hydrology ,A programme of groundwater investigations is to obtain information on the resource through systematic collection, synthesis, interpretation and compilation of data. It seeks information on its occurrence, movement, storage, recharge, discharge, quality & quantity. It includes the study of its geological, environmental, as well as the hydrologic and hydraulic aspects of its flow system. Geologic Methods A geologic investigation begins with the collection, analysis, and hydrogeologic interpretation of existing topographic map, aerial photographs, geologic maps and logs, and other pertinent records. This should be supplemented, when possible, by geologic field reconnaissance and by evaluation of available hydrologic data on: stream flow and springs; well yields; groundwater recharge & levels; and water quality

1. Chapter2: Precipitation
• Precipitation: all forms of water falling from the
atmosphere to the earth.
• The usual forms of precipitation are rainfall, snow fall,
frost and dew.
• Rainfall is the predominant form of precipitation causing
stream flow in majority of the rivers.

2. FORMS OF PRECIPITATION
• Precipitation can be in two forms:
Liquid: rain, drizzle, dew
• Frozen (solid): snow, hail, sleet
• It is measured in terms of depth of water that would
accumulate on a plane, (mm or in.) or its time rate, called
intensity (mm/hr or in/hr).
• Rainfall: precipitation in the form of water with drop size
of 0.5 to 6 mm. Depending on its intensity rainfall is
classified as light, moderate and heavy rain fall.

3. Type Intensity
1. Light rain Trace to 2.5mm/hr
2. Moderator rain 2.5 to 7.5 mm/hr
3. Heavy rain >7.5mm/hr
Types of rainfall intensities

4. • Snow: consists of ice crystals. Show varies in density ranging
from 0.05 to 0.15 g/cc with an average value of 0.1g/cc
• Drizzle: affine sprinkle of numerous water droplets of size less
than 0.5mm and intensity /mm/hr. The droplets are so small that
they float in air.
• Glaze: This is formed when rain or drizzle meets cold ground at
about 0c, the water drops freeze to form an ice coating. This is
called glaze.

5. • Sleet: It is frozen raindrops of transparent grains which
formed when rainfall through air at subfreezing temperature.
• Sometimes sleet denotes precipitation of snow and rain
simultaneously.
• Hail: It is a showery precipitation in the form of irregular
pellets or lumps of ice of size more than 8mm.
• Hails occur in violent thunderstorms in which vertical
currents are very strong.

6. Essential requirement for precipitation to occur
Moisture in the atmosphere
Presence of nuclei around which condensation vapor
takes place
Dynamic cooling responsible for condensation of water
vapor
Precipitation product must reach the ground in some
form

7. TYPES OF PRECIPITATION
1.Cyclonic precipitation:
Based on the direction of the wind cyclonic rainfall is either
frontal or non-frontal.
• Frontal rainfall is further classified as warm frontal and
cold frontal.
• Warm frontal precipitation is formed when the warm air
advances upward over the cold air and when the vice
versa occurs the cold frontal precipitation is formed.

8. Frontal Lifting
• Boundary between air masses with different properties is
called a front
• Cold front occurs when cold air advances towards warm
air
• Warm front occurs when warm air overrides cold air
Cold front (produces cumulus cloud) Cold front (produces stratus cloud)

9. • The uneven heating of the earth’s surface by the sun
results high and low pressure regions, and air masses
move from high pressure regions to low pressure
regions.
• If warm air replaces colder air, the front is called a warm
front. If cold air displaces warm air, its front is called a
cold front

10. 2. Conventional precipitation:
• It is formed when the warm air is moved upwards in cold
air mass due to unequal heating of the earth surface
relative to the top air.
• Heated air near the ground expands and absorbs more
water moisture.
• The warm moisture-laden air moves up and gets
condensed due to lower temperature, thus producing
precipitation

11. Hot earth
surface
Convective precipitation
• occurs when the air near the ground is heated by the earth’s warm surface.
This warm air rises, cools and creates precipitation.

12. • it is caused from mechanical lifting of air mass over the
mountain (due to mountain barriers).
• In orographic rainfall, the rainfall amount is more in the
direction of the wind (in the direction from which the wind
comes), wind ward while the amount is less in the
opposite direction, leeward.
3. Orographic precipitation:

13. Orographic lifting
Orographic uplift occurs when air is forced to rise because of the physical
presence of elevated land.

14. Measurement of Precipitation
• Precipitation is expressed in terms of depth (mm) to
which rainfall water would stand on an area if all the rain
were collected on it.
• Thus 1cm (10mm) of rainfall over a catchment area of
1km 2 represents a volume of water equal to 10 4 m 3 .
• In the case of snow fall, an equivalent depth of water is
used as the depth of precipitation. The precipitation is
collected and measured in rain gauges

15. Rain gauges are of two types.
 Non-recording rain gauges
 Recording rain gauges
• RECORDING RAIN GAUGES
 Tipping bucket type
 Weighing bucket type
 Natural-siphon type
TYPES OF RAIN GAUGES

16. Non recording rain gauge

17. I. Tipping Bucket recording

18. Tipping recording rain-gauge

19. Recording gauges

20. Weighing
Bucket
Type
• Rain is collected in a bucket
supported on a spring balance
• A mechanical Lever arm of the
spring is connected with a pen
which touches a clock mounted
drum with a graph paper
• As it rains, the weight of the
bucket increase and moves the
pen
• The record shows the
accumulation of precipitation over
time
• Can be used for 24 hrs or 7 days
depending on clock & drum size
• When heavy ppt. occurs, the
bucket may overflow
• costly
II. Weighing Bucket Type

21. III. Float Type Rain Gauge
• In Float type rain gauge, as the rain is collected in a
float chamber, the float
• moves up which makes a pen to move on a chart
wrapped round a clock driven drum (Fig.below).
• When the float chamber fills up, the water siphons
out automatically through a siphon tube
• kept in an interconnected siphon chamber.
• The clockwork revolves the drum once in 24 hours.
• The clock mechanism needs rewinding once in a
week when the chart wrapped round the drum is
also replaced.

22. Adequacy of Rain gauge stations

25. Rainfall Network design
• Ideally a basin should have as many as possible number of
gauges to give a clear picture of the Arial rainfall
• However, the following factors govern the density of station
in a country
• Economy
• Topography
• Accessibility
• No definite rule as how many gauges are adequate
• Various countries have different stations density
– Israel has the highest density (1station/26km.sq) and Vietnam has
the lowest (1staion/2600 sq.km)

27. • ESTIMATION OF MISSING DATA
• Given the annual precipitation valves P1, P2,P3---Pm at
neighboring m stations 1,2,3---m respectively, it is required
to find the missing annual precipitation PX at station X, not
included in the above m stations.
• The estimation is based on two basic conditions.
Preparation of rainfall data records

28. • If the normal annual precipitations at various stations are
within about 10% of the normal annual precipitation at
station X, then a simple arithmetic average procedure is
followed to estimate PX.

29. If the normal average precipitations vary considerably (more than 10%), P
X
is estimated
by weighing the precipitation at various stations by the ratio of normal annual
precipitations
Thus, Px= 









m
m
X
N
P
N
P
N
P
M
N
...
2
2
1
1
------------------------------------------------------ (2.3)

30. Example1.
• The normal annual rainfall at stations A, B, C and D in a
basin are 80.97, 67.59, 76.28, and 92.01 cms respectively.
In a year 1975, station D was inoperative and the stations
A, B and C recorded annual precipitations of 91.11,
72.21&79.89 cms respectively. Estimate the rainfall at
station D in that year.
First we have to check whether the normal annual
precipitation variation is more than 10% or not. The maximum
normal annual precipitation valve at station X=92.01cm but
the valve at other station (example at B is 67.59).
Thus the maximum variation is 92.01- 67.59=24.42cm
• When this variation (the maximum variation) is evaluated
with station D, it is given as

31. • This is more than 10%. Hence, as the normal rainfall
values vary more than 10%, the normal ratio method is
adopted.

32. TEST FOR CONSISTENCY OF RECORDS
 Some of the common levels for inconsistency of records
are:
Shifting the rain gauge station to a new location
The Neighborhood of the station undergone a marked
change
Change of the ecosystem due to calamities, such as
forest fire.
Occurrence of observational error from a certain date.

35. Example
A double mass curve is usually used to check the data quality of a specific rain
gauge. A scatter plot is drawn between the interested gauge and a number of
surrounding gauges.

38. Presentation of Precipitation Data
Some common type of these plots include:
1.Curve or Cumulative Rainfall Diagram
- Plot of cumulative rainfall (P) vs. time (t).
 Intensity is depth of rainfall per unit time (mm/hr or in./hr)

40. 2. Hyetograph
• A plot of rainfall depth or intensity(i) versus time, shown in
the form of a histogram.
• Derived from mass curve and represented as a bar chart.
• Representing the characteristics of a storm and is
particularly important in the development of design storms
to predict extreme floods.
• Area under a hyetograph represents the total precipitation
received in the period

42. • Point rainfall is also known as station rainfall, which
refers the rainfall data of the station.
• Depending up on the need data may be listed as daily
weekly monthly, seasonal or annual valves for various
periods.
• As mentioned above point rainfalls are recorded using
rain gauges
3. POINT RAINFALL

43. • 1. Rainfall hyetograph – plot of rainfall depth or
intensity as a function of time
• 2. Cumulative rainfall hyetograph or rainfall mass
curve – plot of summation of rainfall increments as a
function of time
• 3. Rainfall intensity – depth of rainfall per unit time

44. Incremental Rainfall
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150
Time (min)
Incremental
Rainfall
(in
per
5
min)
Rainfall Hyetograph

45. Cumulative Rainfall
0
1
2
3
4
5
6
7
8
9
10
0 30 60 90 120 150
Time (min.)
Cumulative
Rainfall
(in.)
30 min
1 hr
2 hr
3.07 in
5.56 in
8.2 in
Rainfall Mass Curve

46. ESTIMATION OF AVERAGE DEPTH OF RAINFALL OVER A
CATCHMENT
1. THE ARITHMETIC MEAN METHOD
When the rainfall measured at various stations in a catchment
show little variation,
the average precipitation over the catchment is taken as the
arithmetic mean of the station values.
However, this method is used rarely in practice.
2. THE THIESSEN POLYGON METHOD
In this method rainfall records at each station is given a weightage based
on an area closest to the station.

47. 3. THE ISOHYETAL METHOD
• An Isohyet is a line joining points of equal rainfall
magnitude.
• In this method, the catchment area is drawn to scale and
the rain gauge stations are marked on it
4. Inverse distance weighting
Prediction at a point is more influenced by nearby
measurements than that by distant measurements

48. Arithmetic Mean Method
• Simplest method for determining areal
average
P1
P2
P3
P1 = 10 mm
P2 = 20 mm
P3 = 30 mm
• Gages must be uniformly distributed
• Gage measurements should not vary greatly about
the mean



N
i
i
P
N
P
1
1
mm
P 20
3
30
20
10





49. Thiessen polygon method
P1
P2
P3
A1
A2
A3
• Any point in the watershed receives the same
amount of rainfall as that at the nearest gage
• Rainfall recorded at a gage can be applied to
any point at a distance halfway to the next
station in any direction
• Steps in Thiessen polygon method
1. Draw lines joining adjacent gages
2. Draw perpendicular bisectors to the lines
created in step 1
3. Extend the lines created in step 2 in both
directions to form representative areas for
gages
4. Compute representative area for each gage
5. Compute the areal average using the following
formula



N
i
i
i P
A
A
P
1
1
P1 = 10 mm, A1 = 12 Km2
P2 = 20 mm, A2 = 15 Km2
P3 = 30 mm, A3 = 20 km2
mm
P 7
.
20
47
30
20
20
15
10
12








50. 3.Isohyetal method

54. Inverse distance weighting
P1=10
P2= 20
P3=30
• Prediction at a point is more
influenced by nearby
measurements than that by distant
measurements
• The prediction at an ungaged point
is inversely proportional to the
distance to the measurement
points
• Steps
– Compute distance (di) from
ungaged point to all measurement
points.
– Compute the precipitation at the
ungaged point using the following
formula



















N
i i
N
i i
i
d
d
P
P
1
2
1
2
1
ˆ
d1=25
d2=15
d3=10
mm
P 24
.
25
10
1
15
1
25
1
10
30
15
20
25
10
ˆ
2
2
2
2
2
2






p

55. 2.7 FREQUENCY ANALYSIS OF POINT RAINFALL

60. Assignment