runoff processes
DESCRIPTION
Runoff Processes. Reading: Applied Hydrology Sections 5.6 to 5.8 and Chapter 6 for Tuesday of next week. Runoff. Streamflow Generation Excess Rainfall and Direct Runoff SCS Method for runoff amount Examples from Brushy Creek Reading for today: Applied Hydrology sections 5.1 to 5.6 - PowerPoint PPT PresentationTRANSCRIPT
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Runoff Processes
Reading: Applied Hydrology Sections 5.6 to 5.8 and Chapter 6
for Tuesday of next week
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Runoff• Streamflow Generation• Excess Rainfall and Direct Runoff• SCS Method for runoff amount• Examples from Brushy Creek• Reading for today: Applied Hydrology sections 5.1
to 5.6• Reading for Tuesday Feb 19: Applied Hydrology
Sections 5.7 and 5.8, Chapter 6• Review session for Quiz this Thursday Feb 14.
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Surface water• Watershed – area of
land draining into a stream at a given location
• Streamflow – gravity movement of water in channels– Surface and
subsurface flow– Affected by climate,
land cover, soil type, etc.
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Streamflow generation
• Streamflow is generated by three mechanisms
1. Hortonian overland flow2. Subsurface flow3. Saturation overland flow
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Welcome to the Critical Zone
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Denudation
Weathering front advance
Erosion and weathering control the extent of critical zone development
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Sediment
Water, solutes and nutrients
Critical zone architecture influences sediment sources, hydrology, water chemistry and ecology
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fracturezone
5m
5m
bedding
weatheredrock
soil
water flow path
Oregon Coast Range- Coos Bay
Anderson et al., 1997, WRR.Montgomery et al., 1997, WRRTorres et al., 1998, WRR
Channel head
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Hortonian Flow• Sheet flow described by
Horton in 1930s• When i<f, all i is absorbed • When i > f, (i-f) results in
rainfall excess• Applicable in
– impervious surfaces (urban areas)
– Steep slopes with thin soil– hydrophobic or compacted
soil with low infiltration
Rainfall, i
Infiltration, f
i > q
Later studies showed that Hortonian flow rarely occurs on vegetated surfaces in humid regions.
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Subsurface flow• Lateral movement of water occurring through the
soil above the water table• primary mechanism for stream flow generation
when f>i– Matrix/translatory flow
• Lateral flow of old water displaced by precipitation inputs• Near surface lateral conductivity is greater than overall
vertical conductivity• Porosity and permeability higher near the ground
– Macropore flow• Movement of water through large conduits in the soil
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Soil macropores
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Saturation overland flow• Soil is saturated from below by
subsurface flow• Any precipitation occurring over a
saturated surface becomes overland flow• Occurs mainly at the bottom of hill slopes
and near stream banks
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Streamflow hydrograph
• Graph of stream discharge as a function of time at a given location on the stream Perennial river
Ephemeral river Snow-fed River
Direct runoff
Baseflow
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Excess rainfall • Rainfall that is neither retained on the land
surface nor infiltrated into the soil• Graph of excess rainfall versus time is called
excess rainfall hyetograph• Direct runoff = observed streamflow - baseflow• Excess rainfall = observed rainfall - abstractions• Abstractions/losses – difference between total
rainfall hyetograph and excess rainfall hyetograph
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SCS method• Soil conservation service (SCS) method is an
experimentally derived method to determine rainfall excess using information about soils, vegetative cover, hydrologic condition and antecedent moisture conditions
• The method is based on the simple relationship that Pe = P - Fa – Ia
Pe is runoff depth, P is precipitation depth, Fa is continuing abstraction, and Ia is the sum of initial losses (depression storage, interception, ET)
Time
Prec
ipit
atio
n
pt
aI aF
eP
aae FIPP
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Abstractions – SCS Method• In general
• After runoff begins
• Potential runoff
• SCS Assumption
• Combining SCS assumption with P=Pe+Ia+Fa
Time
Prec
ipit
atio
n
pt
aI aF
eP
aae FIPP
StorageMaximumPotentialSnAbstractioContinuing
nAbstractioInitialExcess Rainfall
Rainfall Total
a
a
e
FIPP
PPe
SFa
aIP
a
eaIPP
SF
SIP
IPP
a
ae
2
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SCS Method (Cont.)• Experiments showed
• So
SIa 2.0
SPSPPe 8.0
2.0 2
0
1
2
3
4
5
6
7
8
9
10
11
12
0 1 2 3 4 5 6 7 8 9 10 11 12Cumulative Rainfall, P, in
Cum
ulat
ive
Dir
ect R
unof
f, Pe
, in
10090807060402010
• Surface– Impervious: CN =
100– Natural: CN < 100
100)CN0Units;American(
101000
CN
S
100)CN30Units;SI(
25425400
CNCN
S
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SCS Method (Cont.)• SCS Curve Numbers depend on soil conditions
Group Minimum Infiltration Rate (in/hr)
Hydrologic Soil Group
A 0.3 – 0.45 High infiltration rates. Deep, well drained sands and gravels
B 0.15 – 0.30 Moderate infiltration rates. Moderately deep, moderately well drained soils with moderately coarse textures (silt, silt loam)
C 0.05 – 0.15 Slow infiltration rates. Soils with layers, or soils with moderately fine textures (clay loams)
D 0.00 – 0.05 Very slow infiltration rates. Clayey soils, high water table, or shallow impervious layer
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Hydrologic Soil Group in Brushy Creek
Water
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Land Cover
Interpreted from remote sensing
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CN Table
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Upper Brushy Creek Watershed
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Watersheds upstream of Dam 6
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Subbasin BUT_060
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HEC-HMS simulation of Subbasin
Two questions:• How much of the
precipitation becomes “losses” and how much becomes runoff
• What is the time lag between the time that the rainfall occurs over the subbasin and the time the runoff appears at the outlet?
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Land Use in BUT_060
Park
School
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Imagery and Impervious Cover
42% of land cover is impervious
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Soil Map Units
All soils in this Subbasin are classified as SCS Class D (very limited drainage)
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Flow along the longest path
Sheet Flow
Shallow Flow
Channel Flow
𝑡=∑𝑖=1
𝐼 ∆ 𝑙𝑖𝑣 𝑖
Sum travel times over
each segment
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Time of Concentration• Different areas of a
watershed contribute to runoff at different times after precipitation begins
• Time of concentration– Time at which all parts of
the watershed begin contributing to the runoff from the basin
– Time of flow from the farthest point in the watershed
Isochrones: boundaries of contributing areas with equal time of flow to the watershed outlet
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Modeling Runoff from BUT_060
How much runoff?
How quickly does it move?
How to characterize this subbasin?