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Flow Indication & Control |
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A
Ablation :
The processes by which ice and snow dissipate owing to
melting and evaporation.
Abutment :
The part of a valley or canyon wall against which a dam
is constructed. Right and left abutments are those on
respective sides of an observer looking downstream
Abutment Seepage :
Reservoir water that moves through seams or pores in the
natural abutment material and exits as seepage.
Acre-foot :
The amount of water required to cover one acre to a depth
of one foot. An acre-foot equals 326,851 gallons, or 43,560
cubic feet.
Active Conservation Storage :
The portion of water stored in a reservoir that can be
released for all useful purposes such as municipal water
supply, power, irrigation, recreation, fish, wildlife,
etc. Conservation storage is the volume of water stored
between the inactive pool elevation and flood control
stage.
Active (Usable) Storage Capacity :
The total amount of reservoir capacity normally available
for release from a reservoir below the maximum storage
level. It is total or reservoir capacity minus inactive
storage capacity. More specifically, it is the volume
of water between the outlet works and the spillway crest.
Adirondack Type Snow Sampling Set :
A snow sampler consisting of a 5-foot fiberglass tube,
3 inches in diameter, with a serrated-edge steel cutter
at one end and a twisting handle at the other. This sampler
has a 60-inch snow depth capacity.
ADVIS :
A program which combines the Antecedent Precipitation
Index (API) method of estimating runoff with unit hydrograph
theory to estimate streamflow for a headwater basin.
Aeration Zone :
A portion of the lithosphere in which the functional interstices
of permeable rock or earth are not filled with water under
hydrostatic pressure. The interstices either are not filled
with water or are filled with water that is no held by
capillarity.
AFOS :
Automation of Field Operations and Services
Afterbay :
The tail race of a hydroelectric power plant at the outlet
of the turbines. The term may be applied to a short stretch
of stream or conduit, or to a pond or reservoir.
Agglomerate :
An Ice cover of floe formed by the freezing together of
various forms of ice.
AHOS :
Automatic Hydrologic Observing System
AHOS-S :
Automatic Hydrologic Observing System - Satellite
AHOS-T :
Automatic Hydrologic Observing System - Telephone
Airborne Snow Survey Program :
Center (NOHRSC) program that makes airborne snow water
equivalent and soil moisture measurements over large areas
of the country that are subject to severe and chronic
snowmelt flooding.
Airborne Snow Water Equivalent Measurement Theory :
A theory based on the fact that natural terrestrial gamma
radiation is emitted from the potassium, uranium, and
thorium radioisotopes in the upper eight inches of the
soil. The radiation is sensed from low flying aircraft
500 feet above the ground. Water mass in the snow cover
attenuates the terrestrial radiation signal. The difference
between airborne radiation measurements made over bare
ground and snow-covered ground can be used to calculate
a mean areal snow water equivalent value with a root mean
square error of less than a half inch.
Albedo :
The portion of incoming radiation which is reflected by
a surface.
ALERT Flood Warning System :
A cooperative, community-operated flood warning system;
the acronym stands for Automated Local Evaluation (in)
Real Time.
Alluvial :
An adjective referring to alluvium.
Alluvium :
Sediments deposited by erosional processes, usual by streams.
Anabranch :
A diverging branch of a river which re-enters the main
stream.
Anchor Ice :
Submerged Frazil ice attached or anchored to the river
bottom, irrespective of its formation.
Anchor Ice Dam :
An accumulation of anchor ice which acts as a dam and
raises the water level.
Annual Flood :
The maximum discharge peak during a given water year (October
1 - September 30).
Antecedent Precipitation Index (API) :
An index of moisture stored within a drainage basin before
a storm.
API Method :
A statistical method to estimate the amount of surface
runoff which will occur from a basin from a given rainstorm
based on the antecedent precipitation index, physical
characteristics of the basin, time of year, storm duration,
rainfall amount, and rainfall intensity.
Aquiclude :
A formation which contains water but cannot transmit it
rapidly enough to furnish a significant supply to a well
or spring.
Aquifer :
Permeable layers of underground rock, or sand that hold
or transmit groundwater below the water table that will
yield water to a well in sufficient quantities to produce
water for beneficial use.
Aquifuge :
A geologic formation which has no interconnected openings
and cannot hold or transmit water.
Arch Dam :
A concrete arch dam is used in sites where the ratio of
width between abutments to height is not great and where
the foundation at the abutments is solid rock capable
of resisting great forces. The arch provides resistance
to movement. When combined with the weight of concrete
(arch-gravity dam), both the weight and shape of the structure
provide great resistance to the pressure of water.
Area of Influence :
The area covered by the drawdown curves of a given pumping
well or combination of wells at a particular time.
Area-Capacity Curve :
A graph showing the relation between the surface area
of the water in a reservoir, the corresponding volume,
and elevation.
Area Wide Hydrologic Prediction System (AWHPS) :
A computer system which automatically ingests areal flash
flood guidance values and WSR-88D products and displays
this data and other hydrologic information on a map background.
Arid :
An adjunctive applied to regions where precipitation is
so deficient in quantity, or occurs at such times, that
agriculture is impracticable without irrigation.
Arroyo :
A water-carved channel or gully in arid country, usually
rather small with steep banks, dry most of the time, due
to infrequent rainfall and the shallowness of the cut
which does not penetrate below the level of permanent
ground water.
Artesian Well :
a well drilled into a confined aquifer with enough hydraulic
pressure for the water to flow to the surface without
pumping. Also called a flowing well.
Artificial Control :
A weir or other man-made structure which serves as the
control for a stream-gaging station.
ASAP :
AHOS SHEF Automatic Processing System
ASAPTRAN :
The software component of ASAP.
ASOS :
The ASOS program is a joint effort of the National Weather
Service (NWS), the Federal Aviation Administration (FAA),
and the Department of Defense (DOD). When installation
is completed in the mid-1990s, the ASOS systems will serve
as the nation's primary surface weather observing network.
ASOS is designed to support weather forecast activities
and aviation operations and, at the same time, support
the needs of the meteorological, hydrological, and climatological
research communities.
Automatic Surface Observing System
ASOS
ATDTDCS (Automated Tone Dial Telephone Data Collection
System) :
Data collection system where cooperative observers collect
precipitation, stage, and temperature data then transmit
the data to the NWS ATDTDCS computer through the telephone
lines. The ATDTDCS computer transmits the data to AFOS.
Attenuation :
The process where the flood crest is reduced as it progresses
downs
Automated Event-Reporting Gage:
(see Tipping Bucket Rain Gage); for river stage gages,
IFLOWS pressure transducer type gages can be programmed
to report if water surface rises or falls by a predetermined
amount.
Automated Local Evaluation in Real Time (ALERT) :
A local Flood warning system where river and rainfall
data are collected via radio signals in real-time at an
ALERT base station.
AWIPS :
Advanced Weather Interactive Processing System. A system
that will eventually replace Automation of Field Operations
and Services, AFOS.
B
B-44 Form, Cooperative Station Report :
A Weather Service form documenting station management,
exposure, topography, driving instructions, payment information,
hydrometeorlogic equipment, and observing information.
Backflow :
The backing up of water through a conduit or channel in
the direction opposite to normal flow.
Backsight :
A rod reading taken on a point of known elevation, a benchmark
or a turning point. Backsights are added to the known
elevation to arrive at the height of the instrument. With
a known height of the instrument, the telescope can be
used to determine the elevation of other points in the
vicinity.
Backwater Curve:
The longitudinal profile of the surface of a liquid in
a non-uniform flow in an open channel, when the water
surface is not parallel to the invert owing to the depth
of water having been increased by the interposition of
an obstruction such as a dam or weir. The term is sometimes
used in a generic sense to denote all water surface profiles;
or for profiles where the water is flowing at depths greater
than the critical.
Backwater Effect:
The effect which a dam or other obstruction has in raising
the surface of the water upstream from it.
Backwater Flooding :
Upstream flooding caused by downstream conditions such
as channel restriction and/ or high flow in a downstream
confluence stream.
Bank :
The margins of a channel. Banks are called right or left
as viewed facing in the direction of the flow.
Bank Storage:
Water absorbed and stored in the void in the soil cover
in the bed and banks of a stream, lake, or reservoir,
and returned in whole or in part as the level of water
body surface falls.
Bankfull Stage/Elevation :
An established river stage/water surface elevation at
a given location along a river which is intended to represent
the maximum water level that will not overflow the river
banks or cause any significant damages from flooding.
Bankfull Stage :
An established river stage at a certain point along a
river which is intended to represent the maximum safe
water level which will not overflow the river banks or
cause any significant damage within the reach of the river.
Barrage :
Any artificial obstruction placed in water to increase
water level or divert it. Usually the idea is to control
peak flow for later release.
Base Flood:
The national standard for floodplain management is the
base, or one percent chance flood. This flood has at least
one chance in 100 of occurring in any given year. It is
also called a 100 year flood.
Base Station :
A computer which accepts radio signals from ALERT gaging
sites, decodes the data, places the data in a database,
and makes the data available to other users.
Baseflow :
Streamflow which results from precipitation that infiltrates
into the soil and eventually moves through the soil to
the stream channel. This is also referred to as ground
water flow, or dry-weather flow.
Base Width :
The time duration of a unit hydrograph.
Basin :
An area having a common outlet for its surface runoff.
Basin Boundary :
The topographic dividing line around the perimeter of
a basin, beyond which overland flow (i.e.; runoff) drains
away into another basin.
Basin Lag :
The time it takes from the centroid of rainfall for the
hydrograph to peak.
Basin Recharge :
Rainfall that adds to the residual moisture of the basin
in order to help recharge the water deficit. i.e; water
absorbed into the soil that does not take the form of
direct runoff.
Bed Load :
Sand, silt, gravel, or soil and rock detritus carried
by a stream on or immediately above its bed. The particles
of this material have a density or grain size such as
to preclude movement far above or for a long distance
out of contact with the stream bed under natural conditions
of flow.
Beginning of the Breakup :
Date of definite breaking, movement, or melting of ice
cover or significant rise of water level.
Beginning of Freezup :
Date on which ice forming a stable winter ice cover is
first observed on the water surface.
Benchmark (BM) :
A permanent point whose known elevation is tied to a national
network. These points are created to serve as a point
of reference. Benchmarks have generally been established
by the USGS, but may have been established by other Federal
or local agencies. Benchmarks can be found on USGS maps.
Black Ice :
Transparent ice formed in rivers and lakes.
Border ice :
An ice sheet in the form of a long border attached to
the bank or shore.; shore ice.
Brackish Ice :
Ice formed from Brackish water.
Braided Stream :
Characterized by successive division and rejoining of
streamflow with accompanying islands. A braided stream
is composed of anabranches.
Brash Ice :
Accumulation of floating ice made up of fragments not
more than 2 meters across; the wreckage of other forms
of ice.
Breach :
The failed opening in a dam.
Breakup :
The time when a river whose surface has been frozen from
bank to bank for a significant portion of its length begins
to change to an open water flow condition. Breakup is
signaled by the breaking of the ice and often associated
with ice jams and flooding.
Breakup date :
Date on which a body of water is first observed to be
entirely clear of ice and remains clear thereafter.
Breakup jam :
Ice jam that occurs as a result of the accumulation of
broken ice pieces.
Breakup period :
The period of disintegration of an ice cover.
Bubbler Gage :
A water stage recording device that is capable of attaching
to a LARC for data automation purposes.
Buttress Dam :
Buttress dams are comprised of reinforced masonry or stonework
built against concrete. They are usually in the form of
flat decks or multiple arches. They require about 60 percent
less concrete than gravity dams, but the increased form
work and reinforcement steel required usually offset the
savings in concrete. Many were built in the 1930's when
the ratio of labor cost to materials was comparatively
low. However, this type of construction is not competitive
with other types of dams when labor costs are high.
C
CADAS (Centralized Automated Data Acquisition System)
:
A system of two minicomputers in NWSH that interrogates
LARCs and DARDCs by telephone every 6 hours and transmits
the data to AFOS via HADS.
Calibration :
The process of using historical data to estimate parameters
in a hydrologic forecast technique such as SACSMA, routings,
and unit hydrographs.
Capillarity :
(1)The degree to which a material or object containing
minute openings or passages, when immersed in a liquid,
will draw the surface of the liquid above the hydrostatic
level. Unless otherwise defined, the liquid is generally
assumed to be water. (2)The phenomenon by which water
is held in interstices above the normal hydrostatic level,
due to attraction between water molecules.
Capillary Fringe :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
zone.
Capillary Potential :
The work required to move a unit mass of water from the
reference plane to any point in the soil column.
Capillary Zone :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
fringe.
Catchment Area :
An area having a common outlet for its surface runoff
(also see Drainage Area or Basin, Watershed).
Caution Stage :
Same as alert stage.
CFS (Cubic Feet per Second) :
The flow rate or discharge equal to one cubic foot (of
water, usually) per second. This rate is equivalent to
approximately 7.48 gallons per second. This is also referred
to as a second-foot.
Cfs-Day :
The volume of water discharged in twenty four hours, with
a flow of one cubic foot per second is widely used; 1
cfs-day is 24 x 60 x 60 = 86,000 cubic feet, 1.983471
acre-feet, or 646,317 gallons. The average flow in cubic
feet per second for any time period is the volume of flow
in cfs-days.
Channel (watercourse) :
An open conduit either naturally or artificially created
which periodically, or continuously contains moving water,
or forms a connecting link between two bodies of water.
River, creek, run, branch, anabranch, and tributary are
some of the terms used to describe natural channels. Natural
channels may be single or braided. Canal and floodway
are some of the terms used to describe artificial channels.
Channel Inflow :
Water, which at any instant, is flowing into the channel
system form surface flow, subsurface flow, base flow,
and rainfall that has directly fallen onto the channel.
Channel Lead :
An elongated opening in the ice cover caused by a water
current.
Channel Routing :
The process of determining progressively timing and shape
of the flood wave at successive points along a river.
Channelization :
The modification of a natural river channel; may include
deepening, widening, or straightening.
Closed Basin :
A basin draining to some depression or pond within its
area, from which water is lost only by evaporation or
percolation. A basin without a surface outlet for precipitation
falling precipitation.
Closed Basin Lake Flooding :
Flooding that occurs on lakes with either no outlet or
a relatively small one. Seasonal increases in rainfall
cause the lake level to rise faster than it can drain.
The water may stay at flood stage for weeks, months, or
years.
CNIF :
Calibration Network Information Files
Coastal Flooding :
Flooding that occurs from storms where water is driven
onto land from an adjacent body of water. These can be
hurricanes, "nor'easters," or tropical storms,
but even a severe winter storm or thunderstorm can cause
this type of flooding.
COE :
Corps of Engineers
Columnar Ice :
Ice consisting of columnar shaped grain. The ordinary
black ice is usually columnar-grained.
Composite Hydrograph :
A stream discharge hydrograph which includes base flow,
or one which corresponds to a net rain storm of duration
longer than one unit period.
Cone of Depression :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with the rate of withdrawal of water. Also
called the Cone of Influence.
Cone of Influence :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with rate of withdrawal of water. Also
called the Cone of Depression.
Confined Ground Water :
Ground water held under an aquiclude or an aquifuge called
artesian if the pressure is positive.
Congressional Organic Act of 1890 :
The act that assigned the responsibility of river and
floor forecasting for the benefit of the general welfare
of the Nation's people and economy to the Weather Bureau,
and subsequently the National Weather Service.
Conservation Storage :
Storage of water for later release for usual purposes
such as municipal water supply, power, or irrigation in
contrast with storage capacity used for flood control..
Consolidated Ice Cover :
Ice cover formed by the packing and freezing together
of floes, brash ice and other forms of floating ice.
Contents :
The volume of water in a reservoir. Unless otherwise indicated
reservoir content is computed on the basis of a level
pool and does not include bank storage.
Control Points: Horizontal and Vertical :
Small monuments securely embedded in the surface of the
dam. Any movement of the monument indicates a movement
in the dam itself. Movements in the dam are detected by
comparing control points location to location of fixed
monuments located off the dam using accurate survey techniques.
Cooperative Observer :
An individual (or institution) who takes precipitation
and temperature observations-and in some cases other observations
such as river stage, soil temperature, and evaporation-at
or near their home, or place of business. Many observers
transmit their reports by touch-tone telephone to an NWS
computer, and nearly all observers mail monthly reports
to the National Climatic Data Center to be archived and
published.
Conveyance Loss :
The loss of water from a conduit due to leakage, seepage,
evaporation, or evapo-transpiration.
Corn Snow Ice :
Rotten granular ice.
County Warning Forecast Area (CWFA) :
The area (aggregation of counties, or parishes and sometimes
portions of counties, or parishes) served by a WFO for
which weather forecast and warning services are provided.
Crack :
A separation formed in an ice cover of floe that does
not divide it into two or more pieces.
Creek :
A small stream of water which serves as the natural drainage
course for a drainage basin of nominal, or small size.
The term is a relative one as to size, some creeks in
the humid section would be called rivers if they occurred
in the arid portion.
Crest :
(1)The highest stage or level of a flood wave as it passes
a point. (2)The top of a dam, dike, spillway, or weir,
to which water must rise before passing over the structure.
Crest Gage :
A gage used to obtain a record of flood crests at sites
where recording gages are installed.
Crest (Top) of Dam :
The elevation of the uppermost surface of a dam excluding
any parapet walls, railings, etc.
Crest Width (Top thickness) :
The thickness or width of a dam at the level of the crest
(top) of the dam. The term "thickness" is used
for gravity and arch dams and "width" for other
types of dams.
Critical Depth :
The depth of water flowing in an open channel or conduit,
partially filled, corresponding to one of the recognized
critical velocities.
Critical Flow :
A condition of flow where the mean velocity is at one
of the critical values; ordinarily at Belanger's critical
depth and velocity. Another important usage is in reference
to the Reynolds' critical velocities which define the
point at which the flow changes from streamline or nonturbulent
to turbulent flow.
Critical Rainfall Probability (CRP) :
The Probability that the actual precipitation during a
rainfall event has exceeded or will exceed the flash flood
guidance value.
Cross-sectional area :
Area perpendicular to the direction of flow.
CRP :
Critical Rainfall Probability. The Probability that a
given rainfall will cause a river, or stream to rise above
flood stage.
Cryology :
The science of the physical aspects of snow, ice, hail,
and sleet and other forms of water produced by temperatures
below Zero degrees Celsius.
Cubic Feet Per Second :
A unit expressing rates of discharge. One cubic foot per
second is equal to the discharge through a rectangular
cross section, 1 foot wide by 1 foot deep, flowing at
an average velocity of 1 foot per second. It is also approximately
7.48 gallons per second.
Curtain Drain :
A drain constructed at the upper end of the area to be
drained, to intercept surface or ground water flowing
toward the protected area from higher ground, and carry
it away from the area. Also called an Intercepting Drain.
Current meter :
Device used to measure the water velocity or current in
a river.
Cuttoff :
from passing through a dam's foundation material.An impervious
construction or material which reduces seepage or prevents
it.
C
CADAS (Centralized Automated Data Acquisition System)
:
A system of two minicomputers in NWSH that interrogates
LARCs and DARDCs by telephone every 6 hours and transmits
the data to AFOS via HADS.
Calibration :
The process of using historical data to estimate parameters
in a hydrologic forecast technique such as SACSMA, routings,
and unit hydrographs.
Capillarity :
(1)The degree to which a material or object containing
minute openings or passages, when immersed in a liquid,
will draw the surface of the liquid above the hydrostatic
level. Unless otherwise defined, the liquid is generally
assumed to be water. (2)The phenomenon by which water
is held in interstices above the normal hydrostatic level,
due to attraction between water molecules.
Capillary Fringe :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
zone.
Capillary Potential :
The work required to move a unit mass of water from the
reference plane to any point in the soil column.
Capillary Zone :
The soil area just above the water table where water can
rise up slightly through the cohesive force of capillary
action. This layer ranges in depth from a couple of inches,
to a few feet, and it depends on the pore sizes of the
materials. The capillary zone is also called the capillary
fringe.
Catchment Area :
An area having a common outlet for its surface runoff
(also see Drainage Area or Basin, Watershed).
Caution Stage :
Same as alert stage.
CFS (Cubic Feet per Second) :
The flow rate or discharge equal to one cubic foot (of
water, usually) per second. This rate is equivalent to
approximately 7.48 gallons per second. This is also referred
to as a second-foot.
Cfs-Day :
The volume of water discharged in twenty four hours, with
a flow of one cubic foot per second is widely used; 1
cfs-day is 24 x 60 x 60 = 86,000 cubic feet, 1.983471
acre-feet, or 646,317 gallons. The average flow in cubic
feet per second for any time period is the volume of flow
in cfs-days.
Channel (watercourse) :
An open conduit either naturally or artificially created
which periodically, or continuously contains moving water,
or forms a connecting link between two bodies of water.
River, creek, run, branch, anabranch, and tributary are
some of the terms used to describe natural channels. Natural
channels may be single or braided. Canal and floodway
are some of the terms used to describe artificial channels.
Channel Inflow :
Water, which at any instant, is flowing into the channel
system form surface flow, subsurface flow, base flow,
and rainfall that has directly fallen onto the channel.
Channel Lead :
An elongated opening in the ice cover caused by a water
current.
Channel Routing :
The process of determining progressively timing and shape
of the flood wave at successive points along a river.
Channelization :
The modification of a natural river channel; may include
deepening, widening, or straightening.
Closed Basin :
A basin draining to some depression or pond within its
area, from which water is lost only by evaporation or
percolation. A basin without a surface outlet for precipitation
falling precipitation.
Closed Basin Lake Flooding :
Flooding that occurs on lakes with either no outlet or
a relatively small one. Seasonal increases in rainfall
cause the lake level to rise faster than it can drain.
The water may stay at flood stage for weeks, months, or
years.
CNIF :
Calibration Network Information Files
Coastal Flooding :
Flooding that occurs from storms where water is driven
onto land from an adjacent body of water. These can be
hurricanes, "nor'easters," or tropical storms,
but even a severe winter storm or thunderstorm can cause
this type of flooding.
COE :
Corps of Engineers
Columnar Ice :
Ice consisting of columnar shaped grain. The ordinary
black ice is usually columnar-grained.
Composite Hydrograph :
A stream discharge hydrograph which includes base flow,
or one which corresponds to a net rain storm of duration
longer than one unit period.
Cone of Depression :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with the rate of withdrawal of water. Also
called the Cone of Influence.
Cone of Influence :
The depression, roughly conical in shape, produced in
a water table, or other piezometric surface, by the extraction
of water from a well at a given rate. The volume of the
cone will vary with rate of withdrawal of water. Also
called the Cone of Depression.
Confined Ground Water :
Ground water held under an aquiclude or an aquifuge called
artesian if the pressure is positive.
Congressional Organic Act of 1890 :
The act that assigned the responsibility of river and
floor forecasting for the benefit of the general welfare
of the Nation's people and economy to the Weather Bureau,
and subsequently the National Weather Service.
Conservation Storage :
Storage of water for later release for usual purposes
such as municipal water supply, power, or irrigation in
contrast with storage capacity used for flood control..
Consolidated Ice Cover :
Ice cover formed by the packing and freezing together
of floes, brash ice and other forms of floating ice.
Contents :
The volume of water in a reservoir. Unless otherwise indicated
reservoir content is computed on the basis of a level
pool and does not include bank storage.
Control Points: Horizontal and Vertical :
Small monuments securely embedded in the surface of the
dam. Any movement of the monument indicates a movement
in the dam itself. Movements in the dam are detected by
comparing control points location to location of fixed
monuments located off the dam using accurate survey techniques.
Cooperative Observer :
An individual (or institution) who takes precipitation
and temperature observations-and in some cases other observations
such as river stage, soil temperature, and evaporation-at
or near their home, or place of business. Many observers
transmit their reports by touch-tone telephone to an NWS
computer, and nearly all observers mail monthly reports
to the National Climatic Data Center to be archived and
published.
Conveyance Loss :
The loss of water from a conduit due to leakage, seepage,
evaporation, or evapo-transpiration.
Corn Snow Ice :
Rotten granular ice.
County Warning Forecast Area (CWFA) :
The area (aggregation of counties, or parishes and sometimes
portions of counties, or parishes) served by a WFO for
which weather forecast and warning services are provided.
Crack :
A separation formed in an ice cover of floe that does
not divide it into two or more pieces.
Creek :
A small stream of water which serves as the natural drainage
course for a drainage basin of nominal, or small size.
The term is a relative one as to size, some creeks in
the humid section would be called rivers if they occurred
in the arid portion.
Crest :
(1)The highest stage or level of a flood wave as it passes
a point. (2)The top of a dam, dike, spillway, or weir,
to which water must rise before passing over the structure.
Crest Gage :
A gage used to obtain a record of flood crests at sites
where recording gages are installed.
Crest (Top) of Dam :
The elevation of the uppermost surface of a dam excluding
any parapet walls, railings, etc.
Crest Width (Top thickness) :
The thickness or width of a dam at the level of the crest
(top) of the dam. The term "thickness" is used
for gravity and arch dams and "width" for other
types of dams.
Critical Depth :
The depth of water flowing in an open channel or conduit,
partially filled, corresponding to one of the recognized
critical velocities.
Critical Flow :
A condition of flow where the mean velocity is at one
of the critical values; ordinarily at Belanger's critical
depth and velocity. Another important usage is in reference
to the Reynolds' critical velocities which define the
point at which the flow changes from streamline or nonturbulent
to turbulent flow.
Critical Rainfall Probability (CRP) :
The Probability that the actual precipitation during a
rainfall event has exceeded or will exceed the flash flood
guidance value.
Cross-sectional area :
Area perpendicular to the direction of flow.
CRP :
Critical Rainfall Probability. The Probability that a
given rainfall will cause a river, or stream to rise above
flood stage.
Cryology :
The science of the physical aspects of snow, ice, hail,
and sleet and other forms of water produced by temperatures
below Zero degrees Celsius.
Cubic Feet Per Second :
A unit expressing rates of discharge. One cubic foot per
second is equal to the discharge through a rectangular
cross section, 1 foot wide by 1 foot deep, flowing at
an average velocity of 1 foot per second. It is also approximately
7.48 gallons per second.
Curtain Drain :
A drain constructed at the upper end of the area to be
drained, to intercept surface or ground water flowing
toward the protected area from higher ground, and carry
it away from the area. Also called an Intercepting Drain.
Current meter :
Device used to measure the water velocity or current in
a river.
Cuttoff :
from passing through a dam's foundation material.An impervious
construction or material which reduces seepage or prevents
it.
D
Daily Flood Peak:
The maximum mean daily discharge occuring in a stream
during a given flood event.
Dam :
Any artificial barrier which impounds or diverts water.
The dam is generally hydrologically significant if it
is:
1. 25 feet or more in height from the natural bed of the
stream and has a storage of at least 15 acre-feet.
2. Or has an impounding capacity of 50 acre-feet or more
and is at least six feet above the natural bed of the
stream.
DAMBRK :
The Dam Break Forecasting Model.
Dam Failure :
Catastrophic event characterized by the sudden, rapid,
and uncontrolled release of impounded water.
DAPM :
The Data Acquisition Program Manager.
DATACOL :
The Software System that supports RFC gateway functions.
DATANET :
Hydrologic Data Network Analysis Software.
Day-Second Feet :
Often abbreviated as DSF. See Second-Day Feet.
DCP (Data Collection Platform) :
An electronic device that connects to a river or rainfall
gage that records data from the gage and at pre-determined
times transmits that data through a satellite to a remote
computer.
DDS :
Data Distribution System.
Dead Storage:
The volume in a reservoir below the lowest controllable
level.
Deep Percolation Loss:
Water that percolates downward through the soil beyond
the reach of plant roots.
Deep Seepage:
Infiltration which reaches the water table.
Deep well:
A well whosepumping head is too great to permit use of
a suction pump.
Deformed Ice :
A general term for ice which has been squeezed together
and forced upwards and downwards in places. Subdivisions
are rated ice, ridge ice, hummocked ice, and other similar
deformations.
Degradation :
The geologic process by means of which various parts of
the surface of the earth are worn down and carried away
and their general level lowered, by the action of wind
and water.
Delta :
An alluvial deposit, often in the shape of the Greek letter
"delta", which is formed where a stream drops
its debris load on entering a body of quieter water.
Dendrites :
Thin branch-like growth of ice on the water surface.
Dendritic :
The form of the drainage pattern of a stream and it's
tributaries when it follows a treelike shape, with the
main trunk, branches, and twigs corresponding to the main
stream, tributaries, and subtributaries, respectively,
of the stream.
Density Current:
A flow of water maintained by gravity through a large
body of water, such as a reservoir or lake, and retaining
its unmixed identity because of a difference in density.
Density of Snow:
The ratio, expressed as a percentage, of the volume which
a given quantity of snow would occupy if it were reduced
to water, to the volume of the snow. When a snow sampler
is used, it is the ratio expressed as percentage of the
scale reading on the sampler to the length of the snow
core or sample.
Depletion Curve:
That part of the hydrograph extending from the point of
termination of the Recession Curve to the subsequent rise
or alternation of inflow due to additional water becomming
available for stream flow.
Depression Storage:
The volume of water contained in natural depressions in
the land surface, such as puddles.
Depth of Runoff:
The total runoff from a drainage basin, divided by its
area. For convenience in comparing runoff with precipitation,
the term is usually expressed in inches of depth during
a given period of time over the drainage area or acre-feet
per square mile.
Design Criteria :
The hypothetical flood used in the sizing of the dam and
the associated structures to prevent dam failure by overtopping,
especially for the spillway and outlet works.
Detention Basins :
Structures which are built upstream from a populated area
so that precipitation flows do not flood and cause the
loss of life or property. They are normally dry, but are
designed to detain surface water temporarily during, and
immediately after a runoff event. Their primary function
is to attenuate the storm flows by releasing flows at
a lower flow rate. There are no gates or valves allowed
on the outlet so that water can never be stored on a long-term
basis. Typical detention times in such a basin would be
on the order of 24 to 72 hours although some are as long
as 5 to 10 days.
Detention Storage:
The volume of water, other than depression storage, existing
on the land surface as flowing water which has not yet
reached the channel.
Detritus :
(1) the heavier mineral debris moved by natural watercourses,
usually in bed-load form. (2) the sand, grit, and other
coarse material removed by differential sedimentation
in a relatively short period of detention.
Diffuse Ice :
Poorly defined ice edge limiting an area of dispersed
ice; usually on the leeward side of an area of floating
ice.
Direct Flood Damage:
The damage done to property, structures, goods, etc.,
by a flood as measured by the cost of replacement and
repairs.
Direct Runoff:
The runoff entering stream channels promptly after rainfall
or snowmelt. Superposed on base runoff, it forms the bulk
of the hydrograph of a flood.
Discharge :
The rate at which water passes a given point. Discharge
is expressed in a volume per time with units of L3/T.
Discharge is often used interchangeably with streamflow.
Discharge Curve:
A curve that expresses the relation between the discharge
of a stream or open conduit at a given location and the
stage or elevation of the liquid surface at or near that
location. Also called Rating Curve and Discharge Rating
Curve.
Discharge Table:
(1) A table showing the relation between two mutually
dependant quantities or variable over a given range of
magnitude. (2) A table showing the relation between the
gage height and the discharge of a stream or conduit at
a given gaging station. Also called a Rating Table.
Distribution (Hydro)Graph:
A unit hydrograph of direct runoff modified to show the
proportions of the volume of runoff that occur during
successive equal units of time.
Diversion :
The taking of water from a stream or other body of water
into a canal, pipe, or other conduit.
Divide :
The high ground that forms the boundary of a watershed.
A divide is also called a ridge.
DNR :
Department of Natural Resources.
DOH :
Development and Operations Hydrologist.
Domestic Consumption:
The quantity, or quantity per capita, of water consumed
in a municipality or district for domestic useses or purposes
during a given period, generally one day. It is usually
taken to include all uses included withinthe term Municipal
Use of Water and quantity wasted, lost, or otherwise unaccounted
for.
Domestic Use of water:
The use of water primarily for household purposes, the
watering of livestock, the irrigation of gardens, lawns,
shrubbery, etc., surrounding a house or domicile.
Downstream Slope :
The slope or face of the dam away from the reservoir water.
This slope requires some kind of protection (e.g.; grass)
from the erosive effects of rain and surface flow.
Drainage Area :
An area having a common outlet for its surface runoff
(also see Watershed and Catchment Area).
Drainage Basin:
A part of the surface of the earth that is occupied by
a drainage system, which consists of a surface stream
or a body of impounded surface water together with all
tributary surface streams and bodies of impounded surface
water..
Drainage Density:
The relative density of natural drainage channels in a
given area. It is usually expressed in terms of miles
of natural drainage or stream channel per square mile
of area, and obtained by dividing the total length of
stream channels in the area in miles by the area in square
miles.
Drainage Divide:
The boundary line, along a topographic ridge or along
a subsurface formation, separating two adjacent drainage
basins.
Drains (Relief Wells) :
A vertical well or borehole, usually downstream of impervious
cores, grout curtains or cutoffs, designed to collect
and direct seepage through or under a dam to reduce uplift
pressure under or within a dam. A line of such wells forms
a "drainage curtain".
Drawdown :
The lowering of the surface elevation of a body of water,
the water surface of a well, the water table, or the piezometric
surface adjacent to the well, resulting from the withdrawl
of water therefrom..
Dredging :
The scooping, or suction of underwater material from a
harbor, or waterway. Dredging is one form of channel modification.
It is often too expensive to be practical because the
dredged material must be disposed of somewhere and the
stream will usually fill back up with sediment in a few
years. Dredging is usually undertaken only on large rivers
to maintain a navigation channel.
Drifting Ice :
Pieces of floating ice moving under the action of wind
and/ or currents.
Drought :
A period of abnormally dry weather sufficiently prolonged
from the lack of precipitation to cause a serious hydrologic
imbalance.
Drought Index :
Computed value which is related to some of the cumulative
effects of a prolonged and abnormal moisture deficiency.
(An index of hydrological drought corresponding to levels
below the mean in streams, lakes, and reservoirs.)
Dry Crack :
Crack visible at the surface but not going right through
the ice cover, and therefore it is dry.
Dry Floodproofing :
A dry floodproofed building is sealed against floodwaters.
All areas below the flood protection level are made watertight.
Walls are coated with waterproofing compounds or plastic
sheeting. Openings like doors windows, sewer lines and
vents are closed, wether permanently, with removable shields,
or with sandbags. The flood protection level should be
no more than 2 or 3 feet above the top of the foundation
because the buildings walls and floors cannot withstand
the pressure of deeper water.
Dry Weather Flow :
Streamflow which results from precipitation that infiltrates
into the soil and eventually moves through the soil to
the stream channel. This is also referred to as baseflow,
or ground water flow.
Duration Curve:
A cumulative frequency curve that shows the percent of
time during which specified units of items (e.g. discharge,
head, power,etc.) were equaled or exceeded in a given
period. It is the integral of the frequency diagram.
Duration of Ice Cover :
The time from freeze-up to break-up of an ice cover.
Dynamic Ice :
Pressure due to a moving ice cover or drifting ice. Pressure
occuring at movement of first contact termed Ice Impact
Pressure
Dynamic Wave Routing Model (DWOPER) :
A computerized hydraulic routing program whose algorithms
incorporate the complete one-dimensional equations of
unsteady flow originally developed by Barre' De Saint-Venant
in 1871.
E
E-3, Flood Stage Report :
A form that a Service Hydrologist/ Hydrology Focal Point
completes to document the dates in which forecast points
are above flood stage, as well as the crest dates and
stages. Discussion of the flood event must also be included
in the E-5, Monthly Report of River and Flood conditions.
An E-3 report is sent to Regional Headquarters, the appropriate
RFC, as well as the Office of Hydrology (OH).
E-5, Monthly Report of River and Flood conditions :
A monthly narrative report covering flooding which occurred
over the past month. Flood stage, flood crest and dates
in which flooding occurred is covered within this report
for each data point which was in flood. If the flooding
involved a forecast point, an E-3 must be filled out as
well. If no flooding has occurred within the past month,
a climatic summary of the past month can be included as
well as other interesting non-flood events, such as water
supply, ice jams and the occurrence of drought. An E-5
report is sent to Regional Headquarters, the appropriate
RFC, as well as the Office of Hydrology (OH).
E-7, Flood Damage Report :
A report to be completed anytime there is reported flood
damage or loss of life as a direct result of flooding.
An E-7 report is sent to Regional Headquarters, as well
as the Office of Hydrology (OH).
E-19, Report on River Gage Station :
A report to be completed every 5 years providing a complete
history of a river station and all gages that have been
used for public forecasts since the establishment of the
station.
E-19a, Abridged Report on River Gage Station :
An abridged version of an E-19, an E-19a updates the E-19
as additional information, or changes occur at the station
during the intervening five year period. An E-19a is to
be completed anytime a significant change occurs at a
forecast point. An E-19a is also used to take the place
of an E-19 in documenting any gage history, or information
of any non-forecast point (i.e; data point).
Earthen (or Earthfill) Dam:
An embankment dam in which more than 50% of the total
volume is formed of compacted fine-grained material. A
homogeneous earthen dam is constructed of similar earthen
material throughout. These are the most common type of
dam because their construction involves using materials
in the natural state, requiring little processing.
Effective Porosity:
The ratio, usually expressed as a percentage, of the volume
of water or other liquid which a given saturated volume
of rock or soil will yield under any specified hydraulic
condition, to the given volume of soil or rock.
Effective Precipitation (Rainfall):
(1) That part of the precipitation that produces runoff.
(2) A weighted average of current and antecedent precipitation
that is "effective" in correlating with runoff.
(3) That part of the precipitation falling on an irrigated
area that is effective in meeting the consumptive use
requirements.
Effluent Seepage:
Diffuse discharge of ground water to the ground surface.
Effluent Stream :
Any watercourse in which all, or a portion of the water
volume came from the Phreatic zone, or zone of saturation
by way of groundwater flow, or baseflow.
EIF :
Enhanced IFLOWS Format.
Embankment:
Fill material, usually earth or rock, placed with sloping
sides and usually with length greater than height. All
dams are types of embankments.
Emergency Action Plan :
A predetermined plan of action to be taken to reduce the
potential for property damage and loss of life in an area
affected by a dam break or excessive spillway.
Emergency Services :
Services provided in order to minimize the impact of a
flood that is already happening. These measures are the
responsibility of city, or county emergency management
staff and the owners or operators of major, or critical
facilities. Some examples of emergency services are flood
warning and evacuation, flood response, and post flood
activities.
Energy Dissipator :
A structure which slows fast-moving spillway flows in
order to prevent erosion of the stream channel.
Engineer's Level :
A telescope which is attached to a spirit-tube level,
all revolving around a vertical axis and is mounted on
a tripod. An Engineer's Level is used for determining
the difference in elevation between two points. The telescope
on the level has a vertical cross hair and a horizontal
cross hair. Once the instrument is leveled, the sighting
through the horizontal cross hair represent a horizontal
plane of equal elevation.
Ensemble Hydrologic Forecasting :
A process whereby a continuous hydrologic model is successively
executed several times for the same forecast period by
use of varied data input scenarios, or a perturbation
of a key variable state for each model run. A common method
employed to obtain a varied data input scenario is to
use the historical meteorological record, with the assumption
that several years of observed data covering the time
period beginning on the current date and extending through
the forecast period comprises a reasonable estimate of
the possible range of future conditions.
Equilibrium Drawdown:
The ultimate, constant drawdown for a steady rate of pumped
discharge.
Equilibrium Surface Discharge:
The steady rate of surface discharge which results from
a long-continued, steady rate of net rainfall, with discharge
rate equal to net rainfall rate.
Equilibrium Time:
The time when flow conditions become substantially equal
to those corresponding to equilibrium discharge or equilibrium
drawdown..
Equi-Potential Line:
A line, in a field of flow, such that the total head is
the same for all points on th line, and therefore the
direction of flow is perpendicular to the line at all
points.
Erosion :
Wearing away of the lands by running water, glaciers,winds,
and waves, can be subdivided into three process: Corrasion,
Corrosion, and Transportation. Weathering, although sometimes
included here, is a distant process which does not imply
removal of any material.
ESP :
Extended Streamflow Prediction.
ESPINIT :
ESP Initialization Program
Esturine waters :
Deepwater tidal habitats and tidal wetlands that are usually
enclosed by land but have access to the ocean and are
at least occasionally diluted by freshwater runoff from
the land (such as bays, mouths of rivers, salt marshes,
lagoons).
Esturine Zone :
The area near the coastline that consists of esturaries
and coastal saltwater wetlands.
Estuary :
The thin zone along a coastline where freshwater systems
and rivers meet and mix with a salty ocean (such as a
bay, mouth of a river, salt marsh, lagoon).
Evaporimeter :
An instrument which measures the evaporation rate of water
into the atmosphere.
Evaporation :
Process by which liquid water is converted into water
vapor.
Evaporation Pan:
A pan used to hold water during observations for the determination
of the quantity of evaporation at a given location. Such
pans are of varying sizes and shapes, the most commonly
used being circular or square.
Evaporation Rate:
The quantity of water, expressed in terms of depth of
liquid water, which is evaporated from a given surface
per unit of time. It is usually expressed in inches depth,
per day, month, or year.
Evapotranspiration :
Combination of evaporation from free water surfaces and
transpiration of water from plant surfaces to the atmosphere.
Excess Rain:
Effective rainfall in excess of infiltration capacity.
Exclusive Flood Control Storage Capacity :
The space in a reservoir reserved for the sole purpose
of regulating flood inflows to abate flood damage.
F
Face :
The external surface of a structure, such as the surface
of a dam.
FCEXEC :
A component of the NWSRFS FCST Program.
FCST :
NWSRFS Forecast Program to produce operational forecasts
Federal Emergency Management Agency (FEMA) :
An agency of the federal government having responsibilities
in hazzard mitigation; FEMA also administers the National
Flood Insurance Program.
Federal Snow Sampler:
A snow sampler consisting of five or more sections of
sampling tubes, one which has a steel cutter on the end.
The combined snowpack measuring depth is 150 inches. This
instrument was formerly the Mount Rose Type Snow Sampling
Set.
Fetch :
The effective distance whcih waves have travelsed in open
water, from their point of origin to the point where they
break. (2) The distance of the water or the homogenous
type surface over which the wind blows without appreciable
change in direction.
Field (Moisture) Capacity :
The amount of water held in soil against the pull of gravity
Field Moisture Deficiency:
The quantity of water, which would be required to restore
the soil moisture to field moisture capacity.
Fill Dam :
Any dam constructed of excavated natural materials or
of industrial wastes.
Firn (Snow):
Old snow on top of glaciers, granular and compact and
not yet converted into ice. It is a transitional stage
between snow and ice. Also called Neve.
Firn Line:
The highest level to which the fresh snow on a glacier's
surface retreats during the melting season. The line separating
the accumulation area from the ablation area.
Fischer & Porter Punched Tape Recorder Gage:
A precipitation gage which converts weight into a code
disk position. The code disk position is then punched
on paper tape in a binary decimal format suitable for
automatic machine processing.
Flash Flood :
A flood which follows within a few hours (usually less
than 6 hours) of heavy or excessive rainfall, dam or levee
failure, or the sudden release of water impounded by an
ice jam.
Flash Flood Guidance (FFG) :
An internal product produced by the RFC's containing rainfall
threshold values which must be exceeded in order to produce
a flash flood.
Flash Flood Statement (FFS) :
A statement by the NWS which provides follow-up information
on flash flood watches and warnings.
Flash Flood Table :
A table of pre-computed forecast crest stage values for
small streams for a variety of antecedent moisture conditions
and rain amounts. Soil moisture conditions are often represented
by flash flood guidance values. In lieu of crest stages,
categorical representations of flooding, e.g., minor,
moderate, etc. may be used on the tables.
Flash Flood Warning (FFW) :
A warning by the NWS issued to warn of flash flooding
that is imminent or occurring.
Flash Flood Watch (FFA) :
A statement by the NWS that alerts communities to the
possibility of flash flooding in specified areas.
Flashboards :
A length of timber, concrete, or steel placed on the crest
of a spillway to raise the retention water level but which
may be quickly removed in the event of a flood by a tripping
device, or by deliberately designed failure of the flashboard
or its supports.
Float Recording Precipitation gage :
A rain gage where the rise of a float within the instrument
with increasing rainfall is recorded. Some of these gages
must be emptied manually, while others employ a self-starting
siphon to empty old rainfall amounts.
Floc :
A cluster of frazil particles
Floe :
An accumulation of frazil flocs (also known as a "pan")
or a single piece of broken ice.
Flood :
The inundation of a normally dry area caused by high flow,
or overflow of water in an established watercourse, such
as a river, stream, or drainage ditch ; or ponding of
water at or near the point where the rain fell. This is
a duration type event with a slower onset than flash flooding,
normally greater than 6 hours.
Flood Control Storage:
Storage of water in reservoirs to abate flood damage.
Flood Crest :
The Maximum height of a flood wave as it pases a location.
Flood Frequency Curve:
(1) A graph showing the number of times per year on the
average, plotted as abscissa, that floods of magnitude,
indicated by the ordinate, are equaled or exceeded. (2)
A similar graph but with recurrence intervals of floods
plotted as abscissa.
Flood Loss Reduction Measures :
The strategy for reducing flood losses. There are four
basic strategies. They are prevention, property protection,
emergency services, and structural projects. Each strategy
incorporates different measures that are appropriate for
different conditions. In many communities, a different
person may be responsible for each strategy.
Flood of Record :
The highest observed river stage or discharge at a given
location during the period of record keeping. (Not necessarily
the highest known stage.)
Flood Plain :
The portion of a river valley that has been inundated
by the river during historic floods.
Flood Plain Information Studies :
Reports usually prepared by the U.S. Army Corps of Engineers
(USACE) following a survey of a flood-impacted community.
Flood Potential Outlook (ESF on AFOS) (FPO for Acronym)
:
An NWS outlook that is issued to alert the public of potentially
heavy rainfall that could send area rivers and streams
into flood or aggravate an existing flood.
Flood Prevention :
Measures that are taken in order to keep flood problems
from getting worse. Planning, land acquisition, river
channel maintenance, wetlands protection, and other regulations
all help modify development on floodplains and watersheds
to reduce their susceptibility to flood damage. Preventive
measures are usually administered by the building, zoning,
planning and/ or code enforcement offices of the local
government.
Flood Problems :
Problems and damages that occur during a flood as a result
of human development and actions. Flood problems are a
result from: 1) Inappropriate development in the floodplain
(e.g., building too low, too close to the channel, or
blocking flood flows); 2) Development in the watershed
that increases flood flows and creates a larger floodplain,
or; 3) A combination of the previous two.
Flood Profile:
A graph of elevation of the water surface of a river in
flood, plotted as ordinate, against distance, measured
in the downstream direction, plotted as abscissa. A flood
profile may be drawn to show elevation at a given time,
crests during a particular flood, or to show stages of
concordant flows.
Flood Routing :
Process of determining progressively the timing, shape,
and amplitude of a flood wave as it moves downstream to
successive points along the river.
Flood Stage :
A gage height at which a watercouse overtops its banks
and begins to cause damage to any portion of the defined
reach. Flood stage is usually higher than or equal to
bankful stage.
Flood Statement (FLS) :
A statement issued by the NWS to inform the public of
flooding along major streams in which there is not a serious
threat to life or property. It may also follow a flood
warning to give later information.
Flood Warning (FLW) :
A release by the NWS to inform the public of flooding
along larger streams in which there is a serious threat
to life or property. A flood warning will usually contain
river stage (level) forecasts.
Flood Wave :
A rise in streamflow to a crest and its subsequent recession
caused by precipitation, snowmelt, dam failure, or reservoir
releases.
Flooded Ice :
Ice which has been flooded by melt water or river water
and is heavily loaded by water and wet snow.
Floodproofing :
The process of protecting a building from flood damage
on site. Floodproofing can be divided into wet and dry
floodproofing. In areas subject to slow-moving, shallow
flooding, buildings can be elevated, or barriers can be
constructed to block the water's approach to the building.
These techniques have the advantage of being less disruptive
to the neighborhood. It must be noted that during a flood,
a floodproofed building may be isolated and without utilities
and therefore unusable, even though it has not been damaged.
Floodwall :
A long, narrow concrete, or masonry embankment usually
built to protect land from flooding. If built of earth
the structure is usually referred to as a levee. Floodwalls
and levees confine streamflow within a specified area
to prevent flooding. The term "dike" is used
to describe an embankment that blocks an area on a reservoir
or lake rim that is lower than the top of the dam.
Floodway :
(1) A part of the flood plain, otherwise leveed, reserved
for emergency diversion of water during floods. A part
of the flood plain which, to facilitate the passage of
floodwater, is kept clear of encumbrances.(2) The channel
of a river or stream and those parts of the flood plains
adjoining the channel, which are reasonably required to
carry and discharge the floodwater or floodflow of any
river or stream.
Flow Duration Curve:
A cumulative frequency curve that shows the percentage
of time that specified discharges are equaled or exceeded..
Flowing Well :
a well drilled into a confined aquifer with enough hydraulic
pressure for the water to flow to the surface without
pumping. Also called an Artesian well.
Forebay :
The water behind (upstream) of the dam.
Forecast Crest :
The highest elevation of river level, or stage, expected
during a specified storm event.
Forecast Point :
A location that represents an area (reach of a river),
where a forecast is made available to the public. Each
NWS river forecast point has an associated E-19a, Abridged
Report on River Gage Station, and E-19, Report on River
Gage Station.
Foresight :
A sighting on a point of unknown elevation from an instrument
of known elevation. To determine the elevation of the
point in question, the foresight is subtracted from the
height of the instrument.
Fountainhead:
The upper end of a confined-aquifer conduit, where it
initersects the land surfcae.
Fracture :
Any break or rupture formed in an ice cover or floe due
to deformation.
Fracture Zone :
An area which has a great number of fractures.
Fracturing :
Deformation process whereby ice is permanently deformed,
and fracture occurs.
Frazil Ice :
Fine spicules, plates, or discoids of ice suspended in
water. In rivers and lakes, frazil is formed in supercooled,
turbulent water.
Frazil Slush :
An agglomerate of loosely packed frazil which floats or
accumulates under the ice cover.
Free Ground Water:
Unconfined ground water whose upper boundary is a free
water table.
Freeboard:
The vertical distance between the normal maximum level
of the water surface in a channel, resrvoir, tank, canal,
etc., and the top of the sides of a levee, dam, etc.,
which is provided so that waves and other movements of
the liquid will not overtop the confining structure.
Freezeup date :
Date on which the water body was first observed to be
completely frozen over.
Freezup jam :
Ice jam formed as frazil ice accumulates and thickens.
French Drain:
An underground passageway for water through the interstices
among stones placed loosely in a trench.
Frequency Curve:
A curve that expresses the relation between the frequency
distribution plot, with the magnitude of the variables
as abscissas and the number of occurrences of each magnitude
in a given period as ordinates. The theoretical frequency
curve is a derivative of the probability curve.
Friction Head:
The decrease in total head caused by friction.
G
Gage :
(1) A device for indicating the magnitude or position
of a thing in specific units, when such magnitude or position
undergoes change, for example: The elevation of a water
surface, the velocity of flowing water, the pressure of
water, the amount or intensity of precipitation, the depth
of snowfall, etc. (2) The act or operation of registering
or measuring the magnitude or position of a thing when
these characteristics are undergoing change. (3) The operation,
including both field and office work, of measuring the
discharge of a stream of water in a waterway.
Gage Datum :
The arbitrary zero datum elevation which all stage measurements
are made from.
Gage Height :
The water-surface elevation referred to some arbitrary
gage datum. Gage height is often used interchangeably
with the more general term stage, although gage height
is more appropriate when used with a reading on a gage.
Gage Zero :
The elevation of zero stage. (Same as gage datum.)
Gaging Station :
A particular site on a watercourse where systematic observations
of stage/ and or flow are measured.
Gallery :
A passageway within the body of a dam or abutment.
Gate :
A device in which a leaf or member is moved across the
waterway from an external position to control or stop
flow. There are many different kinds of gates used on
a dam. Some include:
Bulkhead, Crest (or Spillway), Emergency, Fixed Wheel,
Flap, Flood, Guard, Outlet, Radial, Regulating, and Slide
Gates.
Geohydrology:
That branch of hydrology relating to subsurface, or subterranean
waters.
Geophysics:
The study of the physical characteristics and properties
of the earth; including geodesy, seismology, meteorology,
oceanography, atmospheric electricity, terrestrial magnetism,
and tidal phenomena.
Glacier:
Bodies of land ice that consist of recrystallized snow
accumulated on the surface of the ground, and that move
slowly downslope.
Glacier Dammed Lake:
The lake formed when a glacier flows across the mouth
of an adjoining valley and forms an ice dam.
GOES (Geostationary Orbiting Environmental Satellite):
Satellite orbiting at 22,000 miles above the earth surface
that remains above the same location on the earth. DCPs
transmit river and rainfall data to the GOES for relay
to a ground-receive site in Wallops Island, VA.
GOES DCS (Data Collection System):
A data collection system under NESDIS which is comprised
of the DCPs, and the NESDIS Command and Data Acquisition
(CDA) System components. This satellite-based system collects
a variety of environmental data from locations in the
Western Hemisphere. The system is a data relay network
for more than 10,000 DCPs which transmits data to one
of two GOES satellites (East and West). These data are
relayed to the NESDIS CDA ground station located at Wallops
Island, VA. The data are then relayed over to Silver Springs,
MD, where the data is then distributed to the appropriate
recipients.
Gravity Dam :
A concrete structure proportioned so that its own weight
provides the major resistance to the forces exerted on
it.
Ground receive sites:
A satellite dish and associated computer which receives
signals from the GOES satellite, decodes the information,
and transmits it to a another site for further processing.
The GOES satellite ground-receive site is located at Wallops
Island, VA; and the information is relayed to a mainframe
computer at NWSH for processing.
Ground Water:
Water within the earth that supplies wells and springs;
water in the zone of saturation where all openings in
rocks and soil are filled, the upper surface of which
forms the water table. Also termed Phreatic water.
Grounded ice:
Ice that has run aground or is contact with the ground
underneath it.
Ground Water Divide:
A line on a water table where on either side of which
the water table slopes downward. It is analogous to a
drainage divide between two drainage basins on a land
surface.
Ground Water Hydrology:
The branch of hydrology that specializes in ground water;
its occurrence and movements; its replenishment and depletion;
the properties of rocks that control ground water movement
and storage; and the methods of investigation and utilization
of ground water
Ground Water Flow:
Streamflow which results from precipitation that infiltrates
into the soil and eventually moves through the soil to
the stream channel. This is also referred to as baseflow,
or dry-weather flow.
Ground Water Mining:
Pumping ground water from a basin where the safe yield
is very small, thereby extracting ground water which had
accumulated over a long period of time.
Ground Water Outflow:
That part of the discharge from a drainage basin that
occurs through the ground water. The term "underflow"
is often used to describe the ground water outflow that
takes place in valley alluvium (instead of the surface
channel) and thus is not measure at a gaging station.
Ground Water Overdraft:
Pumpage of ground water in excess of safe yield.
Ground Water Runoff:
That part of the runoff which has passed into the ground,
has become ground water, and has been discharged into
a stream channel as spring, or seepage water.
Grout Curtain :
A barrier produced by injecting grout into a vertical
zone, usually narrow (horizontally), and in the foundation
to reduce seepage under a dam.
H
HADS (Hydrometeorological Automated Data System):
Software that replaced GDDS to process and distribute
the GOES DCP data and CADAS data collected from DCP's
and LARCS.
Hanging (ice) dam:
A mass of ice composed mainly of frazil or broken ice
deposited underneath an ice cover in a region of low flow
velocity.
HDRAIN:
An Hourly Digital Rainfall Product of the WSR-88D.
Head:
The difference between the pool height and tailwater height.
Usually expressed in feet of head, or in lbs./sq. inch
Head Loss:
The decrease in total head caused by friction.
Head Race:
A channel which directs water to a water wheel; a forebay.
Headward Erosion:
Erosion which occurs in the upstream end of the valley
of a stream, causing it to lengthen its course in such
a direction.
Headwaters:
Streams at the source of a river.
Headwater Advisory Program (ADVIS) :
A Program which uses the Antecedent Precipitation Index
(API) method of estimating runoff, unit hydrograph theory
and stage-discharge ratings to produce hydrologic forecasts
for headwater basins.
Headwater Advisory Table :
A table developed by a River Forecast Center for a Headwater
Guidance Point; a pre-computed matrix of values allows
a forecaster to ascertain an anticipated crest or rise
on a small river or stream for a variety of rainfall events
and soil moisture conditions.
Headwater basin:
A basin at the headwaters of a river. All discharge of
the river at this point is developed within the basin.
HIC:
Hydrologist in Charge of an RFC.
HIC:
The Hydrometeorological Information Center of the Office
of Hydrology (OH).
Hinge Crack:
Crack caused by significant changes in water level.
HOD:
The Hydrologist on Duty at an RFC.
HOD:
The Hydrologic Operations Division of the Office of Hydrology
(OH).
HRL:
The Hydrological Research Laboratory at the Office of
Hydrology (OH).
HSA (Hydrologic Service Area):
A geographical area assigned to Weather Service Forecast
Office's/Weather Forecast Office's that embraces one or
more rivers.
HSB:
The Hydrologic Systems Branch in the Office of Hydrology
(OH).
HTC:
The Hydrometeorological Training Council
Hummock:
A hillock of broken ice which has been forced upward by
pressure.
Hummocked Ice:
Ice piled haphazardly one piece over another to form an
uneven surface.
Hydraulic Fill Dam :
A dam constructed of materials, often dredged, that are
conveyed and placed by suspension in flowing water.
Hydraulic Grade Line:
A line whose plotted ordinate position represents the
sum of pressure head plus elevation head for the various
positions along a given fluid flow path, such as along
a pipeline or a ground water streamline.
Hydraulic Head:
(1) The height of the free surface of a body of water
above a given point beneath the surface. (2) The height
of the water level at the headworks, or an upstream point,
of a waterway, and the water surface at a given point
downstream. (3) The height of a hydraulic grade line above
the center line of a pressure pipe, at a given point.
Hydraulic Mean Depth:
The right cross-sectional area of a stream of water divided
by the length of that part of its periphery in contact
with its containing conduit; the ratio of area to wetted
perimeter. Also called Hydraulic Radius.
Hydraulic Permeability:
The flow of water through a unit cross-sectional area
of soil normal to the direction of flow when the hydraulic
gradient is unity.
Hydraulic Radius:
The right cross-sectional area of a stream of water divided
by the length of that part of its periphery in contact
with its containing conduit; the ratio of area to wetted
perimeter. Also called Hydraulic Mean Depth.
Hydrograph:
A graph showing the water level (stage), discharge, or
other property of a river volume with respect to time.
Hydrograph Separation:
The process where the storm hydrograph is separated into
baseflow components and surface runoff components.
Hydrographic Survey:
An instrumental survey to measure and determine characteristics
of streams and other bodies of water within an area, including
such things as location, areal extent, and depth of water
in lakes or the ocean; the width, depth, and course of
streams; position and elevation of high water marks; location
and depth of wells, etc.
Hydrologic Budget:
An accounting of the inflow to, outflow from, and storage
in, a hydrologic unit, such as a drainage basin, aquifer,
soil zone, lake, reservoir, or irrigation project.
Hydrologic Cycle:
The natural pathway water follows as it changes between
liquid, solid, and gaseous states.
Hydrologic Equation:
The water inventory equation (Inflow = Outflow + Change
in Storage) which expresses the basic principle that during
a given time interval the total inflow to an area must
equal the total outflow plus the net change in storage.
Hydrologic Model :
A conceptual or physically-based procedure for numerically
simulating a process or processes which occur in a watershed.
Hydrologic Service Area (HSA) :
A geographical area assigned to Weather Service Forecast
Office's/Weather Forecast Office's that embraces one or
more rivers.
Hydrologic Services :
A general Term referring to the operations, products,
verbal communications, and related forms of support provided
by the NWS for the Nation's streams, reservoirs, and other
areas affected by surface water.
Hydrologic Unit:
A geographical area representing part or all of a surface
drainage basin or distinct hydrologic feature such as
a reservoir, lake, etc.
Hydrology:
The applied science concerned with the waters of the earth,
their occurrences, distribution, and circulation through
the unending hydrologic cycle of: Precipitation, consequent
runoff, infiltration, and storage; eventual evaporation;
and so forth. It is concerned with the physical and chemical
reaction of water with the rest of the earth, and its
relation to the life of the earth.
Hydrometeorological Technicians:
Individuals who, at the technical level, have knowledge
in meteorology and hydrology. Among their duties are data
collection, quality control, gage network maintenance,
as well as the gathering and disseminating of data and
products.
Hydrometeorologists:
Individuals who have the combined knowledge in the fields
of both meteorology and hydrology which enables them to
study and solve hydrologic problems where meteorology
is a factor.
Hydrometeorology:
The interdisciplinary science involving the study and
analysis of the interrelationalships between the atmospheric
and land phases of water as it moves through the hydrologic
cycle.
Hydrosphere:
The region that includes all the earth's liquid water,
frozen water, floating ice, frozen upper layer of soil,
and the small amounts of water vapor in the earth's atmosphere.
Hydrostatic Head:
A measure of pressure at a given point in a liquid in
terms of the vertical height of a column of the same liquid
which would produce the same pressure.
Hyetograph:
A graphical representation of rainfall intensity with
respect to time.
I
Ice Boom:
A floating structure designed to retain ice.
Ice Bridge:
A continuous ice cover of limited size extending from
shore to shore like a bridge.
Ice Gorge:
The gorge or opening left in a jam after it has broken.
Ice jam:
A stationary accumulation that restricts or blocks streamflow.
Ice Push:
Compression of an ice cover particularly at the front
of a moving section of ice cover.
Ice Run:
Flow of ice in a river. An ice run may be light or heavy,
and may consist of frazil, anchor, slush, or sheet ice.
Ice Twitch:
Downstream movement of a small section of an ice cover.
Ice twitches occur suddenly and often appear successively.
Ice Shove:
On-shore ice push caused by wind, and currents, changes
in temperature, etcetera.
IFLOWS:
The Integrated Flood Observing and Warning System.
Impermeable:
Material that does not permit fluids to pass through it.
Impervious:
The ability to repel water, or not let water infiltrate.
Import:
Water piped or channeled into an area.
Inactive Storage Capacity - The portion of capacity below
which the reservoir is not normally drawn, and which is
provided for sedimentation, recreation, fish and wildlife,
aesthetic reasons, or for the creation of a minimum controlled
operational or power head in compliance with operating
agreements or restrictions.
Inch-Degrees:
The product of inches of rainfall multiplied the temperature
in degrees above freezing (Fahrenheit Scale), used as
a measure of the snowmelting capacity of rainfall..
Inches of Runoff:
The volume of water from runoff of a given depth over
the entire drainage.
Inclined Staff Gage:
A staff gage that is placed on the slope of a stream bank
and graduated so that the scale reads directly in vertical
depth..
Index of Wetness:
The ratio of precipitation for a given year over the mean
annual precipitation..
Indirect Flood Damage:
Expenditures made as a result of the flood (other than
repair) such as relief and rescue work, removing silt
and debris, etc.
Industrial Consumption:
The quantity of water consumed in a municipality or district
for mechanical, trade, and manufacturing purposes, in
a given period, generally one day. The per capita use
is generally based on the total population of the locality,
municipality, or district.
Infiltration:
Movement of water through the soil surface into the soil.
Infiltration Capacity:
The maximum rate at which water can enter the soil at
a particular point under a given set of conditions.
Infiltration Capacity Curve:
A graph showing the time-variation of infiltration capacity.
A standard infiltration capacity curve shows the time-variation
of the infiltration rate which would occur if the supply
were continually in excess of infiltration capacity.
Infiltration Index:
An average rate of infiltration, in inches per hour, equal
to the average rate of rainfall such as that the volume
of rainfall at greater rates equals the total direct runoff.
Infiltration Rate:
(1) The rate at which infiltration takes place expressed
in depth of water per unit time, usually in inches per
hour. (2) The rate, usually expressed in cubic feet per
second, or million gallons per day per mile of waterway,
at which ground water enters an infiltration ditch or
gallery, drain, sewer, or other underground conduit..
Influent Seepage:
Movement of gravity water in the zone of aeration from
the ground surface toward the water table.
Influent Stream:
Any watercourse in which all, or a portion of the surface
water flows back into the ground namely the, vadose zone,
or zone of aeration.
Initial Detention:
The volume of water on the ground, either in depressions
or in transit, at the time active runoff begins.
Initial Loss:
In hydrology, rainfall preceding the beginning of surface
runoff. It includes interception, surface wetting, and
infiltration unless otherwise specified.
Initial Moisture Deficiency:
The quantity, usually expressed in depth of water in inches
upon a unit area, by which the actual water content of
a given soil zone (usually the root zone) in such area
is less than the field capacity of such zone at the beginning
of the rainy season. Also called Initial Water Deficiency.
Initial Water Deficiency:
The quantity, usually expressed in depth of water in inches
upon a unit area, by which the actual water content of
a given soil zone (usually the root zone) in such area
is less than the field capacity of such zone at the beginning
of the rainy season. Also called Initial Moisture Deficiency.
Inland freshwater wetlands:
Swamps, marshes, and bogs found inland beyond the coastal
saltwater wetlands.
Instantaneous Unit Hydrograph:
The theoretical, ideal, unit hydrograph that has a infinitesimal
duration.
Instream use:
The use of water that does not require withdrawal or diversion
from its natural watercourse; for example, the use of
water for navigation, recreation, and support of fish
and wildlife.
Intangible Flood Damage:
Estimates of the damage done by disruption of business,
danger to health, shock, and loss of life and in general
all costs not directly measurable which require a large
element of judgment for estimating.
Integrated Flood Observing and Warning System (IFLOWS)
:
A 1200 baud wide area network utilizing UHF/VHF radio
and land line communications; IFLOWS components include
rainfall and stage sensors, transceivers, store-forward
repeaters and computer base stations.
Interbasin Transfer:
The physical transfer of water from one watershed to another.
Interception Storage Requirements :
Water caught by plants at the onset of a rainstorm. This
must be met before rainfall reaches the ground.
Intercepting Drain:
A drain constructed at the upper end of the area to be
drained, to intercept surface or ground water flowing
toward the protected area from higher ground, and carry
it away from the area. Also called Curtain Drain.
Interception:
The process by which precipitation is caught and held
by foliage, twigs, and branches of trees, shrubs, and
other vegetation, and lost by evaporation, never reaching
the surface of the ground. Interception equals the precipitation
on the vegetation minus streamflow and through fall.
Interception Storage Requirements:
Water caught by plants at the onset of a rainstorm. This
must be met before rainfall reaches the ground.
Interflow :
The lateral motion of water through the upper layers until
it enters a stream channel. This usually takes longer
to reach stream channels than runoff. This also called
subsurface storm flow.
Intermediate Zone:
The subsurface water zone below the root zone and above
the capillary fringe.
Intermittent Stream:
A stream that flows periodically. Compare perennial stream.
Inundation Map :
A map delineating the area that would be inundated in
the event of a dam failure.
Irrigated Area:
The gross farm area upon which water is artificially applied
for the production of crops, with no reduction for access
roads, canals, or farm buildings.
Irrigation:
The controlled application of water to arable lands to
supply water requirements not satisfied by rainfall.
Irrigation Efficiency:
The percentage of water applied that can be accounted
for in soil moisture increase for consumptive use.
Irrigation Requirement:
The quantity of water, exclusive of precipitation, that
is required for crop production. It includes surface evaporation
and other economically unavoidable wastes.
Isobath:
An imaginary line on the earth's surface or a line on
a map connecting all points which are the same vertical
distance above the upper or lower surface of a water-bearing
formation or aquifer.
Isohyet:
a line that connects points of equal rainfall.
J
Jetty:
A structure (e.g.; a pier, or mole of wood or stone) extending
into a sea, lake, or river to influence the current or
tide or to protect a harbor.
Jokulhlaup:
An Icelandic term meaning glacier dammed lake outburst
flood.
Juvenile Water:
Water formed chemically within the earth and brought to
the surface in intrusive rock.
L
Lag (of a basin) :
The measure of the time between the center of mass of
precipitation to the center of mass of runoff (on the
hydrograph); basin lag is a function of not only basin
characteristics, but also of storm intensity and movement.
Some hydrologic texts define lag from the center of mass
of rainfall to the hydrograph peak.
Lag (Time):
The time it takes a flood wave to move downstream.
Laminar Flow:
Streamline flow in which successive flow particles follow
similar path lines and head loss varies with velocity
to the first power.
LARC (Limited Automatic Report Collector):
An electronic device that interfaces a river or precipitation
gage with a telephone line making it possible for remote
computers to call a gaging site and retrieve data. Eventually
LARCs will Replace DARCs.
length:
The distance in the direction of flow between two specific
points along a river, stream, or channel.
Lentic System:
A nonflowing or standing body of fresh water, such as
a lake or pond. Compare lotic system.
Levee (Dike) :
A long, narrow embankment usually built to protect land
from flooding. If built of concrete or masonary the structure
is usually referred to as a flood wall. Levees and floodwalls
confine streamflow within a specified area to prevent
flooding. The term "dike" is used to describe
an embankment that blocks an area on a reservoir or lake
rim that is lower than the top of the dam.
LFWS:
A generic term for any type of Local Flood Warning System.
Limited Automatic Remote Collector (LARC) :
An electronic device that interfaces a river or precipitation
gage with a telephone line making it possible for remote
computers to call a gaging site and retrieve data.
Limnology:
The branch of hydrology that pertains to the study of
lakes.
Lining :
A coating of concrete, rubber, or plastic to a canal,
tunnel, shaft or reservoir to provide water-tightness,
prevent erosion, reduce friction, or support the periphery
of the structure.
Lithosphere:
That part of the earth which is composed predominantly
of rocks (either coherent or incoherent, and including
the disintegrated rock materials known as soils and subsoils),
together with everything in this rocky crust.
Littoral Zone:
The area on, or near the shore of a body water.
Live Capacity :
The total amount of storage capacity available in a reservoir
for all purposes, from the dead storage level to the normal
water or normal pool level surface level. Does not include
surcharge, or dead storage, but does include inactive
storage, active conservation storage and exclusive flood
control storage.
Local Flooding:
Flooding conditions over a relatively limited (localized)
area.
Local Flood Warning System (LWFS) :
A general designator for a network of stream and rain
gages implemented by a community or local government to
monitor hydrologic events as they occur. LFWS gages are
either read manually by spotters or fitted with radio
transmitter to communicate data to a computerized base
station (see IFLOWS and ALERT).
Log and Safety Boom :
A net-like device installed in a reservoir, upstream of
the principal spillway, to prevent logs, debris and boaters
from entering a water discharge facility or spillway.
Long Term Storage Dams :
Reservoirs used for recreational use or storage of irrigation,
municipal or industrial water. Because water is impounded
on a "permanent" basis, the design of these
dams is more complex than for tailings or flood control
detention dams. A long term storage dam may include an
impermeable core surrounded by shell materia, have many
types of drains and filters, outlet works, with gates
and valves, seepage collection boxes, and possibly several
spillways. The capacity of the spillway is dependant upon
the downstream hazard potential.
Lotic System :
A flowing body of fresh water, such as a river or stream.
Compare lentic system.
Lowland Flooding:
Inundation of low areas near the river, often rural, but
may also occur in urban areas.
Lysimeter:
A device to measure the quantity or rate of downward water
movement through a block of soil usually undisturbed,
or to collect such percolated water for analysis as to
quality.
M
Main Stem:
The reach of a river/stream formed by the tributaries
that flow into it.
Major Flooding:
A general term including extensive inundation and property
damage. (Usually characterized by the evacuation of people
and livestock and the closure of both primary and secondary
roads.)
MAP (Mean Areal Precipitation):
The average rainfall over a given area, generally expressed
as an average depth over the area.
Mass Curve:
A graph of the cumulative values of a hydrologic quantity
(such as precipitation or runoff), generally as ordinate,
plotted against time or date.
Maximum Spillway Discharge :
Spillway discharge (cfs) when reservoir is at maximum
designed water surface elevation.
Mean Depth:
The average depth of water in a stream channel or conduit.
It is equal to the cross-sectional area divided by the
surface width.
Meander:
The winding of a stream channel.
Meander Belt:
The area between lines drawn tangential to the extreme
limits of fully developed meanders.
Meniscus:
The curved surface of the liquid at the open end of a
capillary column.
Meteoric Water:
Water derived from precipitation.
Miners' Inch:
A rate of discharge through an orifice one inch square
under a specific head.
Minor Flooding:
A general term indicating minimal or no property damage
but possibly some public inconvenience.
Mission of the National Weather Service :
To provide weather and flood warnings, public forecasts
and advisories for all of the United States, its territories,
adjacent waters and ocean areas, primarily for the protection
of life and property. NWS data and products are provided
to private meteorologist for the provision of all specialized
services.
Mission of the Hydrologic Services Program :
To provide river and flood forecasts and warnings for
the protection of life and property and to provide basic
hydrologic forecast information for the Nation's economic
and environmental well being.
Moderate Flooding :
The inundation of secondary roads; transfer to higher
elevation necessary to save property -- some evacuation
may be required.
Moisture Equivalent :
The ratio of (1) the weight of water which the soil, after
saturation, will retain against a centrifugal force 1,000
times the force of gravity, to (2) the weight of the soil
when dry. The ratio is stated as a percentage.
Movable Bed :
A stream bed made up of materials readily transportable
by the stream flow.
Moveable Bed Streams :
These are most common in the arid West, where steep slopes
and lack of vegetation result in a lot of erosion. During
a flood, a channel may be eroded more deeply, or it may
become filled with sediment and move to a different location.
Multipurpose Reservoir :
A reservoir constructed and equipped to provide storage
and release of water for two or more purposes such as
flood control, power development, navigation, irrigation,
recreation, pollution abatement, domestic water supply,
etc.
Municipal Use of Water :
The various uses to which water is put to use developed
urban areas, including domestic use, industrial use, street
sprinkling, fire protection, etc. The term is an inclusive
one, applied where the uses are varied.
N
Natural Control :
A stream gaging control which is natural to the stream
channel, in contrast to an artificial control structure
by man.
Navigation Methods :
Three basic methods of providing and managing inland waterways
- 1) Run-of-the-River: no provision of upstream storage;
2) Slack-Water: locks and dams provide slack water or
pools with adequate depth for the draft of heavy barges
and area to prevent excessive velocities; 3) Canalization:
in lieu of a series of dams on the river a canal with
locks adjoins the river.
NCCF :
NOAA Central Computer Facility
National Environmental Satellite, Data, and Information
Service NESDIS :
NESDIS collects, processes, stores, analyzes, and disseminates
various types of hydrologic, meteorologic, and oceanic
data. NESDIS is also responsible for the development of
analytical and descriptive products so as to meet the
needs of it's users.
Net Rainfall :
The portion of rainfall which reaches a stream channel
or the concentration point as direct surface flow.
NOHRSC :
The National Operational Hydrologic Remote Sensing Center.
An organization under the National Weather Service Office
of Hydrology (OH) that mainly deals with snow mapping.
Normal Water Surface Elevation (Normal Pool Level) :
The lowest crest level of overflow on a reservoir with
a fixed overflow level (spillway crest elevation). For
a reservoir whose outflow is controlled wholly or partly
by movable gates, siphons, or other means, it is the maximum
level to which water may rise under normal operating conditions,
exclusive of any provision for flood surcharge.
Normal Year:
A year during which the precipitation or stream flow approximates
the average for a long period of record.
NWSH:
The National Weather Service Headquarters.
NWSRFS V5.0, or Version 5 (National Weather Service River
Forecast Model Version 5):
The system of data entry, data preprocessing, and forecast
programs which are used by RFCs. To make river forecasts,
RFCs run NWSRFS V5.0 on a mainframe computer in NWSH through
Remote Job Entry, or locally via Government Development
Platforms, GDPs.
O
Observation Well:
A non-pumping well used for observing the elevation of
the water table or piezometric surface.
Ogee:
A reverse curve, shaped like an elongated letter S. The
downstream faces of overflow dams are often made to this
shape. (From the French word Ogive).
OH:
The Office of Hydrology, located in Silver Springs, MD.
OML:
An Operations Manual Letter. These serve as updates to
policy and procedure for the National Weather Service
Operations Manual (WSOM).
One Percent Chance Flood (One Hundred Year Flood):
flood magnitude that has one chance in 100 of being exceeded
in any future 1-year period. The occurrence of floods
is assumed to be random in time, or regularity of occurrence
is implied. The exceeding of a 1-percent chance is no
guarantee, therefore, that a similar size flood will not
occur next week. The risk of experiencing a large flood
within time periods longer than 1 year increases in a
nonadditive fashion. For example, the risk of exceeding
a 1-percent chance flood one or more times during a 30-year
period is 25 percent and during a 70-year period is 50
percent.
Orifice :
(1) An opening with closed perimeter, usually sharp edged,
and of regular form in a plate, wall, or partition through
which water may flow, generally used for the purpose of
measurement or control of water. (2) The end of a small
tube, such as a Pitot tube, piezometer, etc.
Orographic Precipitation :
Precipitation which is caused by hills or mountain ranges
deflecting the moisture-laden air masses upward, causing
them to cool and precipitate their moisture.
Outburst Flood :
See Jokulhlaup.
Outflow Channel :
A natural stream channel which transports reservoir releases.
Outlet :
An opening through which water can be freely discharged
from a reservoir.
Outlet Discharge Structure :
Protects the downstream end of the outlet pipe from erosion
and is often designed to slow down the velocity of released
water to prevent erosion of the stream channel.
Overland Flow :
The flow of rainwater or snowmelt over the land surface
toward stream channels. After it enters a watercourse
it becomes runoff.
P
Palmer Drought Severity Index :
An index whereby excesses or deficiencies of precipitation
are determined I relation to average climate values. The
index takes in to account precipitation, potential and
actual evapotransporation, infiltration of water into
the soil, and runoff.
Parametric Data :
Data such as rating curves, unit hydrographs, and rainfall/runoff
curves which define hydrologic variables in models.
Parapet Wall :
A solid wall built along the top of the dam for ornament,
safety, or to prevent overtopping.
Partial-Duration Flood Series :
A list of all flood peaks that exceed a chosen base stage
or discharge, regardless of the number of peaks occurring
in a year.
Pancake Ice :
Circular flat pieces of ice with a raised rim; the shape
and rim are due to repeated collisions.
Peak Discharge :
Rate of discharge of a volume of water passing a given
location. (Usually in cubic feet per second.)
Perched Groundwater :
Local saturated zones above the water table which exist
above an impervious layer of limited extent.
Perched Water Table :
The water table of a relatively small ground-water body
supported above the general ground water body.
Percolation :
The movement of water, under hydrostatic pressure, through
the interstices of a rock or soil, except the movement
through large openings such as caves.
Percolation Deep :
In irrigation or farming practice, the amount of water
that passes below the root zone of the crop or vegetation.
Percolation Path :
The course followed by water moving or percolating through
any other permeable material, or under a dam which rests
upon a permeable foundation.
Percolation Rate :
The rate, usually expressed as a velocity , at which water
moves through saturated granular material. The term is
also applied to quantity per unit of time of such movement,
and has been used erroneously to designate Infiltration
Rate or Infiltration Capacity.
Perennial Stream :
A stream that flows all year round. Compare intermittent
stream.
Permanent Control :
A stream gaging control which is substantially unchanging
and is not appreciably affected by scour, fill, or backwater.
Permanent Monument :
Fixed monuments placed away from the dam which allow movements
in horizontal and vertical control points on the dam to
be monitored by using accurate survey procedures.
Permeability:
The ability of a material to transmit fluid through its
pores when subjected to a difference in head.
Permeability Coefficient:
The rate of flow of a fluid through a cross section of
a porous mass under a unit hydraulic gradient, at a temperature
of 60 degrees Fahrenheit. The standard coefficient of
permeability used in hydrologic work in Meinzer's Units
is defined as the rate of flow of water at 60 degrees
Fahrenheit, in gallons per day, through a cross section
of 1 sq. ft., under a hydraulic gradient of 100%. A related
coefficient, which may be called the Field Coefficient
of Permeability, is defined as the rate of flow of water,
in gallons a day, under prevailing conditions, through
each foot of thickness of a given aquifer in a width of
1 mile, for each foot per mile of hydraulic gradient..
Permeameter:
A laboratory instrument for determining permeability by
measuring the discharge through a sample of the material
when a known hydraulic head is applied.
Pervious Zone :
A part of the cross section of an embankment dam comprising
material of high permeability.
Phiezometer :
An instrument used to measure pressure head in a conduit,
tank, soil, etc. They are used in dams to measure the
level of saturation.
Phreatic surface :
The free surface of ground water at atmospheric pressure.
Phreatic water :
Water within the earth that supplies wells and springs;
water in the zone of saturation where all openings in
rocks and soil are filled, the upper surface of which
forms the water table. Also termed Groundwater.
Phreatic Zone :
The locus of points below the water table where soil pores
are filled with water. This is also called the zone of
saturation.
Phreatophyte :
A plant that habitually obtains its water supply from
the zone of saturation, either directly or through the
capillary fringe.
Piezometer :
An instrument for measuring pressure head in a conduit,
tank, soil, etc. It usually consists of a small pipe or
tube tapped into the side of the container, the inside
end being flush with, and normal to, the water face of
the container, connected with a manometer pressure gage,
mercury of water column, or other device for indicating
pressure head.
Piezometric Level (or Surface) :
Confined groundwater is usually under pressure because
of the weight of the overburden and the hydrostatic head.
If a well penetrates the confining layer, water will rise
to this level, the piezometric level, the artesian equivalent
of the water table. If the piezometric level is above
ground level, the well discharges as a flowing well, artesian
well, or a spring.
Piping :
The progressive development of internal erosion by seepage,
appearing downstream as a hole or seam discharging water
that contains soil particles.
Pitot Tube :
A device for measuring the velocity of flowing water using
the velocity head of the stream as an index of velocity.
It consists essentially of an orifice held to a point
upstream in the water, connected with a tube in which
the rise of water due to velocity head may be observed
and measured. It also may be constructed with an upstream
and downstream orifice, with two water columns, in which
case the difference in height of water column in the tubes
is the index of velocity.
Pluvial :
In hydrology, anything that is brought about directly
by precipitation.
Point Discharge :
Instantaneous rate of discharge, in contrast to the mean
rate for an interval of time.
Point Precipitation :
Precipitation at a particular site, in contrast to the
mean precipitation over an area.
Pondage :
(1) The holding back of water for later release for power
development above the dam of a hydroelectric plant to
(a) equalize daily or weekly fluctuations of streamflow
or (b) to permit irregular hourly use of water by the
wheels to care for fluctuations in the load demand. (2)
In general the holding back of water for later releases.
(3) The storage capacity available for the use of such
water.
Ponding :
In flat areas, runoff collects, or ponds in depression
and cannot drain out. Flood waters must infiltrate slowly
into the soil, evaporate, or be pumped out.
Pool Height :
The height of the water behind a dam. (Various datums
may be used and various pool height may be used, e.g.,
conservation pool, flood control pool, etc.)
Pore Pressure :
The interstitial pressure of water within a mass of soil,
rock, or concrete.
Porosity :
(1) The ratio of pore volume to total volume of the formation.
Sandy soils have large pores and a higher porosity than
clays and other fine-grained soils. (2) An index of the
void characteristics of a soil or stream as pertaining
to percolation; degree of previousness.
Precipitation :
As used in hydrology, precipitation is the discharge of
water, in a liquid or solid state, out of the atmosphere,
generally onto a land or water surface. It is the common
process by which atmospheric water becomes surface, or
subsurface water. The term "precipitation" is
also commonly used to designate the quantity of water
that is precipitated. Precipitation includes rainfall,
snow, hail, and sleet, and is therefore a more general
term than rainfall.
Precipitation Processing System :
The WSR-88D system that generates 1-hour running, 3-hourly,
and running storm total precipitation accumulations. Five
functional steps are performed to calculate the best estimate
of precipitation: 1) development of a sectorized hybrid
scan, 2) conversion to precipitation rate, 3) precipitation
accumulation, 4) adjustment using rain gages, 5) product
update.
Pressure Head :
Energy contained by fluid because of its pressure, usually
expressed in feet of fluid (foot pounds per pound).
Pressure gage :
A device for registering the pressure of solids, liquids,
or gases. It may be graduated to register pressure in
any units desired.
Pressure Relief Pipes :
Pipes used to relieve uplift or pore pressure in a dam
foundation or in the dam structure.
Price Current Meter :
A current meter with a series of conical cups fastened
to a flat framework through which a pin extends. The pin
sets in the framework of the meter, and the cups are rotated
around it in a horizontal plane by the flowing water,
registering the number of revolutions by acoustical or
electrical devices, from which the velocity of the water
may be computed.
Probabilistic Quantitative Precipitation Forecast (PQPF)
:
A form of QPF (see below) that includes an assigned probability
of occurrence for each numerical value in the forecast
product.
Profile :
A graph showing variation of elevation with distance along
a traverse.
Property Protection :
Measures that are undertaken usually by property owners
in order to prevent, or reduce flood damage. Property
protection measures are often inexpensive for the community
because they are implemented by or cost-shared with property
owners. In many cases the buildings' appearance or use
is unaffected, so these measurements are particularity
appropriate for historical sites and landmarks. These
measures include relocation and acquisition, flood proofing,
and buying flood insurance.
Puddle:
(1) The act of compacting earth, soil clay, etc., by mixing
them with water and rolling or tamping the mixture. (2)
A compact mass of earth, soil, clay, or a mixture of material,
which has been compacted through the addition of water,
rolling and tamping. This makes the material less permeable.
(3) A small pool of water, usually a few inches in depth
and from several inches to several feet in it greatest
dimension.
Pumping Head:
Energy given to a fluid by a pump, usually expressed in
feet of fluid (foot pounds per pound).
Q
QPF (Quantitative Precipitation Forecast) :
A spatial and temporal precipitation forecast that will
predict the potential amount of future precipitation for
a specified region, or area.
R
Radioisotope Snow Gage :
A snow water equivalent gage based on the absorption of
gamma radiation by snow; this gage can measure up to 55
inches water equivalent with a 2 to 5 percent error.
Rating Curve :
a graph showing the relationship between the stage, usually
plotted vertically (Y-axis) and the discharge, usually
plotted horizontally (X-axis).
Rating Table :
A table of stage values and the corresponding discharge
for a river gaging site.
Reach :
The distance between two specific points outlining that
portion of the stream, or river for which the forecast
applies. This generally applies to the distance above
and below the forecast point for which the forecast is
valid.
Recession Constant :
Constant used to reduce the API value daily in the API
method of estimating runoff.
Recurrence Interval :
The average amount of time between events of a given magnitude.
For example, there is a 1% chance that a 100- year flood
will occur in any given year.
Reference Mark :
A relatively permanent point of known elevation which
is tied to a benchmark.
Reflectivity :
The measure of the efficiency of a radar target in intercepting
and returning Electro Magnetic Energy. Reflectivity depends
upon the size, shape, aspect and dielectric properties
at the surface of the target.
Regulatory Floodway :
Some maps show an area where construction regulations
require special provisions to account for this extra hazzard.
This is a regulatory floodway.
Remote Observing System Automation (ROSA) :
A type of automated data transmitter used by NWS Cooperative
Program observers.
Reservoir :
A manmade facility for the storage, regulation and controlled
release of water.
Reservoir Surface Area :
The surface area of a reservoir when filled to the normal
pool or water level.
Reservoir Volume :
The volume of a reservoir when filled to normal pool or
water level.
Response Time :
The amount of time in which it will take a watershed to
react to a given rainfall event.
RFC (River Forecast Center) :
Centers that serve groups of Weather Service Forecast
offices and Weather Forecast offices, in providing hydrologic
guidance and is the first echelon office for the preparation
of river and flood forecasts and warnings.
Ridge :
A line or wall of broken ice forced up by pressure. May
be fresh or weathered.
Ridge Ice :
Ice piled haphazardly one piece over another in the form
of ridges or walls.
Riparian Zone :
A stream and all the vegetation on its banks.
River Basin :
Drainage area of a river and its tributaries.
River Flooding :
The rise of a river to an elevation such that the river
overflows its natural banks causing or threatening damage.
River Forecast (RVF) :
An internal product issued by RFCs to other NWS offices.
An RVF contains stage and/ or flow forecasts for specific
locations based on existing, and forecasted hydrometeorologic
conditions. The contents of these products are used by
the HSA office to prepare Flood Warnings (FLW), Flood
Statements (FLS), River Statements (RVS), as well as other
products available to the public.
River Gage :
A device for measuring the river stage.
River Gage Datum :
The arbitrary zero datum elevation which all stage measurements
are made from.
River Ice Statement (RVI) :
A public product issued by the RFC's containing narrative
and numeric information on river ice conditions.
River Observing Station :
An established location along a river designated for observing
and measuring properties of the river.
River Recreation Statement (RVR) :
A statement released by the NWS to inform river users
of current and forecast river and lake conditions. These
statements are especially useful for planning purposes.
River Statement (RVS) :
A product issued to communicate notable hydrologic conditions
which do not involve flooding, i.e., within river bank
rises, minor ice jams, etc.
River Summary (RVA) :
A NWS summary of river and/ or crest stages for selected
forecast points along the river.
River System :
All of the streams and channels draining a river basin.
Rockfill Dam :
An embankment dam of earth or rock in which the material
is placed in layers and compacted by using rollers or
rolling equipment.
Rod :
A graduated staff used in determining the difference in
elevation between two points. The two most common types
of rods are the Philadelphia Rod, graduated in feet and
hundredths of a foot, and a California Rod, graduated
in feet, inches, and eights of an inch.
Rolled Filled Dam :
An embankment dam of earth or rock in which the material
is placed in layers and compacted by using rollers or
rolling equipment.
ROML :
Regional Operations Manual Letter. These serve as updates
to regional policy and procedure for the National Weather
Service Operations Manual (WSOM).
ROSA :
Remote Observation System Automation.
Rotten Ice :
Ice in an advanced stage of disintegration.
Routing :
The methods of predicting the attenuation of a flood wave
as it moves down the course of a river.
Runoff :
That part of precipitation that flows toward the streams
on the surface of the ground or within the ground. Runoff
is composed of baseflow and surface runoff.
S
Sacramento Soil Moisture Accounting Model (SACSMA) :
A model which simulates the movement and occurrence of
water in and on top of the ground.
Satellite Hydrology Program :
A NOHRSC program that uses satellite data to generate
areal extent of snow cover data over large areas of the
western United States.
SCS :
The Soil Conservation Service.
SDM :
Station Duty Manual
Second-Day Feet :
The volume of water represented by a flow of one cubic
foot per second for 24 hours; equal to 84,000 cubic feet.
This is used extensively as a unit of runoff volume. Often
abbreviated as SDF.
Sediment Storage Capacity :
The volume of a reservoir planned for the deposition of
sediment.
Seepage :
The interstitial movement of water that may take place
through a dam, its foundation, or abutments.
Service Hydrologist :
The designated expert of the hydrology program at a WFO.
Sheet Flow :
Flow that occurs overland in places where there are no
defined channels, the flood water spreads out over a large
area at a uniform depth. This also referred to as overland
flow.
SHEF (Standard Hydrologic Exchange Format) :
A documented set of rules for coding data for operational
day-to-day use in a form for both visual and computer
recognition.
SHEFPARS :
A software decoder for SHEF Data.
Shore ice :
An ice sheet in the form of a long border attached to
the bank or shore.; border ice.
Site-Specific :
Term used in conjunction with "forecast" or
"warning" to convey the fact that a hydrologic
(stream) forecast is produced for an individual stream
gage location as opposed to a general area (e.g., a city,
zone, or county) as is commonly done in many types of
weather forecasts.
Site Specific Hydrologic Prediction System (SSHP) :
The WFO hydrologic forecast model for small rivers and
streams that uses RFC soil moisture state variables, stage
and precipitation data. Routing capabilities may be added
to future builds.
SMA :
The Soil Moisture Accounting Model.
Small Stream Flooding :
Flooding of small creeks, streams, or runs.
SMPDBK :
The Simplified Dam Break (DAMBRK) Model
Snow Accumulation and Ablation Model :
A model which simulates snow pack accumulation, heat exchange
at the air-snow interface, areal extent of snow cover,
heat storage within the snow pack, liquid water retention,
and transmission and heat exchange at the ground-snow
interface.
Snow Core :
A sample of either freshly fallen snow, or the combined
old and new snow on the ground. This is obtained by pushing
a cylinder down through the snow layer and extracting
it.
Snow Density :
The mass of snow per unit volume which is equal to the
water content of the snow divided by its depth.
Snow Depth :
The combined total depth of both the old and new snow
on the ground.
Snow Pillow :
An instrument used to measure snow water equivalents.
Snow pillows typically have flat stainless steel surface
areas. The pillow below this flat surface is filled with
antifreeze solution and the pressure in the pillow is
related to the water-equivalent depth of the snow on the
platform. One great advantage of snow pillows over a snow
survey is the frequency of observations, which can be
as high as twice per day.
Snow Stake :
A 1-3/4 inch square, semi-permanent stake, marked in inch
increments to measure snow depth.
Snow Stick :
A portable rod used to measure snow depth.
SNOw TELemetry (SNOTEL) :
An automated network of snowpack data collection sites.
The Natural Resources Conservation Service (NRCS), formerly
the Soil Conservation Service (SCS), has operated the
Federal-State-Private Cooperative Snow Survey Program
in the western United States since 1935. A standard SNOTEL
site consists of a snow pillow, a storage type precipitation
gage, air temperature sensor and a small shelter for housing
electronics.
Snow Water Equivalent :
The water content obtained from melting accumulated snow.
Snowboard :
A flat, solid, white material, such as painted plywood,
approximately two feet square, which is laid on the ground,
or snow surface by weather observers to obtain more accurate
measurements of snowfall and water content.
Snowmelt Flooding :
Flooding caused primarily by the melting of snow.
Snowpack :
The total snow and ice on the ground, including both the
new snow and the previous snow and ice which has not melted.
Soil Conservation Service :
The former name of a branch of the United States Department
of Agriculture, renamed the Natural Resources Conservation
Service (NRCS). NRCS has responsibilities in soil and
water conservation, and flood prevention.
Soil Moisture :
Water contained in the upper regions near the earths surface.
Specific Yield :
The ratio of the water which will drain freely from the
material to the total volume of the aquifer formation.
This value will always be less than the porosity.
Spillway :
A structure over or through which excess or flood flows
are discharged. If the flow is controlled by gates, it
is a controlled spillway, if the elevation of the spillway
crest is the only control, it is an uncontrolled spillway.
Some various types of spillways include:
Auxiliary or Emergency Spillway :
A secondary spillway designed to operate only during exceptionally
large flood flows. Allows inflows from large storms to
be released from the reservoir before the water level
raises high enough to overtop the dam.
Fuse Plug Spillway :
An auxiliary or emergency spillway comprising a low embankment
or a natural saddle designed to be overtopped and eroded
away during flood flows.
Primary (or Principal) Spillway :
The spillway which would be used first during normal inflow
and flood flows.
Shaft or Morning Glory Spillway :
A vertical or inclined shaft into which flood water spills
and then is conducted through, under, or around a dam
by means of a conduit or tunnel. If the upper part of
the shaft is splayed out and terminates in a circular
horizontal weir, it is termed a "bellmouth"
or "morning glory" spillway.
Side Channel Spillway :
A spillway whose crest is roughly parallel to the channel
immediately downstream of the spillway.
Siphon Spillway :
A spillway with one or more siphons built at crest level.
This type of spillway is sometimes used for providing
automatic surface-level regulation within narrow limits
or when considerable discharge capacity is necessary within
a short period of time.
Spillway Crest :
The elevation of the highest point of a spillway.
Spring :
An issue of water from the earth; a natural fountain;
a source of a reservoir of water.
Staff Gage :
A vertical staff graduated in appropriate units which
is placed so that a portion of the gage is in the water
at all times. Observers read the river stage off the staff
gage.
Stage :
The level of the water surface above a given datum at
a given location.
Stage I Precipitation Processing :
The first level of precipitation processing, occurring
within the WSR-88D computer and performance for each volume
scan of the radar. Base reflectivity data are converted
to a precipitation estimate for each grid in the radar
umbrella using a complex algorithm that includes quality
control procedures, a Z/R relationship, and a bias adjustment
using data from a ground-based precipitation gage network.
Several graphical and digital products are produced for
Weather Forecast Offices (WFO) operations and subsequent
processing.
Stage II Precipitation Processing :
The second level of precipitation processing, occurring
within the WFO Advanced Weather Interactive Processing
System (AWIPS) and performed on an hourly basis. Stage
I precipitation estimates are further refined using data
from additional precipitation gages and other sources
such as rain/no rain determinations from satellite imagery.
Stage II may also be executed at RFCs for backup purposes.
Stage III Precipitation Processing :
The third level of precipitation processing, performed
interactively at RFCs. Stage II precipitation estimates
from multiple radars are mosaicked into an RFC area-wide
product for use in river basin hydrologic modeling operations.
RFC forecasters can review the mosaicked product, interactively
edit areas of bad data, and substitute gage-only fields
into portions of the mosaicked radar based product.
Stage IV Precipitation Processing :
The fourth level of precipitation processing, performed
automatically and/or interactively at NCEP. Stage III
precipitation estimates from RFCs are mosaicked into a
Nation-wide product for use in various real-time forecast
activities and forecast verification operations.
Stair Stepping :
The process of continually updating river forecasts for
the purpose of incorporating the effects rain that has
fallen since the previous forecast was prepared. The goal
of using QPF is to minimize "stair-stepping."
Stilling basin :
A basin constructed to dissipate the energy of fast-flowing
water (e.g., from a spillway or bottom outlet), and to
protect the streambed from erosion.
Stoplogs :
Large logs, timbers or steel beams placed on top of each
other with their ends held in guides on each side of a
channel or conduit providing a temporary closure versus
a permanent bulkhead gate.
Storage :
(1) Water artificially impounded in surface or underground
reservoirs for future use. (2) Water naturally detained
in a drainage basin, such as ground water, channel storage,
and depression storage.
Storage Equation :
The Equation for the conservation of mass.
Storm Hydrograph :
A hydrograph representing the total flow or discharge
past a point.
Stormwater Discharge :
Precipitation that does not infiltrate into the ground
or evaporate due to impervious land surfaces but instead
flows onto adjacent land or water areas and is routed
into drain/sewer systems.
Stream Gage :
A site along a stream where the stage (water level) is
read either by eye or measured with recording equipment.
Stream Segment :
Refers to the surface waters of an approved planning area
exhibiting common hydrological, natural, physical, biological,
or chemical processes. Segments will normally exhibit
common reactions to external stresses such as discharge
or pollutants.
Streamflow :
Water flowing in the stream channel. It is often used
interchangeably with discharge.
Subsidence :
Sinking down of part of the earth's crust due to underground
excavation, such as the removal of groundwater
Substation :
A location where observations are taken or other services
are furnished by people not located at NWS offices who
do not need to be certified to take observations.
Subsurface Storm Flow :
The lateral motion of water through the upper layers until
it enters a stream channel. This usually takes longer
to reach stream channels than runoff. This also called
interflow.
Surcharge Capacity :
The volume of a reservoir between the maximum water surface
elevation for which the dam is designed and the crest
of an uncontrolled spillway, or the normal full-pool elevation
of the reservoir with the crest gates in the normal closed
position.
Surface impoundment :
An indented area in the land's surface, such as a pit,
pond, or lagoon.
Surface Runoff :
The runoff that travels overland to the stream channel.
Rain that falls on the stream channel is often lumped
with this quantity.
Surface water :
Water that flows in streams and rivers and in natural
lakes, in wetlands, and in reservoirs constructed by humans.
Sustained Overdraft :
Long-term withdrawal from the aquifer of more water than
is being recharged.
SWE :
Snow Water Equivalent
T
Tailwater Height :
Height of water immediately downstream of the dam. (Various
datums may be used.)
Thalweg :
The line of maximum depth in a stream. The thalweg is
the part that has the maximum velocity and causes cutbanks
and channel migration.
Theodolite :
An instrument used in surveying to measure horizontal
and vertical angles with a small telescope that can move
in the horizontal and vertical planes.
Threshold Runoff :
The runoff in inches from a rain of specified duration
that causes a small stream to slightly exceed bankfull.
When available, flood stage is used instead of slightly
over bankfull.
Tipping-Bucket Rain Gage :
A precipitation gage where collected water is funneled
into a two compartment bucket; 0.01, 0.1 mm, or some other
designed quantity of rain will fill one compartment and
overbalance the bucket so that it tips, emptying into
a reservoir and moving the second compartment into place
beneath the funnel. As the bucket is tipped, it actuates
an electric circuit.
Toe of Dam (Upstream and Downstream) :
The junction of the face of a dam with the ground surface.
Toe Drain (or Outfall) :
A drain which carries seepage away from the dam and can
allow seepage quantities to be measured.
Total Gross Reservoir Capacity :
The total amount of storage capacity available in a reservoir
for all purposes from the streambed to the normal water
or normal water or normal pool surface level. It does
not include surcharge, but does include dead storage.
Trace :
A hydrograph or similar plot for an extended-range time
horizon showing one of many scenarios generated through
an ensemble forecast process.
Trace (of precipitation) :
A rainfall amount less than 0.01 of an inch.
Transpiration :
Water discharged into the atmosphere from plant surfaces.
Trash Rack :
A screen located at an intake to prevent debris from entering.
Travel Time :
The time required for a flood wave to travel from one
location to a subsequent location downstream.
Turbidity :
The thickness or opaqueness of water caused by the suspension
of matter. The turbidity of rivers and lakes increases
after a rainfall.
Turning Point :
A temporary point whose elevation is determined by additions
and subtractions of backsights and foresights respectively.
U
Undercurrent :
A current below the upper currents or surface of a fluid
body.
Underflow :
The lateral motion of water through the upper layers until
it enters a stream channel. This usually takes longer
to reach stream channels than runoff. This also called
subsurface storm flow.
Unit Hydrograph (or Unitgraph) :
The discharge hydrograph from one inch of surface runoff
distributed uniformly over the entire basin for a given
time period.
Unit Hydrograph Duration :
The time over which one inch of surface runoff is distributed
for unit hydrograph theory.
Unit Hydrograph Theory :
Unit Hydrograph Theory states that surface runoff hydrographs
for storm events of the same duration will have the same
shape, and the ordinates of the hydrograph will be proportional
to the ordinates of the unit hydrograph. For example,
the hydrograph for ?" of storm runoff will be half
that of that from the unit hydrograph.
Universal Type Weighting and Recording Gage :
A gage which collects precipitation and then converts
the weight onto an inked pen movement which traces on
graph paper fixed to a clock driven drum.
Upstream Slope :
The part of the dam which is in contact with the reservoir
water. On earthen dams, this slope must be protected from
the erosive action of waves by rock riprap or concrete.
Urban Flash Flood Guidance :
A specific type of flash flood guidance which estimates
the average amount of rain needed over an urban area during
a specified period of time to initiate flooding on small,
ungaged streams in the urban area.
Urban Flooding :
Flooding of streets, underpasses, low lying areas, or
storm drains. This type of flooding is mainly an inconvenience
and is generally not life threatening.
U.S. Army Corps of Engineers (USACE) :
As part of the Department of the Army, the Corps has responsibilities
in civil and military areas. In civil works, the USACE
has authority for approval of dredge and fill permits
in navigable waters and tributaries thereof; the USACE
enforces wetlands regulations, and constructs and operates
a variety of water resources projects, mostly notably
levee, dams and locks.
U.S. Bureau of Reclamation (USBR) :
The Federal agency whose mandate was to reclaim the arid
west of the United States. Operating in 17 western states,
this agency builds, operates and maintains a variety of
irrigation, power, and flood control projects.
USFS :
The U.S. Forest Service
U.S. Geological Survey (USGS) :
The Federal Agency chartered in 1879 by congress to classify
public lands, and to examine the geologic structure, mineral
resources, and products of the national domain. As part
of its mission, the USGS provides information and data
on the Nation's rivers and streams that are useful for
mitigation of hazards associated with floods and droughts.
V
Vadose Zone :
The locus of points just above the water table where soil
pores may either contain air or water. This is also called
the zone of aeration.
Valve :
A device fitted to a pipeline or orifice in which the
closure member is either rotated or moved in some way
as to control or stop flow.
Velocity Zones :
Areas within the floodplain subject to potential high
damage from waves. These sometimes appear on flood insurance
rate maps.
W
Watercourse :
Any surface flow such as a river, stream, tributary.
Watershed :
Land area from which water drains toward a common watercourse
in a natural basin.
Water Equivalent :
The amount of water, in inches, obtained by melting a
snow sample.
Water Pollution :
The alteration of the constituents of a body of water
by man to such a degree that the water loses its value
as a natural resource.
Water Supply Outlook :
A seasonal volume forecast, generally for a period centered
around the time of spring snowmelt (e.g., April-July).
The outlooks are in units of acre-feet and represent the
expected volume of water to pass by a given point during
a snowmelt season. The outlook categories include Most
Probable, Reasonable Maximum, and Reasonable Minimum.
Water Supply Outlook (ESS) Product :
A public product issued by a Forecast Office which contains
narrative and numeric information on current and extended
water supply conditions.
Water Table :
The level below the earth's surface at which the ground
becomes saturated with water. The water table is set where
hydrostatic pressure equals atmospheric pressure.
Water Year :
The time Period form October 1 through September 30.
Weighing-type Precipitation Gage :
A rain gage that weighs the rain or snow which falls into
a bucket set on a platform of a spring or lever balance.
The increasing weight of its contents plus the bucket
are recorded on a chart. The record thus shows the accumulation
of precipitation.
Weir :
(a) A low dam built across a stream to raise the upstream
water level (fixed-crest weir when uncontrolled); (b)
A structure built across a stream or channel for the purpose
of measuring flow (measuring or gaging weir).
Wet Floodproofing :
An approach to floodproofing which usually is a last resort.
Flood waters are intentionally allowed into the building
to minimize water pressure on the structure. Wet Floodproofing
can include moving a few valueable items to a higher place
or completely rebuilding the floodable area. Wet floodproofing
has an advantage over other approaches: not matter how
little is done, flood damage will be reduced. Thousands
of dollars in damage can be avoided just by moving furniture
and appliances out of the flood-prone area.
Wetland :
An area that is regularly wet or flooded and has a water
table that stands at or above the land surface for at
least part of the year.
WFO :
A National Weather Service Weather Forecast Office.
Weighing-type Precipitation Gage :
A rain gage that weighs the rain or snow which falls into
a bucket set on a platform of a spring or lever balance.
The increasing weight of its contents plus the bucket
are recorded on a chart. The record thus shows the accumulation
of precipitation.
Wire Weight Gage :
A river gage comprised of a weight which is lowered to
the water level. The weight is attached to a cable; and
as the weight is lowered, a counter indicates the length
of cable released. The stage is determined from the length
of cable required to reach the water level.
X
Y
Z
Z/R Relationship :
The empirical conversion relationship between radar reflectivity
and precipitation rate.
Zero Datum :
A reference "zero" elevation for a stream or
river gage. This "zero" can be referenced (usually
within ten feet of the bottom of the channel) to mean
sea level, or to any other recognized datum.
Zoned Embankment Dam :
An embankment dam which is comprised of zones of selected
materials having different degrees of porosity, permeability
and density.
Zone of Aeration :
The locus of points just above the water table where soil
pores may either contain air or water. This is also called
the vadose zone.
Zone of Saturation :
The locus of points below the water table where soil pores
are filled with water. This is also called the phreatic
zone.
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Types of Flowmeters |
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