Direct Runoff Hydrograph

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Direct Runoff Hydrograph
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Annual Hydrograph Water Year (Oct Sept)
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Hydrograph Metrics

• Volume (L, L3)
• Peak Discharge (L3/t)
• Flow Duration (t)
• Time to Peak (Rise) (t) ( time_start -time_peak)
• Lag to Peak (t) ( rain_cofm time_peak)
• Lag to Center of Mass (t) ( rain_cofm
  runoff_cofm)

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Flow Paths

• Direct Channel Interception of Precipitation
• Overland Flow
• Shallow Subsurface Flow
• Deep Subsurface Flow
• Groundwater Flow

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Streamflow Response

• Type of Responses from a precipitation event
• No Response Input is least then watershed
  storage
• Baseflow Response Only
• Direct Response Only
• Mixed Response

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Factors at Affect Streamflow Response

• Precipitation Characteristics (most important)
• Antecedent Moisture Conditions
• Watershed Characteristics
• Topography Area, Slope, Drainage Density
• Channel Type, Slope
• Geology/Soils Depth to Bedrock, Infiltration
• Land Cover Type and Condition
• Impervious Area

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Precipitation Characteristics

• Type Rain vs. Snow
• Most flooding events on small watersheds are from
  rain events.
• Large basins have seen major floods from quick
  snowmelt event, usually the result of rain on
  snow.
• Amount Is it greater then watershed storage
• Intensity Is it greater then infiltration
  capacity
• Frequency Affect on antecedent moisture
  conditions
• Direction Relative to drainage.
• Distribution Extent of event over watershed

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Precipitation Process

• Frontal Systems

Small Extents Larger Extents Shorter
Durations Longer Durations Higher
Intensities Lower Intensities
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Precipitation Process

• Orographic

Variable Extents, Durations, and
Intensities Storm tracts through mountains are
common.
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Precipitation Process

• Conventive

Smaller Extents 1-1.5 sq. miles in size,
elliptical shape Short Durations Higher
Intensities
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Precipitation Characteristics

• Duration, Amount and Intensity
• Long Duration, Low Intensity Exceeds Storage
• Flooding in the Gulf Coast
• Short Duration, High Intensity Exceeds
  Infiltration
• Local Flooding in Tucson
• Longer the Duration the lower the average
  Intensity
• Major flooding events have resulted from stagnate
  frontal systems
• Many studies have correlated Qp to rainfall
  intensities
• The basis of the Rational Method
• Qp CIA, where I is rainfall intensity

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AMC
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Direction
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Watershed Factors
Flashy
Flat
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Affect of Geology
Annual hydrographs for two streams in central
Michigan with similar precipitation regimes. One
watershed is on granite and the other is on
limestone.
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Land Use Impacts

• The major impact of management on flooding is the
  increase in routing efficiency. Excess rainfall
  generated on the watershed is routed more quickly
  to the watershed outlet.
• Management has little impact on large return
  period events (e.g. 100 yr) especially at large
  watershed scales.
• Impacts include
• Increase in Overland Flow
• Change in Land Cover and Land Use
• Increase Impervious Area
• Increase in Drainage System (ditches, gullies,
  roads)
• Decrease in Internal Storage (removal of
  wetlands)
• Increase in Drainage System Efficiency
• Change channel characteristics (width/depth,
  roughness)
• Channel confinement (levees)

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Fire

• Vegetation Removal and Mortality
• Sever Fires can have 100 Mortality
• Modification of hydrology cycle
• Decrease in ET and interception
• Increase in snow accumulation
• Increase soil water availability
• Removal of Surface Material
• Creation of bare soil
• Increase potential for overland flow
• Modification of Surface Soil
• Destruction of surface soil aggregates
• Increase in bulk density
• Decrease in porosity
• Create soil hydrophobicity
• Soil Sealing (raindrop impacts, ash)
• Water Repellent Layers

Paired watersheds in Cascade Range in E. WA
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Marshall Gulch (Variable Burned 8.9 sq. km
conifer watershed, C)
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Runoff vs Rainfall for Marshall Gulch (8.6
sq.km.) Before and After Aspen Fire
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Post Fire Peak vs Average Discharge At Marshall
Gulch Before and After Aspen Fire
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Pre- Fire Hydrograph
Post - Fire Hydrograph
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Arizona Example Ponderosa Pine
Unburn Moderate Severe
Area (ha) 17.7 4.0 8.1
Exposed Soil () 7 36 70
ROE (1973-75) () 0.8 2.8 3.6
of Rain/Runoff Events (1973-75) 6 15 25
Largest Peak Discharge (cfs) 6.1 21.5 lt336
Infiltration (cm/hr) 6.9 3.7 2.6
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Forest Management

• The impact of road building on drainage is
  considered important. Increase routing
  efficiency. Roads can decrease the time to peak.
• Changes in available soil water and snow
  accumulations.
• From individual small experimental watersheds
  forest management has been found to change
  stormflow peak discharge from -22 to gt200 and
  stormflow volumes from 0 to 200. Note most of
  the storms that were analyzed were relatively
  small.
• Biggest impacts are on the low return period
  events (lt 50 yr). The impacts are less clear for
  higher return period events. One problem is that
  we have a very small sample of rare large events.
  

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Casper Creek Example Northern CA

• Coastal Redwoods, Rain Dominated
• The average lag to peak time decreased an average
  1.5 hours. The shape of the hydrographs did not
  change.
• Small storm volumes (lt 1200 m3) were increased by
  132 and the peak discharges (lt 566 l/s) were
  increased by 111.
• Storm volumes and peaks for large events were not
  significantly increased.

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Reservoir Response Uncontrolled Outlet
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Urbanization
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Tucson Example
Atterbury Arcadia Wash High School Wash Railroad Wash
Developed 0 22 30 42
Average ROE () 6 22 27 29
of Runoff per Rain Event 13 35 52 41
San Diego, CA The 2-year flood volume would
double on major streams, and increase by four
times on urbanized tributary streams. Similarly,
a 100-year flood volume would increase by 1.3
times on major streams and by 3 times on
tributaries (Prestegaard 1975).
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Desynchronization of Hydrographs

• It has been hard to assess the impact of land use
  on flooding at large watershed scales.
• Because land use can change the lag to peak of
  event hydrographs the impact downstream may
  actually decreased as flows from different
  tributaries are combined.
• Note that even relatively large cities will
  represent a small portion of larger watersheds.
• Flooding control structures like detention basins
  can complicate the issue.

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Desynchronization of Hydrographs

• Hydrographs from different tributaries can have
  different shapes and timing due to management or
  natural factors.
• Factors include
• Watershed shape
• Watershed and channel slope
• Drainage density
• Channel Characteristics
• Internal retention or detention