Transcript NWS-COMET May 1998 Hydrometeorology Course

HYDROLOGY
September 13, 1999
Dr. Arthur C. Miller
Penn State University
Outline
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Introductions
Goals of this Course
Overview of the Hydrologic Cycle
Defining a Watershed
Watershed Reponses
Modeling the Watershed Response
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Unit Hydrographs
Precipitation
Excess Precipitation
Convolution
Routing
Regression Equations
Wrap-up
Purpose of the Hydrology Course
• Increase participants understanding of the Hydrologic
Cycle
• Introduce participants to basic terminology and
concepts of hydrology and hydrologic forecasting as
applied watershed response.
• Establish the course objectives as per the expectations
of the participants.
In the end, it is intended that participants
will have a better understanding of the
hydrologic response of a watershed, the
assumptions in the process, and the
responsibilities associated with
interpreting and issuing a hydrologic
analysis.
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)
Units ??
Area
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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
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.
• 1 m3
Area
Volume
Runoff Volume
Discharge
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
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
Hydrologic Cycle
Topics
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
Precipitation
Precipitation
-Snow
Evaporation
Transpiration
Storage-surface
Infiltration
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
• ... primary "input"
• … 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.
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
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.
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.
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.
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.
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
Subsurface Flow
Precipitation
Evaporation
Transpiration
Storage-surface
Infiltration
-Subsurface
Storage - Subsurface
Runoff
Water Movement
Streamflow
Storage-Reservoirs
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…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
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.
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 than the
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
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
100 feet.
•… Other (Seybert, Kibler, and White 1993)
recommend a distance of 100 feet or less.
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.
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.
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
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•
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… 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
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".
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.
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.
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.
The Watershed
Watershed
•Defining
•Contours
•Topo maps
•Digital Data
• A watershed is an area of land
that drains to a single outlet
and is separated from other
watersheds by a divide.
• Hydrologic analysis and
synthesis focus on the
watershed.
The Watershed
Watershed
-drainage area
-drainage basin
-sub-basin
-sub-area
•Defining
•Contours
•Topo maps
•Digital Data
•Characteristics
•Every watershed has a
drainage area.
• Related terms:
drainage basin, subbasin, sub-area.
Defining a Watershed
Watershed
•Defining
•delineation
•Contours
•Topo maps
•Digital Data
•Characteristics
• Defining a watershed is generally referred to
as delineating the watershed.
• The process involves determining that area
within which water would drain to a common
point.
• It is often easier to visualize the concept by
pretending the ground surface is
impermeable like cement.
Defining a Watershed
Watershed
•Defining
•Contours
•Topo maps
•Digital Data
•Characteristics
Contours are lines of constant
elevation.
Contours “point” or “curve” uphill
at stream crossings.
Contours (generally) have constant
spacing.
Defining a Watershed
Watershed
•Defining
•Contours
•Topo maps
•Digital Data
•Characteristics
• At right, the watershed has been
delineated, using the contours, for
the indicated watershed outlet.
• The streams/channel sections
have also been highlighted within
the watershed.
Defining a Watershed
Watershed
•Defining
•Contours
•Topo maps
•Digital Data
•Characteristics
• The most common form of
mapping used for delineation are
the USGS topographical maps.
• The most common map scale is
the 7.5 minute 1:24,000 scale.
Defining a Watershed
Watershed
•Defining
•Contours
•Topo maps
•Digital Data
•GIS
•DEM’s
•Characteristics
• The use of electronic or digital
data and mapping has become
rather common place in the field
of hydrology.
• GIS or Geographical Information
Systems are used to manage,
manipulate, and analyze digital
data.
• One of the most common GIS
data sets is a Digital Elevation
Models or DEM’s.
Defining a Watershed
Watershed
•Defining
•Contours
•Topo maps
•Digital Data
•Characteristics
• There are a VERY large number of
watershed characteristics and properties
that are used in various aspects of
hydrologic analysis and synthesis.
• Many of these characteristics and properties
may be somewhat ambiguous in nature and
difficult to measure and/or estimate.
• Some of the more common characteristics
are :
Watershed Characteristics
•
•
•
•
•
•
•
Area
Slope
Land Use
Soils
Geology
Climate
Geomorphology
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
Area is measured in units of L2.
Typical units are : acres, square miles, hectares, and
square kilometers.
A GIS automatically calculates the area of the
watershed as it is delineated.
When a GIS is not used - other methods are used to
determine the area.
A plainimeter is a common tool.
The area of the basin is generally thought of as that
area that would or could contribute runoff to the outlet
of the watershed during a rain event.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
The slope is a watershed parameter that may take
on several meanings. Recall that slope is rise over
run or a measure of the change in elevation with
distance.
The slope may be the average slope of the main
channel that drains the outlet.
The slope may be the slope of the watershed as
defined by the change in elevation from the outlet
to the highest point divided by the distance to that
point.
A Note on Slopes
DEM
Contours (5m)
Slopes (>10%)
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
Land Use is a critical element in the hydrologic
response of a watershed.
The land use may determine the amount of runoff, the
timing of the runoff, and the quality of the runoff.
Land use may also drive such factors as evaporation,
transpiration, and heat fluxes between the earth’s
surface and the surrounding atmosphere.
Typical land use classifications may include : forested,
agricultural, urban, etc.. There may also be sub-classes
of these groups : new growth forest, densely populated
urban, row crops, etc…
There are a number of electronic land use data sets or
coverages available.
A site visit to the watershed is ALWAYS a good idea.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
Soils are also very important in the overall
hydrologic cycle, as well as the hydrologic response.
Soils have a variety of properties that are relevant
to the hydrologist : infiltration, infiltration capacity,
conductivity (horizontal and vertical), %
organic,hydrologic soil group, etc..
County, state, and national soil surveys are
available for most areas of the country.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
The underlying geology has great influences on the
infiltration and the ultimate fate of infiltrated water.
Bedrock and depth to bedrock, clay layers, etc..
should be documented.
AS an example, limestone areas are known to have
large cracks or openings. These large openings and
cavities allow for high infiltration rates and storage,
which may result in lower runoff volumes.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
Urbanization is given its own category, even though it
is essentially a land use or land cover.
Urbanization greatly increases the runoff as the
land’s natural ability to infiltrate and retain water
has been severely reduced or eliminated all together.
Urbanization not only increases the volume of runoff,
but it also “speeds up” the response. This is due to
the rapid channelization of the runoff.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
The presence of water bodies such as lakes, ponds,
dams, and other wetlands may have significant
impact on the response of a watershed.
Water bodies have the effect of delaying and storing
runoff and this the overall responseis delayed and
reduced in timing and peak flow, respectively.
Some water bodies are man-made and may be
“controlled” or “regulated”.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
Geomorphology is used to describe the stream netwrok
that drains the watershed.
Metrics such as width, depth, and slope may be used.
Additionally, measurements of sinuosity (a measure of
how “windy” the streams are), as well how many
streams there are and how far apart they are may
greatly affect the response of a watershed.
Stream density (miles of stream/square mile or
drainage area) is another common metric.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
A very common descriptor of the geomorphology of a
drainage basin is the stream order. One of the most
common methods of defining stream order is the
Horton stream ordering system.
In this system, a first order stream is an unbranched
tributary.
Second order streams occur when two first order
streams come together.
A third order stream results from two second order
streams, and so on.
When a first and second order stream come togoether,
the result is still a second order stream.
Watershed Characteristics
Area
Slope
Land Use
Soils
Geology
Urbanization
Water bodies
Geomorphology
Geomorphology is used to describe the stream network
that drains the watershed.
Metrics such as width, depth, and slope may be used.
Additionally, measurements of sinuosity (a measure of
how “windy” the streams are), as well how many
streams there are and how far apart they are may
greatly affect the response of a watershed.
Stream density (miles of stream/square mile or
drainage area) is another common metric.
The Watershed Response
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
The Watershed Response
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
Establish your GOALS & NEEDS
Short term => “Event Model”
Long Term => “Continuous”
The Watershed Response
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
Event Model
•A few hours to a few days
•Initial conditions - CRITICAL
Continuous
•Long Term
•Initial Conditions - maintained?
FOCUS
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
Surface Runoff - recall
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
•
•
•
•
Overland
Gullies & Rills
Swales
Stream Channels
Baseflow
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
• A “slower” response
• For subsurface supplies or storage
• Contribution mostly in long term
or “tail” or event.
• May be an inflow or outflow!
The Runoff Hydrograph
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff 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
Streamflow
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
 Streamflow
Rating Curves
• … 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.
Rating Curves
•
Rating curves establish a relationship between depth and
the amount of flow in a channel.
Rating Curve for a sample watershed
Water Surface Elev. (ft)
Long Term –vs.- Short
Infiltration
Evapotranspiration
Storage
Subsurface Flow
Surface Runoff
Baseflow
The Runoff Hydrograph
 Streamflow
Rating Curves
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208
206
204
0
2000 4000 6000 8000 10000 12000 14000 16000
Q total (cfs)
Factors Affecting the
Hydrologic Response
•
•
•
•
•
Current Conditions
Precipitation Patterns
Land Use
Channel Changes
Others…..