DES 606 :
Watershed Modeling with
HEC-HMS
Module 8
Theodore G. Cleveland, Ph.D., P.E
29 July 2011
Module 8: Average Rainfall
• Unit hydrographs are a tool used to
explain the time re-distribution of
excess precipitation on a watershed.
• Used for design purposes to produce
discharge estimates at a location.
• Different unit hydrograph models
– Empirical
– Parametric
Module 8: Average Rainfall
• FHWA-NHI-02-001 Highway Hydrology
– Chapter 2, Section 2.1; Chapter 3
• Examine spatial distribution of rainfall
and averaging techniques.
• Examine how to put multiple gages into
HMS and assign these gage depths to
a particular watershed.
Module 8: Average Rainfall
• The total amount (measured as depth)
of rainfall that occurs in a storm is the
important input characteristic for
describing the response of a watershed
to rainfall.
Module 8: Average Rainfall
• There are a number of time-related and
space-related factors that are used in
explaining rainfall input.
• The four most important are:
– Intensity (a rate: i.e. inches/hour)
– Duration (a time: 15 minutes)
– Frequency (a probability: 1%)
– Average Depth (a length: inches)
• Actually better thought of as volume/area, but
dimensionally it is a length.
Module 8: Average Rainfall
• HEC-HMS has precipitation input at
“gages” that are assigned to basins.
– The examples so far assume a single
gage is assigned to a sub-basin.
• HEC-HMS inputs are in depths, either
incremental or cumulative
– Intensity x Duration = Depth
• These computations are typically external to HMS.
• Here duration is simply used as a time interval, but the
term really refers to an entire storm length and not
some portion.
Module 8: Average Rainfall
•
Precipitation
– Meterology, Climate
Spatial distribution – these
precipitation arrows may not
be identical.
Unless we wish to route
hydrographs, need some
way to “average” the input.
• Watershed
•
Runoff
– Fraction of precipitation
signal remaining after losses
–
–
–
–
Losses
Transformation
Storage
Routing
Module 8: Average Rainfall
• Averaging is used to generate uniform
inputs for unit hydrograph applications
– One implicit assumption of the UH is spatially
uniform input time series.
• Averaging avoids having to route
hydrographs
– Routing would probably be required on larger
watersheds.
– If the data justify distribution, then would route
subdivided watersheds to capture storm patterns.
Module 8: Average Rainfall
• The entire volume of rainfall applied to
an entire basin is called the
precipitation volume
– If the basin area normalizes this volume
the resulting value is called the equivalent
uniform depth
• Methods to compute equivalent depth
– arithmetic mean
– theissen polygon network
– iso-heyetal method
Module 8: Arithmetic Mean
• The mean value of all nearby
gages is used
– Not all gages actually on the
watershed
Module 8: Polygon Weighting
• A weighted mean based on
polygon area is used
– Not all gages actually on the
watershed
– Polygons formed using Theissen
method
– Can use a minimum distance
algorithm to semi-automatically
generate the weights
Module 8: Polygon Weighting
• A weighted mean based on
polygon area is used
Subarea
A
Subarea
B
Subarea
C
Area ratios are called
Theissen weights
Subarea
D
Subarea
E
Module 8: Iso-Hyetal
• A weighted mean based on
iso-hyetal panel areas is used
– Not all gages actually on the
watershed
– Areas formed by intersection of
isohyetal contours and underlying
drainage area
Module 8: Iso-Hyetal
• A weighted mean based on
isohyetal panel areas is used
Module 8: Averaging Rainfall
• Theissen polygons and arithmetic mean
are probably the most common because
the weights are constants with respect to
geography.
– Arithmetic mean is easiest to automate
Module 8: Example 8
• Illustrate use of multiple gages on Ash
Creek.
– Known Theissen weights are
• 0.12 and 0.88
– Simulate using these known weights.
Summary
• Multiple rain gage data can be used to estimate
an equivalent uniform depth
• Gage weights by a variety of methods
–
–
–
–
Arithmetic mean
Minimum distance (Theissen polygons)
Isoheyetal
Inverse distance methods
Summary
• HEC-HMS models multiple gages in the
Meterological Model Manager
• Example 8 illustrated how to set-up multiple
gages
– Weights were supplied