ENVE455 – Engineering Hydrology
Lecture 3: Precipitation
Prepared by
J. Sui, Ph.D., P.Eng.
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The hydrological cycle
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1. The types of precipitations
Precipitation includes rainfall, snowfall, hail, etc.
✓ The rain comes from the reunion of small droplets into big
droplets. These droplets can subsequently develop, colliding
with each others, reaching 7mm in diameter. The droplets
with a diameter smaller than 0.5mm form drizzle.
✓ The snow is formed from ice crystal masses, which
resulted directly from water vapor, in the regions where the air
temperature is lower than the freezing point.
✓ The sleet is a mixture of rain and snow.
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1. The types of precipitations (Cont.)
✓ The hail (0.5cm~5cm in diameter) usually consisting of
concentric layers of clear ice and compact snow.
➢ The hail falls from cumulonimbus clouds (extremely
dense, vertically developed cumulus, usually producing
heavy rains, thunderstorms, or hailstorms) only, because
of the strong updrafts inside the clouds.
➢ The rain droplets are raised at high altitudes where
they freeze, → form ice grains, → fall by passing
through cloud.
➢ suspended by strong updrafts, hail balls develop
via the accumulation of new droplets of water that
freeze.
➢ Finally, the hail gets free from the air current
and falls on the ground.
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2. Formation of precipitation
The formation of precipitation requires the lifting of an air mass in
the atmosphere, → it cools and some of its moisture condenses.
Lifting causes cooling because as air ascends, the pressure on it
decreases and it expands.
Lifting mechanism
Certain storm characteristics are determined by the manner in
which moist air is lifted to a height where precipitation can
develop. The ability of the atmosphere to produce and sustain
lifting forces depends on dynamic forces that initiate movement
and the stability of the air mass being lifted:
✓ Air is stable if buoyancy forces oppose vertical motion;
✓ Air is unstable if they tend to accelerate vertical motion;
✓ Air is neutral if vertical motion is neither aided nor opposed.
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Lifting is initiated by 4 dynamic forces:
---frontal lifting,
---orographic lifting,
---convective lifting and
---convergence.
Frontal lifting: where warm air
is lifted over cooler air by frontal
passage
A warm air mass displacing a cold one tends to more or less
gently override it, depending on the humidity of the air.
✓ If the air is fairly dry, the atmosphere tends toward stability
and high clouds with little precipitation may occur;
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Frontal lifting (Cont.)
✓If the air is moist, the cloud layer thickens from cirrus
into stratus clouds, yielding slow, steady precipitation
as the surface front approaches.
✓ In moist cases, precipitation accompanying warm
front
➢ is light to moderate in intensity, whatever its
form.
➢ It may last as long as several days and
➢ cover a broad area.
➢ Winds during storms are generally light.
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Frontal lifting (Cont.)
✓ A cold front tends to displace and lift the warmer air mass
more sharply + violently. The rapid lifting causes precipitation
to
➢ be heavier,
➢ more localized, and
➢ of shorten duration.
➢ A narrow band of high winds, heavy showers, and
towering clouds often marks the passage of a cold front.
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Orographic lifting in which an air mass rises to pass over a
mountain range
Behind a topographic barrier, a pronounced rain shadow may occur
as subsiding air masses warm and reverse the condensation
process, preventing further precipitation.
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Convergence is air flowing into a low pressure area from the
higher pressure air surrounding it. The inflowing air displaces
upward the air of lower pressure and density. Most precipitation
is associated with low-pressure system.
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Convective lifting: where air is drawn upwards by convective
action, such as in the center of a thunderstorm cell.
Convective lifting occurs when moist air is unstable and strong
heating of the surface causes expansion and lifting of the air.
➢ Once the lifting is initiated, the air continues to rise as
condensation release heat that prevents cooling to the
temperature of the surrounding air at the same altitude.
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Convective lifting (Cont.)
➢ Raindrops may be held up by the rapidly rising air until they
become large enough to fall.
➢ Some times, hail is formed when raindrops are repeatedly
carried upward and freeze.
➢ Finally, rain/hail falls through the rising warm air. Usually,
✓ an extremely heavy rain,
✓ accompanied by a cold downdraft,
✓ covers only a small area and
✓ lasts for only short time.
Convective storms are usually warm-weather phenomena.
They tend to occur in the afternoon and early evening when
temperatures are the highest.
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The global distribution of precipitation
Precipitation varies in space and with time according to the general
pattern of atmospheric circulation and according to local factors.
Global distribution of mean annual rainfall (cm, Encyclopædia
Britannica, Inc.).
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Influence of the geographic conditions on precipitation
distribution
✓Precipitations are very abundant (+200 cm) in the equatorial zone,
where the high TEMP + vast surfaces of ocean provide large quantities of
water vapors and unstable atmospheric conditions. Almost all of these
precipitations are convective.
✓Precipitations are poor in the subtropical zones of high pressure, due to
the descendent movements of the air and the strong drying process. (i.e.,
the deserts in N.+S.-Africa, Arabian, Iran, Australia, ect.)
✓The monsoon winds in Asia affect the precipitations in the SE side of the
continent. In summer, the humid tropical air from the Indian Ocean comes
across mountains and lots of orographic precipitations occur.
✓In the arctic regions, the annual rainfall is very low. The Temp. is low.
Thus it doesn't contain a lot of water vapors. In addition, the low Temp.
reduce the evaporation process → during the summer there is much of
soil humidity and surface water, and during the winter, snow and ice.
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3. Rainfall Characteristics
Rainfall is the source of the world's fresh-water supplies.
Knowing the nature and characteristics of rainfall, one can
conceptualize and predict its effect in runoff, infiltration,
evapo-transpiration (ET), and water yield.
In general, for the short term:
runoff = rainfall - infiltration
for the long-term:
water yield = rainfall - ET – deep GW recharge
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Rainfall characteristics affect the runoff amount which occurs,.
Specific important characteristics of rainfall are:
✓Size and Shape
✓Intensity and Duration
These are usually inversely related, i.e., high intensity
storms are likely to be of short duration and low intensity
storms can have a long duration.
✓Intensity and Area
We can expect a less intense rainfall (amount also) over a
large area than over a small area.
✓Intensity and Drop Size
High intensity storms have a larger drop size than low
intensity storms.
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Storm Patterns
✓Classified based on the intensity of the storm and the pattern of the
rainfall intensity histogram [rainfall intensity (cm/hr) versus time plot]
✓Storm patterns will determine the shape of the runoff hydrograph
➢ Uniform Pattern: Storm with
nearly constant rainfall intensity
➢ Advanced Pattern: Period of
maximum intensity occurs early
in the storm event
➢ Normal or Intermediate Pattern:
Period of max. intensity occurs
near the center of the storm
event
➢ Delayed pattern: Period of maximum intensity occurs late in the storm event
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Intensity-Duration-Frequency Relationships
Relationships btw. Max. rainfall depth and duration and frequency
of occurrence are relatively consistent over a wide area.
Long term records can be used to estimate, with a reasonable
degree of accuracy, the max. rainfall to be expected for various
durations and frequencies.
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3. The precipitation recorded on the watersheds
It is not possible to know precisely how much precipitation falls
on any watershed over a period of time. There are always two
types of errors involved in determining watershed precipitation:
✓ Measurement errors
✓ Sampling errors
Precipitation estimations
Precipitation estimations often require extrapolation from a single gage
outside a watershed, or interpolation from two widely separated gages.
However, where several gages are found in or near a watershed, the
amount of precipitation can be estimated by several techniques:
---arithmetic mean;
---the method of the Thiessen polygons;
---the isohyets method;
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Point
measurements,
but
need
Point measurements, but need spatial data:
---Thiessen Polygons method
spatial data: Thiessen Polygons
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Isohyetal
method
Point measurements, but need spatial data:
--Isohyetal method
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4. Extreme Rainfall
Rainfall Frequency Analysis
✓ Frequency analysis of annual historical rainfall events of a
specified duration involves exactly the same procedure as the
frequency analysis of peak flows.
✓ In the same way, short duration rainfall has a higher average
intensity than long duration rainfall for a particular return
period.
✓ Therefore, you have to do your frequency analysis for each
duration of interest (e.g., 15 min, 30 min, 2 hrs, 24 hrs, etc).
✓ If you ask for a return period rainfall, such as the 100 year
rainfall, you need to specify the duration.
✓ If you have the depth and the duration, you can compute the
corresponding intensity.
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Where do we get the data? → Environment Canada
What rainfall data are typically available? → Daily data
Thus, you can do your won frequency analysis of daily values.
These can be converted to equivalent 24 hour values using a
standard average ratio of 1.13 (NOAA). (This is because a 24-hr
event can split between two days)
Note: This only gives you one duration → 24 hours
Typically for rainfall, we talk about Intensity, Duration, and
Frequency (IDF)
Where do we get data for other durations?
➢ Different duration data are available for a limited number of
stations across Canada from Environment Canada
Why not at all stations?
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IDF DATA
❖Accessing publically available IDF information
(Environment Canada)
➢ http://climate.weather.gc.ca/index_e.html
➢ Navigate to “Engineering Climate Datasets”
dropdown,
➢ and select “Intensity-Duration-Frequency (IDF)
Files” Open the folder titled “IDF_v3.20_2021_03_26/”
➢ Download IDF_Google_App.zip
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IDF DATA (Cont.)
❖Once downloaded, unzip the file, and then open the
IDF_Google_App file using Internet Explorer.
❖Note that this app may not work with the newest
version of Internet Explorer.
❖The interactive map should display coloured circles
with numbers inside. Navigate the map until you’ve
located a station within your region of interest.
❖► Select the station of interest, and then either select
“IDF Curve” or “IDF Table”.
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Rainfall Frequency Analysis
CI
VL41
8
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26 1
Rainfall Frequency Atlas of Canada (1985)
RAINFALL FREQUENCY ATLAS OF
https://www.worldcat.org/title/rainfall-frequency-atlas-forcanada/oclc/13455195
CANADA (24-hr B.C. - mean)
Rainfall
Frequency
Atlas of
Canada (24-hr
B.C. - mean)
→ →
CIVL 418
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Probable Maximum Precipitation (PMP)
➢Defined by the World Meteorological Association as
“theoretically the greatest depth of precipitation for a
given duration that is physically possible over a given size
storm area at particular location at a certain time of
year”.
➢The PMP is derived by analyzing historical storms and
estimating the increased precipitation that would be
expected if maximum atmospheric moisture were
available.
➢No probability is associated with this value because it is
derived deterministically and theoretically the probability
of exceedance is zero. → typically 104 to 108 years
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PMP in Canada
In the US, government publications are available to provide
guidance for estimating the PMP. There are different reports
and maps for every region in the US.
✓In Canada we don’t have any strong guidelines or
reports to follow.
✓BC Hydro employs specialist meteorologists to
complete detailed PMP studies using the moisture
maximization procedure.
✓Most others use the Herschfield Eq. and the
frequency factor approach.
is mean of the 24 hour annual extremes
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