Transcript NWS-COMET May 1998 Hydrometeorology Course

COMET
University Faculty
Hydrometeorology Course
June 2000
Dennis L. Johnson
Dennis L. Johnson, Asst. Professor
Juniata College
Environmental Science & Studies
(814) 641-5335 (Phone)
(814) 641 – 3685 (Fax)
[email protected] (Email)
Http://www.Juniata.edu/~johnson/
Usual Houghton
Hydrometeorological Operations
in the “Modernized NWS”
The Runoff Picture
• Hydrology is long term and short term….
• In this course we will mainly focus on the
short term:
• Floods & flood flows.
• Generating runoff/high flows.
• Predicting/forecasting flows.
• Space/time scales.
What’s a Flood?
• What is a flood?????
• A rather elusive definition
• Generally contains terms like:
–
–
–
–
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High water
High flows
Normal water course
Human impact(s)
Etc…
Recipe(s) for a Flood
•
•
•
•
What causes a flood?
What are the conditions?
What are the types of flooding situations?
Your area or other areas…..
My Recipes
•
•
•
•
•
•
•
“BIG” heavy soaking rains…
Low infiltration rates
Snow melt
Rain on snow
Very intense precipitation
Dam failure
Others….??
Does a Flood Have to Happen in
a Defined Water Course or
Waterway?
….and If a Flood Does Occur in an
Overland Situation – Does the
Nearest Stream Even Feel It?
FEMA - NFIP…
(www.fema.gov/nfip)
Flood--A general and temporary condition of partial or
complete inundation of normally dry land areas from:
Overflow of inland or tidal waters.
The unusual and rapid accumulation or runoff of
surface waters from any source.
Mudslides (i.e., mudflows) which are proximately caused
by flood, as defined above, and are akin to a river of
liquid and flowing mud on the surface of normally dry
land areas, as when earth is carried by a current of water
and deposited along the path of the current.
The collapse or subsidence of land along the shore of a
lake or other body of water as a result of erosion or
undermining caused by waves or currents of water
exceeding the cyclical levels which result in flood, as
defined above.
What Are the Defining
Characteristics of a Flood?
• Timing – rise time, recession, duration.
• Flows – peak flows, magnitude (statistical).
• Precipitation – intensity, duration,
frequency….
What Controls the Timing, Flow,
and Precipitation?
• The hydrology – short term and long term.
• The meteorology – short term
(weather/storm type) and long term
(climate).
Big Picture
Long term and short term
Long Term
(Climate and the Hydrologic Cycle)
Short Term
Weather (storm type) & “current hydrologic
conditions”
Some of the “Right”
Combinations….
• Precipitation –vs.- infiltration
–
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–
–
–
Precipitation intensity > infiltration rate
Precipitation total > infiltration capacity
“Storage” in the system is full
Human induced high water or flows
Natural alterations to the watershed
Our Focus
• More on the short term..
• The combination(s) of precipitation and
hydrologic conditions that lead up to a
potential flooding situation…
• “basin hydrology” – although basin
hydrology looks at the long term hydrologic
budget, as well.
Let’s Take a Minute to Look at
Hydrology and the Properties,
Units, Concepts, & Terminology
Hydrology
… An Earth Science. It Encompasses the
Occurrence, Distribution, Movement, and
Properties of the Waters of the Earth and Their
Environmental Relationships." (Viessman,
Knapp, Lewis, & Harbaugh, 1977 Introduction to Hydrology, Harper & Row
Publishers, New York)
History of Hydrology
(Hydrometeorology)
Early on….
• Early philosophers speculated on the
hydrologic cycle:
• Homer believed that there existed large
subterranean reservoirs that fed the rivers,
seas, springs, and wells - was he wrong?
• Homer did understand the dependence of
flow in the Greek aqueducts on conveyance
and velocity!
History, Cont....
• In the first century B.C., Marcus Vitruvius in the
treatise de Architectura Libri Decem (the
engineers chief handbook), vol. 8 hypothesized
that rain and snow falling in the mountains
infiltrated into the earth’s surface and appeared in
the lowlands as springs and streams.....
Early Success.....
• 4000 b.C. The Egyptians built a dam on the Nile
to allow barren lands to again be used for
agricultural purposes.
More Early Successes
• 1000’s of years later, a canal to carry fresh water
from Cairo to Suez was built.
• Towns in Mesopotamia were protected by
flooding from high earthen walls.
Early Disputes and Rules
• The cities of Lagash and Umma of Mesopotamia have
documented water disputes.
• The Romans decree:
• Ne quis aquam oletato dolo malo ubi publice saliet si quis
oletarit sestertiorum X mila multa esto.
• It is forbidden to pollute the public water supply; Any
deliberate offender shall be punished by a fine of 10,000
sesterces!
Qualitative Understanding
• Near end of 15th century, Leonardo da Vinci and
Bernard Palissy independently reached
conclusions on the hydrologic cycle - based on a
philosophical understanding.
• There was still a lack of quantitative
understanding of the hydrologic cycle.
The 17th Century
• Perrault, Mariotte, and Halley began quantitative measurements and
applications.
• Perrault measured rainfall and runoff over the seine river drainage
basin for ~ 3 years - he illustrated that rainfall WAS adequate in
quantity to account for river flows.
• Mariotte gauged the velocity of the flow in the river seine and
estimated flows by also estimating river cross sectional areas.
• Halley was an astronomer! He estimated evaporation from the
Mediterranean sea and correlated it to river flows into the med,
concluding that river flows were sufficient enough to provide that
volume of water.
The 18th Century
• Bernoulli - famous for hydraulics and fluid
mechanics - the piezometer, the pitot tube, and
Bernoulli’s theorem.
• The Chezy formula (channel flow).
The 19th Century
•
•
•
•
•
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Hagen-Poiseuille - capillary flow equation.
Darcy’s - flow in porous media.
Duptuit-Thien well formula.
Manning - open channel flow.
Systematic stream gaging.
Mostly empirical in nature.
The 20th Century
– Government agencies began to develop
programs – good or bad?
– Rational analysis begins.
– Sherman - unit hydrograph theory.
– Horton - infiltration theory.
– Snyder - unit hydrograph.
– Clark - unit hydrograph.
– Etc...........
Modern Day
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•
•
•
•
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Very computer and data intensive
High tech instruments
Scale issues
Policy issues
Etc.................
“Diamond edge on an old axe”……
Units & Properties of Water
Property
Density
(mass/volume)
Specific Weight
(weight/volume)
Specific Volume
Specific Gravity
Vapor Pressure
Symbol
Value
~1.94 slugs/ft3

~ 1.0 g/cm3
62.4 Lbs/ft3

9.81 kN/m3

s.g.
1.0 for water
@ 32.9o F
~0.4 psi
Comments
Slug = lb*s2/ft
g

s.g.fluid =
gfluid/gwater
Vapor pressure of
the fluid - not the
atmosphere
The Watershed
• A watershed is an area of
land that drains to a single
outlet and is separated from
other watersheds by a divide.
• Every watershed has a
drainage area.
• Related terms: drainage
basin, sub-basin, sub-area.
Area
•
•
•
•
1 acre = 43,560 ft2
1 mi2 = 640 acres
1 hectare = 100m x 100m = 2.471 acres = 10,000 m2
1 km2 = 0.386 mi2
Volume
• 1 acre-foot = 1 ac-ft = 1 acre of water x 1 foot deep = 43,560
x 1 = 43,560 ft3.
• 1 ac-inch = 1 acre x 1 inch deep = 43,560 x 1/12 = 3,630 ft3.
• 1 ft3 = 7.48 gallons.
• 1 gallon H2O ~ 8.34 lbs.
Runoff Volume
• 1-inch of runoff over 1 square mile :
• 1/12 feet x 1 mi2 x 640 acres/mi2 x 43,560 ft2/mi2 =
2,323,200 ft3
Discharge
• 1 cfs = 1 cubic foot per second
• 1 cfs x 7.48 gal/ft3 x 3600 sec/hr x 24 hrs/day = 646,272 gpd
= 0.646 MGD
• 1 cfs x 3600 sec/hr x 24 hrs/day = 86,400 cfs/day
• 86,400 cfs/day x 1 ac-ft/43,560 ft3 = 1.983 ac-ft/day (~ 2 acft/day)
• 1.983 ac-ft/day x 12 inches/ft x 1 day/24 hrs = 0.992 ac-in/hr
• 1 ac-in/hr x 43,560 ft3/ac-ft x 1 hr/3600 sec x 1 ft/12 inches =
1.008 cfs
Power
• Hp = HQ/550
• 1 hp = 550 ft*lb/sec = 0.7547 kilowatts
Hydrology Terminology
•Streamflow is the movement of water through a channel.
•The cross-sectional area of a stream is the region bounded by the
walls of the stream and the water surface. The cross-sectional area
is illustrated below.
•See also Manning’s “n”.
Cross-sectional Area
Stream Flow
Hydrology Terminology
•Manning’s “n” is a measure of the roughness of a
surface, and in streamflow it is the roughness of the
channel bottom and it’s sides.
Diagram 1
Diagram 2
Diagram 2 will have a higher Manning’s “n”
because it has rougher surface due to the
jagged bottom and pebbles.
Hydrology Terminology
•Routing is used to account for storage and translation effects.
Routing
Hydrologic
Hydraulic
1
1
S 2  S1
I1  I 2   O1  O2  
2
2
t
Hydrology Terminology
Generalized effect of routing
700.0000
600.0000
500.0000
400.0000
300.0000
200.0000
100.0000
0.0000
0.0000
1.0000
2.0000
3.0000
4.0000
5.0000
6.0000
7.0000
8.0000
9.0000
10.0000
Hydrology Terminology
• Snowfall is a form of precipitation that comes down
in white or translucent ice crystals.
•Snowmelt is the excess water produced by the melting
of snow. This leads to flooding possibilities in the
spring when temperatures begin to rise. There is
generally a delay in the snowmelt response of a basin
due to the melting process and travel times.
•Snowpack is the amount of annual accumulation at
higher elevations.
Hydrology Terminology
•Runoff is the excess precipitation and is often
considered a “fast” response.
•Overland flow is the flow of water across the land
surface.
•Sub-surface flow is the flow of water through the soil
layers to the stream.
•Baseflow is the flow in a channel due to ground water
or subsurface supplies. The baseflow is generally
increased by precipitation events that produce enough
infiltration.
Hydrology Terminology
• Infiltration is the movement of water from the
surface into the soil.
•The rate of infiltration is based on a number of
factors, including but not limited to:
•soil types
•current conditions
•precipitation intensity
Hydrology Terminology
•The velocity of the flow is very dependent on the slope
of the stream bottom. The greater the slope the greater
the potential velocity of the flow.
•The “wave” speed is the velocity of the flood wave
down the channel. The speed of this wave affects how
quickly the downstream area will effected.
Hydrology Terminology
•The energy grade line represents the depth of the
water surface and the velocity component of the
Bernoulli equation.
•The hydraulic grade line represents the depth of
the water surface.
Energy Grade Line
2
v1
2g
headloss
Hydraulic Grade Line
(water surface)
Depth1
2
v2
2g
Channel Bottom
Elevation Head
Datum
Dept
h2
Hydrology Terminology
•Karst hydrology is caused by pores and holes in
limestone formations. This increases the infiltration
into the limestone, reducing the runoff potential.
•The slope changes the speed of runoff and therefore
effects collection times.
Hydrology Terminology
•The frequency of a storm event is described by its
return period. For example a two year storm event
has a 1 in 2 chance of occurring in any given year.
•The probability is also affected by the return
period. Thus the probability of a 2 year storm
occurring is 50%. The probability of a 100-year
event occurring is 1/100 or 1%
Basin Hydrology
Precipitation
• ... primary "input" for the hydrologic
cycle (or hydrologic budget).
• … The patterns of the precipitation are
affected by large scale global patterns,
mesoscale patterns, "regional" patterns,
and micro-climates.
• … In addition to the quantity of
precipitation, the spatial and temporal
distributions of the precipitation have
considerable effects on the hydrologic
response.
Snow
• … response mechanisms of snow are at a much slower
time scale than for most of the other forms of
precipitation.
• … The melt takes place and the runoff is "lagged" due
to the physical travel processes.
• … Items to consider in the snowmelt process are the
current "state" of the pack and the snow water
equivalent of the snow pack., as well as the melt
potential of the current climate conditions.
• … A rain-on-snow event may produce very high
runoff rates and is often a difficult situation to predict
due to the integral nature of the runoff and melt
processes. The timing of these events is often very
difficult to predict due to the inherent "lag" in the
responses.
Snow Hydrology
Special Thanks, Credit, and Recognition to
Don Cline
And the
National Operational Hydrologic Remote Sensing Center
Why is Snow Important?
Why is Snow Important?
•
•
•
•
Water Resources
Flooding
Economics
Transportation
Snow Hydrology
• Understanding and predicting the
physical processes of:
• Snow Accumulation
• Ablation
• Melt Water Runoff
Snow Hydrology
• 4 Simultaneous Estimation Problems
– the quantity of water held in snow packs
– the magnitude and rate of water lost to the
atmosphere by sublimation
– the timing, rate, and magnitude of snow melt
– the fate of melt water
Snow Cover Distribution
Snow Cover Distribution
• Three Spatial Scales
– Macroscale
• Areas up to 106 km2
• Characteristic Distances of 10-1000 km
• Dynamic meteorologic effects are important
– Mesoscale
• Characteristic Distances of 100 m to 10 km
• Redistribution of snow along relief features due to wind
• Deposition and accumulation of snow may be related to terrain
variables and to vegetation cover
– Microscale
• Characteristic Distances of 10 to 100 m
• Differences in accumulation result from variations in air flow patterns
and transport
Snow Cover Distribution
• Effect of Topography
– The depth of seasonal snow cover usually increases with elevation
if other influencing factors do not vary with elevation
• This trend is generally due to:
– increase in the number of snowfall events
– decrease in evaporation and melt
• The rate of increase with elevation may vary widely from year-to-year
– However, elevation alone is not a causative factor in snow cover
distribution
• Many other factors must be considered:
– slope, aspect, vegetation, wind, temperature, and characteristics of the
parent weather systems
Snow Cover Distribution
• Effect of Vegetation
– Snow falling into a vegetation canopy is
influenced by two phenomena:
• Turbulent air flow above and within the
canopy
– may lead to variable snow input rates and
microscale variation in snow loading on the
ground
• Direct interception of snow by the canopy
elements
– may either sublimate or fall to the ground
– Processes are related to vegetation type,
vegetation density, and the presence of
nearby open areas
Snow Cover Distribution
• Forested Environments
– Differences in snow accumulation between
different species of conifers is usually small
compared to between coniferous and
deciduous stands
• coniferous stands are all relatively efficient
snow interceptors
• Once intercepted, cohesion between snow
particles helps keep snow in the canopy for
extended time periods
– snow is more susceptible to sublimation
losses in the canopy than on the forest floor
» High surface area to mass ratio
Snow Cover Distribution
• Forested Environments
– Most studies show greater snow
accumulation in clearings than in the forest
– Most of the difference develops during
storms, not between storms
• redistribution of intercepted snow by wind to
clearings is not typically a significant factor
– Interception and subsequent sublimation are
the major factors contributing to the
difference
20-45%
Greater Snow
Accumulation
Snow Cover Distribution
• Open Environments
– Over highly exposed terrain, the effects of meso- and microscale differences in vegetation and terrain features may produce
wide variations in accumulation patterns.
Snow Cover Distribution
• Open Environments
– Relative accumulation on
various landscapes in an
open grassland
environment
• Normalized to snow
accumulation on level
plains under fallow
Landscape
Level Plains
Fallow
Stubble
Pasture (grazed)
Gradual Hill and Valley Slopes
Fallow
Stubble, hayland
Pasture (ungrazed)
Steep Hill and Valley Slopes
Pasture (ungrazed)
Brush
Ridge and Hilltops
Fallow, ungrazed pasture
Stubble
Small Shallow Drainageways
Fallow, stubble, pasture (ungrazed)
Wide Valley Bottoms
Pasture (grazed)
Farm Yards
Mixed Trees
Relative
Accumulation
1.00
1.15
0.60
1.0 – 1.10
1.0 – 1.10
1.25
2.85
4.20
0.40 – 0.50
0.75
2.0 – 2.15
1.30
2.40
Blowing Snow
• Sublimation Losses
– Snow particles are more exposed to atmosphere
during wind transport
– Sublimation losses can be very high as a result
• depends on transport rate, transport distance,
temperature, humidity, wind speed, and solar radiation
Blowing Snow
• Sublimation Losses
Mean Annual Blowing Snow Sublimation
CANADA, 1970-1976
Loss in mm SWE over 1 km
22
16
20
25
30
22
25
50
Snow Pack Characteristics
Snow Pack Characteristics
• What is a Snow Pack?
– Porous Medium
• ice + air (+ liquid water)
– Generally composed of layers of different types
of snow
• more or less homogeneous within one layer
– Ice is in form of crystals and grains that are
usually bonded together
• forms a texture with some degree of strength
Snow Pack Characteristics
• Snow Water Equivalent (SWE)
– The height of water if a snow cover is
completely melted, on a corresponding
horizontal surface area.
• Snow Depth x (Snow Density/Water Density)
Density of Snow Cover
Snow Type
Density (kg/m3)
Snow Depth for
One Inch Water
Wild Snow
10 to 30
98” to 33”
Ordinary new snow immediately
after falling in still air
50 to 65
20” to 15”
Settling Snow
70 to 90
14” to 11”
Average wind-toughened snow
280
3.5”
Hard wind slab
350
2.8”
New firn snow
400 to 550
2.5” to 1.8”
Advanced firn snow
550 to 650
1.8” to 1.5”
Thawing firn snow
600 to 700
1.6” to 1.4”
Snow Pack Characteristics
• Liquid Water Content
– Wetness, Percentage by volume
Term
Dry
Moist
Remarks
Approximate Range
Usually T < 0oC, but can occur at any temperature up to 0oC.
Little tendency for snow grains to stick together.
T = 0oC. The water is not visible even at 10x magnification.
Has a distinct tendency to stick together.
0%
<3%
Wet
T = 0oC. The water can be seen at 10x magnification by its
miniscus between grains, but cannot be pressed out by
squeezing snow (pendular regime).
3-8%
Very Wet
T = 0oC. The water can be pressed out by squeezing snow,
but there is an appreciable amount of air (funicular regime).
8-15%
T = 0oC. The snow is flooded with water and contains a
relatively small amount of air.
>15%
Slush
Snow Characteristics
• Diurnal Temperature Gradients
o
Temperature ( C)
0
-5
-10
Snow Surface
140
120
Day
Evening
100
Snow Pack
80
60
40
20
Ground Surface
0
Temperature
Profile
Water Flow Through Snow
Water Flow through Snow
• Wide Range of Flow Velocities
– 2 - 60 cm/min
– Depends on several factors
• internal snow pack structure
• condition of the snow pack prior to introduction of water
• amount of water available at the snow surface
Water Flow Through Snow
• Flow through Homogeneous
Snow
– At melting temperature, a thin film of
water surrounds each snow grain
• Much of the water can flow through this
film
– Once pores are filled, laminar flow can
occur
• Very efficient mechanism for draining the
snow pack
Water Flow through Snow
• Four Liquid Water Regimes
• Capillary: < 1% free water
– water doesn’t drain due to capillary tension
• Unsaturated: 1-14% free water
– water drains by gravity, but air spaces are continuous
– Pendular Regime
• Saturated: > 14% free water
– water drains by gravity, but air spaces are discontinuous
– Funicular Regime
• Melt/Freeze
– water melts and refreezes, possible several times, before it drains from
the snow pack
Water Flow Through Snow
• Flow through Heterogeneous
Snow
– Preferential Flow Paths
Ice Lens
with Ponding
• Dye studies reveal vertical channels or
macropores in most natural snowpacks
– Ice Layers
• Develop from surface melt or refreezing
Preferential Flow Paths
• Relatively impermeable
• Forces ponding of water and lateral flow
Water Flow
Ice Lens
Water Flow Through Snow
• Liquid Water Transmission
Niwot Ridge, Colorado
May 2-30, 1995
Melt and rain water are
lagged and attenuated
as they move through
the snow cover.
Function of depth,
density, ice layers, grain
size, and refreezing.
6
4
2
0
122
6
4
2
0
130
6
4
2
0
138
6
4
2
0
146
Snow Melt at Surface
Outflow from Base
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
Day of Year
151
152
153
154
Rain
Snow Measurement
Snow Measurement
• Snow Water Equivalent (SWE)
– Ground Observations
• Snow Pillows
– SNOTEL Sites (Western U.S.)
• Snow Courses
– Transects with snow depth and density
• Snow Tubes
– measure volume and mass of snow cores
• Snow Pits
– Measure vertical profiles of SWE, and other snow pack
variables.
Snow Measurement
• Airborne Snow Survey Program (SWE)
Atmosphere
Cosmic Rays
Radon Daughters
in Atmosphere
Gamma Radiation
reaches
Detector in Aircraft
Uncollided
6000
Scattering
Gamma Radiation
Absorbed by Water
in the Snow Pack
Natural Gamma Sources
238
U Series,
232
Th Series, 40K Series
5000
Background
(No Snow)
4000
K40
3000
Snow
Tl208
2000
1000
Over-Snow
Soil
0
400
800
1200
1600
ENERGY (keV)
2000
2400
2800
Snow Measurement
• Satellite Areal Extent of Snow Cover
Snow Measurement
• NOAA-16 1.6 Micron Channel
Snow Measurement
• NOAA-16 1.6 Micron Channel
Snake River Valley, Idaho
SNOW
Visible Channel
1.6 micron Channel
Evaporation
• … Evaporation is a process that allows water to
change from its liquid phase to a vapor.
• … Hydrologists are mostly interested in the
evaporation from the free water surface of open
water or subsurface water exposed via the
capillary action; however, precipitation that is
intercepted by the vegetative canopy may also be
evaporated and may be a significant amount in
terms of the overall hydrologic budget.
• … Factors that affect evaporation are temperature,
humidity and vapor pressure, radiation, and wind
speed.
• … A number of equations are used to estimate
evaporation. There are also a number of
published tables and maps providing regional
estimates of annual evaporation.
Transpiration
• … Water may also pass to the atmosphere by
being "taken up" by plants and passed on
through the plant surfaces.
• … Transpiration varies greatly between
plants or crops, climates, and seasons.
• … Evaporation and transpiration are often
combined in a term - evapotranspiration.
• … In many areas of the country and during
certain seasons evapotranspiration is a major
component of the hydrologic budget and a
major concern in water supply and yield
estimates.
Storage - Surface
• ... Storage - Surface is used to describe the
precipitation that reaches the ground surface;
however, is not available for runoff or
infiltration.
• … It is instead, held in small quantities on the
surface in areas, such as the leafy matter and
small depressions.
• … In general, surface storage is small and only
temporary in terms of the overall hydrologic
budget; however, it may have an effect on a
storm response as it is effectively "filled" early
on a storm event.
Infiltration
• … Soils, depending on current conditions, have a
capacity or ability to infiltrate precipitation,
allowing water to move from the surface to the
subsurface.
• ... "physically based” -> soil porosity, depth of soil
column, saturation levels, and soil moisture.
• … The infiltration capacity of the soil column is
usually expressed in terms of length per time (i.e.
inches per hour).
• … As more water infiltrates, the infiltration
generally decreases, thus the amount of water that
can be infiltrated during the latter stages of a
precipitation event is less than that at the beginning
of the event.
Infiltration cont.
•
•
•
•
… Storms that have high intensity levels may
also cause excess precipitation because the
intensity (inches per hour) may exceed the
current infiltration capacity (inches per hour).
… periods of low rainfall or no rainfall will
allow the soil to "recover" and increase the
capacity to infiltrate water.…
Infiltrated water replenishes soil moisture and
groundwater reservoirs. Infiltrated water may
also resurface to become surface flow.
… attempt to account for infiltration by
estimating excess precipitation (the difference
between precipitation and excess being
considered infiltration), for example, the Soil
Conservation Service (SCS) runoff curve
number method
Subsurface Flow
•
•
•
•
…water may move via several paths.
…subsurface flow can be evaporated if there is a
well maintained transfer mechanism to the
surface. This is particularly true for areas of high
ground water table (the free water surface of the
groundwater) which is within the limits of the
capillary action or transport abilities.
…Vegetation may also transpire or use the water.
…The subsurface flow may also continue to
move with the groundwater table as a subsurface
reservoir, which the natural system uses during
periods of low precipitation.
Storage - Subsurface
• … The infiltrated water may continue
downward in the vertical, may move through
subsurface layers in a horizontal fashion, or a
combination of the two directions.
• … Movement through the subsurface system
is much slower than the surface and thus there
are storage delays. The water may also reach
an aquifer, where it may be stored for a very
long period of time.
Runoff
• … runoff will be used to collectively describe the
precipitation that is not directly infiltrated into the
groundwater system.
• … is generally characterized by overland, gully and
rill, swale, and channel flows.
• … is that portion of a precipitation event that
"quickly" reaches the stream system. The term
"quickly" is used with caution as there may be great
variability in response times for various flow
mechanisms.
• … Runoff producing events are usually thought of as
those that saturate the soil column or occur during a
period when the soil is already saturated. Thus
infiltration is halted or limited and excess
precipitation occurs. This may also occur when the
intensity rate of the precipitation is greater than the
infiltration capacity.
Overland Flow
•… Overland flow or surface flow is that precipitation
that either fails to penetrate into the soil or that
resurfaces at a later point due to subsurface
conditions.
•… often referred to as "sheet" flow.
•… for the purposes of this discussion, overland flow
(sheet and surface flow, as well) is considered to be
the flow that has not had a chance to collect and begin
to form gullies, rills, swales
Overland Flow (cont.)
•… will eventually reach defined channels and the
stream system.
•… may also be infiltrated if it reaches an area that
has the infiltration capacity to do so.
•… Overland flow distances are rather limited in
length - National Engineering Handbook (1972) overland flow will concentrate into gullies in less than
1000 feet.
•… Other (Seybert, Kibler, and White 1993)
recommend a distance of 100 feet or less.
Gullies & Rills
•
... sheet flow or overland flow will
soon concentrate into gullies and rills
in the process of flowing towards the
stream network. The location of these
gullies and rills may vary from storm
to storm, depending on storm patterns,
intensities, current soil and land use
conditions.
Swales
•
… swales are of a more constant or permanent
nature.
•
… do not vary in location from storm to storm.
•
… Swales are a natural part of the landscape or
topography that are often more apparent than
gullies and rills.
•
… Flow conditions and behaviors in swales are
very close to that which is seen in channels.
Channel Flow
•
•
•
•
•
•
… Excess precipitation ultimately reaches the
stream channel system.
… the stream system is generally more defined,
it is by no means a constant or permanent entity.
… The stream bed is constantly changing and
evolving via aggredation and degradation.
… Stream channels convey the waters of the
basin to the outlet and into the next basin.
… attenuation of the runoff hydrograph takes
place.
… Stream channel properties (flow properties)
also vary with the magnitude of the flow.
Stream Channels
•
•
•
•
•
… Channels are commonly broken into main
channel areas and overbank areas.
… overbank areas are often referred to as
floodplains.
… Stream gaging stations are used to
determine flows based on elevations in the
channel and/or floodplain.
… Bank full is often thought of as flood stage
although more rigorous definitions are more
applicable as they pertain to human activity
and potential loss of life and property.
… It is worth noting that the 2-year return
interval flow is often thought of as "bankfull".
Streamflow
• … in the public eye -> the most important aspect
of flooding and hydrology.
• … flooding from streams and rivers have the
greatest potential to impact human property and
lives; although overland flow flooding, mudslides,
and landslides are often just as devastating.
• … Subsurface flow also enters the stream;
although in some instances and regions, stream
channels lose water to the groundwater table regardless, this must be accounted for in the
modeling of the stream channel.
• … Channels also offer a storage mechanism and
the resulting effect is most often an attenuation of
the flood hydrograph.
Storage - Reservoirs
• … Lakes, reservoirs, & structures, etc. are given
a separate category in the discussion of the
hydrologic cycle due to the potential impact on
forecasting procedures and outcomes.
• … provide a substantial storage mechanism and
depending on the intended purpose of the
structure will have varying impacts on the final
hydrograph, as well as flooding levels.
• … This effect can vary greatly depending on the
type of reservoir, the outlet configuration, and the
purpose of the reservoir.
Storage - Reservoirs (cont.)
• … Flood control dams are used to attenuate
and store potentially destructive runoff events.
• … Other structures may have adverse effects.
For example, bridges may cause additional
"backwater" effects and enhance the level of
flooding upstream of the bridge.
• … a catastrophic failure of a structure often
has devastating effects on loss of life and
property.
Simulating the Hydrologic
Response
Model Types
Precipitation
Losses
Modeling Losses
Model Components
Model Types
Model Types
Precipitation
Losses
Modeling Losses
Model Components
•
•
•
•
Empirical
Analytical
Lumped
Distributed
General Goal of Most Models
Infiltration
Basin Process
Representation
Excess Precip.
Interception
Storage
Time
Series
Time
Series
We must begin to think of the basin as a “whole”
The Basic Process
Excess Precip.
Model
Excess Precip.
Runoff
Hydrograph
Excess Precip.
Basin “Routing”
Runoff
Hydrograph
Stream
“Routing”
Downstream
Hydrograph
From A Basin View
Excess Precip.
Excess Precip.
Model
Basin “Routing”
Stream
“Routing”
Runoff
Hydrograph
Precipitation Input
•
•
•
•
•
Precipitation is generally “pre-processed
Uniform in space and time – never!
Gages
Radar
satellite
Precipitation
•
… magnitude, intensity, location, patterns, and
future estimates of the precipitation.
•
… In lumped models, the precipitation is input in
the form of average values over the basin. These
average values are often referred to as mean aerial
precipitation (MAP) values.
•
… MAP's are estimated either from 1) precipitation
gage data or 2) NEXRAD precipitation fields.
Precipitation (cont.)
•
… If precipitation gage data is used, then the
MAP's are usually calculated by a weighting
scheme.
•
… a gage (or set of gages) has influence over an
area and the amount of rain having been recorded at
a particular gage (or set of gages) is assigned to an
area.
•
… Thiessen method and the isohyetal method are
two of the more popular methods.
Thiessen
•Thiessen method is a method for areally weighting rainfall through graphical
means.
Isohyetal
•Isohyetal method is a method for areally weighting rainfall using
contours of equal rainfall (isohyets).
NEXRAD
•Nexrad is a method of areally weighting rainfall using satellite imaging of
the intensity of the rain during a storm.
Excess Precip. Models
•
•
•
•
•
Physically Based
Empirical
Analytical
Conceptual
Generally Lumped
Losses
•
•
•
•
•
… modeled in order to account for the destiny of the
precipitation that falls and the potential of the precipitation
to affect the hydrograph.
… losses include interception, evapotranspiration,
depression storage, and infiltration.
… Interception is that precipitation that is caught by the
vegetative canopy and does not reach the ground for
eventual infiltration or runoff.
… Evapotranspiration is a combination of evaporation and
transpiration and was previously discussed.
… Depression storage is that precipitation that reaches the
ground, yet, as the name suggests, is stored in small surface
depressions and is generally satisfied during the early
portion of a storm event.
Modeling Losses
•
•
•
•
… simplistic methods such as a constant loss method
may be used.
… A constant loss approach assumes that the soil can
constantly infiltrate the same amount of precipitation
throughout the storm event. The obvious weaknesses are
the neglecting of spatial variability, temporal variability,
and recovery potential.
Other methods include exponential decays (the
infiltration rate decays exponentially), empirical
methods, and physically based methods.
… There are also combinations of these methods. For
example, empirical coefficients may be combined with a
more physically based equation. (SAC-SMA for
example)
SCS Curve Number
(P - 0.2S )2
Q=
(P + 0.8S)
Estimating “S”
•
•
•
The difficult part of applying this method to a watershed is the estimation of
the watershed’s potential maximum retention, S.
SCS developed the concept of the dimensionless curve number, CN, to aid in
the estimation of S.
CN is related to S as follows :
S=
1000
- 10
CN
CN ranges from 1 to 100 (not really!)
Determine CN
•
•
•
•
•
•
The Soil Conservation Service has classified over 8,500 soil series into four
hydrologic groups according to their infiltration characteristics, and the proper
group is determined for the soil series found.
The hydrologic groups have been designated as A, B, C, and D.
Group A is composed of soils considered to have a low runoff potential. These
soils have a high infiltration rate even when thoroughly wetted.
Group B soils have a moderate infiltration rate when thoroughly wetted,
while group C soils are those which have slow infiltration rates when
thoroughly wetted.
Group D soils are those which are considered to have a high potential for
runoff, since they have very slow infiltration rates when thoroughly wetted
(SCS, 1972).
Determine CN, cont….
•
Once the hydrologic soil group has been determined, the curve number of the
site is determined by cross-referencing land use and hydrologic condition to
the soil group - SAMPLE
Land use and treatment
or Hydrologic
practice
condition
Fallow
Straight row
Row Crops
Straight row
Straight row
Contoured
Hydrologic soil group
A
B
C
----
77
86
91
94
Poor
Good
Poor
72
67
70
81
78
79
88
85
84
91
89
88
D
Initial Conditions
5-day antecedent rainfall, inches
Antecedent moisture
Dormant Season
Growing Season
I
Less than 0.5
Less than 1.4
II
0.5 to 1.1
1.4 to 2.1
III
Over 1.1
Over 2.1
Adjust CN’s
Corresponding CN’s
CN for AMC II
AMC I
AMC III
100
100
100
95
87
98
90
78
96
85
70
94
80
63
91
75
57
88
70
51
85
65
45
82
60
40
78
55
35
74
50
31
70
Sort of the other end of the
scale….
SAC-SMA
• … The Sacramento Soil Moisture Accounting
Model (SAC-SMA) is a conceptual model of
soil moisture accounting that uses empiricism
and lumped coefficients to attempt to mimic
the physical constraints of water movement in
a natural system.
Upper Zone
Tension
Free
Lower Zone
Tension
Free - Primary
Free - Supplemental
Runoff
•
•
•
… Runoff is essentially the excess precipitation - the
precipitation minus the losses.
… Runoff must be transformed to streamflow at the
basin outlet via a unit hydrograph.
… In actuality, all forms of surface and subsurface flow
that reach a stream channel and eventually the outlet are
modeled through the use of the unit hydrograph for the
general hydrologic model…
Unit Hydrograph Theory
• Sherman - 1932
• Horton - 1933
• Wisler & Brater - 1949 - “the hydrograph of surface runoff
resulting from a relatively short, intense rain, called a unit
storm”
• The runoff hydrograph may be “made up” of runoff that is
generated as flow through the soil (black, 1990)
126
Linearity of Unit Hydrograph
•
… In addition, when unit hydrograph theory is applied, it is
assumed that the watershed responds uniformly.
•
… Meaning that peak flow from 2 inches of excess will be
twice that of 1 inch of excess
700.0000
600.0000
500.0000
400.0000
300.0000
200.0000
100.0000
0.0000
0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
4.0000
Unit Hydrograph “Lingo”
•
•
•
•
•
•
•
•
•
•
Duration
Lag Time
Time of Concentration
Rising Limb
Recession Limb (falling limb)
Peak Flow
Time to Peak (rise time)
Recession Curve
Separation
Base flow
Graphical Representation
Duration of
excess
precipitation.
Lag time
Time of
concentration
Base flow
Methods of Developing UHG’s
• From Streamflow Data
• Synthetically
– Snyder
– SCS
– Time-Area (Clark, 1945)
• “Fitted” Distributions
Unit Hydrograph
• The hydrograph that results from 1-inch of excess precipitation (or
runoff) spread uniformly in space and time over a watershed for a
given duration.
• The key points :
 1-inch of EXCESS precipitation
 Spread uniformly over space - evenly over the watershed
 Uniformly in time - the excess rate is constant over the time
interval
 There is a given duration
Derived Unit Hydrograph
700.0000
600.0000
500.0000
Surface
Response
400.0000
300.0000
Baseflow
200.0000
100.0000
0.0000
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 .16 .32 .48 .64 .80 .96 .12 .28 .44 .60 .76 .92 .08 .24 .40 .56 .72 .88 .04 .20 .36 .52 .68
0.
0
0
0
0
0
0
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
Derived Unit Hydrograph
700.0000
Total
Hydrograph
600.0000
500.0000
Surface
Response
400.0000
300.0000
Baseflow
200.0000
100.0000
0.0000
0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
4.0000
Derived Unit Hydrograph
Rules of Thumb :
æ… the storm should be fairly uniform in nature and the
excess precipitation should be equally as uniform throughout
the basin. This may require the initial conditions throughout
the basin to be spatially similar.
æ… Second, the storm should be relatively constant in time,
meaning that there should be no breaks or periods of no
precipitation.
æ… Finally, the storm should produce at least an inch of
excess precipitation (the area under the hydrograph after
correcting for baseflow).
Deriving a UHG from a Storm
sample watershed = 450 mi2
25000
0.8
0.7
20000
0.5
Flow (cfs)
15000
0.4
10000
0.3
0.2
5000
0.1
0
0
0
8
16
24
32
40
48
56
64
72
80
Time (hrs.)
88
96 10 4 11 2 12 0 12 8
Precipitation (inches)
0.6
Separation of Baseflow
... generally accepted that the inflection point on the recession limb of a
hydrograph is the result of a change in the controlling physical processes
of the excess precipitation flowing to the basin outlet.
In this example, baseflow is considered to be a straight line connecting
that point at which the hydrograph begins to rise rapidly and the
inflection point on the recession side of the hydrograph.
the inflection point may be found by plotting the hydrograph in semilog fashion with flow being plotted on the log scale and noting the time
at which the recession side fits a straight line.
Semi-log Plot
100000
Recession side of hydrograph
becomes linear at approximately hour
64.
10000
Flow (cfs)
1000
100
10
Time (hrs.)
99
10
4
10
9
11
4
11
9
12
4
12
9
13
4
89
94
79
84
69
74
59
64
49
54
39
44
29
34
1
Hydrograph & Baseflow
25000
20000
Flow (cfs)
15000
10000
5000
133
119
126
105
112
91
98
77
Time (hrs.)
84
63
70
56
42
49
28
35
14
21
0
7
0
Separate Baseflow
25000
20000
Flow (cfs)
15000
10000
5000
0
0
7
14
21
28
35
42
49
56
63
70
77
Time (hrs.)
84
91
98 10 5 11 2 11 9 12 6 13 3
Sample Calculations
• In the present example (hourly time step), the flows are summed and
then multiplied by 3600 seconds to determine the volume of runoff in
cubic feet. If desired, this value may then be converted to acre-feet by
dividing by 43,560 square feet per acre.
• The depth of direct runoff in feet is found by dividing the total volume
of excess precipitation (now in acre-feet) by the watershed area (450
mi2 converted to 288,000 acres).
• In this example, the volume of excess precipitation or direct runoff for
storm #1 was determined to be 39,692 acre-feet.
• The depth of direct runoff is found to be 0.1378 feet after dividing by
the watershed area of 288,000 acres.
• Finally, the depth of direct runoff in inches is 0.1378 x 12 = 1.65
inches.
Obtain UHG Ordinates
• The ordinates of the unit hydrograph are
obtained by dividing each flow in the direct
runoff hydrograph by the depth of excess
precipitation.
• In this example, the units of the unit
hydrograph would be cfs/inch (of excess
precipitation).
Final UHG
25000
Storm #1 hydrograph
Storm#1 direct runoff
hydrograph
20000
Flow (cfs)
15000
Storm # 1 unit
hydrograph
10000
Storm #1
baseflow
5000
Time (hrs.)
126
133
119
105
112
98
91
84
77
70
63
56
49
42
35
28
21
14
7
0
0
Determine Duration of UHG
• The duration of the derived unit hydrograph is found by examining the
precipitation for the event and determining that precipitation which is
in excess.
• This is generally accomplished by plotting the precipitation in
hyetograph form and drawing a horizontal line such that the
precipitation above this line is equal to the depth of excess
precipitation as previously determined.
• This horizontal line is generally referred to as the F-index and is based
on the assumption of a constant or uniform infiltration rate.
• The uniform infiltration necessary to cause 1.65 inches of excess
precipitation was determined to be approximately 0.2 inches per hour.
Estimating Excess Precip.
0.8
0.7
Precipitation (inches)
0.6
0.5
Uniform loss rate of
0.2 inches per hour.
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19
Time (hrs.)
Excess Precipitation
1
0.9
Derived unit hydrograph is the
result of approximately 6 hours
of excess precipitation.
0.8
Excess Prec. (inches)
0.7
0.6
0.5
Small amounts of
excess precipitation at
beginning and end may
be omitted.
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19
Time (hrs.)
Average Several UHG’s
• It is recommend that several unit hydrographs be derived and averaged.
• The unit hydrographs must be of the same duration in order to be
properly averaged.
• It is often not sufficient to simply average the ordinates of the unit
hydrographs in order to obtain the final unit hydrograph. A numerical
average of several unit hydrographs which are different “shapes” may
result in an “unrepresentative” unit hydrograph.
• It is often recommended to plot the unit hydrographs that are to be
averaged. Then an average or representative unit hydrograph should be
sketched or fitted to the plotted unit hydrographs.
• Finally, the average unit hydrograph must have a volume of 1 inch of
runoff for the basin.
One Step Shy of a Full Derivation?
•
•
•
•
•
•
•
•
•
You could part of the previous analysis for a very useful tool.
Take a storm
Plot streamflow
Determine volume of runoff
Divide by basin area
Get depth of runoff
Estimate total basin (mean) precipiation
Compare!
Do this for a variety of storm over a variety of conditions and seasons.
Synthetic UHG’s
• Snyder
• SCS
• Time-area
Snyder
• 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.
Basic Relationships
tLAG  Ct ( L  Lca )0.3
tduration 
tLAG
5.5
talt.lag  tLAG  0.25(talt.duration  tduration)
tbase  3 
q peak 
t LAG
8
640 AC p
t LAG
What are the L & Lca Doing?
Final Shape
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:
SCS
SCS Dimensionless UHG Features
1
Flow ratios
Cum. Mass
0.8
Q/Qpeak
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
T/Tpeak
3
3.5
4
4.5
5
Dimensionless Ratios
Time Ratios
(t/tp)
0
.1
.2
.3
.4
.5
.6
.7
.8
.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.5
5.0
Discharge Ratios
(q/qp)
.000
.030
.100
.190
.310
.470
.660
.820
.930
.990
1.000
.990
.930
.860
.780
.680
.560
.460
.390
.330
.280
.207
.147
.107
.077
.055
.040
.029
.021
.015
.011
.005
.000
Mass Curve Ratios
(Qa/Q)
.000
.001
.006
.012
.035
.065
.107
.163
.228
.300
.375
.450
.522
.589
.650
.700
.751
.790
.822
.849
.871
.908
.934
.953
.967
.977
.984
.989
.993
.995
.997
.999
1.000
Triangular Representation
D
SCS Dimensionless UHG & Triangular Representation
Excess
Precipitation
1.2
Tlag
1
0.8
Flow ratios
Q/Qpeak
Cum. Mass
Triangular
0.6
Point of
Inflection
Tc
0.4
0.2
0
0.0
Tp
1.0
2.0
Tb
3.0
T/Tpeak
4.0
5.0
Triangular Representation
Tb  2.67 x Tp
D
SCS Dimensionless UHG & Triangular Representation
Excess
Precipitation
1.2
Tlag
1
Tr  Tb - Tp  1.67 x Tp
0.8
Flow ratios
Q/Qpeak
Cum. Mass
Triangular
0.6
Point of
Inflection
Tc
Q=
qpT p
2
+
qpT r
2
=
qp
2
0.4
( T p +T r )
0.2
0
0.0
2Q
qp=
T p +T r
qp =
654.33x 2 x A x Q
T p +T r
qp=
484 A Q
Tp
Tp
1.0
2.0
Tb
3.0
4.0
5.0
T/Tpeak
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).
484 ?
qp=
484 A Q
Tp
Comes from the initial assumption that 3/8 of the volume
under the UHG is under the rising limb and the remaining 5/8
is under the recession limb.
General Description
Peaking Factor
Urban areas; steep slopes
Typical SCS
Mixed urban/rural
Rural, rolling hills
Rural, slight slopes
Rural, very flat
575
484
400
300
200
100
Limb Ratio
(Recession to Rising)
1.25
1.67
2.25
3.33
5.5
12.0
Time of Concentration
• Regression Eqs.
• Segmental Approach
A Regression Equation
. (S  1)0.7
L08
Tlag 
.
1900(%Slope)05
where : Tlag = lag time in hours
L = Length of the longest drainage path in feet
S = (1000/CN) - 10 (CN=curve number)
%Slope = The average watershed slope in %
Segmental Approach
• More “hydraulic” in nature
• The parameter being estimated is essentially the time of concentration
or longest travel time within the basin.
• In general, the longest travel time corresponds to the longest drainage
path
• The flow path is broken into segments with the flow in each segment
being represented by some type of flow regime.
• The most common flow representations are overland, sheet, rill and
gully, and channel flow.
A Basic Approach
K
0.25
0.5
0.7
0.9
1.0
1.5
2.0
Land Use / Flow Regime
Forest with heavy ground litter, hay meadow (overland flow)
Trash fallow or minimum tillage cultivation; contour or strip
cropped; woodland (overland flow)
Short grass pasture (overland flow)
Cultivated straight row (overland flow)
Nearly bare and untilled (overland flow); alluvial fans in
western mountain regions
Grassed waterway
Paved area (sheet flow); small upland gullies
Flow Type
Small Tributary - Permanent or intermittent
streams which appear as solid or dashed
blue lines on USGS topographic maps.
Waterway - Any overland flow route which
is a well defined swale by elevation
contours, but is not a stream section as
defined above.
Sheet Flow - Any other overland flow path
which does not conform to the definition of
a waterway.
V  kS
1
2
McCuen (1989) and SCS
(1972) provide values of k
for several flow situations
(slope in %)
K
2.1
1.2
0.48
Sorell & Hamilton, 1991
Time-Area
Time-Area
100%
Q
Time
of conc.
% Area
Time
Time
Time-Area
Hypothetical Example
• A 190 mi2 watershed is divided into 8 isochrones of travel time.
• The linear reservoir routing coefficient, R, estimated as 5.5 hours.
• A time interval of 2.0 hours will be used for the computations.
Rule of Thumb
R - The linear reservoir routing coefficient
can be estimated as approximately 0.75
times the time of concentration.
Basin Breakdown
Map
Area #
1
2
3
4
5
6
7
8
TOTAL
Bounding
Isochrones
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
Area
2
(mi )
5
9
23
19
27
26
39
40
190
Cumulative
2
Area (mi )
5
14
37
58
85
111
150
190
190
Cumulative
Time (hrs)
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
8.0
Incremental Area
40
Incremental Area (sqaure miles)
35
30
25
20
15
10
5
0
1
2
3
4
5
Time Increment (hrs)
6
7
8
Cumulative Time-Area Curve
9
Cumulative Area (sqaure miles)
8
7
6
5
4
3
2
1
0
0
20
40
60
80
100
Time (hrs)
120
140
160
180
200
Trouble Getting a Time-Area
Curve?
TAi  1.414Ti1.5
1  TAi  1.414(1  Ti )1.5
for (0  Ti  0.5)
for (0.5  Ti  1.0)
Synthetic time-area curve The U.S. Army Corps of
Engineers (HEC 1990)
Instantaneous UHG
IUH i  cI i  (1  c) IUH ( i 1)
c
2 t
2 R  t
ü t = the time step used n the
calculation of the translation unit
hydrograph
ü The final unit hydrograph may be
found by averaging 2
instantaneous unit hydrographs
that are a t time step apart.
Computations
Time
(hrs)
(1)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
Inc.
Area
(mi2)
(2)
0
14
44
53
79
0
Inc.
Translated
Flow (cfs)
(3)
0
4,515
14,190
17,093
25,478
0
Inst.
UHG
(4)
0
1391
5333
8955
14043
9717
6724
4653
3220
2228
1542
1067
738
510
352
242
168
116
81
55
39
26
19
13
IUHG
Lagged 2
hours
(5)
0
1,391
5,333
8,955
14,043
9,717
6,724
4,653
3,220
2,228
1,542
1,067
738
510
352
242
168
116
81
55
39
26
19
13
2-hr
UHG
(cfs)
(6)
0
700
3,360
7,150
11,500
11,880
8,220
5,690
3,940
2,720
1,890
1,300
900
630
430
300
200
140
100
70
50
30
20
20
Incremental Areas
90
Area Increments (square miles)
80
70
60
50
40
30
20
10
0
0
2
4
6
Time Increments (2 hrs)
8
10
Incremental Flows
30000
Translated Unit Hydrograph
25000
20000
15000
10000
5000
0
1
2
3
4
Time Increments (2 hrs)
5
6
Instantaneous UHG
16000
14000
Flow (cfs/inch)
12000
10000
8000
6000
4000
2000
0
0
10
20
30
Time (hrs)
40
50
60
Lag & Average
16000
14000
Flow (cfs/inch)
12000
10000
8000
6000
4000
2000
0
0
10
20
30
Time (hrs)
40
50
60
Let’s talk about Modeling Issues
Weaknesses, strengths, etc…
Factors Affecting the
Hydrologic Response
•
•
•
•
•
Current Conditions
Precipitation Patterns
Land Use
Channel Changes
Others…..
Channel Changes
• Slopes
• Storage
• Rating Curve
Variable Source Area Concept
Not all of the watershed is contributing during an
event......
Example
And so on...
And the recession...
Small Basin Hydrology
and
Distributed Models
Why do we need
DISTRIBUTED MODELS?
Non-homogeneity!
Causes of Non-homogeneity
• Small scale precipitation
• Spatially diverse precipitation patterns
• Small scale basin changes – i.e. soil
moisture, slope, etc….
• Sub-basin changes – urbanization
• Others????
Hydrology Terminology
•Precipitation can fall in many different patterns,
which influences the hydrologic response.
•For example, a storm may be:
•Uniform over the entire watershed
•A storm may move up the watershed
•A storm may move down the watershed
•A storm may only rain on a portion of
the watershed.
Approaches
• Many sub-basins – at least more than you
currently have…
• Hillslope processes
• TIN’s
• Grids - raster
Common with Lumped
• Still must compute excess
• Can still use empirical, analytical,
conceptual, etc….
Computationally
• Huge demands computationally
• Must now keep track of flow, precip,
moisture, etc.. On hundreds to thousands
of pixels, sub-basins, etc….
Moving water off basin
• Lumped we tended to use the UHG
• Now we tend to be more physically
based:
– Hydraulic equation
– Hydrologic routing
– Etc….