Runoff Generation Mechanisms

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Runoff Generation Mechanisms
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Overland Flow Occurrence

• On road surfaces and other impermeable areas
• bedrock outcrops, city parks, lawns
• On hydrophobic soils (fire and seasonality)
• On trampled and crusted soils
• On low permeable soils
• Silt-clay soils without macropores
• On saturated soils (SOF)
• Riparian zone
• Waterlogged soils

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Fig. 5.3
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Runoff Pathways
Slide from Mike Kirkby, University of Leeds, AGU
Chapman Conference on Hillslope Hydrology,
October 2001
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Hydrograph Dissection
Rainfall
Falling limb or recession
Rising Limb
Discharge
Antecedent flow rate
0
Time
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Fig. 5.14
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The old water paradox

• streamflow responds promptly to rainfall
  inputs, but fluctuations in passive tracers are
  often strongly damped. This indicates that storm
  flow in these catchments is mostly old water
• Kirchner 2003 Hydrological Processes

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Rainfall intensity and amount
Pearce et al. circa 1970s
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A whole litany of controls on runoff generation

• Broad conceptual controls
• Rainfall intensity or amount
• Antecedent conditions
• Soils and vegetation
• Depth to water table (topography)
• Geology

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Fig. 5.13
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Fig. 5.1
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Storm Runoff

• Factors affecting runoff
• Hydrographs
• Volume, Peak Rate, Timing

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Figure 28. Schematic of typical porous pavement
section (Young et al., 1996)
Porous pavement is gaining wider acceptance in
Washington. Seattle's High Point neighborhood
is home to the city's first residential
porous-pavement street, at 32nd Avenue
Southwest.
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Prediction-Frequency Distributions

• To plan and design projects must be able to
  predict probability of rainfall events
• Duration, Intensity, Return Period
• Often must estimate Return Periods
• Use Hazen method to develop intensity-duration-fre
  quency curve (Example 2.5).

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You have determined that more than 23.4 of
annual rainfall will result in a net Economic
loss for your crop. Now, you need to predict
How often this will occur.
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Hazen
Probability of Occurrence, Fa ()
100 (2n-1)
2y
n the rank of each event y the total number
of events
For Example Year 1938--23.4 in--Rank 3
100 (23-1)
12.5
220
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Weibull
Probability of Occurrence, Fa ()
n
100
y1
n the rank of each event y the total number
of events
For Example Year 1938--23.4 in--Rank 3
100 (3)
15.0
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100
Return Period
Fa
Fa probability of occurrence ()
For Example Year 1938-- Fa 12.5
100
8 yrs OR 100/15 6.7 yrs
12.5
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Frequency-Magnitude Graph
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Predict probability of a given return
period storm occurring within a given number of
years
What is the probability that the 8-year event
will happen in LA within the next 5 years? Or,
what The probability of an economic loss due to
reduced Harvest in the next 5 years?
equation 2.4, page 47
1
P(T,n) 1 - ( 1 - )n
T
1
(49) 0.49 1 - ( 1 - )5
8
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Q V A vol/time length/time x (length x
length)
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Case Study Yellowstone River at Livingston, MT
http//water.usgs.gov/waterwatch/

• Manually measured occasionally to define rating
  curve QVA
• Continuously monitored stage

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Weibull Distribution
Discharge Value Rank , m Recurrence
interval 10,000 1 (n1/m) 6,200 2 5,000 3
500 50 m n
Q
Return Interval
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Yellowstone River II

• Synthesized for floods / droughts
• Summarized for risk / geomorphic work

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What if there isnt a gaging station?

• Predict discharge from physiographic variables
• drainage area, annual precip. (inches),
  forested area,
• Q f(A)
• SW Montana
• Q2 2.48 A0.87 (HE10)0.19
• Omang, 92 USGS WRIR 92-4048

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GROUNDWATER
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http//www.uiowa.edu/c012003a/14.20Groundwater.p
df
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GROUNDWATER
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Porosity

• Ratio of void volume to total volume
• V Va Vw Vs
• Voids are spaces filled with air and water
• Range of porosity values
• granular mass of uniform spheres with loose
  packing, n47.6
• granular mass of uniform spheres with tight
  packing, n 26
• unconsolidated material like sandstones and
  limestones, n 5-15
• Vv Va Vw

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n Sy Sr
Specific yield (effective porosity) measure of
gw that drains by gravity storage
characteristics of aquifer Specific retention
measure of gw that doesnt drain under gravity
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Calculating groundwater flow

• Darcys law
• Q K A (dh/L)
• Transmissivity
• T Kb
• Groundwater flow velocity
• QVA
• KA (dh/L)/A V
• V K (dh/L)/ne

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GROUNDWATER
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GROUNDWATER
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GROUNDWATER
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GROUNDWATER
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GROUNDWATER
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GROUNDWATER
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Questions?
next time..basin morphometry/denudation and
fluvial process