Module 1 introduction

Prepared By:
Assistant Professor Ankit Patel
ME ( CPM)

INRODUCTION:
HYDROLOGY:
 Hydro – water and logos – science
 Hydrology means science of water.
 The flow of water across and through near surface environments
OR
 It is the science that deals with the occurrence, circulation and
distribution of water on the earth.
HYDROLOGY DEALS WITH:
 Estimation of water resources
 The study of processes such as precipitation, runoff,
evapotranspiration and their interaction.
 The study of problems such as floods, droughts and strategies to
combat them
BITS Edu CampusProf. Ankit Patel

ENGINEERING HYDROLOGY APPLICATOINS:
 The maximum probable flood that may occur at a given sites and its
frequency
 Ground water development
 The maximum intensity off rainfall and its intensity
 Various methods of flood forecasting and flood control
 Selection of suitable site for a dam, reservoir and hydro electric power
generation.
 The capacity of storage structures such as reservoir

WATER FROM
OCEANS
ATMOSPHERE
GROUND
SUN
THE HYDROLOGIC CYCLE

The water cycle
is also called the
cycle.
BITS Edu Campus Prof. Ankit Patel

Water that is stored
in the oceans and
lakes can
and become a gas.

As the water
rises through the
atmosphere, it cools,
condenses and becomes

When the water gets
heavy enough it can fall
to the ground in the form
of different types of

If the lithosphere (ground) is
saturated, the water that
has fallen can become
and flow directly into
streams, rivers, or lakes.BITS Edu Campus Prof. Ankit Patel

If the lithosphere is not
saturated, the water will
the lithosphere and move
into the zone of
or the zone of

The interface (boundary)
between these two zones
is called the

The roots of plants can
reach into the zone of
soak up the water, and the
water can then re-enter the
atmosphere through the
process of

evaporation
condensation
infiltration
transpiration
precipitation
evaporation

THE HYDROLOGIC CYCLE:
 The hydrologic cycle is the general continuous circulation
of water from the oceans to the atmosphere, to the
ground and back to the oceans again.
 Sun is the main source of energy for hydrologic cycle.
 Let us consider the cycle begins with oceans

The various stages of hydrologic cycle are
 Evaporation
 Precipitation
 Infiltration
 Transpiration
It is a continuous process
Each path of hydrologic cycle may have one or more
of the following
 Transportation of water
 Temporary storage
 Change of state

Catchment area: The area of a land draning into a stream or a water
course at a given location is known as catchment area.
It is also called as drainage area. In usa it is known as watershead.

• The quantification of the hydrologic cycle which is an
open system, can be represented by a mass balance
equation, where inputs minus outputs are equal to the
change in storage.
I - O = DS
Water Budget Equation

Inflow:
1. Precipitation
2. Import defined as water channeled into a given area.
3. Groundwater inflow from adjoining areas.
Outflow:
1. Surface runoff outflow
2. Export defined as water channeled out of the same area.
3. Evaporation
4. Transpiration
Change in Storage: This occurs as change in:
1. Groundwater
2. Soil moisture
3. Surface reservoir water and depression storage
Water Balance Components

Precipitation Introduction
• All forms of water that reach the earth from the
atmosphere is called Precipitation.
• The usual forms are rainfall, snowfall, frost, hail,
dew. Of all these, the first two contribute
significant amounts of water.
• Rainfall being the predominant form of
precipitation causing stream flow, especially the
flood flow in majority of rivers. Thus, in this
context, rainfall is used synonymously with
precipitation.

FORMS OF PRECIPITATION:
 Rain the condensed water vapour of the atmosphere falling in
drops (>0.5 mm, maximum size—6 mm) from the clouds.
 Snow ice crystals resulting from sublimation (i.e., water vapour
condenses to ice)
 Drizzle a light steady rain in fine drops (0.5 mm) and intensity
<1 mm/hr
 Glaze freezing of drizzle or rain when they come in contact with
cold objects.
 Sleet frozen rain drops while falling through air at subfreezing
temperature.
 Hail small lumps of ice (>5 mm in diameter) formed by
alternate freezing and melting, when they are carried up and
down in highly turbulent air currents.

Forms of precipitation

TYPES OF PRECIPITATION
PRECIPITATION
CONVECTIVE OROGRAPHIC CYCLONIC
NON-FRONTALFRONTAL
WARM FRONT COLD FRONT

Types of Precipitations.
1. Cyclonic Precipitation:
It is caused by the lifiting of an air mass due to the pressure difference.
If low pressure occurs in an area, air will flow horizontally from the
surrounding area, causing the air in the low pressure to uplift.
It is divided into
1. Frontal Precipitation
2. Non Frontal Precipitation

A. Frontal Precipitation
 When two air masses due to contrasting temperatures and densities
clash with each other, condensation and precipitation occur at the
surface of contact. This surface of contact is called a ‘front’ or ‘frontal
surface’.
 If a cold air mass drives out a warm air mass’ it is called a ‘cold front’
and if a warm air mass replaces the retreating cold air mass, it is
called a ‘warm front’. On the other hand, if the two air masses are
drawn simultaneously towards a low pressure area, the front
developed is stationary and is called a ‘stationary front’. Cold front
causes intense precipitation on comparatively small areas, while the
precipitation due to warm front is less intense but is spread over a
comparatively larger area. Cold fronts move faster than warm fronts
and usually overtake them, the frontal surfaces of cold and warm air
sliding against each other

B. Non Frontal Precipitation
In case of non frontal precipitation the moist warm air mass is stationary
and the moving cold air mass meets it. Thus due to lightness of the
warm air mass there is passive ascent of warm air over cold air owing to
the active under cutting. When the lifted warm air cools down at higher
altitude precipitation occurs.
2. Convective Precipitation:
It is caused by natural rising of warmer lighter air in colder, denser
surroundings. The difference in temperature may result from unequal
heating at the surface, unequal cooling at the top of the air layer.
Generally this kind of precipitation occurs in tropics where on a hot day
the ground surface gets heated unequally causing the warmer air to lift
up as the colder air come to take its place.

3. Orographic Precipitation:
It is caused by moist air masses, which strike some natural topographic
barriers like mountains, rise up causing condensation and precipitation.
The greatest amount of precipitation falls on the windward side and the
leeward side often has very little precipitation

Causes of PrecipitationBITS Edu CampusProf. Ankit Patel

RAINFALL MEASURMENT:
 Rainfall is the main source of water used for various purpose.
 Instrument used to collect and measure the precipitation is called rain
gauge.
 Rainfall at a place can be measured by a rain guage, usually in
cm.
 Rain guage is a cylindrical vessel assembly kept in open to
collect rain.

 TYPES OF RAINGUAGE
TYPES OF RAINGUAGE
NON-RECORDING RAINGUAGE SELF RECORDING(AUTOMATIC)
SYMON’S GUAGE
STANDARD NON RECORDING
•TIPPING BUCKET
•WEIGHING TYPE
•FLOAT TYPE

1. Nonrecording Gauges:
 The nonrecording gauge used in India is Symons gauge.
 Consists of a circular collecting area of 12.7 cm diameter
connected to the funnel and the rim of the collector is set in a
horizontal plane at a height of 30.5 cm above the ground level.

 The funnel discharges the rainfall catch into a receiving vessel
which is housed in a metallic container.
 The water in the receiving vessel is measured by a graduated
measuring glass with an accuracy of 0.1 mm.
 The rainfall is measured at 8:30 a.m. and is recorded as the
rainfall of that day.
 The collecting bottle cannot hold more than 10 cm of rain and
thus in case of heavy rainfall frequent readings must be taken.
 However last reading must be taken at 8:30 a.m. and the sum
of previous readings in the past 24 hours is the rainfall of that
day.

(a) Tipping-Bucket type:
 This is a 30.5 cm size raingauge used by US Weather Bureau.
 The catch from the funnel falls onto one of a pair of small
buckets.
 These buckets are so balanced that when .25 mm of rainfall
collects in one bucket it tips and brings the other bucket in
position and the water is collected in storage can.
 The tipping actuates an electrically driven pen to plot the
intensity of rainfall with time.
 The water in storage can is measured regularly to give total
rainfall.

(b) Weighing-Bucket type
 The weighing bucket rainguage essentially consisits of a
receiver bucket supported by a spring or lever balance or any
other weighing mechanism. The movement of bucket due to
its on the clock driven recording drum.
 The rainguage produces a graph of cumulative rainfall versus
time.

(c) Float type:
 In this type of rainguage a funnel is provided at one end of the
rectangular container and a rotating recording drum is provided at
the other end.
 The working of a float type rainguage is similar to the weighing
bucket type guage.
 A funnel receives the rain water which is collected In a
rectangular container. A float is provided at the bottom of
container the float is raised as the water level rises in the container
its movement is being recorded by a pen moving on the graph
paper wrapped on the recording clock driven drum.
 It consist of syphon which starts functioning when flot rises at
some definite height and the container goes on emptying
gradually.

The graphic rain gauge
1-receiver
2-floater
3-siphon
4-recording needle
5-drum with diagram
6-clock mechanism
The rainguages that automatically record the intensity of
rainfall over a period of time in the form of pen trace or a
clock driven chart.

Errors in precipitation measurement
by Rain Gauges
 Instrumental errors
 Errors in scale reading
 About 0.25mm of water is initially required to wet the
surface of gauge
 Loss of water by evaporation
 Leakage in measuring cylinder
 Wind speed reduces measured amount of rain in the
rain gauges.

COMPUTATION OF AVERAGE RAINFALL OVER A
BASIN:
 In order to compute the average Rainfall over a basin or catchment
area, the rainfall is measured at a number of stations located in that
area.
 If the basement area contains more than one rainguage station then
following methods are used for computation of average rainfall.
 Arithmetic mean method
 Thiessen polygon method
 Isohytel method

• When the area is physically and climatically
homogenous and the required accuracy is small,
the average rainfall ( ) for a basin can be
obtained as the arithmetic mean of the hi values
recorded at various stations.
• Applicable rarely for practical purpose
Arithmetic Mean Method




N
i
i
ni
P
NN
PPPP
P
1
21 1..........
P

Thiessen Polygon Method:
This is the weighted mean method. The rainfall is
never uniform over the entire area of the basin or
catchment, 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. This method is
more suitable under the following conditions:
- For areas of moderate size.
- When rainfall stations are few compared to the size
of the basin.

Thiessen polygons ……….

A1
A2
A3
A4
A5
A6
A7
A8
P1
P2
P3
P4
P5
P6
P7
P8

 m
mm
AAA
APAPAP
P



.....
.....
21
2211





M
i
i
i
total
i
M
i
i
A
A
P
A
AP
P
1
1
Generally for M station
The ratio is called the weightage factor of station i
A
Ai

• An isohyet is a line joining points of equal rainfall
magnitude.
Isohyetal Method
F
B
E
A
C
D
12
9.2
4.0
7.0
7.2
9.1
10.0
10.0
12
8
8
6
6
4
4
a1a1
a2
a3
a4
a5

• P1, P2, P3, …. , Pn – the values of the isohytes
• a1, a2, a3, …., a4 – are the inter isohytes area respectively
• A – the total catchment area
• - the mean precipitation over the catchment
Isohyetal Method
P
A
PP
a
PP
a
PP
a
P
nn
n 




 





 





 



2
...
22
1
1
32
2
21
1
The isohyet method is superior to the other two methods
especially when the stations are large in number.
NOTE

COMPARISON BETWEEN METHODS FOR
CALCULATING AVERAGE RAINFALL
 Arithmetic mean method
 Assumes uniform rainfall distribution
 Very seldom occurs
 Easiest to use but least accurate
 Thiessen polygon method
 Assumes linear variation
 Use when gages are not uniformly distributed
 Can use gages outside of watershed
 Isohyetal method
 Theoretically the most accurate
 Most time consuming method
 Can use gages outside of the watershed
BITSEduCampusProf.AnkitPatel

Test for consistency record
• Let a group of 5 to 10 base stations in the
neighbourhood of the problem station X is selected
• Arrange the data of X stn rainfall and the average of the
neighbouring stations in reverse chronological order
(from recent to old record)
• Accumulate the precipitation of station X and
the average values of the group base stations
starting from the latest record.
• Plot the against as shown on the next
figure
• A decided break in the slope of the resulting plot is
observed that indicates a change in precipitation
regime of station X, i.e inconsistency.
• Therefore, is should be corrected by the factor shon on
the next slide
  xP
  avgP  xP
  avgP

(Double mass curve techniques)
Double Mass Curve Analysis
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5 2 2.5
Accumulated annual rainfall of neigbouring stns in 10^3 cm
accumulatedannualrainfallofXstnin10^3cm

a
c
M
M
a
c

a
c
xcx
M
M
PP 
Pcx – corrected precipitation at any time period t1 at
stationX
Px – Original recorded precp. at time period t1 at
station X
Mc – corrected slope of the double mass curve
Ma – original slope of the mass curve

A rainfall at a palce can be completely described if its intensity,
duration and frequency are known. The intensity of rain is the
rain at which it is falling. The duration is the time for which
rainfall falling with the given intensity. Frequency is the
number of times rainfall falls.
A few commonly used methods are
 Mass curve of rainfall
 Hyetograph

Mass curve of rainfall
0
10
20
30
40
50
60
0 20 40 60 80 100 120
Time, hour
accumulatedprecipitation,mm
Mass Curve of Rainfall:
The total accumulated precipitation is plotted against time

Hyetograph of a storm
0
0.1
0.2
0.3
0.4
0.5
0 – 8 8 – 16 16 – 24 24 – 32 32 – 40 40 – 48
Time, hours
Intensity,cm/hr
Hyetograph
-A hyetograph is a graphical representation of the relationship
between the rainfall intensity and time

 Evaporation is a process by which water is converted
into gaseous state and is returned to the atmosphere as
Vapour.
 It is the process by which water in the liquid form
transforms into vapour through the transfer of energy.
 When water is converted from solid state to vapour
state without passing through liquid state then it is
called Sublimition.
 Due to evaporation loss of water in the form of vapour
from soil, snow, lakes, rivers, seas, reservoirs.
EVAPORATION

• Rate of evaporation dependent on
• Rate of evaporation increase with increase of temperature as
well as increase in wind velocity, but decrease with the increase
in humidity as well as with increase of dissolved salts in water.
• Further as open area of water and soil is more evaporation also
more and vice versa.

 John dalton was first scientist to describe the
process of evaporation scientifically.
Evaporation rate varies directly with difference
of vapour pressure between air and water.
E= C (ew – ea)
where,
E= rate of evaporation (mm/day)
ew &ea=vapour pressure in water and in air.
C = constant
This equation is called john dalton’s law of
evaporation.
Dalton’s law of Evaporation

i. The vapour pressure at the water surface and air above
ii. Air and water temperatures
iii. Wind speed
iv. Atmospheric pressure
v. Quality of water
vi. Size of water body
vii. Depth of water in the water body
viii. Humidity
ix. Radiation
Factor affecting evaporation

1. Empirical formulae
2. Water budget method
3. Energy budget method
4. Mass transfer method
5. Actual observations
6. Pan observations
Measurement of Evaporation

 Transpiration is the process by which water leaves the
water body of a living plants and reaches the
atmosphere as water vapour. The water is taken up by
the plant root system and escape through the leaves.
 The soil water used by crops is through the process of
evaporation. This loss of water in combined form of
evaporation and transpiration is called
EVAPOTRANSPIRATION (ET). Water use by
evapotranspiration is known as consumptive use of
crops.
 Evapotranspiration of water by a crop is the depth of
water consumed by evaporation and transpiration during
crop growth.
EVAPOTRANSPIRATION

 When consumptive use of crop is known, the water use
of large unit can be calculated.
 Consumptive use of for each crop is can be determined
which gives water requirement of crop. The value of
consumptive use of water varies from crop to crop and
also for the same crop it varies with time as well as
place.
 Detailed study of evapotranspiration is essential in in
the design of reservoir, irrigation canals, water balance
on earth surface and projects relating to water.

 Factors affecting evapotranspiration are same that
affect evaporation and transpiration. Some of the factor
are as follows.
1. Metrological factors : potential evapotranspiration is
controlled essentially in metrological factor.
Evapotranspiration increase as temperature, sunshine,
and wind velocity increase but decrease as the
humidity decrease.
2. Plant and soil factors:
 Evapotranspiration is less if adjoining land is
cropped because air becomes cool and more humid.
Factor affecting Evapotranspiration

1) Direct measurement of consumptive use of water:
a) Tank or lysimeter method
b) Field experimental plots
c)soil moisture studies
d)Integration method
e)Inflow and outflow method
2) Use of Empirical Formulae :
a) Blaney – Criddle equation
b) Thornthwaite equation
c) Christiansen equation
Measurement of Evapotranspiration or
Consumptive use of water

 INFILTRATION:
 Water entering the soil at the ground surface is called infiltration. It
replenishes the soil moisture deficiency and the excess moves
downward by the force of gravity called deep seepage or
 percolation and builds up the ground water table. The maximum rate
at which the soil in any given condition is capable of absorbing water
is called its infiltration capacity (fp).
 Infiltration (f) often begins at a high rate (20 to 25 cm/hr) and
decreases to a fairly steady state rate (fc) as the rain continues, called
the ultimate fp (= 1.25 to 2.0 cm/hr) (Fig. 3.6). The infiltration rate (f)
at any time t is given by Horton’s equation.

 The infiltration takes place at capacity rates only when the
intensity of rainfall equals or exceeds fp; i.e., f = fp when i ≥ fp;
but when i < fp, f < fp and the actual infiltration rates are
approximately equal to the rainfall rates.
 The infiltration depends upon the intensity and duration of
rainfall, weather (temperature), soil characteristics, vegetal
cover, land use, initial soil moisture content (initial wet- ness),
entrapped air and depth of the ground water table. The vegetal
cover provides protection against rain drop impact and helps to
increase infiltration.

 The downward flow of water from the land surface into
the soil medium is called infiltration.
 The part of precipitation that is not available as surface
runoff is referred as precipitation loss or abstraction.
Infiltration is a dominant abstraction.
 Infiltration first replenishes the soil moisture deficiency,
the excess water moves downward by force of gravity.
This downward movement under gravity is called
percolation (seepage).
 The rate at with water enters the ground surface and
then flow downwards is known as infiltration rate.
INFILTRATION

 Infiltration rate is higher in the beginning because it has
to meet the requirement of dry soil. It attains steady
constant value as passage of time.
 Infiltration rate is used for computation of water loss
due to infiltration for the determination of surface
runoff.
 Unit of infiltration rate is mm/hour.
 Maximum rate at which soil in any given condition is
capable of absorbing water is called its infiltration
capacity(f).
 Infiltration has great influence on rainfall, runoff,
transpiration of plants and evaporation from soil
surface.
 Infiltration is responsible for subsurface flow and
ground water flow. BITS Edu CampusProf. Ankit Patel

1. Soil moisture
2. Intensity of rainfall
3. Temperature
4. Duration of the rainfall
5. Texture of soil
6. Vegetal cover
7. Condition of soil surface
8. Compaction of soil
9. Depth of surface detention
Factor affecting Infiltration

 Infiltration capacity of soil does not remain constant, it
decrease with an increase of duration of rainfall.
 In hydrological computations for computing surface
runoff and flood discharge, use an average constant
value of infiltration rate called infiltration index.
Infiltration Indices

 Figure is superimposition of a rainfall hyetograph and
infiltration rate curve appropriate to the soil of
catchment.
 The area of hyetograph above infiltration curve
represent the surface runoff and that of below
represent the infiltration.
 If the rainfall intensity is always greater than the
infiltration rate, results are satisfactory, but if rainfall
fluctuate above and below the problems becomes
complicated. In this case is assumed to be
constant(average infiltration) throughout the rainfall
time.
 Infiltration indices are extremely useful for the analysis
of major floods when the soil is wet and infiltration rate
becomes constant.

1. Ø-index (phi-index)
2. W-index (dablu-index)
3. Wmin.-index.
Types of infiltration indices

(i) φ-index—The φ-index is defined as that rate of rainfall above which
the rainfall volume equals the runoff volume. The φ-index is relatively
simple and all losses due to infiltration, interception and depression
storage (i.e., storage in pits and ponds) are accounted
for; hence,
φ-index = P-Q
Where p = Precipitation
Q= Surface runoff
provided i > φ throughout the storm. The bar graph showing the time
distribution of rainfall, storm loss and rainfall excess (net rain or storm
runoff) is called a hyetograph, Fig. 3.12. Thus, the φ- index divides the
rainfall into net rain and storm loss.

(ii) W-index—The W-index is the average infiltration rate during the
time rainfall intensity exceeds the infiltration capacity rate, i.e.,

The W-index attempts to allow for depression storage, short rainless
periods during a storm and eliminates all rain periods during which i <
fp. Thus, the W-index is essentially equal to the φ-index minus the
average rate of retention by interception and depression storage,
i.e., W < φ.
(iii) Wmin-index—In this method, an average infiltration loss is
assumed throughout the storm, for the period i > f.

Measurement Of Infiltration
(1) Flooding type – simple and double ring type infiltrometers (as shown in
below
Figure) and (2) Rainfall simulator.

 The ring infiltrometer is a simple device used to measure
the infiltration capacity of a soil. An open cylindrical ring
is forced a few centimeters into the soil and filled with
water. By measuring the rate at which the level of ponded
water decreases, one can obtain an estimate of the
infiltration capacity as a function of time. In simple/single
ring infiltrometer infiltrated water spreads out in outer
adjoining area leading to over-estimation.
 In double ring type the outer ring provides water jacket to
the infiltrating water from the inner ring and hence
prevents the spreading out of the infiltrating water from
the inner ring. The water depths in both rings are kept the
same during the observation period. Because of high
spatial variability, several measurements must be taken to
get a reasonable estimate of the infiltration capacity of a
soil.

Thank You

Module 1 introduction

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Module 1 introduction