Module 1 Hydrology-Precipitation

HYDROLOGY
Abhishek R, Asst. Prof., Dept. of Civil Engg., JSS ATE - B'lore.

Hydrology
• Hydrology is the branch of earth science which means the science of water. It is the science which
deals with the occurrence, circulation and distribution of water of the earth and earth’s
atmosphere.
Hydrology deals with,
• Estimation of water resources. i.e. to know the water yield from basin/catchment which is
essential for the design of dams.
• The study of processes such as runoff, precipitation and their interaction.
• The study of problems such as floods, droughts and strategies to overcome them. Like safe
design of bridges, dams and flood control structures.
• To know the maximum rainfall intensity and its frequency over a given basin.

Practical Applications
• Design of Hydraulic Structure: The design of hydraulic structure such as spill ways, dams,
culvert, bridges etc.
• Municipal and industrial water supply: The check with the availability of water to meet the
sufficient needs of the municipal city or the industry.
• Hydropower: Absolute min. flow decides the prime capacity of the plant while the additional
flow data is useful in estimating the amount of power that will have to be obtained.
• Flood Control: Reservoir, levees, channel improvements.
• Navigation: To maintain min amount of water without affecting the navigation structure.
• Erosion & Sediment Control: The reservoir may loose their capacity. Measures like cropping,
afforestation, formation of counter bunds etc.
• Irrigation Abhishek R, Asst. Prof., Dept. of Civil Engg., JSS ATE - B'lore.

Precipitation
It is the return of atmospheric moisture to the ground in solid or liquid form. Solid form- snow,
sleet, snow pellets, hailstones. Liquid form- drizzle, rainfall.
The following are the main characteristics of rainfall,
Amount or quantity- The amount of rainfall is usually given as a depth over a specified area,
assuming that all the rainfall accumulates over the surface and the unit for measuring amount of
rainfall is cm. The volume of rainfall = Area x Depth of Rainfall (m3).
The amount of rainfall occurring is measured with the help of rain gauges.
Intensity- This is usually average of rainfall rate of rainfall during the special periods of a storm
and is usually expressed as cm/hour.

Duration of Storm- In the case of a complex storm, we can divide it into a series of storms of
different durations, during which the intensity is more or less uniform.
Aerial distribution- During a storm, the rainfall intensity or depth etc. will not be uniform over
the entire area. Hence we must consider the variation over the area i.e. the aerial

Other Terminologies
Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate
in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is
most often measured in millimeters per hour or inches per hour. The rate decreases as the soil
becomes saturated.
Surface runoff is water, from rain, snowmelt, or other sources, that flows over the land surface
and is a major component of the water cycle. Runoff that occurs on surfaces before reaching a
well-defined stream is also called overland flow.
Interflow is the lateral movement of water in the unsaturated zone, that first returns to the
surface or enters a stream prior to becoming groundwater. This is known as interflow.

Ground water flow is the flow of water in the soil occurring below the ground water table. It is
defined as the part of stream flow that has infiltrated the ground, has entered the saturated zone.
Hence we say that runoff is the portion of precipitation which enters a well-defined stream and has
three components; namely- surface runoff, interflow runoff and ground water runoff or base flow.

Catchment is an area where water is
collected by the natural/man made
landscape. Such as river, dam, lake,
ocean.
Watershed is an area of land that
feeds all the water running under it
and draining off of it into a body of
water. It combines with other
watersheds to form a network of
rivers and streams that progressively
drain into larger water areas.

Evaporation- This is the process by which
state of substance (water) is changed from
liquid state to vapor form. Evaporation
occurs constantly from water bodies, soil
surface and even from vegetation.
Transpiration – This is the process by which
the water extracted by the roots of the plants
is lost to the atmosphere through the surface
of leaves and branches by evaporation.

Engineering Hydrologic CycleAbhishek R, Asst. Prof., Dept. of Civil Engg., JSS ATE - B'lore.

Cyclonic Precipitation- This is the precipitation associated with cyclones or moving masses of
air and involves the presence of low pressures.
Frontal cyclonic precipitation- FRONT is a barrier region between two air masses having
different temperature, densities, moisture, content etc.
Warm fronts occur where the warm air pushes out a previously lodged cold air mass. The warm
air overrides the cooler air and moves upward. Warm fronts are followed by extended periods of
light rain and drizzle.
Cold fronts occur when a mass of cooler air dislodges a mass of warm air. This type of
transition is sharper, since cold air is more dense than warm air. The rain duration is shorter, and
generally more intense, than that which occurs ahead of warm fronts.

Convective precipitation- This is due to the lifting of warm air which is lighter than the
surroundings. Generally this type of precipitation occurs in the tropics where on a hot day, the
ground surface gets heated unequally causing the warmer air to lift up and precipitation occurs
in the form of high intensity and short duration.
Orographic Precipitation- It is the most important precipitation and is responsible for most of
heavy rains in India. Orographic precipitation is caused by air masses which strike some natural
topographic barriers like mountains and cannot move forward and hence the rising amount of
precipitation. The greatest amount of precipitation falls on the windward side and leeward side
has very little precipitation.

Precipitation
• Term precipitation denotes all forms of water that reaches the earth from the atmosphere.
Forms of Precipitation
• Rain
• Principal form of precipitation
• Water drops of size larger than 0.5 mm up to 6 mm
Type of Rain Intensity
Light up to 2.5 mm/h
Moderate 2.5 to 7.5 mm/h
Heavy >7.5 mm/h

• Snow
• Snow Consists on ice crystals which usually combine
to form flakes.
• Density varies from 0.06 to 0.15 g/cm3.
• Drizzle
• Fine sprinkle of numerous water droplets.
• Size less than 0.5 mm and intensity less than 1 mm/h.
• Glaze
• When rain or drizzle come in contact with cold ground
at around 00C, the water drops freeze to form an ice
coating called glaze or freezing rain.

• Sleet
• It is frozen raindrops which forms rainfall through air at subfreezing temperature.
• Hail
• Showery precipitation in form of irregular pellets or lumps of ice size more than 8 mm.

Measurement of Precipitation
• Expressed in terms of depth to which rainfall water would stand on an area.
• 1 cm of rain fall over a catchment area of 1 km2 collects a volume of water equal to 104 m3
• Precipitation is collected and measured in a rain gauge.
• Rain gauge consist of a cylindrical vessel assembly kept in open to collect rain.
For siting a rain gauge the following considerations are important.
• Ground must be level and in the open and the instrument must present a horizontal catch surface.
• Instrument must placed such that not affected by wind, flood, splashing etc.
• Instrument be surrounded by an open fenced area at least 5.5m x 5.5m
• No object should be near to instrument than 30m or twice the height of the obstruction.

Non Recording Gauge – Symon’s Gauge
• Consists circular collecting area of 12.7 cm dia.
• Connected to funnel and funnel discharges rainfall water into receiving vessel.
• Entire arrangement is housed in a metallic container.
• Water collect is measured using a graduated measuring glass, with an accuracy 0.1 mm.
• The total rainfall collected in past 24hrs is summed up everyday at 8.30 AM and entered as days
rainfall.

• When snow is expected the funnel & receiving bottle are removed and is collected in the
outer metal container.
• The snow is then melted & the depth of resulting water is measured. Sometimes antifreeze
agents are used.

Recording type rain gauge – Syphon Rain gauge
• A funnel receives the water which is collected in a rectangular container. A float is provided at
the bottom of container, and this float raises as the water level rises in the container. Its
movement being recorded by a pen moving on a recording drum actuated by a clock work.
• When water rises, this float reaches to the top floating in water, then syphon comes into
operation and releases the water outwards through the connecting pipe, thus all water in box is
drained out.

Optimum number of rain gauge
• Catchment area per rain gauge is very small compared to the areal extent of a storm.
• Factors to be considered are economy, topography, accessibility etc.
• Number of rain gauge stations according to World Meteorological Organization (WMO)
• Ten percent of rain gauges should be of recording gauges.
Regions Ideal Acceptable As per IS (4987-
1968)
Flat 1 for 600 to 900 km2 1 for 900 to 3000 km2 1 per 520 km2
Mountainous 1 for 100 – 250 km2 25 – 1000 km2 1 per 260-390 km2
Polar 1 for 1500 to 1000 km2 1 per 130 km2

Preparation of the Data
• First to check with the data for continuity and consistency.
• If any missing data due to damage or fault in a rain gauge, then normal rainfall is used as a
standard comparison to estimate the missing data.
• The normal rainfall is the average value of rainfall at a particular date, month or year over
specified 30 year period
• The 30 year normal are computed every decade.

Estimation of Missing Data
• Px be the missing annual precipitation value of station X
• P1, P2,P3…Pm be the annual precipitation values of neighbouring M stations.
• N1,N2,N3..Nm be the normal annual precipitation value of (M+1) station including station
X. Then,
• If values of N of the neighbouring station are within 10% of Nx . Then Px can be calculated
using.
Px = (P1+P2+P3+…Pm) / M
• If the values vary considerably
Px = (Nx / M) [ (P1 / N1) + (P2 / N2) +…+ (Pm / Nm) ]

Test for Consistency of data:
• Common cause for inconsistency of record are.
• Shifting of rain gauge station to new location.
• Forest fire, land slides.
• Occurrence of observational error.
Double mass curve method
• Group of 5-10 base stations in neighbourhood of problem station is selected.
• Annual/monthly/seasonal rainfall of station X [Px] and also average rainfall of group of
station [Pav] is arranged in reverse chronological order.
• The cumulative value of Px and Pav i.e, ∑Px and ∑Pav respectively is determined.
• Value of ∑Px along Y-axis is plotted against ∑Pav along X-axis

• A decided break in slope of the resulting plot indicates a change in precipitation regime of
station X.
• More homogeneous the base station records are more accurate will be the corrected value at
station X.
• Change in slope is normally taken only where it persists for more than five years.
• The corrected value is obtained by,
Pcx = Px (Mc / Ma)
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 double mass curve.

Presentation of Rainfall Data
1. Mass Curve of Rainfall:
• The mass curve of rainfall is a plot of the accumulated precipitation against time, plotted in
chronological order.
• Records of float type and weighing-bucket type gauges are of this form.
• Mass curves of rainfall are very useful in extracting the information on the duration and
magnitude of a storm.
• Also, intensities at various time intervals in a storm can be obtained by the slope of the curve.
• For non-recording rain gauges , mass curves are prepared from a knowledge of the
approximate beginning and end of a storm and by using the mass curves of adjacent
recording gauge stations as a guide.

2. Hyetograph
• A hyetograph is a plot of the intensity of rainfall against the time.
• The hyetograph is derived from the mass curve and is usually represented as a bar chart.
• It is a very convenient way to represent characteristics of a storm and is particularly
important in the development of a design storms to predict extreme floods.
• The area under a hyetograph represents the total precipitation received in that period.
• The time interval used depends on the purpose; in urban-drainage problems small durations
are used while in flood-flow computations in larger catchments areas the intervals of about
6 h.

3. Moving average
• Moving average is a technique for smoothening out the high frequency fluctuations of a time
series and to enable the trend, if any, to be noticed. The basic principle is that a window of
time range m years is selected. Starting from the first set of m
• years of data, the average of the data for m years is calculated and placed in the middle year of
the range m .The window is next moved sequentially one time unit (year) at a time and the
mean of the m terms in the window is determined at each window location .The value of m
can be 3 or more years; usually an odd value. Generally the larger size of the range m, the
greater is the smoothening .There are many ways of averaging and the method described
above is called CENTRAL SIMPLE MOVING AVERAGE.

Mean Precipitation over an Area:
• Rain gauges represent only point sampling of the areal distribution of a storm.
• In practice, hydrological analysis requires a knowledge of the rainfall over an area, such
as over a catchment.
To convert the point rainfall values at various stations into an average value over a
catchment, the following 3 methods are in use:
• Arithmetical-mean method
• Thiessen-mean method
• Isohyetal method

Arithmetic-mean Method:-
• When the rainfall measured at various stations in a catchment show little variation over
catchment area I taken as the arithmetic mean of the station values. Thus, if P1,P2……Pi…Pn
are the rainfall values in a given period in N stations within catchment then value of mean
precipitation




N
i
Pi
N
P
N
PPPP
P
ni
1
1
......21

Thiessen-mean Method:
• In this method, the rainfall recorded at each station is given a weightage on the basis of an
area closest to the station.
• Consider the catchment area as in Fig. below containing six rain gauge stations.
• Stations 1 to 6 are joined to for a network of triangles. Perpendicular bisectors for each of the
sides of the triangle are drawn.
• These bisectors form a polygon around each station. These bounding polygons are called
Thiessen polygons.









M
i
M
i
A
A
P
A
AP
P
AAA
APAPAP
P
i
i
ii
1
1
621
662211
...
...

Isohyetal Method:
•An isohyet is a line joining points of equal rainfall magnitude.
•In the isohyetal method, the catchment area is drawn to scale and the rain-gauge stations are
marked.
•The recorded values for which areal average is to be determined are then marked on the plot at
appropriate stations.
•The isohyets are then drawn by considering point rainfalls as guided and interpolating between
them.
•The area between two adjacent isohyets are then determined with a planimeter. If the isohyets
go out of catchment, the catchment boundary is used as the bounding line.

A
PP
a
PP
A
PP
A
P
nn
n )
2
(...)
2
()
2
(
)1(
)1(
3
2
21
1
2 








Adequacy of rain gauge stations
• If there are already some raingauge stations in catchment, the optimal number of stations that
should exist to have an assigned % of error in the estimation of mean rainfall is obtained by
N =
𝐶𝑣
є
2
𝐶𝑣 =
100 𝑋𝜎 𝑚−1
𝑃
𝜎 𝑚 −1 = 1
𝑚 𝑃𝑖− 𝑃
𝑚−1
• Where N = optimal number of stations, є = allowable degree of error in the estimate of the mean
rainfall (є = 10%) , Cv = Co-efficient of variation of the rainfall values at the existing stations (in
%), Pi = Precipitation magnitude i th station, 𝑃 = mean precipitations.
• According to WMO, at least 10% of the total raingauges should be self recording type.

Module 1 Hydrology-Precipitation

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