Hydrograph Modeling

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Hydrograph Modeling

• Goal Simulate the shape of a hydrograph given a
  known or designed water input (rain or snowmelt)

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Hydrograph Modeling The input signal

• Hyetograph can be
• A future design event
• What happens in response to a rainstorm of a
  hypothetical magnitude and duration
• See http//hdsc.nws.noaa.gov/hdsc/pfds/
• A past storm
• Simulate what happened in the past
• Can serve as a calibration data set

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Hydrograph Modeling The Model

• What do we do with the input signal?
• We mathematically manipulate the signal in a way
  that represents how the watershed actually
  manipulates the water
• Q f(P, landscape properties)

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Hydrograph Modeling

• What is a model?
• What is the purpose of a model?
• Types of Models
• Physical
• Analog
• Mathematical
• Physically based vs Empirical

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Hydrograph Modeling

• Physically Based, distributed

Physics-based equations for each process
In each grid cell
See dhsvm.pdf Kelleners et al., 2009a Kelleners
et al., 2009b
Pros and cons?
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Hydrologic ModelingSystems Approach
A transfer function represents the lumped
processes operating in a watershed -Transforms
numerical inputs through a black box to numerical
outputs -Predictions at outlet only
P
Mathematical Transfer Function
Q
t
t
Pros and Cons?
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Transfer Functions

• Can be developed to simulate any component of a
  hydrograph
• Response factor (Weff/W) Volume only
• SCS curve number method
• Estimates Weff
• Rational Method (page 443)
• Qpeak CIA

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Transfer Functions

• 2 Basic steps to rainfall-runoff transfer
  functions
• 1. Estimate losses.
• W minus losses effective precipitation (Weff)
• Determines the volume of streamflow response
• 2. Distribute Weff in time
• Gives shape to the hydrograph

Recall that Qef Weff
Event flow (Weff)
Base Flow
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Transfer Functions

• General Concept

Task Draw a line through the hyetograph
separating loss and Weff volumes
W
Weff Q
W
?
Losses
t
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Loss Methods

• Methods to estimate effective precipitation
• You have already done it one wayhow?
• However,

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Loss Methods

• Physically-based infiltration equations
• Chapter 6
• Green-ampt, Richards equation, Darcy
• Kinematic approximations of infiltration and
  storage

Exponential Weff(t) W0e-ct c is unique to
each site
W
Uniform Werr(t) W(t) - constant
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Loss Methods

• SCS curve (page 445-447)
• Calculates the VOLUME of effective precipitation
  based on watershed properties (soils)
• Assumes that this volume is lost

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SCS Concepts

• Precipitation (W) is partitioned into 3 fates
• Vi initial abstraction storage that must be
  satisfied before event flow can begin
• Vr retention W that falls after initial
  abstraction is satisfied but that does not
  contribute to event flow
• Qef Weff event flow
• Method is based on an assumption that there is a
  relationship between the runoff ratio and the
  amount of storage that is filled
• Vr/ Vmax. Weff/(W-Vi) where Vmax is the maximum
  storage capacity of the watershed
• If Vr W-Vi-Weff,

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SCS Concept

• Assuming Vi 0.2Vmax (??)
• Vmax is determined by a Curve Number

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Curve Number
The SCS classified 8500 soils into four
hydrologic groups according to their infiltration
characteristics
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Curve Number

• Related to Land Use

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Transfer Function

• 1. Estimate effective precipitation
• SCS method gives us Weff
• 2. Estimate temporal distribution

Volume of effective Precipitation or event flow
-What actually gives shape to the hydrograph?
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Transfer Function

• 2. Estimate temporal distribution of effective
  precipitation
• Various methods route water to stream channel
• Many are based on a time of concentration and
  many other rules
• SCS method
• Assumes that the runoff hydrograph is a triangle

On top of base flow
Tw duration of effective P Tc time
concentration
Q
How were these equations developed?
Tb2.67Tr
t
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Transfer Functions

• Time of concentration equations attempt to relate
  residence time of water to watershed properties
• The time it takes water to travel from the
  hydraulically most distant part of the watershed
  to the outlet
• Empically derived, based on watershed properties

Once again, consider the assumptions
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Transfer Functions

• 2. Temporal distribution of effective
  precipitation
• Unit Hydrograph
• An X (1,2,3,) hour unit hydrograph is the
  characteristic response (hydrograph) of a
  watershed to a unit volume of effective water
  input applied at a constant rate for x hours.
• 1 inch of effective rain in 6 hours produces a 6
  hour unit hydrograph

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Unit Hydrograph

• The event hydrograph that would result from 1
  unit (cm, in,) of effective precipitation
  (Weff1)
• A watershed has a characteristic response
• This characteristic response is the model

1
Qef
1
t
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Unit Hydrograph

• How do we Develop the characteristic response
  for the duration of interest the transfer
  function ?
• Empirical page 451
• Synthetic page 453
• How do we Apply the UH?
• For a storm of an appropriate duration, simply
  multiply the y-axis of the unit hydrograph by the
  depth of the actual storm (this is based
  convolution integral theory)

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Unit Hydrograph

• Apply For a storm of an appropriate duration,
  simply multiply the y-axis of the unit hydrograph
  by the depth of the actual storm.
• See spreadsheet example

In this picture, what duration is 2.5 hours
Referring to? Where does 2.4 come from?
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Unit Hydrograph

• How do we derive the characteristic response
  (unit hydrograph)?
• Empirical

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Unit Hydrograph

• How do we derive the characteristic response
  (unit hydrograph)?
• Empirical page 451
• Note 1. approximately equal duration
• What duration are they talking about?
• Note 8. adjust the curve until this area is
  satisfactorily close to 1unit
• See spreadsheet example

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Unit Hydrograph

• Assumptions
• Linear response
• Constant time base

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Unit Hydrograph

• Construction of characteristic response by
  synthetic methods
• Scores of approaches similar to the SCS
  hydrograph method where points on the unit
  hydrograph are estimated from empirical relations
  to watershed properties.
• Snyder
• SCS
• Clark

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Snyder Synthetic Unit Hydrograph

• Since peak flow and time of peak flow are two of
  the most important parameters characterizing a
  unit hydrograph, the Snyder method employs
  factors defining these parameters, which are then
  used in the synthesis of the unit graph (Snyder,
  1938).
• The parameters are Cp, the peak flow factor, and
  Ct, the lag factor.
• The basic assumption in this method is that
  basins which have similar physiographic
  characteristics are located in the same area will
  have similar values of Ct and Cp.
• Therefore, for ungaged basins, it is preferred
  that the basin be near or similar to gaged basins
  for which these coefficients can be determined.

The final shape of the Snyder unit hydrograph is
controlled by the equations for width at 50 and
75 of the peak of the UHG
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SCS Synthetic Unit Hydrograph
Triangular Representation
The 645.33 is the conversion used for delivering
1-inch of runoff (the area under the unit
hydrograph) from 1-square mile in 1-hour (3600
seconds).
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Synthetic Unit Hydrograph

• ALL are based on the assumption that runoff is
  generated by overland flow
• What does this mean with respect to our
  discussion about old water new water?
• How can Unit Hydrographs, or any model, possibly
  work if the underlying concepts are incorrect?

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Other Applications

• What to do with storms of different durations?

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Other Applications

• Deriving the 1-hr UH with the S curve approach

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