UNIT 1 - PRECIPITATIONS AND ABSTRACTIONS (1).ppt

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1. AI3404 - HYDROLOGY AND
WATER RESOURCES
ENGINEERING
R.SHANMUGASUNDARAM, AP CE, PSNACET 1

2. SYLLABUS
UNIT I PRECIPITATION AND ABSTRACTIONS
Hydrological cycle - Meteorological measurements – Types and forms of precipitation - Rain gauges
- Spatial analysis of rainfall data using Thiessen polygon and Iso-hyetal methods - Interception –
Evaporation: Measurement, Evaporation suppression methods – Infiltration: Horton s equation -
‟
Double ring infiltrometer - Infiltration indices.
UNIT II RUNOFF
Catchment: Definition, Morphological characteristics - Factors affecting runoff - Run off estimation
using Strange’s table and empirical methods - SCS-CN method – Stage discharge relationship - Flow
measurements - Hydrograph – Unit Hydrograph – IUH.
UNIT III HYDROLOGICAL EXTREMES
Natural Disasters - Frequency analysis - Flood estimation - Flood management - Definitions of
drought: Meteorological, Hydrological, Agricultural and Integrated - IMD method - NDVI analysis -
Drought Prone Area Programme (DPAP).
R.SHANMUGASUNDARAM, AP CE, PSNACET 2

3. UNIT IV RESERVOIRS
Classification of reservoirs - Site selection - General principles of design - Spillways -Elevation-Area
Capacity curve - Storage estimation - Sedimentation - Life of reservoirs – Rule curve.
UNIT V GROUNDWATER AND MANAGEMENT
Origin - Classification and types - Properties of aquifers - Governing equations – Steady and unsteady
flow - Artificial recharge - RWH in rural and urban areas.
TEXT BOOKS:
1.Subramanya K, "Engineering Hydrology"- Tata McGraw Hill, 2010
2.2. Jayarami Reddy P, "Hydrology", Tata McGraw Hill, 2008.
REFERENCES
1.David Keith Todd. "Groundwater Hydrology", John Wiley & Sons, Inc. 2007
2.Ven Te Chow, Maidment, D.R. and Mays, L.W. "Applied Hydrology", McGraw Hill International Book
Company, 1998.
3.Raghunath. H.M., "Hydrology", Wiley Eastern Ltd., 1998.
4.Bhagu R. Chahar, Groundwater Hydrology, McGraw Hill Education (India) Pvt Ltd, New Delhi, 2017
R.SHANMUGASUNDARAM, AP CE, PSNACET 3

4. UNIT I PRECIPITATION AND ABSTRACTIONS
HYDROLOGY:
•Hydrology is defined as the science that is concerned with all the aspects of water
available on Earth.
•It deals with the study of the occurrence of water, its distribution, its properties, and
circulation of water on Earth including water resources, water cycle, and watershed
sustainability.
•Hydrology is the science that encompasses the study of water on Earth's surface and
underneath the Earth surface, the occurrence and circulation of water, the physical and
chemical properties of water, and its relationship with the human and material components
of the environment.
•A professional of hydrology is known as a hydrologist. Using different scientific
techniques and analytical methods, hydrologists collect and analyse data to resolve water-
related problems such as natural disasters, environmental preservation, and water
management.
R.SHANMUGASUNDARAM, AP CE, PSNACET 4

5. Hydrology as a Profession
•A profession is a “calling requiring specialized knowledge, which has
as its prime purpose the rendering of a public service”
•What hydrologists do:
•Water Use – water withdrawal and instream uses
•Water Control – flood and drought mitigation
•Pollution Control – point and nonpoint sources
R.SHANMUGASUNDARAM, AP CE, PSNACET 5

6. Application of Hydrology
•Hydrology helps to calculate the rainfall
•Hydrology helps to calculate the surface runoff and precipitation
•Designing bridges, sewers, irrigation schemes, and
  urban drainage systems
•Provides clean drinking water
•Designing dams for hydroelectric power irrigation or water supply
•Real-time flood warning and forecasting
•Determining the water balance of a region and agricultural water balance.
•Estimating or predicting the flood, landslide, or drought risk
•Helping to analyze the impacts of antecedent moisture on the sanitary sewer system
•Helps to predict geomorphology changes such as erosion or sedimentation
•Managing agricultural productivity, and many more.
R.SHANMUGASUNDARAM, AP CE, PSNACET 6

7. Scope of Hydrology:
•The study of hydrology helps us to know
1. Maximum probable flood that may occur at a given site and its
frequency; this is required for the safe design of drains and culverts, dams
and reservoirs, channels and other flood control structures.
2. Water yield from a basin —its occurrence, quantity and frequency, etc;
this is necessary for the design of dams, municipal water supply, water
power, river navigation, etc.
3. Ground water development for which a knowledge of the hydrogeology
of the area, i.e., of the formation soil, recharge facilities like streams and
reservoirs, rainfall pattern, climate, cropping pattern, etc. are required.
4. Maximum intensity of storm and its frequency for the design of a
drainage project.
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8. Branches of Hydrology
HYDROLOGY
Surface
Water
Ground
Water
ice and
Snow
Limnolo
gy
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9. Sources of Water:
R.SHANMUGASUNDARAM, AP CE, PSNACET 9
•Water resources
 are natural resources of water that are potentially useful for
humans. For example as a source of drinking
 water supply or irrigation water.
•97% of the water on Earth is
 salt water and only three percent is fresh water;
slightly over two-thirds of this is frozen in
 glaciers and polar ice caps.

•The remaining unfrozen freshwater is found mainly as groundwater, with only
a small fraction present above ground or in the air.
 
•Natural sources of
 fresh water include surface water, under river
flow,
 groundwater and frozen water.
•Non-natural or
 
human-made 
sources of fresh water can include wastewater
that has been treated for reuse options, and
 desalinated seawater.

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11. Global Water Resources
105,000 km
3
or
0.0076% of total
water
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13. Hydrological Cycle:
•Journey of water from the ocean to atmosphere and back to the earth and
ultimately to the ocean through the processes of evaporation, precipitation,
percolation, runoff and return to the ocean is called hydrologic cycle.
•The hydrologic cycle is the continuous, unsteady circulation of water from
the atmosphere to and under the land surface and back to the atmosphere by
various processes.
•It is dynamic in that the quantity and quality of water at a particular location
may vary greatly with time.
•Temporal variations may occur in the atmosphere, on land surface, in surface
waters, and in the groundwater of an area.
•Within the hydrologic cycle, water may appear in all three of its states; solid,
liquid, and gas.
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15. Hydrologic Cycle
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16. Global water balance
Land (148.7 km
2
)
(29% of earth area)
Ocean (361.3 km
2
)
(71% of earth area)
Precipitation
800 mm (31 in)
Evaporation
480 mm (19 in)
Outflow
320 mm (12 in)
Precipitation
1270 mm (50 in)
Evaporation
1400 mm (55 in)
Atmospheric moisture flow
316 mm (12 in)
(Values relative to land
area)
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17. Precipitation: Includes rain, snow and other forms of water falling from the
atmosphere into the land & oceans.
Evaporation Physical process by which water is vaporized into the atmosphere from
free water surface and land areas.
Transpiration: Water from the soil is absorbed by plant roots and eventually
discharged into the atmosphere through little pores in the leaves called stomata.
Evapotranspiration: Combined processes by which water is transferred to the
atmosphere from open water surfaces and vegetation.
Infiltration: Movement of water from the land surface to the upper layers of the soil.
Percolation: Movement of water through the subsurface down to the water table.
Overland flow: Portion of runoff that travels over the surface of the ground to reach
a stream
channel
Surface runoff: Includes all overland flow as well as precipitation falling directly
onto stream channels.
Components of the Hydrologic Cycle
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18. Precipitation types
•In
 meteorology, the different types of precipitation often include the character,
formation, or
 phase of the precipitation which is falling to ground level. There are
three distinct ways that precipitation can occur.
 
•Convective 
precipitation is generally more intense, and of shorter duration,
than
 
stratiform 
precipitation. 
Orographic 
precipitation occurs when moist air is
forced upwards over rising terrain and
 condenses on the slope, such as a mountain.
•Precipitation can fall in either
 liquid or solid phases, is mixed with both,
or
 transition between them at the freezing level.
•Liquid forms of precipitation include rain and drizzle and dew. Rain or drizzle which
freezes on contact with a surface within a subfreezing
 air mass gains the preceding
adjective "freezing", becoming the known
 freezing rain or freezing drizzle.
•Slush is a mixture of both liquid and solid precipitation. Frozen forms of precipitation
include
 snow, ice crystals, ice pellets (sleet), hail, and graupel. Their respective
intensities are classified either by rate of precipitation, or by visibility restriction
R.SHANMUGASUNDARAM, AP CE, PSNACET 18

19. Phases
•Liquid precipitation:
•Drizzle
 (DZ)
•Rain
 (RA)
•Cloudburst
 (CB)
•Freezing/Mixed precipitation:
•Freezing drizzle
 (FZDZ)
•Freezing rain
 (FZRA)
•Rain and snow mixed
 / Slush (RASN)
•Drizzle and snow mixed
 / Slush (DZSN)
•Frozen precipitation:
Snow
 (SN) Snow grains (SG)
Ice crystals
 (IC)
Ice pellets / Sleet
 (PL)
Snow pellets / Graupel
 (GS)
Hail
 (GR)
Megacryometeors
 (MC)
R.SHANMUGASUNDARAM, AP CE, PSNACET 19

20. FORMS OF PRECIPITATION:
Drizzle: a light steady rain in fine drops (0.5 mm) and intensity <1 mm/hr
Rain: the condensed water vapour of the atmosphere falling in drops
(>0.5 mm, maximum size—6 mm) from the clouds.
Glaze: freezing of drizzle or rain when they come in contact with cold objects.
Sleet: frozen rain drops while falling through air at subfreezing temperature.
Snow: ice crystals resulting from sublimation (i.e., water vapour condenses to ice)
Hail: small lumps of ice (>5 mm in diameter) formed by alternate freezing and melting,
when they are carried up and down in highly turbulent air currents.
Dew: moisture condensed from the atmosphere in small drops upon cool surfaces.
Frost: a feathery deposit of ice formed on the ground or on the surface of exposed
objects by dew
Fog: a thin cloud of varying size formed at the surface of the earth by condensation of
atmospheric vapour
Mist: a very thin fog
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21. Types of Precipitation:
1.Convectional Precipitation
It results from the heating of the earth's surface. Due to this, the warm air rises rapidly into
the atmosphere. As the air rises, it cools. Water vapour in the air condenses into clouds and
precipitation.
2.Orographic Precipitation:
It results when warm moist air moving across the ocean is forced to rise by large mountains.
As the air rises, it cools. As air cools, the water vapour in the air condenses and water
droplets form. Precipitation occurs on the windward side of the mountain. The air is now
dry and rises over top the mountain. As the air moves back down the mountain, it collects
moisture from the ground via evaporation.
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22. Convectional
Precipitation
Orographic
Precipitation
Frontal
Precipitation
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23. 3.Cyclonic
Cyclonic or Frontal precipitation results when the leading edge of a warm, moist air
mass
•Frontal precipitation is the result of frontal systems surrounding
 extratropical
cyclones
 or lows, which form when warm and tropical air meets cooler, subpolar
air. Frontal precipitation typically falls out from
 nimbostratus clouds.
•When masses of air with different densities (moisture and temperature
characteristics) meet, the less dense warmer air overrides the more dense colder air.
•The warmer air is forced to rise and, if conditions are right, creates an effect of
saturation and condensation, causing precipitation. In turn, precipitation can
enhance the temperature and dewpoint contrast along a frontal boundary, creating
more precipitation while the front lasts.
• Passing weather fronts often result in sudden changes in environmental
temperature, and in turn the
 humidity and pressure in the air at ground level as
different air masses switch the local weather.
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24. •Warm fronts
 occur where advancing warm air pushes out a previously
extant cold air mass. The warm air overrides the cooler air and moves
upward.
•Warm fronts are followed by extended periods of light rain and drizzle
due to the fact that, after the warm air rises above the cooler air (which
remains on the ground), it gradually cools due to the air's expansion
while being lifted, which forms clouds and leads to precipitation.
•Cold fronts
 occur when an advancing mass of cooler air dislodges and
plows through a mass of warm air. This type of transition is sharper
and faster than warm fronts, since cold air is more dense than warm air
and sinks through in gravity's favor.
•Precipitation duration is often shorter and generally more intense than
that which occurs ahead of warm fronts.
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26. The Effect of Soil Use Toward Hydrological Cycle
•When development occurs, the resultant alterations to the land can lead to
dramatics changes to the hydrology or the way water is transported and stored,
•Impervious man-made surfaces (asphalt, concrete, rooftops) and compacted earth
associated with development create a barrier to percolation of rainfall into the soil,
increasing surface runoff and decreasing ground water infiltration.
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27. This disruption of the natural water cycle
leads to a number of changes, including:
1.Increased volume and velocity”of
runoff
2.Increased frequency.and severity
of flooding
3.Peak (Storm) flows many times greater
than in natural basins
4.Loss of natural runoff storage capacity in
vegetation, wetlands, and soil
5.Reduced groundwater recharge
6. Decreased base flow (the ground
water contribution to stream flow). This
can result in streams becoming
intermittent or dry, and also affects water
temperature.
Impacts on Stream Form anal
Function
R.SHANMUGASUNDARAM, AP CE, PSNACET 27

28. Relationships between impervious cover
and surface runoff
Natural Ground Cover
0%
ImperviousSurface
Urban Residential
35%-50% Impervious
Surface
Low Density Residential
10%-20% Impervious
Surface
Commercial Industrial
75%-100% Impervious
Surfa
R.SHANMUGASUNDARAM, AP CE, PSNACET 28

29. The Effect of Soil Use Toward Hydrological
Cycle
Rainwater runs off the land into water
bodies. It also percolates into the soil,
Percolation recharges groundwater and
filters pollutants. Through both pathways,
water makes its way into our creeks, ponds,
wetlands, rivers, and oceans.
Development puts impervious surfaces,
roads, sidewalk, and roofs, that prevent
percolation. Most of the rainwater runs
off the land carrying pollutants into
water bodies. Without percolation,
flooding is more frequent and severe.
R.SHANMUGASUNDARAM, AP CE, PSNACET 29

30. Meteorological Measurements
•The hydrometeorological observatory was established in the
department of hydrology with the aim of collecting data on various
hydrometeorological parameters and training students on various
instruments.
•On a daily basis, various hydrometeorological parameters such as
precipitation, temperature, evaporation, humidity, wind speed
and direction, solar radiation, and atmospheric pressure are
measured.
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31. CHARACTERISTICS OF PRECIPITATION
IN INDIA
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32. CHARACTERISTICS OF PRECIPITATION IN
INDIA
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37. M
O
N
S
O
O
N
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39. November
>> The Bay of Bengal
>> The Arabian Sea
Tropical cyclone
Strike the coastal area cause :Intense rainfallHeavy damage
Life
Property
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42. Cv = 100 x standard deviation
mean
Annual rainfall varies between 15 and 70
from place to place
Average value of about 30
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43. RAINFALL MONITORING
Rainfall monitoring is crucial for several reasons:
1. Water Resource Management
Accurate rainfall data is crucial for managing water resources for household,
agricultural, and industrial purposes. It also supports the design and operation of irrigation
systems and the management of reservoirs and dams.
2. Flood Prevention
Monitoring rainfall allows authorities to forecast and reduce the risk of flooding. This
is crucial for protecting lives and properties, particularly in areas susceptible to flooding.
3. Climate Research
Rainfall patterns are key indicators in climate studies. Tracking these patterns over
time helps scientists understand changes in the climate and aids in modelling future weather
conditions.
R.SHANMUGASUNDARAM, AP CE, PSNACET 43

44. 4. Agricultural Planning
Farmers rely on rainfall data for planting and harvesting schedules,
choosing crop types, and applying irrigation. This information is essential
for maximizing crop yield and managing resources efficiently.
5. Drought Assessment
Regular monitoring of rainfall helps identify the early signs of
drought, enabling timely responses such as water rationing or the
implementation of drought management strategies.
R.SHANMUGASUNDARAM, AP CE, PSNACET 44

45. Rainfall Measurement
Storm (Heavy Rain) Characteristics:
The characteristics of a storm, namely depth, duration, intensity and distribution,
affect the
watershed response to the rainfall event.
Depth – Amount of precipitation that falls (usually in or cm).
Duration – Length of a storm (usually min, hr or day)
Intensity – Depth of rainfall per unit time (usually in/hr or cm/hr). Rainfall intensity
Changes continuously throughout a storm, but it may be averaged over
short timeintervals or over the entire storm duration.
Distribution – Describes how rainfall depth or intensity varies in space over an
area orwatershed
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46. Intensity
•Precipitation is measured using a
 rain gauge, and more recently remote sensing techniques
such as a
 weather radar.
• When classified according to the rate of precipitation, rain can be divided into categories.
•Light rain describes rainfall which falls at a rate of between a
 trace and 2.5 millimetres
(0.098
 in) per hour. Moderate rain describes rainfall with a precipitation rate of between
2.6 millimetres (0.10
 in) and 7.6 millimetres (0.30 in) per hour.
•Heavy rain describes rainfall with a precipitation rate above 7.6 millimetres (0.30
 in) per
hour, and violent rain has a rate more than 50 millimetres (2.0
 in) per hour.
•Snowfall intensity is classified in terms of
 visibility instead.
•When the visibility is over 1 kilometre (0.62
 mi), snow is determined to be light.
• Moderate snow describes snowfall with visibility restrictions between 0.5 kilometres
(0.31
 mi) and 1 kilometre (0.62 mi).
•Heavy snowfall describes conditions when visibility is restricted below 0.5 kilometres
(0.31
 mi).
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46

47. Measurement of precipitation
Raingauge is used to collect and measure the precipitation
Pluviometer , ombrometer and hyetometer also used as a raingauge
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49. Rainfall Measurement
Measurement of rainfall and optimum number of rain gauges:
The basic instrument for rainfall measurement is rain gauge, which samples the incidence of
rainfall at a specific point, through an orifice of known area.
Types of Rain Gauges:
1.Non-Recording Rain Gauge
The non-recording rain gauge used in India is the Symon’s rain gauge. The rainfall is measured every
day at 08.30 hours IST. During heavy rains, it must be measured three or four times in the day, lest the
receiver fill and overflow, but the last measurement should be at 08.30 hours IST.
2.Recording Rain Gauge
a)Tipping Bucket Rain Gauge
b)Weighing Type Rain Gauge
c)Floating Type Rain Gauge
This type of gauge is used by Indian Meteorological Department (IMD).
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50. Measurement of precipitation
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51. Proper care, maintenance and
inspection :-
Especially during dry weather
to keep the instrument free
from dust and dirt
Non Recording or Original Rain Gauges
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52. Recording Type Automatic Rain Gauges
•In the recording gauge, it plots the rainfall against time. From that plot of data, we can easily
extract information about the intensity and duration of rainfall, on the basis of which we may
make a hydrological analysis of storms.
•Recording-type rain gauges are those rain gauges, which can give for permanent automatic
rainfall records without any bottle reading.
•In this type of rain gauge, no man is required to measure or read the amount of rainfall from the
rain gauge. The record was started and gets recording was recorded automatically on graph
paper.
•The gauge, focuses on a record of humidity range versus (VS) time, in the form of a graph,
which is known as the mass curve of rainfall. The figure below shows the mass curve of rainfall.
•The following are some of the commonly used
 
recording rain gauges:
•Tipping (or tilting) bucket type
•Siphon float type
•Weighing bucket type
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53. Produce a continuous plot of rainfall against time
Provide valuable data of intensity and duration of rainfall for hydrological analysis of storm
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55. Tipping or tilting bucket-type rain gauges:
•The tipping or tilting bucket-type rain gauge design principle is very simple.
 A container is taken and divided into
two vertical compartments and is balanced in an unstable equilibrium about a horizontal axis. In its normal position.
•The instrument arrangement is in the given ways:
•It consists of a 30 cm diameter sharp-edge receiver. At the end of the receiver, a funnel is provided. A pair of buckets
are pivoted on the funnel in such a way that when one bucket receives 0.2 mm of precipitation it tips, discharging its
contents into a tank and bringing the other bucket under the funnel. The teaching of the work completes an electric
circuit coming to the movement of a pen to mark on a clock-driven revolving drum that carries a record sheet.
Disadvantages of the Tipping or tilting bucket type rain gauges:
•When tipping of pockets takes place, rainfall at that instant is not recorded.
•Very high-intensity of rainfall gives close signals, which can make it difficult to record the number of tips and.
•Calibration of tips may change due to rusting and dirt accommodation.
•It does not produce the mass curve of rainfall.
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57. Weighing bucket type rain gauges:
•This type of rain gauge can be used for recording rainfall as well as snowfall.
•Rain is collected in a receiver vacate supported on a spring balance. A
 
mechanical lever
arm 
of the balance is connected without pain which touches a clock-mounted drum with the
graph paper.
•As it rains, the weight of the bucket gradually increases, which changes the position of the
pen of the balance. With the time the pen marks a line Cu continuously moves graph paper.
The recorded shows the accumulation of precipitation our time. The recording can be
chicken after
 
24 hours or 7 days 
depending on the clock and drum size.
•These gauges are generally
 
used in the USA
 
and are becoming increasingly popular but
not used in India and South Asia.
Disadvantages of weighing bucket type rain gauges:
•When very heavy precipitation occurs, there is a good chance that the bucket will overflow
and
•Expensive instruments are used
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59. Syphon ( Float) type automatic rainfall recorder
•The working principle of float-type rain gauge is similar to the waiting bucket-type rain gauge.
The reason water enters into the container of the gauge through the funnel. A float is provided at
the bottom of the container as shown in the figure below. This float is lifted off by the rainwater
which is collected in the container. This type of recording rain gauge is also known as a float type
gauge.
•Here, rainfall collected by funnel-shaped collection is led into a float chamber causing the upload
to rise. When the float rises, a pen (pointer) attached to the float through a level system records
the elevation of the float on a rotating drum driven by a clock mechanism. When the float has
reached a preset maximum level, the siphon arrangement empties the float chamber.
•All types of information about the storm can be obtained from the accumulator plot of graph
paper. The beginning and end of the storm, its intensity, duration, distribution of rain, and the
depths of total storm precipitation can easily e be obtained from the plot of the graph.
Disadvantages of syphon type automatic rainfall recorder:
•Cost instruments than other recording-type instruments.
•Mechanical defects sometimes give enormous results
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61. Rainfall of 53.8 mm in 30 hr
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63. Form of precipitation accumulate over a surface for sometime before it melts
and cause runoff
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64. Depth of water that result in melting of a unit of snow
Impotant : to assess the seasonal water resource of a catchment in estimates
>> stream flow
>> flood
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66. Optimum number of rain gauges:
•Statistics has been used in determining the optimum number of rain gauges required to be
installed in a given catchment.
•The basis behind such statistical calculations is that a certain number of rain gauge
stations are necessary to give average rainfall with a certain percentage of error.
•If the allowable error is more, lesser number of gauges would be required. The optimum
number of rain gauges (N) can be obtained using:
Where,
Cv = Coefficient of variation of rainfall based on the existing rain gauge stations;
= Mean / Standard Deviation of rainfall
E = Allowable percentage / degree of error in the estimate of basic mean
rainfall
R.SHANMUGASUNDARAM, AP CE, PSNACET 66

67. EVAPORIMETER
•Class A
 Evaporation Pan
•ISI Standard Pan
•Colorado Sunken Pan
•US Geological Survey Floating Pan.
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68. R.SHANMUGASUNDARAM, AP CE, PSNACET 68
Class A
 Evaporation
Pan

69. •A pan of diameter 1210mm and depth 255mm
•Depth of water is maintained between 18 and 20cm
•The pan is made of unpainted GI sheet
•The pan is placed on a wooden platform of height 15cm above ground
level to allow free air circulation below the pan
•Evaporation is measured by measuring the depth of water in a stilling
well with a hook gauge
R.SHANMUGASUNDARAM, AP CE, PSNACET 69

70. R.SHANMUGASUNDARAM, AP CE, PSNACET 70
ISI Standard Pan

71. •A pan of diameter 1220mm and depth 255mm
•The pan is made of copper sheet 0.9mm thick, tinned inside and painted white outside
•The pan is placed on a square wooden platform of width 1225mm and height 100mm above
ground level to allow free air circulation below the pan
•A fixed point gauge indicates the level of water
•Water is added to or removed from the pan to maintain the water level at a fixed mark using a
calibrated cylindrical measure
•The top of the pan is covered with a hexagonal wire net of GI to protect water in the pan from
birds
•Presence of the wire mesh makes the temperature of water more uniform during the day and
night
•Evaporation from this pan is about 14% lower as compared to that from an unscreened pan
R.SHANMUGASUNDARAM, AP CE, PSNACET 71

72. R.SHANMUGASUNDARAM, AP CE, PSNACET 72
Colorado Sunken Pan

73. •920mm square pan made of unpainted GI sheet, 460mm deep, and
buried into the ground within 100mm of the top
•Main advantage of this pan – its aerodynamic and radiation
characteristics are similar to that of a lake
•Disadvantages – difficult to detect leaks, expensive to install, extra
care is needed to keep the surrounding area free from tall grass, dust
etc
R.SHANMUGASUNDARAM, AP CE, PSNACET 73

74. R.SHANMUGASUNDARAM, AP CE, PSNACET 74
US Geological Survey Floating Pan

75. •A square pan of 900mm sides and 450mm deep
•Supported by drum floats in the middle of a raft of size 4.25m x
4.87m, it is set afloat in a lake with a view to simulate the
characteristics of a large body of water
•Water level in the pan is maintained at the same level as that in the
lake, leaving a rim of 75mm
•Diagonal baffles are provided in the pan to reduce surging in the pan
due to wave action
•Disadvantages – High cost of installation and maintenance, difficulty
in making measurements
R.SHANMUGASUNDARAM, AP CE, PSNACET 75

76. Analytical Methods of Evaporation
Estimation
•Water Budget Method
•Energy Budget Method
•Mass Transfer Method
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77. R.SHANMUGASUNDARAM, AP CE, PSNACET 77

78. (2) Energy Budget Method
•It involves application of the law of conservation of energy
•Energy available for evaporation is determined by considering the
incoming energy, outgoing energy, and the energy stored in the water
body over a known time interval
•Estimation of evaporation from a lake by this method has been found to
give satisfactory results, with errors of the order of 5%, when applied to
periods less than a week
R.SHANMUGASUNDARAM, AP CE, PSNACET 78

79. R.SHANMUGASUNDARAM, AP CE, PSNACET 79

80. R.SHANMUGASUNDARAM, AP CE, PSNACET 80

81. R.SHANMUGASUNDARAM, AP CE, PSNACET 81

82. Comparison
 of Methods
•Analytical methods can provide good results. However, they involve
parameters that are difficult to assess.
•Empirical equations can at best give approximate values of the correct
order of magnitude.
•In view of the above, pan measurements find wide acceptance in
practice.
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83. R.SHANMUGASUNDARAM, AP CE, PSNACET 83
LYSIMETER

84. Infiltrometer
•An
 
infiltrometer 
is a device used to measure the rate of water
infiltration
 into soil or other porous media.
•Commonly used infiltrometers are single-ring and double-ring
infiltrometers,
 disc permeameters, and falling head infiltrometers.
R.SHANMUGASUNDARAM, AP CE, PSNACET 84

85. Single Ring Infiltrometer
•A single-ring infiltrometer involves driving a ring into the soil and
supplying water in the ring either at constant
 head or falling head
condition. Constant head refers to condition where the amount of
water in the ring is always held constant.
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86. R.SHANMUGASUNDARAM, AP CE, PSNACET 86

87. Double Ring Infiltrometer
•A double ring infiltrometer requires two rings: an inner and outer ring. The
purpose is to create a one-dimensional flow of water from the inner ring, as the
analysis of data is simplified.
•An inner ring is driven into the ground, and a second bigger ring around that to
help control the flow of water through the first ring.
•Water is supplied either with a constant or falling head condition, and the operator
records how much water infiltrates from the inner ring into the soil over a given
time period.
•The ASTM standard method

specifies inner and outer rings of 30 and 60 cm
diameters, respectively.
R.SHANMUGASUNDARAM, AP CE, PSNACET 87

88. R.SHANMUGASUNDARAM, AP CE, PSNACET 88