Transcript NWS-COMET May 1998 Hydrometeorology Course

NWS-COMET
Hydrometeorology 00-1
13-28 October, 1999
Hydrology Primer
1
Dr. Russ Qualls
University of Colorado
Department of Civil Engineering
(303) 492-5968 (phone)
(303) 492-7317 (fax)
[email protected] (email)
2
Where is MTU?
3
Topographic Relief of Great
Lakes Region
4
Some Lake Superior Facts
•
•
•
•
•
•
•
Surface Area = 31,700 square miles
Land Drainage Area = 49,300 square miles
Lake Superior covers ~ 39% of the total basin area!!
More than 200 rivers enter with 1000’s of streams.
Max. depth = 1,330 feet
Volume = 2,900 cubic miles
More than half of all the great lakes and ~ 10% of
earth’s flowing surface fresh waters!!
5
Usual Houghton
6
Purpose of the Hydrometeorology Course
•
Increase the participants knowledge and understanding of the interaction
between meteorology and hydrology in watersheds:
•
Increase participants understanding of the functional aspects of
watersheds;
•
Enhance the participants knowledge of the capabilities, limitations, and
applications of new hydrometeorological observing systems;
•
Improve the participants ability to identify significant mesoscale
meteorological events and to produce Quantitative Precipitation Forecasts;
•
Increase participants understanding of the effectiveness of the NWS
forecast and warning methodologies and plan future enhancements; and
•
Build awareness of the need for close ties between RFC's and WFO's.
7
Purpose of the PRIMER
• Provide an introduction between participants & establish
backgrounds.
• Introduce participants to basic terminology and concepts of
hydrologic forecasting that will be used throughout the hydrology
portion of the COMET Hydromet course. The primer introduces
these concepts and specific detail will be provided in week 3.
• Establish the course objectives as per the expectations of the
participants.
• Establish hydrologic concerns in the various participants' regions.
8
In the end, it is intended that participants
will understand the hydrologic forecast
process, the assumptions in the process,
and the responsibilities associated with
interpreting and issuing the forecast.
9
Mission of NOAA's NWS
Hydrologic Services Program
To provide river and flood forecasts and
warnings for protection of life and
property
Provide basic hydrologic forecast
information for the nation's economic
and environmental well being.
10
Modernized NWS
• “It is essential to emphasize the
complementary aspects of operational
hydrology and meteorology in the
modernized NWS, while recognizing the
uniqueness of RFC and WFO
operations. “
11
New or Improved Products
• ...the production of a variety of hydrologic
forecast products for an increased number of
river locations across the country, including
ESP-based products
12
What is ESP?
13
What is ESP?
• Ensemble Streamflow Production (ESP)
• Inputs the current moisture level of soil and the precipitation from
previous years into a model which produces the diagram seen
above.
• For example, the moisture content of today would be inputted,
along with the precipitation that occurred over the next week, but
50 years ago.
• This would then be repeated for 49 years ago, 48, etc., and then an
average discharge based on history can be determined.
14
NWS
Office of Hydrology
Dr. Danny Fread
Director, Office of Hydrology
Front
Office Staff
Chief Scientist
Special Asst. to the Director
Secretary
Program Analyst
Program Support Assistant
Administrative Support Technician
Dr.
Ed Johnson
Chief, Hydrologic Operations Division
??
Chief, Hydrologic Research Laboratory
15
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)
16
Hydrometeorology
… an interdisciplinary science involving the
study and analysis of the interrelationships
between the atmospheric and land phases of
water as it moves through the hydrologic cycle."
(Hydrometeorological Service Operations for
the 1990's, Office of Hydrology, National
Weather Service, NOAA, 1996).
17
Hydrometeorology - Links
Hydrology
Engineering/Fluid
Mechanics
 In-depth hydrologic
analysis
 Execution of complex
hydrologic models.
Adjustment of
model parameters, and
the derivation of
hydrologic forecasts for
all time scales
 Applied hydrologic
research
 Development and
calibration of
hydrologic models
 Development of
hydrologic applications
procedures.
Hydrometeorology
Interdisciplinary
Orientation
 Assimilation/use of
WSR-88D based
precip. estimates
 Production and/or
use of QPF's and
other hydromet.
forecasts
 Use of RFC guidance
(e.g. flash flood) in
hydrologic warning
operations
 Use of soil moisture
states from
hydrologic model in
atmospheric model
 Applied
hydrometeorological
research.
Meteorology
Thermodynamics/atmospheric
physics orientation
 In-depth meteorological
analysis
 Weather forecast and
warning operations
 Climatological forecasting
 Applied meteorological
and climatological
research.
 Development and calibration
of meteorological models
 Development of
meteorological applications
and procedures.
18
19
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
20
Common
Unit Conversions
Area
Volume
Runoff Volume
Discharge
21
Power
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
Runoff Volume
Discharge
22
Power
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.
Area
Volume
Runoff Volume
Discharge
23
Power
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
Area
Volume
Runoff Volume
Discharge
Power
24
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
Area
Volume
Runoff Volume
Discharge
25
Power
Power
• 1 hp = 550 ft*lb/sec = 0.7547 kilowatts
Area
Volume
Runoff Volume
Discharge
Power
26
Hydrologic Cycle
Topics
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
27
Precipitation
Precipitation
-Snow
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• ... 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.
• … Knowing and understanding the general,
regional, and local precipitation patterns greatly
aids forecasters in determining QPF values.
• … In addition to the quantity of precipitation, the
spatial and temporal distributions of the
precipitation have considerable effects on the
hydrologic response.
28
Snow
Precipitation
-Snow
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• ... nature of the modeling efforts that are required.
• … 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
29
responses.
Evaporation
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• … 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.
30
Transpiration
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• … 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.
31
Storage - Surface
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• ... 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.
32
Infiltration
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
-Subsurface
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• … 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.
33
Infiltration cont.
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
-Subsurface
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
•
•
•
•
… 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
34
Subsurface Flow
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
-Subsurface
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
•
•
•
•
…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.
35
Storage - Subsurface
Precipitation
• … The infiltrated water may continue downward in the
Evaporation
vertical, may move through subsurface layers in a
Transpiration
horizontal fashion, or a combination of the two
Storage-surface
directions.
Infiltration
• … Movement through the subsurface system is much
Storage - Subsurface
slower than the surface and thus there are storage
Runoff
delays. The water may also reach an aquifer, where it
Water Movement
may be stored for a very long period of time.
Streamflow
• … In the NWS River Forecast System (RFS), the
Storage-Reservoirs
subsurface storage is represented by imaginary zones or
"tanks". These tanks release the stored water at a given
or calibrated rate. The released water from the
subsurface zones is added to the surface runoff for
convolution with the unit hydrograph.
36
Runoff
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• … 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 than37the
infiltration capacity.
Overland Flow
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
-Overland flow
-Gullies and Rills
-Swales
-Channel Flow
-Stream Channels
Streamflow
Storage-Reservoirs
•… 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
38
Overland Flow (cont.)
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
-Overland flow
-Gullies and Rills
-Swales
-Channel Flow
-Stream Channels
Streamflow
Storage-Reservoirs
•… 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.
39
Gullies & Rills
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
-Overland flow
-Gullies and Rills
-Swales
-Channel Flow
-Stream Channels
Streamflow
Storage-Reservoirs
•
... 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.
40
Swales
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
-Overland flow
-Gullies and Rills
-Swales
-Channel Flow
-Stream Channels
Streamflow
Storage-Reservoirs
•
… 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.
41
Channel Flow
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
-Overland flow
-Gullies and Rills
-Swales
-Channel Flow
-Stream Channels
Streamflow
Storage-Reservoirs
•
•
•
•
•
•
… 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.
42
Stream Channels
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
-Overland flow
-Gullies and Rills
-Swales
-Channel Flow
-Stream Channels
Streamflow
Storage-Reservoirs
•
•
•
•
•
… 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".
43
Streamflow
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• … 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.
44
Storage - Reservoirs
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
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.
45
Storage - Reservoirs (cont.)
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• … Flood control dams are used to attenuate
and store potentially destructive runoff events.
• … Other structures may 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.
46
NWS - Forecast Terminology
47
Hydrology Terminology
Topics
Watershed
Stream flow
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
48
Hydrology Terminology
Watershed
-drainage area
-drainage basin
-sub-basin
-sub-area
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
• 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, subbasin, sub-area.
49
Hydrology Terminology
Watershed
Streamflow
-cross-section area
-Manning’s “n”
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•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
50
Hydrology Terminology
Watershed
Streamflow
-cross-section area
-Manning’s “n”
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•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.
51
Hydrology Terminology
Watershed
Streamflow
Routing
•Routing is used to account for storage and
translation effects.
-Hydrologic
-Hydraulic
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
Routing
Hydrologic
Hydraulic
52
Hydrology Terminology
Watershed
Streamflow
Generalized effect of routing
Routing
-Hydrologic
-Hydraulic
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
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
53
10.0000
Hydrologic Routing
Watershed
Streamflow
Routing
-Hydrologic
-Hydraulic
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•Hydrologic routing is the more simple of the two
techniques.
•Based on the continuity equation which says
Inflow - Outflow = Change in Storage - or -
dS
I O 
dt
•A second relationship is also required which relates
storage to discharge. This relationship is usually
assumed, empirical, or analytical in nature.
•Two types of hydrologic routing, River and Reservoir
Routing.
54
Hydraulic Routing
Watershed
Streamflow
Routing
-Hydrologic
-Hydraulic
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•Hydraulic routing is more complex and generally
considered more accurate than hydrologic routing.
•Based on the simultaneous solution of the continuity
equation and the momentum equation, commonly
called the St. Venant equations.
55
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
-Storage
-routing
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•Reservoir storage attenuates the flow and delays
the impact of flood waters. Reservoirs are generally
used for flood control, drinking water supply,
hydropower, and recreation.
56
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
-Storage
-routing
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•Reservoir routing is generally easier to perform
than river routing because storage-discharge
relations for pipes, weirs, and spillways are singlevalued functions independent of flow.
•Storage indication method or Puls Method
 2 sn
  2sn1

( I n  I n1 )  
 On   
 On1 
 t
  t

•Other Methods: Runge-Kutta Method
57
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
-Muskingum
-Muskingum-Cunge
-dynamic
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
• Channel routing can be broken into hydrologic and
hydraulic methods.
• Hydrologic routing again uses the storage or
continuity equation:
1
1
S 2  S1
I1  I 2   O1  O2  
2
2
t
• This formula subtracts the average outflow from an average inflow to
determine the change in storage over a given time period.
58
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
-Muskingum
-Muskingum-Cunge
-dynamic
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
• Common methods of hydrologic routing :
•Lag & K
•Tatum
•Mod-Puls
•Kinematic Wave
•Muskingum
• Muskingum-Cunge**
59
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
-Muskingum
-Muskingum-Cunge
-dynamic
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•Hydraulic river routing includes solving the
continuity equation and the momentum equation
simultaneously.
•Dynamic routing is an example of this.
•DAMBRK & FLDWAV, as well as, UNET are
dynamic routing models
60
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
-excess
-intensity
-patterns
•Precipitation is water that falls to the earth in the form
of rain, snow, hail or sleet.
•Excess precipitation is the precipitation that is not
infiltrated into the soil and becomes available as a
rapid runoff component in the hydrologic response of a
basin.
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
61
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
•The intensity of the precipitation is the rate at which it
is raining, and is measured in length/time. A radar
picture of rainfall intensity can be seen below.
Precipitation
-excess
-intensity
-patterns
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
62
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
-excess
-intensity
-patterns
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•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.
63
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
-snowfall
-snowmelt
-snowpack
• 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.
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
64
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
-overland flow
-sub-surface flow
-baseflow
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
•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.
65
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
• 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
•The are many methods to estimate infiltration and/or
excess precipitation. To name a few :
• f index
•Horton’s
•Green-Ampt
•SCS - curve number *
•Continuous simulations (SAC-SMA)
66
Hydrology Terminology
Unit hydrograph
-derived
-synthetic
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
• The unit hydrograph is the hydrograph for 1 unit
of runoff in a given specified time or duration of
runoff.
700.0000
600.0000
500.0000
Surface
Response
400.0000
300.0000
Baseflow
200.0000
100.0000
0.0000
0.
00
0
0. 0
16
00
0.
32
0
0. 0
48
00
0.
64
0
0. 0
80
00
0.
96
0
1. 0
12
0
1. 0
28
00
1.
44
0
1. 0
60
00
1.
76
0
1. 0
92
00
2.
08
0
2. 0
24
00
2.
40
0
2. 0
56
0
2. 0
72
00
2.
88
0
3. 0
04
00
3.
20
0
3. 0
36
00
3.
52
0
3. 0
68
00
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
67
Hydrology Terminology
Unit hydrograph
-derived
-synthetic
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
• The unit hydrograph is a “transfer” mechanism for
transforming excess precipitation into streamflow.
700.0000
600.0000
500.0000
Surface
Response
400.0000
300.0000
Baseflow
200.0000
100.0000
0.0000
0.
00
0
0. 0
16
00
0.
32
0
0. 0
48
00
0.
64
0
0. 0
80
00
0.
96
0
1. 0
12
0
1. 0
28
00
1.
44
0
1. 0
60
00
1.
76
0
1. 0
92
00
2.
08
0
2. 0
24
00
2.
40
0
2. 0
56
0
2. 0
72
00
2.
88
0
3. 0
04
00
3.
20
0
3. 0
36
00
3.
52
0
3. 0
68
00
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
68
Derived Unit Hydrograph
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
-derived
-synthetic
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
Rules of Thumb
•… 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.
•… Second, the storm should be relatively
constant in time, meaning that there should be no
breaks or periods of no precipitation.
•
•… Finally, the storm should produce at least an
inch of excess precipitation (the area under the
hydrograph after correcting for baseflow).
69
Synthetic Unit Hydrograph
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
• SCS
• Snyder
• Clark - (time-area)
Unit hydrograph
-derived
-synthetic
Timing
Flooding
Flow
Grade lines
Land Use
Frequency
70
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
•Lag Time is the time from the center of mass of
the rainfall to the peak of the unit hydrograph.
•Time of concentration is the time at which
outflow from a basin is equal to the inflow. It is
often considered the longest travel time from any
point in the watershed.
•Duration is the time span of the rainfall.
Duration of
excess
precipitation
Timing
-lag time
-time of concentration
-duration
Flooding
Flow
Grade lines
Land Use
Frequency
Lag
time
Time of
concentration
71
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
•Flooding is the main concern of forecasters.
•Bank-full flooding is often thought of as the twoyear return flow or Q2.
•The effects of flooding can drastically effect an
ecosystem, which can be seen in the next two
pictures.
Flooding
-bank-full
Flow
Grade lines
Land Use
Frequency
72
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Before
Flooding
-bank-full
Flow
Grade lines
Land Use
Frequency
After
73
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
•The flow and its effect on the environment and the
human population depends on quantity, timing,
velocity, and wave speed.
•The quantity of the flow is the volume of water, while
the peak flow is generally of greatest interest.
•The timing of the flow is based on when a storm event
occurs. If it occurs when a river is already close to
flood stage, it will have a greater impact than if it
occurred over a river that was relatively low. The time
to peak, time of concentration, lag time, response time,
and duration are all of great concern.
-quantity
-timing
-velocity
-”wave” speed
Grade lines
Land Use
Frequency
74
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
•The velocity of the flow is based 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.
Flow
-quantity
-timing
-velocity
-”wave” speed
Grade lines
Land Use
Frequency
75
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
-EGL
-HGL
Land Use
Frequency
•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
76
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
•Land Use is a major contributor to runoff behavior.
•If the land is covered by trees, it will behave
differently than if it was a pasture or a meadow.
•Urbanization also changes runoff patterns by the
increase in artificial materials which decrease
infiltration and increase flow response time.
Land Use
-land cover
-urbanization
-karst
-slope
Frequency
77
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
•Karst hydrology is caused by pores and holes in
limestone formations. This increases the infiltration
into the limestone, reducing the runoff potential.
Land Use
-land cover
-urbanization
-karst
-slope
Frequency
•The slope changes the speed of runoff and therefore
effects collection times.
78
Hydrology Terminology
Watershed
Streamflow
Routing
Reservoirs
Channel
Precipitation
Snow
Runoff
Infiltration
Unit hydrograph
Timing
Flooding
Flow
Grade lines
Land Use
•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%
Frequency
-return period
-probability
79
Fluid Concepts
Topics
Energy Head
Momentum
Open Channel
80
Energy or Energy Head
Energy Head
-Elevation Head
-Velocity Head
• Elevation head
• Velocity head
• Total head
-Total Head
Momentum
Open Channel
81
Energy or Energy Head
Energy Head
-Elevation Head
-Velocity Head
-Total Head
Momentum
Open Channel
•The total energy of water moving through a channel
is expressed in total head in feet of water.
•This is simply the sum of the the elevation above a
datum (elevation head), the pressure head and the
velocity head.
•The elevation head is the vertical distance from a
datum to a point in the stream.
•The velocity head is expressed by:
V2
Velocityhead 
2g
82
Energy Head
Energy Head
-Elevation Head
Graphical depiction of elevation head, velocity
head, and total head. Total head is the sum of
velocity head, depth and elevation head.
-Velocity Head
-Total Head
2
Momentum
Open Channel
Energy Grade Line
v1
2g
Hydraulic Grade Line
(water surface)
Depth1
Channel Bottom
Elevation Head
headloss
Velocity
head
Depth2
Datum
83
Momentum Equation
Energy Head
Momentum
-Equation
 Forces  Q(V V )
x
-Forces
2
1
Open Channel
 Forces  hydrostatic forces friction weight  externalforces
x
Hydrostatic Forces
Friction Forces
Weight
External84Forces
Hydrostatic Forces
Energy Head
Momentum
-Equation
•Hydrostatic Forces are the forces placed on a control volume by
the surrounding water.
•The strength of the force is based on depth and can be seen in
the following relationship:
-Forces
Open Channel
P=H
H
Hydrostatic
Forces
Control Volume
Hydrostatic Forces
Friction Forces
Weight
External 85
Forces
Friction Forces
Energy Head
Momentum
-Equation
The friction force on a control volume is due to the
water passing the channel bottom and depends on the
roughness of the channel.
-Forces
Control Volume
Open Channel
Friction Force
Hydrostatic Forces
Friction Forces
Weight
External 86
Forces
Weight
Energy Head
Momentum
The weight of a control volume is due to the gravitational
pull on the its mass.
-Equation
-Forces
Weight = mg
Control Volume
Open Channel
Weight
Hydrostatic Forces
Friction Forces
Weight
External 87
Forces
External Forces
Energy Head
Momentum
External Forces (Fd) the forces created by a control volume
striking a stationary object. External Forces can be explained
by the following equation:
-Equation
-Forces
Fd=1/2CdAv2
Open Channel
Streamflow direction
Fd
Top View of Control Volume
Hydrostatic Forces
Friction Forces
Weight
External 88
Forces
Steady vs. Unsteady Flow
Energy Head
Momentum
Open Channel
-Steady -vs- Unsteady
-Uniform -vs- Nonuniform
-Supercritical -vs- subcritical
-Equations
•Fluid properties including velocity,
pressure, temperature, density, and
viscosity vary in time and space.
•A fluid it termed steady if the depth
of flow does not change or can be
assumed constant during a specific
time interval.
•Flow is considered unsteady if the
depth changes with time.
89
Uniform and Nonuniform Flow
Energy Head
Momentum
Open Channel
-Steady -vs- Unsteady
-Uniform -vs- Nonuniform
-Supercritical -vs- subcritical
•Uniform Flow is an equilibrium flow
such that the slope of the total energy
equals the bottom slope.
•Nonuniform Flow is a flow of water
through a channel that gradually
changes with distance.
-Equations
90
Super -vs.- Sub Critical
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
-Sub/Supercritical
-Equations
91
Critical flow: a demonstration
Energy Head
Momentum
Open Channel
If a stone is dropped into a body of water, with no
velocity, the waves formed by the water are fairly
circular. This is similar to sub-critical flow.
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
-Sub/Supercritical
-Equations
No velocity
92
Critical flow: a demonstration
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
Now, if a velocity is added to the body of water,
the waves become unsymmetrical, increasing to
the downstream side. This happens as the velocity
approaches critical flow. Notice that the wave still
moves upstream, though slower than the
downstream wave.
-Sub/Supercritical
-Equations
Small velocity
93
Critical flow: a demonstration
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
Now if a large velocity is added to the body of
water, the wave patterns only go in one
direction. This represents the point when flow
has gone beyond critical, into the supercritical
region.
-Uniform-vs. Nonuniform
-Sub/Supercritical
-Equations
Large velocity
94
Froude number
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
The Froude number is a numerical value that
describes the type of flow present (critical,
supercritical, subcritical), and is represented
by the following equation for a rectangular
channel:
-Uniform-vs. Nonuniform
-Sub/Supercritical
-Froude number
NF 
v
gdm
-Equations
NF = Froude number
v = mean velocity of flow
g = acceleration of gravity
dm = mean (hydraulic) depth
95
Froude number
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
The generalized formula for the Froude
number is as follows:
2
QT
Fr 
3
9A
2
-Sub/Supercritical
-Froude number
-Equations
Fr = Froude number
Q = Flow rate in the channel
T = Time
A = Area of the channel
96
Froude number - mean depth
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
•Mean depth is a ratio of the width of the free
water surface to the cross-sectional area of the
channel.
A
dm 
B
-Sub/Supercritical
-Froude number
B=width of the free water surface
-Equations
A=cross-sectional area of the channel
97
Froude number
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
-Sub/Supercritical
-Froude number
-Equations
The Froude number can then be used to quantify
the type of flow.
•If the Froude number is less than 1.0, the flow is
subcritical. The flow would would be
characterized as tranquil.
•If the Froude number is equal to 1.0, the flow is
critical.
•If the Froude number is greater than 1.0, the
flow is supercritical and would be characterized
as rapid flowing. This type of flow has a high
velocity which can be potential damaging.
98
Super-vs.-Subcritical
Energy Head
Momentum
Open Channel
-Steady-vs.-Unsteady
-Uniform-vs. Nonuniform
•Critical depth can also be determined by
constructing a Specific Energy Curve.
•The critical depth is the point on the curve with
the lowest specific energy.
•Any depth greater than critical depth is
subcritical flow and any depth less than is
supercritical flow.
-Sub/Supercritical
-Equations
99
Super-vs.-Subcritical
Specific Energy Curve
Critical depth
Depth, d
Subcritical depth
3.5
3
2.5
2
1.5
1
0.5
0
0
Supercritical depth
2
4
6
8
10
Specific Energy, E
100
Open Channel Equations
Energy Head
Momentum
Open Channel
-Steady -vs- Unsteady
-Uniform -vs- Nonuniform
-Supercritical -vs- subcritical
-Equations:
•
•
•
•
Chezy Equation
Manning’s Equation
Bernoulli Equation
St. Venant Equations
Chezy
Manning
Bernoulli
St. Venant
101
Chezy Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
•In 1769, the French engineer Antoine
Chezy developed the first uniform-flow
formula.
V  C RS
•The formula was derived based on two
assumptions. First, Chezy assumed that the
force resisting the flow per unit area of the
stream bed is proportional to the square of
the velocity (KV2), with K being a
proportionality constant.
102
Chezy Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
•The second assumption was that the
channel was undergoing uniform flow.
•The difficulty with this formula is
determining the value of C, which is the
Chezy resistance factor. There are three
different formulas for determining C, the
G.K. Formula, the Bazin Formula, and the
Powell Formula.
103
Chezy Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
•Later on, when Manning's equation was
developed in 1889, a relationship between
Manning’s “n” and Chezy’s “C” was
established.
1.49 1/ 6
C
R
n
•Finally in 1933, the Manning equation was
suggested for international use rather than
Chezy’s Equation.
104
Manning’s Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
•In 1889 Robert Manning, an Irish engineer,
presented the following formula to solve open
channel flow.
1.49 2 3 1 2
V
Rh S f
n
V = mean velocity in fps
R = hydraulic radius in feet
S = the slope of the energy line
n = coefficient of roughness
The hydraulic radius (R) is a ratio of the water area to
the wetted perimeter.
105
Manning’s Equation
Energy Head
Momentum
Open Channel
•This formula was later adapted to obtain a
flow measurement. This is done by multiplying
both sides by the area.
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
1.49
Q
ARh2 3 S 1f 2
n
•Manning’s equation is the most widely used of
all uniform-flow formulas for open channel
flow, because of its simplicity and satisfactory
results it produces in real-world applications.
106
Manning’s Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
•Note that the equation expressed in the
previous slide was the English version of
Manning’s equation.
•There is also a metric version of Manning’s
equation, which replaces the 1.49 with 1. This
is done because of unit conversions.
•The metric equation is:
1
23 12
Q  ARh S f
n
107
Bernoulli Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
•The Bernoulli equation is developed from
the following equation:
2
2
V1
V2
z1  y1  1
 z 2  y2   2
 hL
2g
2g
-Bernoulli
-St. Venant
This equation states that the elevation (z)
plus the depth (y) plus the velocity head
(V12/2g) is a constant. The difference being
the headlosses - hL
108
Bernoulli Equation
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
•This equation was then adapted by making
a few assumptions.
•First, the head loss due to friction is equal to
zero. This means the channel is perfectly
frictionless surface.
•Second, that alpha1 is equal to alpha2 which
is equal to 1. The alpha’s are in the original
equation to account for a non-uniform
velocity distribution. In this case we will
assume a uniform distribution which
produces the following equation:
2
2
V1
V2
z1  y1 
 z 2  y2 
 constant
2g
2g
109
Bernoulli Equation
Energy Head
Momentum
A simplified version of the formula is given
below:
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
-St. Venant
2
2
V
V
i  P  z  constant  i  wsel
i
2g γ
2g
110
Bernoulli Equation
Energy Head
Momentum
Some comments on the Bernoulli equation
Open Channel
•Energy only
-Chezy Equation
-Manning’s
•Headloss in terms of energy
-Bernoulli
•Cannot calculate forces
-St. Venant
•Limited Effect in “rapidly varying flow”
111
St. Venant Equations
Energy Head
Momentum
Open Channel
-Chezy Equation
-Manning’s
-Bernoulli
The two equations used in modeling are
the continuity equation and the
momentum equation.
A Q

q
t x
Continuity equation
-St. Venant
y v v 1 v
S f  So  

x g x g t
Momentum Equation
112
St. Venant Equations
Energy Head
Momentum
The Momentum Equation can often be
simplified based on the conditions of the
model.
Open Channel
-Chezy Equation
S f = So -
y v v 1 v
x g x g t
Unsteady -Nonuniform
S f = So -
y v v
x g x
Steady - Nonuniform
S f = So -
y
x
Diffusion or noninertial
-Manning’s
-Bernoulli
-St. Venant
S f  So
Kinematic
113
Simulating the Hydrologic
Response
Model Types
Precipitation
Losses
Modeling Losses
Model Components
114
Model Types
Model Types
Precipitation
Losses
Modeling Losses
Model Components
• Empirical
• Lumped
• Distributed
115
Precipitation
Model Types
Precipitation
-Thiessen
-Isohyetal
-Nexrad
Losses
Modeling Losses
Model Components
• … 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.
116
Precipitation (cont.)
Model Types
Precipitation
-Thiessen
-Isohyetal
-Nexrad
Losses
Modeling Losses
Model Components
• … 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.
117
Thiessen
Model Types
Precipitation
-Thiessen
-Isohyetal
-Nexrad
Losses
Modeling Losses
Model Components
•Thiessen method is a method for areally weighting rainfall through graphical
means.
118
Isohyetal
Model Types
Precipitation
-Thiessen
-Isohyetal
-Nexrad
Losses
Modeling Losses
Model Components
•Isohyetal method is a method for areally weighting rainfall using
contours of equal rainfall (isohyets).
119
NEXRAD
Model Types
Precipitation
-Thiessen
-Isohyetal
-Nexrad
Losses
Modeling Losses
Model Components
•Nexrad is a method of areally weighting rainfall using satellite imaging of
the intensity of the rain during a storm.
120
Losses
Model Types
Precipitation
Losses
Modeling Losses
Model Components
• … 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.
121
Modeling Losses
Model Types
Precipitation
Losses
Modeling Losses
-SAC-SMA
Model Components
• … 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)
122
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Infiltration or “losses” - this section
describes the action of the precipitation
infiltrating into the ground. It also covers
the concept of initial abstraction, as it is
generally considered necessary to satisfy
the initial abstraction before the
infiltration process begins.
123
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Initial Abstraction - It is generally
assumed that the initial abstractions must
be satisfied before any direct storm
runoff may begin. The initial abstraction
is often thought of as a lumped sum
(depth). Viessman (1968) found that 0.1
inches was reasonable for small urban
watersheds.
Would forested & rural watersheds be
more or less?
124
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Forested & rural watersheds would
probably have a higher initial
abstraction.
The Soil Conservation Service (SCS)
now the NRCS uses a percentage of
the ultimate infiltration holding
capacity of the soil - i.e. 20% of the
maximum soil retention capacity.
125
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Infiltration is a natural process that we
attempt to mimic using mathematical
processes. Some of the mathematical
process or simulation methods are
conceptual while others are more
physically based.
126
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Constant Infiltration Rate :
A constant infiltration rate is the most simple of
the methods. It is often referred to as a phi-index
or f-index.
In some modeling situations it is used in a
conservative mode.
The saturated soil conductivity may be used for
the infiltration rate.
The obvious weakness is the inability to model
changes in infiltration rate.
The phi-index may also be estimated from
individual storm events by looking at the runoff
hydrograph.
127
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Constant Percentage Method :
Another very simplistic approach - this method
assumes that the watershed is capable of
infiltrating or “using” a value that is
proportional to rainfall intensity.
The constant percentage rate can be “calibrated”
for a basin by again considering several storms
and calculating the percentage by :
%f  1 
Qexcess
Ptotal
128
Constant Percentage Example
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
2
1
77.5%
infiltrates
0
129
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Exponential Decay: This is purely a
mathematical function - of the following
form:
f  f  ( f  f )e kt
i
c
o c
130
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Exponential Decay:
Effect of fo or fc
131
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
Exponential Decay:
Effect of K
132
Simulating Watershed Response
Infiltration
Long Term –vs.- Short
Infiltration
Evapotranspiration
Unit Hydrograph
Timing
Routing
SCS Curve Number:
Soil Conservation Service is an
empirical method of estimating
EXCESS PRECIPITATION
We can imply that :
P - Pe = F
133
SCS (NRCS)
Runoff Curve Number
• The basic relationships used to develop the curve number runoff
prediction technique are described here as background for subsequent
discussion. The technique originates with the assumption that the
following relationship describes the water balance of a storm event.
F Q

S P
where F is the actual retention on the watershed, Q is the
actual direct storm runoff, S is the potential maximum
retention, and P is the potential maximum runoff
134
More Modifications
• At this point in the development, SCS redefines S to be the
potential maximum retention
• SCS defines Ia in terms of S as : Ia = 0.2S
• and since the retention, F, equals effective precipitation minus
runoff : F = (P-Ia) - Q
• Substituting gives the familiar SCS rainfall-runoff
(P - 0.2S )2
Q=
(P + 0.8S)
135
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!)
136
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).
137
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
138
SAC-SMA
Model Types
Precipitation
Losses
Modeling Losses
-SAC-SMA
Model Components
• … 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
139
Runoff
Model Types
Precipitation
Losses
Modeling Losses
Model Components
-Runoff
-Unit Hydrograph
• … Runoff is essentially the excess precipitation the precipitation minus the losses.
• … In the NWSRFS, runoff is modeled through the
use of the SAC-SMA or an antecedent precipitation
index (API) model.
• … Runoff is 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.
140
Unit Hydrograph
Model Types
Precipitation
Losses
Modeling Losses
Model Components
-Runoff
-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
141
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
142
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
143
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
144
Derived Unit Hydrograph
•Rules of Thumb :
•… 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.
•… Second, the storm should be relatively constant in time,
meaning that there should be no breaks or periods of no
precipitation.
•… Finally, the storm should produce at least an inch of
excess precipitation (the area under the hydrograph after
correcting for baseflow).
145
Synthetic Unit Hydrograph
• SCS
• Snyder
• Clark - (time-area)
146
SCS - Dimensionless UHG
147
SCS - Dimensionless UHG
148
SCS - Dimensionless UHG
149
Time-Area
150
Time-Area
100%
Q
Time
of conc.
% Area
Time
Time
151
Time-Area
152
Stream Routing
• ... stream routing is used to account for storage and translation effects
as a runoff hydrograph travels from the outlet of one basin through the
next downstream basin.
• … Most of the time, channels act as reservoirs and have the effect of
attenuating the hydrograph.
• … 2 basic types of flow or channel routing :
• hydrologic
• hydraulic
153
Typical 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
154
Lakes, Reservoirs, Impoundments,
• ...have the effect of storing flow and attenuating hydrographs.
• … Reservoirs (and impoundments) are modeled with some form of
routing.
• … hydrologic and hydraulic routing may be applicable; although most
often, hydrologic routing is used in reservoir routing for normal flow
conditions.
• … During failure scenarios an unsteady flow model (hydraulic routing)
is usually necessary due to the nature of the flow, which is rapidly
changing.
155
Factors Affecting the
Hydrologic Response
•
•
•
•
•
Current Conditions
Precipitation Patterns
Land Use
Channel Changes
Others…..
156
Current Conditions
•
•
•
•
Wet
Dry
Update model states
subjective
157
Precipitation Patterns
• … The pattern is both temporal and spatial.
• … A storm moving away from an outlet will have a very
different result than the identical storm pattern (spatially)
moving towards the outlet.
• … Lumped hydrologic models have a very difficult time
in simulating spatially and temporally varied storm
events.
• … The very nature of MAP values - indicates one of the
problems.
• … A forecaster must understand the potential of
precipitation patterns to affect the forecast
158
Land Use
• Urban
• Agricultural
• Anything that changes the infiltration, runoff, etc...
600.0000
500.0000
After
"urbanization"
400.0000
300.0000
Before
"urbanization"
200.0000
100.0000
0.0000
0.0000
2.0000
4.0000
6.0000
8.0000
10.0000
12.0000
159
Channel Changes
•
•
•
•
Slopes
Storage
Rating Curve
Ice!!!
160
Rating Curves
• Rating curves establish a relationship between depth
and the amount of flow in a channel.
Water Surface Elev. (ft)
Rating Curve for a sample watershed
218
216
214
212
210
208
206
204
0
2000 4000 6000 8000 10000 12000 14000 16000
Q total (cfs)
161