Estimating Catchment Runoff

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Estimating Catchment Runoff
Direct runoff or Effective rainfall
Storm
Initial abstraction
Infiltration
Losses
Surface depressionstorage
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Infiltration methods

• Soil Conservation Service (SCS) Curve Number
  (CN) method
• Hortons equation moving curve method
• Green Ampt model

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SCS Curve Number method
CN depends on soil type and pre-wetting
inches
P(t) depth of rainfall Q(t) depth of
runoff Ia initial abstraction S
potential storage CN curve number ? 100
mm
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Horton equation
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Green Ampt model
where Mmoisture deficit S suction head K
hydraulic conductivity
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Rainfall-Runoff models (1)
Rainfall
Effectiverainfall
Infiltration Model
Runoff
Losses
Catchment Model
Losses subtracted from rainfall to get effective
rainfall which is then applied to catchment.
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Rainfall-Runoff models (2)
Losses and infiltration calculated along with
runoff as part of Runoff Model
Rainfall
Runoff
Catchment Model
SurfaceDepressionStorage
Losses andinfiltration
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Calculating the Runoff (1)
Runoff from pervious and impervious fractions
computed and added together
Flow lengths can be- (a) equal (b)
proportional (c) user supplied
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Calculating the Runoff (2)
Symmetrical catchmentArea 2.2 ha
Overland flow length can be estimated as area
divided by length of stream bank available for
inflow.
75m
96m
63m
One-sided catchmentArea 2.4 ha
192m
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Calculating the Runoff (3)

• Overland flow routing choices
• Combine effective rainfall with
• triangular response function
• rectangular response function
• single linear reservoir response function
• Combine infiltration other losses with outflow
  from idealized inclined plane.(Similar to SWMM
  RUNOFF method)