CE 411 - Hydrology
PRECIPITATION
O
U
T
L
I
N
E
Formation of Precipitation
Different Types of
Precipitation
Rainfall Characteristics
Point Rainfall Measurement
Different Types of
Raingauges
Sources of errors in
measurement
Estimation of missing
rainfall data
Conversion of point rainfall
to areal rainfall
Double mass analysis
surface
Conclusion
CE 411 - Hydrology
CE 411 - Hydrology
https://gpm.nasa.gov/education/water-cycle
CE 411 - Hydrology
EVAPORATION
Water vapors absorb enough heat to be energized
and break away from the surface as water vapor.
Oceans are the chief source of
moisture for precipitation.
Continental exaporation only
contribute to about 10% to
precipitation.
Other factors influencing
Orographic barriers
EVAPORATION
Shifting of monsoon winds
CE 411 - Hydrology
Evaporation enables water to
enter the atmosphere
Then it condenses to form clouds
and lead to precipitation
CE 411 - Hydrology
CE 411 - Hydrology
CONDENSATION NUCLEI
Particles in the atmosphere that provide a
surface for water vapor to cling to.
usually 0.1 to 10 picometer in size
examples:
sea salt spray, dust, smoke,
pollen, volcanic materials
wave action contributes to sea salt
spray, giant condensation nuclei that
are very moisture-attracting
CE 411 - Hydrology
FREEZING NUCLEI
Freezing nuclei only serve to nucleate the liquid
phase and initiate the growth of ice crystals
It usually consists of clay materials like kaolin.
Water can stay liquid even until temperatures of
negative 40-degrees Celcius but the presence of
these supercooled droplets initiate ice crystal
formation
examples:
carbon dioxide, silver iodide (AgI), etc.
GROWTH OF WATER DROPLETS & CRYSTALS
1. Diffusion
2. Collision and coalescence
DIFFUSION of water vapor
Movement and scattering of water vapor, then clinging to the
condensing nuclei results to the formation of droplets
Diffusion alone leads to formation of generally smaller than 10
picometer in diameter, though some reach 50 picometer.
Most effective when ice crystals and liquid droplets are in the cloud
Growth of water droplets and crystals
Condensation enlargens water droplets and ice crystals at the
same rate
Differences in size result mainly from
the differences in size of the nuclei
on which they are fomed
However, they are still very light that an slight upward lift
brough by the wind (even less than 0.5 cm/s) keep them
from falling. Ice crystals of the same weight require even
lower velocities due to their shape.
For precipitation to occur, cloud elements must increase in size until
their falling speeds exceed the ascencional rate of air.
They must also be large enough to penetrate unsaturated air and resist
evaporation
Because of their differences in size,
heavier droplets fall and rise slower
than lighter droplets.
This movement causes
COLLISION
Growth of water droplets and crystals
COLLISION AND COALESCENCE
The collision of cloud elements result to a significant increase in the
size of droplets. Collision happens due to the difference in the falling
speeds of droplets brought by their sizes.
This process is repeated several times
Raindrops can grow up to 6mm in diameter.
Ice crytals coalesce to form snowflakes.
CE 411 - Hydrology
TERMINAL VELOCITY
As a particle is accelerated due to gravity,
its motion is increasingly resisted by by
friction due to wind. The final speed of the
particle is called the terminal velocity.
Maximum falling speeds tend to level off as
drops approaches to its maximum size due to
air resistance brought by flattening. Further
than this, the droplet may break off.
CE 411 - Hydrology
MAXIMUM LIQUID-WATER CONTENT OF
CLOUDS
Maximum liquid-water conent for nonprecipitating clouds:
- thin stratus clouds: 0.5 g/cu.m
- thick cumulus: 4 g/cu.m (max)
~2 g/cu.m (average)
some records even reach up to 30 g/cu.m
Clouds having concentration greater than 4 g/cu.m usually
produce precipitation that reach the ground
CE 411 - Hydrology
MAXIMUM LIQUID-WATER CONTENT OF
CLOUDS
In vigorous convective systems, speeds of ascending air
can exceed the terminal velocities of raindrops, preventing
precipitation to fall thus water continue to accumulate in
the atmosphere.
Radar observations indicate that the
accumulation depend on the updraft speed.
height
of
updraft
CE 411 - Hydrology
MAXIMUM LIQUID-WATER CONTENT OF
CLOUDS
The accumulated water will eventually be precipitated by:
(1) Weakening of the updraft
(2) Horizontal displacement of the cloud from the
supporting updraft to a weaker one or, as often happens, a
downdraft
TYPES AND FORMS
OF
PRECIPITATION
CE 411 - Hydrology
TYPES OF
PRECIPTATION
01.
CYCLONIC
PRECIPITATION
CONVECTIVE
02.
PRECIPTATION
OROGRAPHIC
03.
PRECIPITATION
CE 411 - Hydrology
CYCLONIC
PRECIPTATION
Is a type of precipitation typical brought by
cyclones or low pressure zones.
They can either be frontal or non-frontal.
CE 411 - Hydrology
FRONTAL
PRECIPITATION
Is a type of precipitation that occurs when two air masses
of different temperature meet and interact eventually
resulting to precipitation.
There are two types of frontal temperature the warm-front
and the cold-front.
NON - FRONTAL
PRECIPITATION
Is a type of precipitation in which the formation of
clouds that leads to precipitation is due to the
convergence of air masses within the cyclone.
CE 411 - Hydrology
WARM - FRONT
PRECIPITATION
Is frontal precipitation in which a warm air mass rises to a
colder air mass in which the warm air gradually cools and
condense forming clouds that will lead to precipitation.
They tend to result in precipitation that are prolonged,
steady, and low in intensity.
COLD - FRONT
PRECIPITATION
Is frontal precipitation is formed when a cold air
mass advanced towards a warm air mass forcefully
making it rise up.
They tend to result to precipitation which are
intense but short lived.
CE 411 - Hydrology
CONVECTIVE
PRECIPITATION
Is a type of precipitation that results when
the ground surface is heated up forcing
water molecules to transform into water
vapor and rise up into the air where they
cool down, condense, and later on
precipitate.
CE 411 - Hydrology
OROGRAPHIC
PRECIPITATION
Is a type of precipitation that occur when
air masses encounter elevated objects
,such as mountains, and is forced to rise up
the mountain. As they rise up they cool
down and condense due to the low
temperature.
FORMS OF
PRECIPTATION
01. RAIN
02. SNOW
03. HAIL
04. SLEET
05. FREEZING RAIN
CE 411 - Hydrology
FORMS OF
PRECIPTATION
06. SHOWER
07. DRIZZLE
08. VIRGA
CE 411 - Hydrology
RAIN
It is a type of precipitation that is in the form
of water droplets. It is formed when water
vapor from the surface rises and cools in the
atmosphere.
SNOW
It is a type of precipitation that is in the form
of ice crystals. It is formed when water vapor
from the surface rises and freezes into ice
crystals that clump together typically
forming a complex structure.
HAIL
It is a type of precipitation that is in the form
of big clumps of ice. They are formed when
falling ice crystals continue to clump
together as they fall to the ground.
CE 411 - Hydrology
SLEET
IIt is type of precipitation that falls from the
atmosphere unto the ground as a ice crystal
that melts but refreezes into small ice pellets.
FREEZING RAIN
It is a type of precipitation that falls from the
atmosphere unto the ground as rain but
freezes upon contact with the ground
surface.
SHOWER
IIt is a brief period of rain having high
intensity but low in duration.
CE 411 - Hydrology
DRIZZLE
IIt is also known as a very light rain with
droplets similar to rain droplets but smaller
in size.
VIRGA
It is a type of precipitation that evaporates
before it hits the ground.
RAIN CHARACTERICS
AND
THE HYETOGRAPH
RAIN
CHARACTERISTICS
01.
DEPTH
02. INTENSITY
03. DURATION
RAIN
CHARACTERISTICS
04. FREQUENCY
02. SEASONALITY
03. AMOUNT
DEPTH
It pertains to the liquid depth of the
precipitation covering a horizontal surface for a
period given that no evaporation, drainage, or
percolation will exist in the surface.
INTENSITY
It is the average rainfall rate over a certain
duration and frequency. It have the unit of
mm/h or mm/min.
DURATION
It is the length of time in which the rainfalls in
a specific location as a continuous
downpour or an intermittent downpour.
FREQUENCY
It pertains to how often rainfall occurs in a
specific time frame.
SEASONALITY
It pertains to time periods where rainfall
occurs more frequently or less frequently.
AMOUNT
It is the total amount of rainwater produce in
a given area from a certain rain event. It is
measured in mm or inches.
CE 411 - Hydrology
HYETOGRAPH
It is a graph representing the relation
between the intensity of rainfall with that of
time.
It is often represented as a bar graph with
time as the abscissa and intensity or
amount as the vertical axis.
POINT RAINFALL
MEASUREMENT
&
DIFFERENT TYPES OF
RAINGAUGES
CE 411 - Hydrology
MEASUREMENT OF
PRECIPITATION
A variety of instruments and techniques have been
developed for gathering information on precipitation.
Instruments for measuring amount and intensity of
precipitation are the most important. Other instruments
include devices for measuring raindrop-size distribution
and for determining the time of beginning and ending of
precipitation. All forms of precipitation are measured on
the basis of the vertical depth of water that would
accumulate on a level surface if the precipitation
remained where it fell. In the metric system precipitation is
measured in millimeters and tenths.
CE 411 - Hydrology
TWO TYPES OF
RAINGAUGES
NON-RECORDING TYPE
These are called non-recording
rain gauges because they do not
record the rain, but only collect
the rain.
RECORDING TYPE
Gauges which can give
permanent and automatic
rainfall record without any bottle
reading.
CE 411 - Hydrology
NON-RECORDING RAIN GAUGE: SYMON'S RAIN GAUGE
The non-recording rain gauge commonly
used in the world is the Symon's rain
gauge.
It consists of a funnel with a circular rim of
12.7 cm diameter and a glass bottle as a
receiver.
The cylindrical metal casing is fixed
vertically to the masonry foundation with
the level rim 30.5 cm above the ground
surface.
The rain falling into the funnel is collected
in the receiver and is measured in a
special measuring glass graduated in mm
of rainfall; when full it can measure 1.25 cm
of rain.
CE 411 - Hydrology
NON-RECORDING RAIN GAUGE: SYMON'S RAIN GAUGE
During heavy rains, it must be measured
three or four times in the day.
Thus the non-recording or the Symon's
rain gauge gives only the total depth of
rainfall for the previous 24 hours (i.e., daily
rainfall) and does not give the intensity
and duration of rainfall during different
time intervals of the day.
It is often desirable to protect the gauge
from being damaged by cattle and for this
purpose a barbed wire fence may be
erected around it.
RECORDING RAIN GAUGE
This is also called self-recording, automatic or integrating
rain gauge. This type of rain gauge has an automatic
mechanical arrangement consisting of a clockwork, a drum
with a graph paper fixed around it and a pencil point, which
draws the mass curve of rainfall.
The gauge is installed on a concrete or masonry platform 45
cm square in the observatory enclosure by the side of the
ordinary rain gauge at a distance of 2-3 m from it. The gauge
is so installed that the rim of the funnel is horizontal and at a
height of exactly 75 cm above ground surface.
CE 411 - Hydrology
RECORDING RAIN GAUGE: TIPPING BUCKET RAIN
GAUGE
This consists of a cylindrical receiver 30 cm
diameter with a funnel inside.
Just below the funnel a pair of tipping
buckets is pivoted such that when one of the
bucket receives a rainfall of 0.25 mm it tips
and empties into a tank below, while the
other bucket takes its position and the
process is repeated.
The tipping of the bucket actuates on
electric circuit which causes a pen to move
on a chart wrapped round a drum which
revolves by a clock mechanism. This type
cannot record snow.
CE 411 - Hydrology
RECORDING RAIN GAUGE: WEIGHING TYPE RAIN GAUGE
In this type of rain-gauge, when a
certain weight of rainfall is collected in
a tank, which rests on a spring-lever
balance, it makes a pen to move on a
chart wrapped round a clock-driven
drum.
The rotation of the drum sets the time
scale while the vertical motion of the
pen
records
the
cumulative
precipitation.
CE 411 - Hydrology
RECORDING RAIN GAUGE: FLOAT TYPE RAIN GAUGE
In this type, as the rain is collected in a
float chamber, the float moves up which
makes a pen to move on a chart
wrapped round a clock driven drum.
When the float chamber fills up, the water
siphons out automatically through a
siphon tube kept in an interconnected
siphon chamber.
The clockwork revolves the drum once in
24 hours. The clock mechanism needs
rewinding once in a week when the chart
wrapped round the drum is also
replaced.
MASS CURVE OF PRECIPITATION
The weighing and float type rain gauges can store a moderate snow
fall which the operator can weigh or melt and record the equivalent
depth of rain. The snow can be melted in the gauge itself (as it gets
collected there) by a heating system fitted to it or by placing in the
gauge certain chemicals such as Calcium Chloride, ethylene glycol,
etc.
THE PRECIPITATION-GAGE NETWORK
The spatial variability of precipitation
and the intended uses of the data
should determine network density.
A relatively sparse network of stations
would suffice for studies of large
general storms or for determining
annual averages over large areas of
level terrain.
THE PRECIPITATION-GAGE NETWORK
A network should be planned to yield a
representative picture of the areal
distribution of precipitation.
The cost of installing and maintaining
a network and accessibility of the
gage site to an observer are always
important considerations.
THE PRECIPITATION-GAGE NETWORK
The following minimum densities of precipitation networks have
been recommended for general hydrometeorological purposes:
1. For flat regions of temperate, Mediterranean, and tropical
zones, 600 to 900 km² per station.
2. For mountainous regions of temperate, Mediterranean, and
tropical zones, 100 to 250 km² per station.
3. For small mountainous islands with irregular precipitation, 25
km² per station.
4. For arid and polar zones, 1500 to 10,000 km² per station.
SOURCES OF ERROR IN
MEASUREMNENT
MEASUREMENT OF
PRECIPITATION ERRORS
•Dents in the collector.
• Moistening of inside-surface of the funnel and the tube.
• Rain drops splashing from the collector.
• For very intense rain some water is still pouring into the already
filled bucket.
• Inclination of the gauge may result in catching less or more rain
than the actual amount.
• Error in measurement due to wind.
MEASUREMENT OF
PRECIPITATION ERRORS
 emedial measures for Error in Precipitation measurement
R
Removal of error due to dents obviously needs repair of the instrument.
For rain recorded with dents a correction should be applied.
Errors such as moistening of the inside surfaces of the gauge,
splashing of rainwater from the collector and pouring of water into the
already filled bucket during an intense rain can only be corrected by
some correction factor.
Inclined instrument needs to be reinstalled. The correction factor
however can be calculated from the angle of inclination.
For wind protection certain wind shields are designed and used which
are called Splash Guards. Proper setting of gauge above ground level is
necessary.
EXAMPLE 1
A rain gauge recorded 125 mm of precipitation. It was found later
that the gauge was inclined at an angle of 20 degree with the
vertical. Find the actual precipitation
Solution:
P(measured) = 125 mm
Angle of inclination (0) = 20° with the vertical
P(actual) = (measured /cos(0) = 125/cos20° = 133 mm
ESTIMATING
MISSING
PRECIPITATION
DATA
ARITMETIC AVERAGE
NORMAL RATIO METHOD
ESTIMATING MISSING
PRECIPITATION DATA
•Why is it Important to obtain these data?
Accurate measurement of precipitation is crucial for
understanding the water cycle and its effects on the
environment. Predicting the quantity of rainfall in a particular
area is essential for a wide range of purposes, including
agricultural planning and flood prevention.
ESTIMATING MISSING
PRECIPITATION DATA
•Causes of missing precipitation data
Instrument Failure - technical issues with measurement
instruments such as rain gauges or weather stations
Records of absences
human error during data recording or transmission
extreme weather conditions such as high winds or heavy
snowfall, which can interfere with accurate measurement.
ESTIMATING MISSING
PRECIPITATION DATA
Data Service, precipitation amounts are estimated from observations at three
stations as close to and as evenly spaced around the station with the missing record
as possible.
If NA (Normal Annual Precipitation) differs with only less than 10% of that for the
station with the missing record, then we can calculate the unknown by getting the
average of the data.
If NA (Normal Annual Precipitation) differs by more than 10% then we can calculate
the missing data using Normal Ratio Method
Example:
The average annual rainfall at stations A. B, C and X in a basin are
80.97, 67.59, 76.28, and 92.0l cm, respectively. In the year 1975, the
station X was inoperative and the stations A, B, and C recorded
annual rainfall of 91.11, 72.23, and 79.89 cm, respectively. Estimate
the rainfall at station D in that year.
POINT RAINFALL TO
AREAL RAINFALL
POINT RAINFALL TO
AREAL RAINFALL
In a hydrological analysis, the empirical data needed is the
rainfall over an area, such as a catchment. Although our
main way of measuring rainfall is through rain gauges,
which only measure a single point sample. To make use of
the data provided by the rain gauges certain adjustments
are needed to modify the point sample into an areal
sample.
To convert the point rainfall values over various
stations into an average value of areal rainfall, the
following methods can be used:
01.
Arithmetic Mean
Method
Thiessen Polygon
02.
Method
03. Isohyetal Method
ARITHMETIC MEAN METHOD
This is the simplest method of computing
rainfall over an area.
As the name suggests, the result is obtained by
the division of the sum of rain depths at
different rain gauge stations by the number of
stations
Where: Pi = rainfall at the nth rain gauge station
N = total no. of rain gauge station
EXAMPLE 1
Compute the average rainfall over
the given area using arithmetic
mean method.
SOLUTION:
EXAMPLE 2
Using the Arithmetic Mean Method, find the average
rainfall over a catchment. The rain gauge data is:
12.6mm, 18.8mm, 14.8mm, 10.4mm, and 16.2mm.
SOLUTION:
Thiessen Polygon Method
This is the weighted mean version.
The rainfall is never uniform over the entire catchment area
but varies in intensity and duration from place to place.
Thus the rainfall recorded by each rain gauge station should
be weighted according to the area, it represents.
For the construction of the polygon, the procedure is to be
followed:
1. Draw the catchment area to a scale and mark the rain
gauge stations on it.
2. Join each station by straight line to create a triangulated
network
3. Draw perpendicular bisectors on each side of each
triangle. Extend the bisectors to meet the other bisectors
and the catchment boundary
4. Delineate the formed polygons and measure their area
using a planimeter or by converting them into smaller
regular shapes.
5. Compute the average rainfall using the following formula:
Where: A = total area of basin
Ai = area of the particular polygon
Pi = rainfall data of particular rain gauge
EXAMPLE 3
Compute the average rainfall over
the given area using thiessen
polygon method.
SOLUTION:
EXAMPLE 4
Using the Thiessen Polygon Method, find average rainfall
over a catchment using the given data:
Rain Gauge Station:
Polygon Area (km^2):
Precipitation (mm):
A
40
30.8
B
45
33.4
C
38
34.6
D
30
32.6
E
43
24.6
SOLUTION:
Station
A
B
C
D
E
P(mm)
30.8
33.4
34.6
32.6
24.6
A(km^2)
40
45
38
30
43
Total Catchment Area (km^2) = 196
Pave = 31.06mm
Ai/At
0.20
0.23
0.19
0.15
0.22
Pi(Ai/At)
6.29
7.67
6.71
4.99
5.40
An Isohyetal is a line joining places where the rainfall
amounts are equal on a rainfall map of a basin
An Isohyetal map showing contours of equal rainfall is more
accurate picture of the rainfall over the basin.
Isohyets are drawn on the map by the method of
interpolation, after the rainfall at each station is marked. The
area between the adjacent Isohyets are measured using
planimeter.
Let, A1, A2, A3...... An are the area between each pair of
Isohyets. P1, P2, P3......Pn are the Average precipitation for
each pair of adjacent isohvets.
Then, mean rainfall on whole basin is given by,
The isohyetal method is superior to the other
two methods especially when the stations are
large in number.
EXAMPLE 5
CASE 1: The isohyets due to storm in a catchment together with the area of the
catchment bounded by the isohyets are given below. Estimate the mean
precipitation due to the storm using Isohyetal Method.
SOLUTION:
EXAMPLE 6
CASE 2: The isohyets due to a storm in a catchment together with the area of
the catchment bounded by the isohyets are given below. Estimate the mean
precipitation due to the storm using Isohyetal Method.
SOLUTION:
EXAMPLE 7
CASE 3: Given a 300 km² watershed with 10 rain gauge stations below, compute
the mean precipitation using Isohyetal Method.
SOLUTION:
DOUBLE MASS
ANALYSIS SURFACE
DOUBLE MASS ANALYSIS SURFACE
Double mass analysis used for checking consistency of a
hydrological or meteorological record and is considered to be
an essential tool before taking it for analysis purpose
It is used to determine whether there is a need for corrections
to the data to account for changes in data collection
procedures or other local conditions.
COMMON CAUSES OF INCONSISTENCY OF RECORD
1. Shifting of rain gauge station at a new location.
2. The neighborhood of a station is undergoing a
marked change.
3. Change in the ecosystem due to calamities such
as Forest fires, Land slides etc.
4. Occurrence of observational error from a certain
date.
HOW DOES IT WORK?
The theory of the double-mass curve is based on the fact
that a plot of the two cumulative quantities during the
same period exhibits a straight line so long as the
proportionality between the two remains unchanged. The
slope of the line represents the proportionality. This method
can smooth a time series and suppress random elements
in the series, and thus show the main trends of the time
series.
DOUBLE MASS CURVE
For the construction of the curve the following procedure is to be
followed:
1. Let X be the station where inconsistency in rainfall records is observed.
2. Select a group of about 10 or more base stations in the neighborhood
of station X.
3. Data of annual or monthly mean rainfall of station X as well as the
average rainfall of the group of base stations over a long time period is
arranged in reverse chronological order i.e. the latest record is the first
entry and the oldest record is the last entry in the list.
DOUBLE MASS CURVE
4. Accumulated precipitation at station X (EP) and the accumulated
values of the average precipitation of the group of base stations (Pav)
are computed from the latest records.
5. A plot of (EP) v/s (Pav) for various consecutive time periods is
prepared.
6. A break in the slope of this plot indicates a change in the precipitation
of station X.
7. Precipitation values at X beyond the period of change of regime is
corrected as shown in the next slide.
DOUBLE MASS CURVE
Where:
Pcx - corrected precipitation at any
time period t₁ at station X
Px - Original recorded precipitation
at time period t₁ at station X
Mc - corrected slope of the double
mass curve
Ma - original slope of the mass
curve
EXAMPLE 8
Check consistency of the data and correct it if inconsistent
SOLUTION:
Corrected Precipitation
Applicable to the data before the year
1950
THANK YOU
for listening!
References
Linsley, R.K., Kohler, M., Paulhus, J. (1988). Hydrology for Engineers - SI Metric Edition.
McGraw - Hill Book Company.
https://www.youtube.com/watch?v=EK_UOLsN72Q
https://www.slideshare.net/AbdulHakimMobin/1-introduction-to-hydrology-108132250?
fbclid=IwAR3TuN5GVym99Shn1Oiy15f_ksYSfkoC4vj14TJb-dI6hjhQ-PMfhtPi6ho