DES 606 :
Watershed Modeling with
HEC-HMS
Module 2
Theodore G. Cleveland, Ph.D., P.E
29 June 2011
Module 2: Hydrologic Principles
• Course Objectives
• Hydrologic Principles
– Watershed
– Hydrologic Cycle
• Overview of HEC-HMS
– Projects
– Components
Course Objectives
1. Explain the fundamentals of watershed
runoff computations, including infiltration,
unit hydrograph computations, and
channel routing.
2. Simulate performance of water control
measures, including detention and
retention ponds and diversions.
Course Objectives
3. Use the HEC-HMS interface and menus
to set up rainfall-runoff models, routing
models, and urban and reservoir
controlled basin models.
Course Objectives
4. Use HEC-HMS to solve both routine and
complex hydrologic engineering problems
encountered by TxDOT designers, both in
normal design and in cooperative projects
with other state and local and federal
government agencies.
Course Objectives
5. Troubleshoot HEC-HMS projects.
6. Evaluate and review consultant projects
that use HEC-HMS.
7. Develop a more complete scope of work
for consultant projects using HEC-HMS.
Hydrologic Principles
• The Watershed
• The Hydrologic cycle
– Precipitation
– Losses
– Runoff
• HEC-HMS
– History
– Program structures
Watershed
• The fundamental unit in surface water hydrology
is the watershed.
– It can be as small as a parking lot draining to an inlet,
or continental sized (Bedient et al., 2008).
• A watershed is defined as the area on the
surface of the earth that drains to a specific
location.
– The watershed is therefore defined both by the
drainage location as well as topographic features that
govern flow to that point.
Watershed
• Watershed physical properties are
characteristics such as:
– Area
– Main channel length (if a main channel exists)
– Slope (requires the specification of path),
– Soil moisture/permeability, and similar
measurable characteristics.
Watershed
• Watersheds also have descriptive
properties such as:
– %-developed
– %- polluted, and so forth.
• These properties are certainly physical, but are
called descriptive because there will be analyst
interpretation in the specification of the values.
Watershed
• Physical properties are those things than
can be measured from a topographic map.
– Area, slope, length
• Descriptive properties are everything else.
– Soil texture (and infiltration rate)
– Fraction developed
Watershed
• An analysis or even design will likely
start with watershed delineation.
– Aerial imagery
– Topographic map
– Sewer drawings
Watershed
• What is the process of delineating
watersheds?
– Manual delineation involves drawing lines on a
topographic map, and connecting the slope or
ridge tops.
• Assuming the water will drain away from those
points, the watershed is delineated by enclosing a
polygon.
Watershed
• What is the process of delineating
watersheds?
– Automated delineation involves some technical
skills with GIS-like databases.
• Digital Elevation Model (DEM)
• Stream network, and stream outlets.
– The DEM and stream outlets can either be downloaded or
constructed.
• If a new DEM must be constructed, consider costsharing with the U.S. Geological Survey to do so.
• Manual delineation example in course
reference CD
– Topowatershed.pdf
Watershed
• Watershed physical and descriptive
characteristics determined after delineation.
– Areas and lengths
• Use Acrobat Pro tools to find areas and lengths on
map image, convert to actual lengths using map
scale.
– Slopes
• Change in elevation along a path
– Special concerns
• Sewers can cross topographic watershed
boundaries
• Flat terrain – channels hard to define
Watershed
• Watershed physical and descriptive
characteristics determined after delineation.
– Descriptive characteristics
• Google Earth/aerial imagery can be used to define
cover types and fractions.
• Soil maps for selected properties.
Watershed
• Minimal descriptions
– Watershed boundary on a map
• Outlet
• Subsurface storm sewer network
– Area that drains to the outlet
– Main channel length
• Outlet to highest point in watershed
Watershed
• Minimal descriptions
– Slope(s)
– Descriptive (any or all)
• Soil type
• Fraction developed/impermeable etc.
Hydrologic Cycle
Precipitation (Input)
Loss
Runoff (Output)
Precipitation
• Precipitation
– Rainfall (by far most important in Texas)
– Snow, Sleet, Hail
• Meteorology
– Synoptic storms
– Cyclonic storms
• Data
– NWS, local networks, SAO, NCDC (historical)
Losses
• Losses
– Infiltration
•
•
•
•
Hortonian Loss Model
Green-Ampt Loss Model
NRCS Runoff Generation Model
Initial Abstraction, Constant Rate Model
– Evapotranspiration
• Thornwaithe
• Energy Balance Models
Transformation
• Transform the spatially distributed
precipitation input to the outlet
– Unit Hydrograph
Storage and Routing
• Storage
– Reservoirs, ponds, depressions store water
and release later in time (as compared to the
input)
• Routing
– Moving water from one location to another on
the watershed occurs over a path (route).
– Routing develops the temporal relationship of
input to the outlet from this process
Rainfall-Runoff Process
•
Precipitation
– Meterology, Climate
• Watershed
•
Runoff
– Fraction of precipitation
signal remaining after losses
–
–
–
–
Losses
Transformation
Storage
Routing
HEC-HMS Overview
• History
– Evolved from HEC-1 as part of “newgeneration” software circa 1990
– Integrated user interface to speed up data
input and enhance output interpretation
• HMS is a complex and sophisticated tool
– Intended to be used by a knowledgeable and
skilled operator
– Knowledge and skill increase with use
HEC-HMS
• Conceptualizes precipitation, watershed
interaction, and runoff into major elements
– Basin and sub-basin description
• Supply how the system components are
interconnected
– Loss model
• Supply how rainfall is converted into excess rainfall
– Transformation model
• Supply how the excess rainfall is redistributed in
time and moved to the outlet
HEC-HMS
• Conceptualizes precipitation, watershed
interaction, and runoff into major elements
– Meterological model
• Raingage specifications and assignment to
different sub-basins
– Time-series models
• Supply input hyetographs
• Supply observed hydrographs
– Simulation control
• Supply instructions of what, when, how to simulate
Summary
• Watershed is fundamental unit
– Area, Length, Slope, etc.
• Rainfall is the input function
• Losses and Storage are Watershed functions
– Excess rainfall is fraction of rainfall signal remaining
after storage and losses are satisfied
– Runoff is the excess rain redistributed in time
• HEC-HMS is a computer program for rainfallrunoff modeling