Hydrology

UNIT 2
HYDROLOGY
January 18 NAMRATA AGNIHOTRI

HYDROLOGY
 Hydrology is the science which deals with the
occurance, circulation and distribution of water
upon over and beneath the earth surface. It is
the science concerned with the transportation
of water vapour through the air, the
precipitation occuring on ground as rainfall.

The Geologic Cycling of Water

THE HYDROLOGICAL CYCLE

 The hydrological cycle is the descriptive term
applied to the circulation of water from the
ocean to the atmosphere to the ground and
back to the ocean again. Thus hydrological cycle
is the earths water circulatory system.

 The water from the surface sources like lakes, river,
ocean etc converts to vapour by evaporation due to
solar heat. The vapour goes on accumulating
continuously in the atmosphere. This vapour is
again condensed due to sudden fall in temperature
and pressure. Thus clouds are formed. These
clouds again cause the precipitation (i.e. rainfall).
 Some of the vapour is converted to ice at the peak
of the mountains. The ice again melts in summer
and flows as rivers to meet the sea or ocean. These
processes of evaporation, precipitation and melting
of ice go on continuously like an endless chain and
thus a balance is maintained in the atmosphere.
This phenomenon is known as hydrologic cycle.

Precipitation

Definition
 All types of moisture reaching the surface of earth
from atmosphere.
Precipitation is the basic input to the hydrology.
 Factors determining
precipitation or the
amount of atmospheric
moisture over a region
 Climate
 Geography
 Ocean surfaces is the
chief source of moisture
for precipitation

Forms of precipitation

Rain
 Rain is the most common type of
precipitation in our atmosphere. Rain
is when liquid droplets fall to the
surface of the Earth.
 There are two different forms of rain,
either in the form of
 showers
 drizzles
 Showers are heavy, large drops of rain
and usually only last a period of time.
 Drizzles however usually last longer
and are made up of smaller droplets
of water.
 Rain can either be formed as ice
crystals melt or it can be smaller water
droplets.
Light
I = 2.5mm/hr
Moderate
I = 2.8-7.6mm/hr
Heavy
I > 7.6 mm/hr

Snow
 Snow is the second most common
precipitation in the North East.
 Snow forms when water vapor turns directly
into ice without ever passing through a liquid
state. This happens as water condenses
around an ice crystal.
Density of freshly fallen
snow varies between 125-
500mm of snow required
to equal 25mm of liquid
water
Average density (specific
gravity) = 0.1

Hail
 Hail is created when moisture and wind are together.
Inside the cumulonimbus clouds ice crystals form, and
begin to fall towards the surface of Earth. When this starts
to happen wind gusts start to pick up the ice crystals
pushing them up high into the clouds. As they start to fall
down again they continue to grow in size. A wind gust
might catch the hail stone again which will push it back up
into the cloud. This whole process gets repeated several
times before the hail stone becomes so big that it is too
heavy for the wind to carry so it must fall towards Earth.
Shapes of hail particles
1. Spherical
2. Conical
3. Irregular
Diameter range 5 to 125 mm
Specific gravity = 0.8
Average density (specific gravity) =
0.1 January 18 NAMRATA AGNIHOTRI

Fog
 There are four main types of fog,
 radiation fog
 advection fog
 upslope fog
 evaporation fog
 There is really no different between fog and the
clouds that are high in the sky. In simple terms
fog is; a cloud that has formed near the surface
of the Earth.

Dew
 The small drops of water which can be found
on cool surfaces like grass in the morning.
 This is the result of atmospheric vapor
condensing on the surface in the colder night
air.
 Dew Point is the temperature in which
condensation starts to take place or when dew
is created.

Mist / Drizzle
 Mist is a bunch of small droplets of water which are
in the air. This occurs with cold air when it is above
a warm surface, for example water.
 Fog and mist are very similar, the only difference is
their visibility.
 If you cannot see 1 kilometer or less you know you're
dealing with fog.
 You can see visuals through mist and it is
more haze looking than a thicker substance.
Diameter range between 0.1
and 0.5 mm/hr

Glaze
 Glaze is the ice coating, generally clear
and smooth, formed on exposed
surfaces by the freezing of super cooled
water deposited by rain or drizzle.

Sleet
 Sleet consists of transparent, globular,
solid grains of ice formed by the freezing of
raindrops or freezing of largely melted ice
crystals falling through a layer of sub
freezing air near the earth’s surface.

Arithmetic Mean Method
 Arithmetic mean method is used when
normal annual precipitation is within 10%
of the gauge for which data are being
reconstructed. This method is least
accurate however.
Where:
Pm = precipitation at the missing location
Pi = precipitation at index station I
N = number of rain gauges

Theissen Polygon Method
 Divide the region (area A)
into sub-regions centred
about each rain gauge;
 Determine the area of each
sub-region (Ai) and compute
sub-region weightings (Wi)
using: Wi = Ai/A
 Compute total aerial rainfall
using Rainfall recorded at
each station is given a
weight age based on the
area closest to the station.

Theissen Polygon Method
Consider a catchment area with say, 3
rain gauge stations. Let there be 3
stations outside the catchment, but
in its neighborhood. Catchment
area is drawn to scale and position
of these 6 stations is plotted on it.
Stations are joined so as to get a
network of triangles. Perpendicular
bisectors are drawn to each of the
sides of these triangles. These
bisectors form a polygon around
each station. If the boundary of
catchment cuts the bisectors, then
boundary is taken as outer limit of
polygon. These bounding polygons
are called Thiessen Polygons. The
area of these polygons is measured
with a planimeter or by grid overlay

Isohytal Method
 Plot gauge locations
on a map;
 Subjectively
interpolate between
rain amounts
between gauges at a
selected interval;
 Connect points of
equal rain depth to
produce lines of
equal rainfall
amounts (isohyets);

Isohytal Method
Compute aerial rain using Isohyets –
It is a line joining points of equal rainfall
magnitude.
The catchment area is drawn to scale
and the rain gauge stations are marked
on it. The recorded rainfall values for
which aerial average is to determined
are marked at the respective stations.
Neighboring stations outside the
catchment are also considered. Taking
point rainfall values as the guide,
isohyets of different rainfall values are
drawn (similar to drawing contours
based on spot levels.
The area between adjacent isohyets is
measured using a planimeter. If
isohyets go out of the catchment, the
catchment boundary is used as the
bounding line.
It is assumed that the average value of
rainfall indicated by two isohyets acts
over the inter isohytal area

RUNOFF:
 Runoff is that part of precipitation that is not
evaporated.
 When moisture falls to the earth’s surface as
precipitation a part of it is evaporated from the water
surface soil and vegetation and through transpiration by
plants and the remainder precipitation is available as
runoff.

Factors Affecting Runoff:
The main factors affecting runoff are:
1. Precipitation characteristics.
2. Shape and size of the catchment.
3. Topography
4. Geological characteristics of the catchment.
5. Meterological characteristics
6. Character of the catchment surface
7. Storage characteristics of the catchment.

Precipitation characteristics:
The important characteristics of precipitation are:
i. Intensity of rainfall
ii. Duration of rainfall
iii. Form of precipitation
iv. Direction of storm movement etc.
Runoff increases with the intensity of rainfall. The more the
rainfall, more will be the runoff. Also the rainfall for a longer
duration would produce greater runoff.
The precipitation may occur either in the form of rain or
snow. If the precipitation occurs in the form of rain, it will
immediately produce the runoff but if the precipitation occurs
in the form of snow, it will produce the runoff much later.

Shape and size of the
catchment:
The runoff is considerably affected by the shape and size
of the catchment.
A drainage basin may be either fan shaped of fern-
leaf shaped. In case of fan shaped catchment, all the
tributaries are approximately of the same size. Such
catchment gives greater runoff since peak flood from the
tributaries is likely to reach the main stream
approximately at the time.
In the case of fern-leaf shaped catchment the
tributaries are generally of different lengths, and meet the
main stream at the regular intervals. In such a narrow
catchment, the peak flood intensity is reduced.

Topography
The topography of the catchment area has
significant effect on the runoff. A steeply sloping
drainage basin produces greater runoff because a
steep slope helps quicker drainage no depression
storage and less evaporation losses.
If the catchment is mountainous and is located on
the windward side the intensity of rainfall is more
and hence the runoff is more.

Geological characteristic of
the catchment
 If the surface soil and sub surface is pervious
seepage will be more and this in turn reduce
the peak flood. If the surface is rocky the water
absorption will be less and runoff will be more.

Meteorological characteristic
Higher temperature and higher wind velocity
cause more losses due to evaporation and
transpiration and reduces the runoff. On the
other hand greater humidity decrease
evaporation and transpiration losses resulting in
greater runoff.

Computation of runoff
 The runoff from the catchment can be
computed daily, monthly or yearly.
 By empirical formulae and table:
runoff coefficient:
R= kP
k= R/P
R= runoff
P= precipitation
k= runoff corfficient

Evaporation : it is the process in which water is
changed to vapours at the free surface below the
boiling point of water through the transfer of heat
it is the continuous process by which a substance
changes from liquid to gaseous state.
Transpiration : it is the process of water being
lost from the leaves of the plants from their
pores.

Factors affecting evaporation
1) Temperature of air:
the evaporation increase with
an increase in air temperature. Thus, in summer
seasons or in hot countries the evaporation will
be more as compared to that in the winter season
or in clod countries.
2) Wind velocity:
if the velocity of air is more, the
saturated film of air containing the water vapour
will move easily, causing evaporation.

3) Atmospheric pressure:
a decrease in
atmospheric pressure increase the rate of
evaporation because there are fewer
molecules in the air above the free surface
to cause the interference.
4) Nature of evaporating surface:
different evaporating
surfaces like soil, barren land, forest area,
houses, lakes, affect evaporation to the
extent they have the potential.

5) Area of water surface:
the amount of evaporation is
directly proportional to the area of evaporation. If
the exposed area is large the evaporation will be
more and vise versa.
6) Depth of the body:
the depth of water influences the
evaporation considerably. Deep water body
evaporate slower than the shallow water bodies
in summer while in winter season they evaporate
faster.

7) Humidity :
evaporation is inversely proportional to
humidity. If the humidity in the atmosphere is
more evaporation will be less.
8) Impurities in water:
any dissolved salts in water
reduces the saturated vapour pressure of water
which consequently reduces the rate of
evaporation

Hydrology

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