This document provides an introduction to hydrology. It discusses the hydrologic cycle and its components like evaporation, transpiration, infiltration, etc. It also discusses different types of precipitation like rain, snow, drizzle and methods of precipitation classification. Measurement of rainfall using rain gauges and estimation of rainfall for areas between gauges using methods like arithmetic mean, Thiessen polygon and isohyetal maps are described. Optimum density of rain gauges for different terrains is also mentioned.

1. Introduction to Hydrology
Prepared by
Bahnisikha Das
Assistant Professor
Civil Engg. Department
MEFGI

3. HYDROLOGY | applications
Determining the
water balance
for a region
Determining
agricultural
water balance
Designing buffers
Predicting
floods
Designing
irrigation
schemes
Designing
drainage
systems Designi
ng
Urban
drinking
water
and
sewer
systems
Assessing
export of
sediment
&
nutrients
from fields
to water
systems
Assessing
impacts of
natural and
human induced
environmental
change on water
resources

4. HYDROLOGY | the hydrologic cycle
1
2
3
4
5
6
7
Evaporation
Transpiration
Infiltration
Condensation
Runoff
Precipitation
Subsurface flow

5. 1) Evaporation of water from the (a) surface sources like
river, lakes, oceans, (b) from the surface of soil (c) plants
through transpiration. By this process water is converted
into vapour.
2) Condensation :- It is the process by which vapour is
converted into solid form. (Clouds)
3) Precipitation in the form of rain, snow, sleet, drizzle etc.
4) Interception :- When precipitation occurs it is intercepted
by vegetation . So, this part do not contribute to the
surface water.

6. 6) Infiltration :- After water reaches the ground it infiltrates
into the ground.
7) Surface Detention :- Some part of the water is stored in
depression present on the ground.
8) Surface runoff :- The remaining water which flows on
ground or surface and joins channel is called surface
runoff.
9) Ground water flow or inter flow :- The water which
infiltrated into the ground joins the stream on the later
stage is called inter flow.

8.  Rain – It is the main form of precipitation in India. When the size of
the drop is larger than 0.5 mm, it is called rainfall.
The rainfall is classified into
Light rain – if intensity is trace to 2.5 mm/h.
Moderate rain – if intensity is 2.5 mm/hr to 7.5 mm/hr.
Heavy rain – above 7.5 mm/hr.
 Snow- Snow is made up of one or more tiny ice crystals that come
together to form the unique shapes of a snowflake.
 Drizzle- Drizzle is a light liquid precipitation consisting of liquid
water drops smaller than those of rain generally smaller than 0.5 mm
in diameter and having intensity less than 1mm/hr.

10.  Sleet- a mixture of rain and snow, when the temperature of
atmosphere and that near the ground is near 0 degree Celsius than
precipitation will be mixture of rain and snow.
 Dew - forms by condensation on the ground during night when the
surface has been cooled by outgoing radiation.

11.  The main phenomenon responsible for the precipitation
is lifting of air mass.
 Based on the factors responsible for lifting, precipitation
is classified as follows :
1. Cyclonic precipitation
2. Convective precipitation
3. Orographic precipitation

12.  Cyclonic precipitation is caused by lifting of air mass due
to the pressure difference created in an area.
 It is responsible for most of the winter rain in Haryana
and Punjab.
 Cyclonic precipitation may be of two types.

13. 1. Frontal Precipitation – When the moving warm moist air
mass is obstructed by the zone of cold air mass , the
warm air mass rises up to higher altitude where it gets
condensed and heavy rainfall occurs.
2. Non-Frontal Precipitation – When air mass rushes into
low pressure area , the air mass from low pressure area
rises to the higher altitude. At higher altitude this air
mass gets condensed and heavy rainfall occurs.

16.  In a tropical countries where on a particular hot day
ground surface heated unequally , the warm air is lifted
to the higher altitude and cooler air takes its place.
 The warm moist air is condensed at higher altitude and
rainfall of heavy intensity and of short duration.

18.  Orographic precipitation is the most important
precipitation and is responsible for most of the
precipitation in India.
 It occurs when air masses laid with moisture strikes any
topographic barriers and cant move further so rises up
and condensed at higher altitude and cause precipitation.

19. Orographic precipitation

20.  In hydrology water balance equation describes the flow of
water in and out of a system.
 A system can be one of several hydrological systems such
as column of soil or a whole drainage basin.
 The water balance equation can be used for many basic
analysis of water availability in the area. For example how
much water is available for various purposes from the
system.

21.  The water balance equation for a basin states that all the
water that enter in a system in specified period of time
must be consumed, stored or go out as surface or sub-
surface flow.
Inflow = Outflow + Change in storage
I = O + ΔS
Where,
I= Inflow in system
O= Outflow in system
Δ = Change in storage in a given time period

22.  The inflow into the system includes :
 Precipitation
 Surface inflow
 Subsurface inflow
 The outflow from the system includes :
 Surface outflow
 Subsurface outflow
 Evaporation
 Transpiration
 Evapo -transpiration

23.  The change in storage includes :
 Change in soil moisture storage
 Change in groundwater storage
 Snow cover
 Surface Storage
 Depression storage
 Channel storage

24.  For a specified area and for specified period of time , the
water balance equation can be written as,
P = Q + E+ ΔS
Where,
P= Precipitation
Q = Runoff
E= Evapotranspiration
ΔS = Change in storage

25.  The measurement of rainfall is generally done by instrument
called rain gauge.
 Rainfall and other forms of precipitation are measured in
terms of depth, the values being expressed in millimeters.
 It works on the principle that amount of rainfall collected in a
small area will represent the total amount of rainfall in large
area provided the meteorological conditions of the both areas
are identical.

26.  Non-Recording type Rain gauges
 Recording/ Automatic Rain gauges
1) Weighing bucket type
2) Tipping bucket type
3) Float- type rain gauge

27.  It is also known as Symon’s rain gauge.
 It is used for all Government rain gauge stations throughout
India.
 The capacity of the glass bottle is 100 mm of rainfall which
is placed in the casing.
 A funnel with brass rim is placed in the top of the bottle.
 The rainfall collected in the bottle is measured at every 24
hours.
 It is generally taken at 8 am in India.

28. Symon’s Raingague

29.  In this type of rain gauges the rainfall is automatically
recorded on a graph paper by some mechanical
arrangement.
 It records the cumulative rainfall over the specified
period of time.

30.  It is the most common type of self-recording
gauge.
 The weighing bucket rain gauge essentially
consists of a receiver bucket supported by a spring
or any other weighing mechanism.
 The movement of the bucket due to its increasing
weight is transmitted to a pen, which traces the
record on a clock-driven chart.

32.  It consists a sharp edge receiver.
 At the end of the receiver is provided a funnel.
 A pair of buckets are pivoted under the funnel in
such a way that when one tipper bucket receives
2.54 mm of precipitation it tips, discharging its
contents into a reservoir bringing the other bucket
under the funnel.
 Tipping of bucket completes an electric circuit
causing the movement of pen to mark on clock
driven revolving drum, which carries a record sheet.

34.  The working of a Float type rain gauge is similar to the weighing bucket
type gauge.
 A funnel receives the rainwater, which is collected in a rectangular
container.
 A float is provided at the bottom of the container. The float is raised as
the water level rises in the container, its movement being recorded by a
pen moving on a recording drum actuated by clockwork.
 When the water level in the container rises so that the float touches the
top, the siphon comes into operation, and releases the water; thus the
entire box is drained out.

35. Float type Raingague
The graphic rain gauge
1-receiver
2-floater
3-siphon
4-recording needle
5-drum with diagram
6-clock mechanism

36. 1. Arithmetic Average Method: Normal annual precipitation of the
adjacent stations are within 10% of the normal annual
precipitation of the station under consideration
2. Normal Ratio Method:
• Px = missing precipitation of station X
• P1, P2, P3,… Pm = precipitation values at m neighboring
raingauges.
• Nx = normal annual precipitation at station X.
• N1, N2, N3,…, Nm,= normal annual precipitation at m surrounding
raingauges
Estimation of Missing Rainfall Data
 mx PPP
m
P  ...
1
21







m
mx
x
N
P
N
P
N
P
m
N
P ...
2
2
1
1

37.  Sometimes a significant change may occur in and around
a particular rain gauge station.
 Then it becomes necessary to check the rainfall data from
that particular station.
 Causes of inconsistency of records are:
1. Shifting of rain gauge to a new location
2. Change in instrument
3. Change in surrounding of the rain gauge

38. 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 neighbourhood 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.
4. Accumulated precipitation at station X (∑Px) 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 (∑Px) 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.

39. Pcx – corrected precipitation at any time period t1 at station X
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
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
c
a
a
c
M
M
a
c

a
c
xcx
M
M
PP 

40.  Rain gauges rainfall represent only point sampling of the
areal distribution of a storm.
 The important rainfall for hydrological analysis is a rainfall
over an area, such as over the catchment.
 To convert the point rainfall values at various stations in to
average value over a catchment, the following methods are
used:
◦ Arithmetic mean method
◦ Thiessen polygon method
◦ Isohyets method

41.  This is the simplest method of computing the average rainfall
over a basin.
 As the name suggests, the result is obtained by the division of
the sum of rain depths recorded at different rain gauge stations
of the basin by the number of the stations.
Where,
Pi : rainfall at the ith rain gauge station
N : total no of rain gauge stations




N
i
i
ni
P
NN
PPPP
P
1
21 1..........

42.  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.
 For the construction of the polygon, the following
procedure is to be followed:

43. Step 1: Draw the area concerned to a suitable scale, showing its
boundary, locations of the rain gauges in the area and outside
but close to the boundary

44. Step 2: Join location of the rain gauges to form a network of
triangles

45. Step 3: Draw perpendicular bisectors to the triangle sides. These
bisectors form polygons around the stations

46. Step 4: Delineate the formed polygons and measure their areas
using a planimeter or by converting them into smaller regular
geometric shapes (i.e. triangles, squares, rectangles, etc.)

47. Step 5: compute the average rainfall using following formula:
Where,
A= total area of basin
Ai = area of the particular polygon
Pi = rainfall data of particular rain gauge
 m
mm
AAA
APAPAP
P



.....
.....
21
2211





M
i
i
i
total
i
M
i
i
A
A
P
A
AP
P
1
1
The ratio is called the weightage factor of station i
A
Ai

48.  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 isohyets.

50.  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 isohyets.
 Then, mean rainfall on whole basin is given by,
A
PP
A
PP
A
PP
A
P
nn
n 




 





 





 



2
...
22
1
1
32
2
21
1
The isohyetal method is superior to the other two methods
especially when the stations are large in number.

51.  To obtain the reliable rainfall data, the rain gauges in the
catchment area should be evenly and uniformly distributed.
 Their number should be neither too many nor too less as to
give unreliable results.
 Optimum numbers of rain gauges are required in catchment
area to give correct average rainfall.
 However the density of rain gauge may vary from region to
region.

52.  According to Indian Standard (IS :4987-1968) the following
terrain gauge density is required.
1) In plain terrains, 1 station / 520 km2
2) In region of average elevation of 1000 m , 1 station /
260-360 km2
3) Hilly areas with heavy rainfall , 1 station / 130 km2
Optimum Numbers of Rain gauges
N = (Cv / E)2
Cv = Coefficient of variation of rainfall values at the existing
station
E = Allowable percentage error in the estimate of basic mean
rainfall.

53.  A rainfall at a place can be described if its intensity, duration
and frequency are known.
 The intensity of the rainfall is the rate at which it is falling.
 The duration is the time for which rain is falling with given
intensity.
 Frequency is the number of times the rainfall is falling.

54.  It is a plot of accumulated precipitation against time,
plotted in chronological order.
 It gives information on duration and magnitude of a storm.
 Intensity at various time intervals in a storm = slope of the
curve.

55.  It is a plot of rainfall intensity against time interval.
 Hyetograph can be derived from the mass curve of rainfall.
 To draw the hyetograph, a convenient time period is chosen and
corresponding accumulated rainfall is noted from the mass curve of the
rainfall.
 From it, the intensity for that time period is computed and it is plotted
against the time period to get the hyetograph.
 The area under the hyetograph represents the total rainfall received in that
period.
 It is useful in estimation of design storm for predicting extreme floods.

57.  Point rainfall, also known as station rainfall refers to the rainfall
data of a station.
 It can be in the form of daily, weekly, monthly, seasonal or
annual of rainfall.
 These are also represented graphically in the form of bar
diagrams.

59. EVAPORATION
■ Evaporation is the
process whereby liquid water is converted to
water vapour by the transfer of water molecules
to the atmosphere.
■ Evaporation (and Transpiration) are small for a
runoff event and can be neglected.
■ The bulk of these abstractions take place during
the time between runoff events, which is usually
long.
■ Hence, these are more important during this time
interval.

60. Factor Affecting Evaporation
 Vapor pressure between the water surface and air above
 Temperature
 Atmospheric pressure
 Wind
 Depth of water in the water body
 Water quality
 Size of the water body
 Radiation
 Humidity

61. Vapor-pressure difference
 The rate of evaporation is proportional to the difference
between the saturation vapour pressure at the water
temperature, ew and the actual vapour pressure in the air, ea
EL = C(ew-ea)
Where;
EL= rate of evaporation (mm/day)
C= constant
ew and ea are in mm of mercury
 Evaporation continues till ew= ea
Dalton’s law of evaporation
Temperature
 Other factors remaining the same, the rate of evaporation
increases with an increase in the water temperature.
 Increase in evaporation rate with increasing temperature
Atmospheric pressure
 A decrease in the barometric pressure, as in high altitudes,
increases evaporation.

62. Wind speed
 Wind aids in removing the evaporated water vapour from the zone
of evaporation and consequently creates greater scope for
evaporation.
Water depth/ Heat storage in water Bodies
 Deep water bodies have more heat storage than shallow ones.
 A deep lake may store radiation energy received in summer and
release in winter causing less evaporation in summer and more
evaporation in winter compared to a shallow lake exposed to a
similar situation.
 More exposed area leads to more evaporation and vice-versa.
Size of water body

63. Water quality
 When solute is dissolved in water, the vapour pressure of
solution is less than that of pure water.
 Hence causes reduction in the rate of evaporation.
 Thus, under identical condition evaporation from sea water is about
2-3 % less than that from fresh water.
 Turbidity also affects the rate of evaporation by affecting the heat
transfer within the depth of water body.

64. Measurement/ Estimation of Evaporation
The amount of water evaporated from a water surface is
estimated by the following methods:
■ Empirical evaporation equation
■ Water budget method
■ Energy budget method
■ Mass transfer method
■ Actual observations
■ Pan observations

65. Empirical
Formulae Various empirical formulae have been developed by different
investigators to estimate the evaporation.
 Most of them are dependent on wind velocity, temperature and
atmospheric pressure.
 Fitzgerald’s equation (1886):
E =( 0.4 + 0.124 V ) (es - ea)
 Rohwer’s equation (1931):
E = 0.771 (1.465 – 0.000732 Pa) ( 0.44 + 0.0733 V ) (es - ea)
 Meyer’s equation (1915):
E =C ( 1 + V10 / 16 ) (es - ea)
 Lake Mead’s equation:
E = 0.0331 V (es - ea) [1-0.03 (T a-T w)]

66. The various variables used in the formulae are as follows:
E = Evaporation in mm/day
es = Saturated vapor pressure in mm Hg
ea = Actual vapor pressure in mm Hg
Pa = Mean atmospheric pressure in mm Hg
V = Wind velocity at the water surface in km/hr
Ta = Average air temperature in °C
Ta = Average water temperature in °C

67. Surface runoff - Qr
Subsurface
runoff - Qs
Inflow- Q
Outflow- Q0
Evaporation- E
Subsurface seepage losses- Qd
    


EEQQQQQP
t
S
dsr 0
Precipitation - P

69.  When wind flows on the surface, a boundary is formed.
 This method is based on turbulent mass transfer in the
boundary layer to calculate the mass of water vapor
transferred from surface to the surrounding atmosphere.
The evaporation is expressed as
E =
Where
E = Evaporation in mm/h
z1 & z2 = Arbitrary levels above water surface
e1 & e2 = Vapor pressure at z1 & z2 in km/h
v1 & v2 = wind velocity at in km/h
T = Average temperature in C between z1 & z2.
)ln()273(
))((08.46
2
1
1221
z
z
T
vvee



70.  Atmometers are provided with special surface which
are kept wet from where the evaporation takes place.
 There is continuous supply of water to the surface
for measuring the evaporation.
 A variety of Atmometers are used in the world. The
most frequently used one are Piche and Bellani
Atmometer.
The different types of atmometers indicate different
amount of evaporation under different meteorological
conditions
 However, they are not common because of their
small size.

71.  The previous methods are not directly applicable in design
problems.
 In most design problems, evaporation is measured by
evaporation pans which are called evaporimeters.
 A pan is a metal container (square or circular) with diameter
varying from 300 – 1500 mm.
 It is filled water and the water loss is measured in a specified
period.
The rate of evaporation is then correlated to the evaporation
from a reservoir.
 The most commonly used evaporimeter in India is US Weather
Bureau Class A Pan.

72. 120 cm
15 cm
Wooden
support
Galvanized
steel
25 cm
US Weather Bureau Class A Pan

73. Advantages:
 Cost of installation is reasonably low.
 It is easy for measurement.
Disadvantages:
 The pan gives higher rate of evaporation than that of large free water
surface.
 Effects of wind and radiation are more which overestimate the
evaporation rate.

74. Transpiration and its
Measurement

75. Environmental factors that
affect the rate of transpiration
1. Light
Plants transpire more rapidly in the light
than in the dark. This is largely because
light stimulates the opening of the stomata
(mechanism). Light also speeds up
transpiration by warming the leaf.

76. 2. Temperature
Plants transpire more rapidly at
higher temperatures because water
evaporates more rapidly as the
temperature rises. At 30°C, a leaf
may transpire three times as fast
as it does at 20°C.

77. 3. Humidity
At high humidity (moist air), the
stomata tends to close and thus limit the
exit of water vapour from the plant. In
addition, at high humidity the
atmosphere contains more water and
has low atmospheric demand, meaning
that it has limited capacity to absorb
more water.

78. 4. Wind
When there is no breeze, the air
surrounding a leaf becomes increasingly
humid thus reducing the rate of
transpiration. When a breeze is present,
the humid air is carried away and replaced
by drier air.

79. 5. Soil water
A plant cannot continue to transpire
rapidly if its water loss is not made up by
replacement from the soil. This
immediately reduces the rate of
transpiration (as well as of
photosynthesis).

80. 6. Stage of plant development
Transpiration depends upon plant
growth as the water requirement is
different at different stage of its growth.

81. EVAPOTRANSPIRATION

83.  Evapotranspiration or consumptive use of water is the depth of
water consumed by evaporation and transpiration during crop
growth, including water consumed by accompanying weed growth.
 Consumptive use of water includes the water deposited by rainfall
and subsequently evaporating without entering the plant system.
 Its study is important in the design of reservoir, irrigation canals,
water balance on earth surface and projects relating to water.
 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.

84. Factors Affecting Evapotranspiration
Meteorological factors:
It increases with the increase in temperature,
sunshine and wind velocity but decreases with
humidity.
Plant and soil factors:
• Greater the density of vegetation, greater is the
evapotranspiration.
• When the vegetative surface becomes dry and the
soil moisture decreases, the evaporation decreases.
• Evapotranspiration depends upon the stage of the
plant growth.

85. Measurement (or Estimation) of Evapotranspiration (or Consumptive use)
The various methods adopted are broadly classified into:
a) Direct measurement of consumptive use of water.
o Lysimeter Method
o Field Experimental Method
o Soil Moisture Studies
o Integration Method
b) Empirical formula

86. Lysimeter
Method
 Lysimeter is an evapotransporimeter, which is a circular tank
with pervious bottom whose diameter may be extended to 5m.
 Tanks are watertight cylindrical containers open at one end and
are set into ground with their rim flush with the surface.
 Consumptive use is determined by the difference of the total
water applied to the tank and that draining through the pervious
bottom and collected in a pan.
 This method is time consuming and expensive.

87. Field Experimental
Plots
 In this method the irrigation water is applied to the selected
field experimental plots.
 In the plot, all the elements of water budget are measured in a
known time interval and the evapotranspiration is determined as
Evapotranspiration = Precipitation + Irrigation input –
Runoff – Increase in soil storage – Groundwater loss
 Since it is difficult to determine the ground water loss due to
deep percolation so it can be neglected by maintaining the
moisture condition in the plot at the field capacity.

88. In this method the consumptive use of water is determined by
the summation of the products of
i. Consumptive use of water for each crop times its area.
ii. Consumptive use of water for natural vegetation time its
area.
iii. Evaporation from water surface times water surface area.
iv. Evaporation from bare land times its area.
Note: It is necessary to know the division of total area under
irrigation crops, natural vegetation, water surface area bare land
area.

89. 1. Blaney-Cridddle Equation:
 It is based on the data collected from the arid Western Zone
of the United States.
 It is based on the assumption that the potential
evapotranspiration depends only on the mean monthly
temperature and the monthly daylight hours.
 ET = evapotranspiration (cm).
 t = mean monthly temperature (°C).
 p = monthly percentage of hours of bright day.
 k = monthly consumptive use coefficient.

90. 2. Thornthwaite Equation:
The equation was derived from the data obtained from the eastern
USA.
 The monthly heat index i:
 The monthly heat indices for a year I:
 Potential evapotranspiration (cm/month) is calculated from
Where, a = (67.5 x 10-8) I3 – (77.1 x 10-6) I2 – (0.01791) I + 0.492

91. 3. Christiansen Equation:
The Christiansen equation for estimation of potential
evaporation
PET = 0.473 Qo C
Where
Qo = Solar radiation at the top of the atmosphere
converted to mm of equivalent evaporation.
C = Coefficient derived from series of climatic
measurements like temperature, humidity, wind,
sunshine, elevation etc.

92.  The Penman-Monteith equation requires daily mean
temperature, wind speed, relative humidity, and solar
radiation to predict net evapotranspiration.
 In addition to weather uncertainties, this equation is
sensitive to vegetation specific parameters (stomatal
resistance).