Hydrological cycle- Meteorological measurements – Requirements, types and forms of Precipitation-Rain Gauges-Spatial analysis of rainfall data using Thiessen and Isohyetal methods Infiltration-Infiltration Index-Interception-Evaporation, Watershed, catchment and basin - Catchment characteristics - factors affecting runoff – Runoff estimation using empirical

1. Water molecules have been
transferred time and time again
from the oceans and the land
surface into the atmosphere by
evaporation, dropped on the
land as precipitation, and
transferred back to the sea by
rivers and groundwater. This
endless circulation is known as
the "hydrologic cycle".

2. 0- Evaporation from Ocean
1- Raindrop evaporation
2- Interception
3- Transpiration
4- Evaporation from land
5- Evaporation from water
bodies
6- Surface runoff
7- Infiltration
8- Ground water
Deep percolation

3. Precipitation: The term precipitation denotes all forms of water
that reached the earth’s surface from atmosphere. Rain,
drizzle, hail and snow are examples of precipitation.
Causes of precipitation: It is necessary that the moist air masses
to cool in order to condense. This is generally accomplished by
adiabatic cooling of moist air through a process of being lifted to
higher altitudes.
Types of precipitation:
 Frontal precipitation: This is the precipitation that is caused by
the expansion of air on ascent along or near a frontal surface.

4. Convective precipitation: Precipitation caused by the
upward movement of air which is warmer than its
surroundings. This precipitation is generally showery
nature with rapid changes of intensities.
Orographic precipitation: Precipitation caused by the
air masses which strike the mountain barriers and rise
up, causing condensation and precipitation. The
greatest amount of precipitation will fall on the
windward side of the barrier and little amount of
precipitation will fall on leave ward side.

5. Forms of precipitation:
• Rain- It is used to describe precipitation in the form of
water drops of sizes in the range of 0.5mm to 6mm.
• Snow- It consists of ice crystals which usually combine
to form flakes.
• Drizzle- It is fine sprinkle of numerous water droplets of
size less than 0.5mm and intensity less than 1.5mm/hr.

6. • Glaze- When rain or drizzle comes in contact with cold ground at
around 0° C, the water drop freezes to form an ice coating called as
Glaze.
• Sleet- It is frozen raindrops of transparent grains which forms when
rain falls through air at subfreezing temperature.
• Hail- It is a showery precipitation in the term of ice of size more than
8mm.
• Dew- It forms directly by conensastion on the ground mainly during
the night when the surface has been cooled by outgoing radiation.

8. 2- Weighting Bucket rain gauge.

9. Measurement of Precipitation:
(a)- Recording Type :
1- Tipping Bucket rain gauge.

10. 3- Siphon Rain gauge.

11. (b)- Non-Recording Type

12. Analysis and Interpretation of precipitation Data:
1- Intensity: It is a measure of quantity of rain falling in a given
time in a specific area. It is usually expressed in mm/hr.
2- Duration: It is a period of time during which Rainfall occurs.
3- Frequency: This refers to the probability that given depth of
Rainfall which will fall in a given time.
4- Areal extent: This concerns the area over which a point
Rainfall can be held to apply.

13. 5- Rainfall mass curve: It is plot of cumulative Rainfall against Time.
Slope of the tangent drawn to the rainfall mass curve at any time will
gives intensity of rainfall.

14. 6- Rainfall Hyetograph: It is a plot between Rainfall Intensity Vs Time.
The area under Hyetograph represents the total precipitation received in that
period.

15. Raingauge network:
Adequacy of Raingauge stations:

19. Theissen Polygon method:

21. Interception: When it rains over a catchment, not all the
precipitation falls directly onto the ground. Before it reaches the
ground, a part of it may be caught by the vegetation and
subsequently evaporated. The volume of, water so caught is called
Interception.
DEPRESSION STORAGE: When the precipitation of a storm reaches
the ground, it must first fill up all depressions before it can flow over
the surface. The volume of water tripped in these depressions is
called depression storage. This amount is eventually lost to runoff
through processes of infiltration and evaporation and thus form a
part of the initial loss. Depression storage depends on a vast number
of factors the chief of which are:

22. • The type of soil
• The condition of the surface reflecting the amount and
nature of depression
• The slope of the catchment
• The antecedent precipitation, as a measure of the soil
moisture.
INFILTRATION: It is the flow of water into the ground through
the soil surface. The distribution of soil moisture within the soil
profile during the infiltration process is illustrated in Fig. When
water is applied at the surface of a soil, four moisture zones in
the soil, as indicated in Fig. can be identified.

23. Zone 1: At the top a thin layer of saturated zone is created.
Zone 2: Beneath zone 1, there is a transition zone.
Zone 3: Next lower zone is the transmission zone where the
downward motion of the moisture takes place. The moisture
content in this zone is above field capacity but below
saturation.
Zone 4: The last zone is the wetting zone. The soil moisture in
this zone will be at or near field capacity and the moisture
content decreases with the depth. The boundary of the wetting
zone is the wetting front where a sharp discontinuity exists
between the newly wet soil and original moisture content of
the soil. Depending upon the amount of infiltration and
physical properties of the soil, the wetting front can extend
fron1 a few centimetres to metres.

25. INFILTRATION CAPACITY: The maximum rate at which a given soil at a
given time can absorb water is defined as the infiltration capacity. It is
designated as fp and is expressed in units of cm/h. The actual rate of
infiltration f can be expressed as:
f = fp when i > fp or i = fp ,
and f = i when i <j
Where, i = intensity of rainfall.
The infiltration capacity of a soil is high at the beginning of a storm and
has an exponentially decay as the time elapses. The infiltration capacity
of an area is dependent on a large number of factors:

26.  Characteristics of the soil
 Condition of the soil surface
 Current moisture content
 Vegetative cover
 Soil temperature
CHARACTERISTICS OF SOIL : The type of soil, viz. sand, silt or clay, its texture,
structure, permeability and under drainage are the important characteristics
under this category. A loose permeable sandy soil will have a larger infiltration
capacity than a tight clayey soil. A soil with good underdrainage, i.e. the facility
to transmit the infiltered water downward to a groundwater storage would
obviously have a higher infiltration capacity. When the soils occur in layers, the
transmission capacity of the layers determines the overall infiltration rate.

27. Infiltration indices: The two commonly used
infiltration indices are the following -
• φ – index: This is defined as the rate of
infiltration above which the rainfall volume
equals runoff volume. The method to
determine the φ - index would usually involve
some trial. Since the infiltration capacity
decreases with a prolonged storm, the use of
an average loss rate in the form of φ - index is
best suited for design storms occurring on
wet soils in which case the loss rate reaches a
final constant rate prior to or early in the
storm.

28. • W – index : This is the average infiltration rate during the time when
the rainfall intensity exceeds the infiltration rate. It may be
mathematically calculated by dividing the total infiltration
(expressed as a depth of water) divided by the time during which
the rainfall intensity exceeds the infiltration rate.
Total infiltration = Total precipitation – Surface runoff – Effective
storm retention
The W – index can be derived from the observed rainfall and runoff
data. It differs from the φ - index in that it excludes surface storage
and retention.

29. Evapotranspiration: It consists of evaporation from
soil and water bodies and loss of water from plant
leaves, which is called transpiration.
It is a major component of the hydrologic cycle and its
information is needed to design irrigation projects and
for managing water quality and other environmental
concerns.
In urban development, evapotranspiration
calculations are used to determine safe yields from
aquifers and to plan for flood control.
For a given set of atmospheric conditions,
evapotranspiration depends on the availability of
water.

30. If sufficient moisture is always
available to completely meet the
needs of vegetation fully covering the
area, the resulting evapotranspiration
is called potential evapotranspiration
(PET).
The real evapotranspiration occurring
in a specific situation is called actual
evapotranspiration (AET).

31. The term consumptive use is also sometimes used to
denote the loss of water molecules to atmosphere by
evapotranspiration.
The term consumptive use is also sometimes used to
denote the loss of water molecules to atmosphere by
evapotranspiration.
For a given set of atmospheric conditions,
evapotranspiration depends on the availability of water.

32. If sufficient moisture is always available to
completely meet the needs of vegetation fully
covering the area, the resulting
evapotranspiration is called potential
evapotranspiration (PET).
The real evapotranspiration occurring in a
specific situation is called actual
evapotranspiration (AET).

33. CATCHMENT AREA: The area of land
draining into a stream or a water course at a
given location is known as catchment area.
lt is also called as drainage area or drainage
basin.
In USA, it is known as watershed.
A catchment area is separated from its
neighbouring areas by a ridge called divide
in USA and Watershed in UK.

34. The areal extent of the catchment is obtained by tracing
the ridge on a topographic map to delineate the
catchment and measuring the area by a Planimeter.

40. RUNOFF: Runoff means the draining or flowing off of precipitation from a
catchment area through a surface channel. It thus represents the output from the
catchment in a given unit of time.
For a given precipitation, the evapotranspiration, initial loss, infiltration and
depression storage requirements will have to be first satisfied before the
commencement of runoff. When these are satisfied, the excess precipitation
moves over the land surfaces to reach smaller channels. This portion of the runoff
is called Overland flow and involves building up of a storage over the surface and
draining off of the same.
Flows from several small channels join bigger channels and flows from these in
turn combine to form a larger stream, and so on, till the flow, reaches the
catchment outlet. The flow in this mode, where it travels all the time over the
surface as overland flow and through the channels as open-channel flow and
reaches the catchment outlet is called surface runoff.

41. A part of the precipitation that
infilters moves laterally through
upper crusts of the soil and returns to
the surface at some location away
from the point of entry into the soil.
This component of runoff is known as
interflow.
Based on the time delay between the
precipitation and the runoff, the
runoff is classified into two
categories: as 1. Direct runoff and 2.
Base flow