This document discusses hydrology and the hydrological cycle. It defines hydrology as the science dealing with the occurrence, distribution, and movement of water on Earth. The hydrological cycle involves the constant circulation of water between the atmosphere and Earth's surface through evaporation, precipitation, and runoff. Factors like precipitation characteristics, catchment shape and size, topography, and geology affect the amount of runoff from a catchment area. Accurate measurement of rainfall and runoff is important for irrigation engineering design and management.

1. IRRIGATION ENGINEERING
-P.A.Pandya Sr.Lecturer G.P. Himatnagar

2. HYDROLOGY

3. INTRODUCTION
Irrigation Engineer is not only concern
with the collection and distribution of
water for irrigation , but it also
essential for him to know about the
occurrence ,distribution and
movement of water on the earth.

4. HYDROLOGY
What is Hydrology???
 It is the science which deals with occurrence,
distribution and movement of water on the earth
including that in the atmosphere and below the
surface of the earth.
 Water occurs in the atmosphere in the form of water
vapour, on the surface as water, snow or ice and below
the surface as ground water occupying all the voids
within a geological stratum.

5. HYDROLOGY
Except for the deep ground water , the
total water supply of earth is in
constant circulation from earth to
atmosphere and back to earth.
The earth’s water circulatory system is
known as hydrological cycle.

6. HYDROLOGICAL CYCLE
 Hydrological cycle is the process of transfer of
moisture from the atmosphere to the earth in the
form of precipitation, conveyance of precipited
water by streams and rivers to the oceans and lakes
etc. and evaporation of water back to the
atmosphere complete the hydrological cycle.

7. HYDROLOGICAL CYCLE

8. HYDROLOGICAL CYCLE
Evaporation and Transpiration (E)
Precipitation(P)
Runoff (R)

9. Evaporation and Transpiration
Evaporation: (E)
 Surface Evaporation
 Water Surface Evaporation From River
From Oceans
 Evaporation from Plants and
Leaves(Transpiration)
 Atmospheric Evaporation.

10. Precipitation
It may be defined as fall of moisture from the
atmosphere to the earth’s surface in any form.
Precipitation may be in two forms:
(1)Liquid Precipitation: Dizzel –Size<0.5 mm
(2)Frozen precipitation: Rain- Size >0.5 mm
 Snow-Precipitation in the form of ice crystal.
 Hail – Lump of ice over 5 mm
 Sleet- frozen rain drops cooled to the ice stage.
 Snow Flakes-Number of ice crystal fused together.

11. Run off
It is that portion of precipitation that is not
evaporated.
 Run off = Precipitation – Evaporation
i.e. R = P – E or P= E + R
 Run off ultimately runs to ocean through surface or
sub –surface streams.
 Thus it may be classified as
Surface run off ,Subsurface run off, Ground water flow
or base flow.

12. Types of Run off
 Surface run off : Water flows over the land and is first
to reach the streams or rivers which ultimately
discharge the water to the sea.
 Sub Surface Run off: A portion of precipitation
infiltrates in to surface soil and depending upon the
geology of basin runs as sub-surface soil run off and
reaches the streams and rivers.
 Ground Water Flow: A portion of precipitation which
after infiltrate percolate down and joins the ground
water reservoir which is ultimately connected to ocean.

13. Terms related to Rainfall
 Intensity of Rainfall: It is rain fall in unit of time
e.g if 18 cm rain fall in 3 hours,
Intensity of rain fall= 6 cm
 Daily rainfall:
Rain fall in 24 hours at any rain gauge St.
 Annual rain fall:
Rain fall in 1 year at any rain gauge St.
 Mean annual rainfall:
Rain fall in 35 year at any rain gauge St.

14. Rainfall Measurement
 Rain fall is source of all water used for irrigation
purpose and therefore the knowledge of its amount
character season or period the effects produced by it is
of prime importance to irrigation engineer to design,
carry out, improve or maintain irrigation works.
 The amount of precipitation is expressed as the depth
in cm or inches which fall on the level surface, and is
measure by rain-gauge.

15. Types of Rain gauge
 Non –Automatic or Non-recording rain gauge.
e.g. Symon’s Rain-gauge.
 Automatic or Recording rain gauge.
e.g. (1) Weighing bucket rain gauge.
(2) Tipping Bucket rain gauge.
(3) Float type rain gauge.

16. Symon’s Rain-gauge
 Figure to be attached

17. Symon’s Rain-gauge
 Used at all Govt. Rain gauge stations through out
India.
 It consist of cylindrical vessel 127mm dia.with a base
enlarged to 210mm dia.

18. Weighing Bucket Rain-gauge

19. Tipping Bucket Rain-gauge

20. Recording Rain-gauge
Advantages over non recording rain-gauge:
 Rainfall recorded automatically.
 Also gives the intensity of rain fall at any time, while
non recording type rain gauge gives total rain fall in
particular interval of time.
 As no attendant required can be installed in far-off
places.
 Possibilities of human error is obviated.

21. Recording Rain-gauge
Disadvantages:
Costly
Fault may be developed in
electrical/mechanical mechanism/
recording rain fall.

22. Sources of Error in recording the
Measurement
 Mistake in reading the scale of gauge.
 Some amount of water displaced by measuring stick.
(this may increase error by 1%)
 Dent in the collector rim may change its receiving
area.
 Funnel and inside surface required about 2.5 mm of
rain to get moistened when gauge is initially dry.
( this may extent of amt of 25mm/yr in some areas)
 Deficiency of measurement due to wind.

23. Sources of Error in recording the
Measurement
 Inclination of Gauge may cause lesser collection.
(10 % inclination = 1.5 % low catch)
 Tipping of bucket may be affected due to rusting
or accumulation of dirt at the pivot.
 No rainfall is recorded during tipping of the
bucket.

24. Computation of Av.Rainfall
 Required no. of gauges in a catchment area:
Area in square km No. of rain gauge station
0 to 80 1
80 to 160 2
160 to 320 3
320 to 520 4
560 to 800 5
800 to 1200 6

25. Computation of Av.Rainfall
If a basin or catchment area contain more than 1 rain
gauge station the average rainfall may be done by the
following methods:
 Arithmatic Average method
 Thiessen Polygon method
 Isohytal Method

26. Arithmatic Average Method
If P1 , P2, P3……etc are the Precipitation or rain fall
values measured at n rain gauge stations then,
The average rain fall,
Pav= (P1 + P2 +P3+…………+Pn)/n
= ∑P/n

27. Thiessen Polygon Method
Pav= (P1A1 + P2A2+P3A3+…….PnAn)/(A1 + A2+…An)
= ∑(P x A)/∑A
Here A1, A2,A3……An= Area of Polygon

28. Isohytal Method
 Pav = [∑A * {(P1 +P2)/2}]/∑A
 Here A= Area enclosed by two successive isohytes.
 P1 & P2 = Precipitation Values at two isohytes.

29. Run off
The runoff of a catchment area in any specified period
is the total quantity of water draining in to a stream or
in to a reservoir in that period.
 It can be expressed as
(1) cm of water in a catchment area
(2) The total water in m3 or hectare- metre for a given
catchment.

30. Factors affecting run off
 Precipitation Characteristics.
 Shape and size of the catchment.
 Topography
 Geological Characteristics of the catchment.
 Meteorological Characteristics.
 Character of the catchment surface.
 Storage Characteristics of the catchment

31. Precipitation Characteristics
More the rainfall more will be run off.
Duration of rainfall.
Type of rainfall
Intensity of rainfall-more intensity-
more run off.

32. Shape and size of the catchment.
 Fan shape catchment Area- More run off
 Fern leaf shape catchment-Less run off.