Hydrology means science of water. It is the science that deals with the occurance, circulation and distribution of water on the earth. Hydrology is a broad subject of an inter-disciplinary nature drawing support from allied sciences.

2. INRODUCTION:
HYDROLOGY:
 Hydrology means science of water.
 It is the science that deals with the occurance, circulation
and distribution of water on the earth.
 Hydrology is a broad subject of an inter-disciplinary
nature drawing support from allied sciences.
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

3. ENGINEERING HYDROLOGY(APPLIED HYDROLOGY):
 A study concerned with engineering applications
ENGINEERING HYDROLOGY APPLICATOINS:
 In design
 Operations of projects dealing with water supply
 Irrigation and drainage
 Water power
 Flood control
 Navigation
 Costal works
 Salinity control
 Recreational uses of water etc.

4. WATER FROM
OCEANS
ATMOSPHERE
GROUND
SUN
THE HYDROLOGIC CYCLE

7. 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 oceas again.
 Sun is the main source of energy for hydrologic cycle.
 Let us consider the cycle begins with oceans

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

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

10. FORMS OF PRECIPITATION:
 Rain
 Snow
 Drizzle
 Glaze
 Sleet
 Hail

11. RAINFALL MEASURMENT:
 Rainfall is expressed interms of the depth to which
rain water would stand on an area if allthe rain
were collected in it.
 Rainfall is measured by rainguage
 Rainguage is a cylindrical vessel assembly kept in
open to collect rain

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

13. NON RECORDING TYPE RAINGUAGES:
 It does not record the rainfall directly but only
collect the rain water, which when measured gives
the total amount of rainfall at the given point of
time.

14. Precipitation gauge
1 - pole
2 - collector
3 - support-
galvanized
metal sheet
4 – funnel
5 - steel ring
1. Non recording gauge

15. 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.

16. COMPUTATION OF AVERAGE RAINFALL OVER A
BASIN:
 Inorder 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

17. Arithmetic mean method:
 Average rainfall is calculated by arithmetic average
of recorded rainfall at various stations of selected
area
 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.




N
i
i
ni
P
NN
PPPP
P
1
21 1..........

18. 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.
- In moderate rugged areas.

20.  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 Ai/A is called the weightage factor of
station i

21. isohyetal Method:
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. This method is more suited under the following
conditions:
- For hilly and rugged areas.
- For large areas over 5000 km2.
- For areas where the network of rainfall stations within
the storm area is sufficiently dense, isohyetal method
gives more accurate distribution of rainfall.

22. • 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

23. F
B
E
A
C
D
12
9.2
4.0
7.0
7.2
9.1
10.0
12
8
6
a1a1
a2
a3
a4
a5
The isohyet method is superior to the other two methods
especially when the stations are large in number.
• An isohyet is a line joining points of equal
rainfall magnitude.

24. • Given annual precipitation values – P1, P2, P3,… Pm
at neighboring M stations of station X 1, 2, 3 & m
respectively
• The normal annual precipitation given by N1, N2,
N3,…, Nm, Ni… (including station X)
• To find the missing precipitation, Px , of station X








m
mx
x
N
P
N
P
N
P
M
N
P ...
2
2
1
1
 Before using rainfall data, it is necessary to check the
data for continuing and consistency
◦ Missing data
◦ Record errors
Estimation of Missing Data

25. 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

26. (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

27. 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

28. A few commonly used methods are
 Mass curve of rainfall
 Hytograph

29. 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
- is a plot of the accumulated precipitation against time, plotted in
chronological order

31. 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 percipitation is plotted against time

32. RUNOFF:
It is a part of precipitation which is
transmitted through natural surface channels,
streams etc.
 Runoff is overlandflow and interflow which enters
stream immediately after precipitation
 Runoff includes
 surface flow
 interflow
 groundwater flow

33. RUNOFF
DIRECT RUNOFF
BASE FLOW
RUNOFF

34.  Rainfall characteristics
 Metrological factors
 Water shed factor
 Storage characteristics

35. The Rational Method properly understood and applied can
produce satisfactory results for urban storm sewer and small on-
site detention design.
Rational Formula:
The Rational Method is based on the Rational Formula:
RATIONAL METHOD
Q =CIA
Q = the maximum rate of runoff (cfs)
C = a runoff coefficient that is the ratio between the runoff
volume from an area and the average rate of rainfall depth
over a given duration for that area
I = average intensity of rainfall in inches per hour for a
duration equal to the time of concentration, tc
A = area (acres)

36.  Sevaral empherical formulae, tables, curves
relating to runoff and rainfall have been
developed to estimate runoff over catchment
area.
 Binnie’s percentage
 Barlow’s tables
 Strange’s tables and curves
 Inglis and Desouza formula
 Lacy formula
 Khosla’s formula etc