HYDROLOGY- HYDROLOGICAL CYCLE- RAUFALL MEASUREMENT

1. HYDROLOGICAL CYCLE
RIYA SUSAN
DEPARTMENT OF CIVIL ENGINEERING
AL AZAHAR COLLEGE OF ENGINEERING AND POLYTECHNIC

2. HYDROLOGY
It is science dealing with the occurrence, distribution and movement of water on the earth’s
surface and as well as below the earth surface.
Water occurs in the atmosphere in the form of vapour, on the surface as water and snow mainly
and below the surface as ground water.
The total water supply of earth will be in constant circulation from earth to atmosphere and
back to earth(except ground water)
This circulatory system of water is called hydrological cycle.

4. In hydrological cycle transfer of moisture to earth in the form of precipitation and this
precipitated water flows to the sea by streams and rivers.
Similarly the water in the lakes, rivers and sea evaporates and goes back into the
atmosphere.

5. PROCESSES OF HYDROLOGIC CYCLE
● PRECIPITATION : It can be called as
the fall of moisture from atmosphere
to the earth surface in any form.
Mainly two forms are there
1. LIQUID PRECIPITATION:
Eg : rainfall
1. FROZEN PRECIPITATION:
2. Eg: snow, hail etc

6. ● EVAPORATION: The water from the surfaces of streams, lakes, rivers and seas evaporates
and vapours are carried by air in the form of clouds.
● TRANSPIRATION: it is the process of water being lost from the leaves of trees from their
pores.
The total evaporation consists of 1) Surface evaporation and 2)water surface evaporation and
3)Atmospheric evaporation

7. Evapotranspiration (ET) is the sum of water
evaporation and transpiration from a surface
area to the atmosphere. Evaporation accounts
for the movement of water to the air from
sources such as the soil, canopy interception,
and water bodies

8. ● RUNOFF : Runoff is the portion of precipitation which is not evaporated.
When the rain falls down to earth’s surface a part of it is evaporated and the
remaining is available as runoff, which ultimately runs to the sea through streams and
rivers.
Runoff can be classified as:
● Surface runoff: in this water flows on the surface of the land and is the first to reach
the streams and rivers and finally to the sea.
● Sub -surface runoff: a portion of precipitation infiltrates into the surface soil and as
per the geological conditions of the basin runs as sub -surface runoff and reaches the
streams and rivers
● Groundwater flow: this is the portion of precipitation which after infiltration
percolates down and joins the ground water reservoir which is finally connected to
the sea.

9. The total hydrological cycle can be written in the form as follows
PRECIPITATION (P)= EVAPORATION + RUNOFF

10. RAINFALL (PRECIPITATION)
● Rainfall is a natural process in which vapour in the atmosphere changes into water.
● The water so formed then travels from atmosphere to earth .
FORMATION OF PRECIPITATION:
RAINFALL occurs when the capacity of an air mass to hold the vapour exceeds for the following reasons ;
1. HOT AIR Has high capacity to hold the vapour particles under suspension. When the air gets cooled
the capacity of air mass to hold vapour particles is much reduced and the vapour precipitates in the
form of water drops.
2. At times variations in pressure bring about the changes of state from vapour to rainfall.

11. EXPRESSION OF RAINFALL
The yearly, monthly or daily rainfall is expressed in cm, or millimeters of depth over a particular area which
receives the precipitation.
One centimetre of rainfall mean that the water precipitated on a certain plain area in the form of rainfall
which is not lost in any manner and there is no runoff and evaporation then all the water accumulates on the
surface in the form of layer. When the layer of this deposited water is 1cm thick it is supposed that 1 cm
rainfall has occurred.

12. MEASUREMENT OF RAINFALL- Rain gauges
Rain gauges
Non Automatic (non
recording type)
Automatic (Recording
type)
tipping
bucket
Weighing
bucket
Symon’s rain gauge Float
type

13. SYMON’S RAIN GAUGE

14. ● It consist of a funnel and a cylindrical zinc bottle.
● The diameter of the bottle and the top of the funnel is 127mm
● The funnel is fitted in the bottle neck , both are then placed in a metal casing
● The base of metal casing is enlarged to 210mm
● The capacity of bottle is such as to collect the rainfall likely to occur in 24 hours time
● The gauge is provided with graduated jar which measures water in mm
● The smallest division in jar is 0.2mm
● Installation : at the site where the rainfall is to be measured a concrete block of
cubical shape of 60cm size is constructed. The casing is fixed in the block
permanently in such a way that the top of casing is about 30cm above the natural
surface level. While fixing the gauge precaution is to be taken to perfectly level the
top of the funnel

15. Measurement
● The rain gauge is adjusted every day for measurement of rainfall
● Rainfall is measured every day at 8.30 AM
● In case of heavy rainfall it is measured for more times and is recorded as the total rainfall
of that day
Points to observed while fixing the site for rain gauge station:
1. The rain gauge should be set up in an open place by at least 30 meters away from
obstructions.
2. The rain gauge should not be set up on the top of a hill.
3. If it is not possible to find a levelled surface the site should be properly shielded from
gusty winds.
4. A fence should be erected to protect the gauge from cattle

16. FLOAT TYPE

17. ● This type of rain gauge consists of a funnel rotating drum, a float, float rod which connects the float to
the pen carrier
● The pen carrier , carries a pen which moves up and down on the graph paper fixed to the rotating drum
● It also consists of a syphon and as such this type of rain gauge is also called syphon rain gauge.
● The drum rotates around the vertical axis once in 24 hours.
● When rainfalls water enters into the box raises the float by a suitable mechanism.
● It causes the movement of the pen on a revolving chart
● The pen will mark line on the graph and the inclination of the line will depend on the movement of the
float which again depends on the intensity of rainfall
● When the float reaches upto a certain level, syphoning action starts and the box is emptied quickly
● The curve obtained on the graph is called MASS CURVE OF RAINFALL

18. INFORMATIONS FROM FLOAT TYPE RAIN
GAUGE
1. DURATION OF RAINFALL
2. INTENSITY OF RAINFALL
3. TOTAL AMOUNT OF RAINFALL

19. FACTORS AFFECTING RAINFALL
HEIGHT OF THE PLACE ABOVE SEA LEVEL: At high altitudes temperature is very low and when clouds reach
these areas they are eroded and there will be heavy rainfall
NEARNESS TO FORESTS
NEARNESS TO SEA
DIRECTION OF WINDS

20. WEIGHING BUCKET TYPE

21. ● Weighing bucket type rain gauge is most common self-recording rain gauge.
● It consists of a receiver bucket supported by a spring or lever balance or some other weighing
mechanism.
● The movement of bucket due to its increasing weight is transmitted to a pen which traces
record or some marking on a clock driven chart.
● Weighing bucket type rain gauge instrument gives a plot of the accumulated (increased)
rainfall values against the elapsed time and the curve so formed is called the mass curve.

22. TIPPING BUCKET TYPE
In this type a pair of tipping buckets is placed below a funnel.
The bucket gets filled up by 0.25 mm of rainfall and immediately it tips and empties the
water into a chamber below.
At that very instant other bucket comes below the funnel to receive rainwater.
The tipping of the bucket actuates an electrical circuit which moves a pointer to register
the rainfall on a graph.
The water collected in the chamber below could also be measured by a measuring jar.

24. RAINFALL RECORDS
The number of rain gauge stations in a catchment basin mostly depend on the extent of the area, climate and
topographic conditions.
AREA IN SQ.KM NO OF RAIN GAUGE STATIONS
Less than 125 1
126- 250 2
251-500 3
501-1000 4

25. ● Rainfall measured at every 24 hours
● Usually at 8 AM IST
● The total rainfall occurred in past 24 hours is entered against on which the measurement is done.
● It means that rainfall recorded on a particular day is the rainfall of 8 hours of the day and 16 hours of
the previous day
● Records are maintained on daily, monthly , yearly basis

26. USE OF RAINFALL RECORDS
1. To calculate the run off
2. To predict the floods
3. To study the trend of rainfall
4. To determine the water requirements of crops

27. COMPUTING AVERAGE RAINFALL OVER AN
AREA
The average rainfall over a catchment area can be computed by measuring the rainfall
A network of rain gauges should be so planned to have a correct representative
There should not be concentration of gauges in areas of heavy rainfall
3 methods:
1. Arithmetic average method
2. Thiessen polygon method
3. Isohyetal method

28. ARITHMETIC AVERAGE METHOD
● 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.
● If the rain gauges are uniformly distributed over the area and the rainfall varies in a very
regular manner, the results obtained by this method will be quite satisfactory and will not differ
much than those obtained by other methods.
● This method can be used for the storm rainfall, monthly or annual rainfall average
computations.

29. ● AVERAGE DEPTH OF RAINFALL OVER THE BASIN = TOTAL RAINFALL/ NO OF RAIN GAUGE STATIONS
RAIN GAUGE STATIONS RAINFALL IN MM
A 54
B 48
C 50
D 59
E 52
5 263
AVG RAINFALL = 263/5 = 52.6mm

30. If 2 of rain gauge stations are located in a forest region which recorded the
highest rainfall when compared to the other stations the method of computation
has to be done in this way
Rain gauge stations Rainfall in mm
A 54
B 126
C 50
D 120
E 52

31. Sum of rainfalls recorded in stations A,C E which are nearer to each other =54+50+52=156mm
Average rainfall in these stations=156/3 = 52mm
Sum of rain fall recorded in stations B and D which are nearer to each other = 126+120= 246mm
Average rainfall in these two stations = 246/2= 123mm
Average rainfall in the entire basin = 52+123 /2 = 175/2 =87.5mm

32. THIESSON’ 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.

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

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

36. 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.)
Step 5: Compute the average rainfall using the
following formula
P1xA1 + P2 xA2 +... + PnxAn
(A1 + A2 + .... + A)

37. Station No. Bi-sectional
areas (Ai)
[km2]
Measured
precipitation (Pi)
[mm]
(Col. 2 * Col. 3)
(Ai *Pi)
P1 25 10 250
P2 125 15 1875
P3 80 20 1600
P4 90 17 1530
P5 120 25 3000
P6 115 40 4600
P7 130 12 1560
Total 685 14415

38. ISOHYTEL 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.

39. Step 1: Draw the area under study to scale and
mark rain gauges on it. Put at each of the rain
gauge location the recorded values of rainfall at the
station, for the period within which the average is
required to be determined.
Step 2:DRAW the isohyets of various values of the
point rainfall data as guidelines and interpolating
between them.

40. Step 3: Determine the area between each pair of the isohyet lines, either by a planimeter or
by converting the areas into smaller regular geometric shapes
Step 4: Calculate the average rainfall using the following formula:
A1 (P1 + P2)/2 + A2 (P2 + P3)/2 + . . . + An-1(Pn-1 + Pn)/2
P
av = (A1 + A2 + . . . + An)
Pi = Value of Isohyet lines
Ai = Area between pair of isohyet lines.

41. COMPARISON BETWEEN 3 METHODS
ARITHMETIC MEAN METHOD THIESSEN POLYGON METHOD ISOHYETAL METHOD
● This is the simplest and
easiest method to compute
average rainfall.
● If the recording stations and
rainfall is uniformly
distributed over the entire
catchment, then this method
is equally accurate
In this method the rainfall stations
located at a short distance beyond
the boundary of drainage are also
used to determine the mean
rainfall of the basin, but their
influence diminishes as the
distance from the boundary
increases.
1It is the most accurate method if
the contours are drawn correctly.
However to obtain the best results
good judgment in drawing the
isohyets and in assigning the
proper mean rainfall values to the
area between them is required.
In this method every station has
equal weight regardless its
location.
commonly used for flat and low
rugged areas
1It is the best method for rugged
areas and hilly regions.