work
Data:
Whipkey
, 1965
Highly preferential
Sidle et al 2001 HP
Tarboton web course
Stable isotopes reveal the importance of stored water
Not a new idea
Pinder and Jones 1969 WRR
Two component mixing model
 Solve two simultaneous mass-balance equations for Qold
and Qnew
Qstream = Qold + Qnew
CstreamQstream = ColdQold+ CnewQnew
 To yield the proportion of old water
pol d
Qol d
Cst ream  Cne w


Qst ream
Col d  Cne w
Hooper (2001)
Weiler et al. 2004, WRR
Qpe/Qs = (Cs-Ce)/(Cpe-Ce)
Groundwater
Surface Water
Interactions
Variations in stream
discharge, dD, and
electrical conductivity
at M8
(Sklash et al., 1986
WRR)
“Groundwater”
is the main
component of
flood
hydrographs
Runoff Pathways
R
Hortonian OF
a i n
f a l l
Infiltration
Capacity
Percolation
Saturation OF
Saturation
Regolith
Regolith Subsurface Flow
Percolation
Bedrock
Aquifer
Aquifer Subsurface Flow
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
Groundwater Ridging
Abdul and Gillham, 1984
The Soil-Water Interface and the
Effect of Suction
Abdul and Gillham, 1984
Abdul and Gillham, 1984
Groundwater Ridging
Capillary
Fringe
Precipitation
Seepage
face
Equipotential
lines
Flow Lines
...a Swedish view on the subject
Rodhe, 1987 Transmissivity feedback
From Grip and Rodhe; Seibert et al. 2002
Rodhe, 1987 Transmissivity feedback
Runoff Pathways
Putting it all together
R
Hortonian OF
a i n
f a l l
Infiltration
Capacity
Percolation
Saturation OF
Saturation
Regolith
Regolith Subsurface Flow
Percolation
Bedrock
Aquifer
Aquifer Subsurface Flow
Slide from Mike Kirkby, University of Leeds, AGU Chapman Conference on Hillslope Hydrology, October 2001
Storm Precipitation
Saturation Overland Flow
Hortonian Overland Flow
Channel Precip.
+
Overland Flow
Soil
Mantle
Storage
Overland
Flow
Subsurface
Stormflow
Baseflow
Interflow
Basin Hydrograph
Re-drawn from Hewlett and Troendle, 1975
Dominant processes of hillslope
response to rainfall
Arid to sub-humid
climate; thin vegetation
or disturbed by humans
Thin soils; gentle
concave footslopes;
wide valley bottoms;
soils of high to low
permeability
Variable source
concept
Subsurface stormflow
dominates hydrograph
volumetrically; peaks
produced by return
flow and direct
precipitation
Steep, straight
hillslopes; deep,very
permeable soils;
narrow valley
bottoms
Topography and soils
Horton overland flow
dominates
hydrograph;
contributions from
subsurface stormflow
are less important
Direct precipitation
and return flow
dominate hydrograph;
subsurface stormflow
less important
Humid climate;
dense vegetation
Climate, vegetation and land use
(Dunne and Leopold, 1978)