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9.1 Runoff Processes
flow of water over land in thin layers & channels includesmany direct & indirect components:
river discharge, Q [L3 T-1] in channel as m3s-1
channel precipitation, Qp often small river area 1-2% of basin
overland flow, Qo (aka excess ppt, Pe)
sheetflow, rills, gullies ppt impact & erosion
throughflow, Qt lateral downslope flow above water table
groundwater flow, Qg
emergent baseflow in rivers & springs
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Hortons (1933) overland flow
first conceptual model to partition stream flow Q into runoff
(Qo) & groundwater (Qg) components
land surface partitions water into infiltration vs. surface runoff
components
when rainfall intensity (i) > infiltration capacity (f), excess ppt (Pe) will
flow as Qo
infiltration excess overland flow or Hortonian overland flow (HOF)
all other moisture percolates to groundwater table
based on observations in arid basins with low permeability
limitations? rarely occurs in forested basins!
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Hortonian overland flow (HOF)
i = rainfall intensity
f = infiltration rate
t = point in time
Pe= excess P
Qo = runoff
rainstorm onto
semi-arid
landscape
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Partial Area Concept
Building upon the scarcity of observed infiltration excess, the
partial area concept (Betson 1964) was proposed
Horton overland flow from only a small area of the catchment
is required to produce the storm hydrograph
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Subsurface Flow
By 1960s recognized that most of precipitation
infiltrates the soil surface of forested catchments and
the subsurface flow is the major source of stormflow
number terms all basically denoting same process
subsurface stormflow (Hursh 1936; Hewlett & Hibbert 1967) Interflow (Hertzel 1936; Beven 1989)
saturated interflow (Betson et al. 1968)
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Hewletts (1961) saturated overland flow
on land surfaces, most P infiltrates very little Qo
via HOF infiltration & throughflow (Qt) cause soil zone to reach
saturation, water table rises & eventually emerges as
return flow combines with surface ppt inputs as saturated overland
flow, Qo(s)
thus, only saturated areas provide quickflow allothers absorb & infiltrate very little HOF
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Pressure potential in a straight hillslopeduring course of storm
Soil water close to gravity drainage
Surface layer become wetted
Saturated layer start to form
Flow from upper area
Throughflow (Qt) (fig. 6.21)
especially pronounced
on sloped surfaces re-emergence at base as
return flow
Qo(s)
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Hewletts (1961) Saturated OF
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Variable Source Area Concept (Hewlett & Hibbert 1967)
As a result of infiltration and
subsurface flow the water table in thenear-stream areas rises to ground
surface and expanding the saturated
stream channel area
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Contributions of Hewletts model
more suitable for vegetated, humid-temperate basins
saturated overland flow, Qo(s) as major component of runoff up to 85% of total
50-60% total storm discharge (quickflow) in rivers
30-40% of delayed storm discharge (baseflow)
very small portion (
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Groundwater ridging (fig. 7.9)
originally, Hewlett suggested all
flow infiltrates & travels via Qt
Qo
only on lower slopes by
saturated return flow Qo(s)
later modified to include
groundwater ridge on lower
riparian slopes (Slash &Farvolden 1979)
migrates toward channel &
contributes groundwater pulse
to river flow
delayed baseflow
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Subsurface Preferential Flow (Beven & German 1982)
Flow through soil matrix in forested soils may not be entirely
uniform
preferential flow pathways (macropores) that bypass the soil matrixrapidly transmit subsurface flow to stream channel
Water conducted ahead of wetting front & flow turbulent
Darcian matrix flow uniform laminar flow through soil
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Hillslope Runoff Measurement
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Relative importance of pathways and mechanisms involved in
streamflow generation affected by:
Basin characteristics
climate
topography
pedology
geology
Spatial and seasonal characteristics
maximum saturation contribution in northern areas
occurs during spring when stream valley areas
saturated as result of large water input by snowmelt
quantity of runoff produced will also vary depending on
the moisture within basin prior to storm - wet or dry
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9.2 Stream discharge, Q
volume of water moved per unit time (m3 s-1) through cross-
section of a river channel
calculated using the area-velocity method
Q = A v
where A is area m2 and
v is velocity m s-1
generation, timing & amount of Qs depends on how runoff is
partitioned into Qo, Qo(s) Qt, Qg each has different response time
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Streamflow (Q) Hydrograph
Sto rm d i s ch a rge a t R i t he t C reek We i r ( Jan . 28 -Feb . 17 , 1995 )
0
20
40
60
80
100
120
140
160
1/28/95 2/2 /95 2/7 /95 2/12 /95 2 /17 /95
D at e
Discharge(m
gd)
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Streamflow (Q) Hydrograph
A n n u a l D is c h ar g e fo r R i th e t C r eek (1 995)
0
100
200
300
400
500
600
700
1/1 /95 1/31 /95 3 /2 /95 4/1 /95 5 /1 /95 5/31 /95 6/30 /95 7/30 /95 8/29 /95 9/28 /95 10 /28 /95 11/27 /95 12/27 /95
D at e /t i m e
Discharge(m
gd)
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Measurement of Q: Weirs
Discharge rating curve for Judge Creek - triangular profile flat V weir
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0
discharge (m3
s-1
)
stage(m)
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Other examples of weirs
Weirs also used to raise water level
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Rating curves: summer vs winter
208
210
212
214
216
218
220
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000
Discharge (m3 s-1)
Waterlevel(masl)
1964
1967
1968
1980
1990
1996
1972
Peace R. @ Peace Pt. Peace R. @ Peace Pt.
Athabasca R. @ Ft MacMurray
Peace R. @ Peace Pt.
What about a large river?
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Measurement of Q: velocity-area method
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Estimate of Q: Slope Area Method
Difficult or impossible to make measurements during flood
events
Possible to estimate discharge by taking measurements of high
water lines, cross sectional area, and channel slope Use this data in an equation such as Mannings to estimate flow
Manning equation
Q = C/n A R 2/3 S 1/2
Q - discharge (m3 s-1)
C - 1.0 for SI units
S - energy (surface water) slope (m m-1) [head loss per unit length of channel]
R - hydraulic radius [area (m2) divide by length wetted perimeter (m)]
A area (m2)
n roughness coefficient (Mannings)
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0.009-0.011Polyvinyl Chloride (PVC) with smooth inner walls d,e
0.018-0.025Corrugated Polyethylene (PE) with corrugated inner walls c
0.009-0.015Corrugated Polyethylene (PE) with smooth inner walls a,b
0.013Unplaned Wood
0.012Planed Wood0.025Masonry
0.025Earth0.016Asphalt
0.029Gravel0.015Brickwork
0.014UnfinishedConcrete
0.014Clay Tile
0.012Finished Concrete0.010Glass
Non-Metals
0.15Trees0.022Corrugated Metal
0.075Heavy Brush0.012Smooth Steel
0.050Light Brush0.013Cast Iron
0.035Pasture, Farmland0.011Brass
FloodplainsMetals
0.035Stony, Cobbles
0.030Weedy0.040Sluggish with Deep
Pools
0.025Gravelly0.035Major Rivers
0.022Clean0.030Clean and Straight
Excavated Earth ChannelsNatural Streams
Manning nMaterialManning nMaterial
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Hydrograph separation
Conventional hydrograph separation methods rely onsome type of straight-line extrapolation drawn under thestorm hydrograph to partition it into stormflow (alsoreferred to as quickflow & direct runoff) and baseflow(delayed flow & groundwater runoff) components
Q = QB + QS
where Q is the monitored total discharge, QB and QS are the inferredbaseflow and stormflow components of discharge; all have units ofvolume per time
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Hydrograph separation
Baseflow - streamflow generated from relatively slow,long-term drainage of groundwater and delayedsubsurface flow, which sustains streamflow betweenstorms and also provides a continuing flow component
during the storm hydrograph
Stormflow - streamflow generated by a number of rapidrunoff processes (ie. Horton/saturation overland flow and
rapid subsurface flow) which can deliver water to thestream channel in time to contribute to the stormhydrograph
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Hydrograph separation
Line 4
a horizontal line is drawn from the initial rise until it intersects therecession limb of the hydrograph
Use method explained
in lab similar to #3