WATER RESOURCES ENGINEERING MODULE 1 NOTES

WATER RESOURCES ENGINEERING: MODULE 1
Prepared by RMR Page 1
SYLLABUS
Hydrologic cycle-precipitation-mechanism, types and forms. Measurement of rainfall using rain
gauges-optimum number of rain gauges. Estimation of missing precipitation. Representation of
rainfall data-mass curve and hyetograph. Computation of mean precipitation over a catchment.
Design rainfall - probable maximum rainfall. Infiltration-measurement by double ring infiltrometer.
Horton’s model. Evaporation-measurement by IMD land pan, control of evaporation
Prepared by :
Reshma M. Raju
KTU-F34378

HYDROLOGIC CYCLE
Definition: The water which goes in atmosphere by evaporation and transpiration again comes back
in the form of precipitation under favourable climatic conditions is known as hydrological cycle of
water.
 The hydrologic cycle, also known as water cycle is the global scale, endless re-circulatory
process linking water in the atmosphere, land and in the oceans.
 The cycle has no beginning. The three fundamental process of the hydrological cycle are – a)
Evaporation and transpiration b) Precipitation and c) Runoff as shown in fig.1.1.
Fig.1.1 Hydrologic Cycle
 Description of the hydrologic cycle can start with the evaporation of water from the oceans,
which is driven by energy from the sun.
 The evaporated water, in the form of water vapour, rises by convection, condenses in the
atmosphere to form clouds, and precipitates onto land and ocean surfaces, predominantly
as rain or snow.
 Rainfall on land surfaces is partially intercepted by surface vegetation, partially stored in
surface depressions, partially infiltrated into the ground, and partially flows over land into
drainage channels and rivers that ultimately lead back to the ocean.
 Rainfall that is intercepted by surface vegetation is eventually evaporated into the
atmosphere; water held in depression storage either evaporates or infiltrates into the
ground;

 Water that infiltrates into the ground contributes to the recharge of groundwater, which is
either utilized by plants, evaporates, is stored, or becomes subsurface flow that ultimately
emerges as recharge to streams or directly to the ocean.
 Snowfall in mountainous areas typically accumulates in the winter and melts in the spring,
thereby contributing to larger-than-average river flows during the spring.
 The hydrologic cycle may be expressed by the following simplified equations.
Precipitation = Evaporation + Runoff
[ ] [ ] [ ]
1. Evaporation and Transpiration (E)
 The water from the surfaces of ocean, rivers, and lakes and also from the moist soil
evaporates.
 The vapours are carried over the land by air in the form of clouds.
 Transpiration is the process of water being lost from the leaves of the plants from their
pores.
 Thus the total evaporation (E), inclusive of the transpiration consists of
o Surface evaporation
o Water surface evaporation from rivers and oceans
o Evaporation from plants and leaves (transpiration)
o Atmospheric evaporation
2. Precipitation (P)
 Precipitation maybe defined as the fall of moisture from the atmosphere to the earth’s
surface in any form. Precipitation maybe of two forms:
o Liquid precipitation – e.g. rainfall
o Frozen precipitation – e.g. snow, hail, sleet etc.
3. Runoff (R)
 Runoff is that portion 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 run off which ultimately runs to the ocean through
surface or sub-surface streams.
 Thus the runoff may be classified as follows:
o Surface runoff: - Water flows over the land and is first to reach the
streams and rivers, which ultimately discharge the water to the sea.

o Inter-flow or subsurface runoff: - A portion of precipitation infiltrates
into surface soil and depending upon the geology of the basins, runs as
sub-surface runoff and reaches the streams and rivers.
o Ground water flow or base flow: - it is that portion of precipitation,
which after infiltration, percolates down and joins the ground water
reservoir which is ultimately connected to the ocean.
PRECIPITATION
 Precipitation is one of the main phases of hydrologic cycle.
 It includes all moisture that reaches earth’s surface in liquid or solid form due to
condensation of the atmospheric vapour.
 The atmospheric air always contains moisture. Evaporation from the oceans is the major
source (about 90%) of the atmospheric moisture for precipitation. Continental evaporation
contributes only about 10% of the atmospheric moisture for precipitation.
 The atmosphere contains the moisture even on days of bright sun-shine. However, for the
occurrence of precipitation, some mechanism is required to cool the atmospheric air
sufficiently to bring it to (or near) saturation.
 This mechanism is provided by either convective system (due to unequal radiative heating or
cooling of the earth’s surface and atmosphere) or by orographic barriers (such as mountains
due to which air gets lifted up and consequently undergoes cooling, condensation, and
precipitation) and results into, respectively, convective and orographic precipitations.
 Alternatively, the air lifted into the atmosphere may converge into a low-pressure area (or
cyclone) causing cyclonic precipitation.
 Artificially induced precipitation requires delivery of dry ice or silver iodide or some other
cloud seeding agent into the clouds by aircrafts or balloons.
Types of precipitation
Depending upon the way in which the air is cooled, as to cause precipitation, we have three kinds of
precipitation; there are three types of precipitation.
1. Cyclonic Precipitation: -
 Caused by the lifting of an air mass due to the pressure difference.
 The large whirling mass of air, at the centre of which the barometric pressure is low, is
known as a cyclone.

 The central portion acts like a chimney, through which the air gets lifted, expands cools and
finally gets condensed causing precipitation.
 Cyclonic precipitation can occur in the form of drizzle, intermittent rain, or steady rain.
 If low pressure occurs in an area, air will flow horizontally from the surrounding area,
causing the air in the low pressure area to lift.
 The precipitation that results is called non-frontal precipitation.
 If one air mass lifts over another mass, the precipitation is called frontal precipitation.
 The boundary between these two air masses of different temperatures and densities (one
warm air mass and the other colder) is known as a front or frontal surface.
1.Frontal Precipitation
2.Non-Frontal Precipitation
Fig.1.2 Cyclonic Precipitation
 If a cold air mass drives out a warm air mass’ it is called a ‘cold front’ and if a warm air mass
replaces the retreating cold air mass, it is called a ‘warm front’.
 On the other hand, if the two air masses are drawn simultaneously towards a low pressure
area, the front developed is stationary and is called a ‘stationary front’.

 Cold front causes intense precipitation on comparatively small areas for short duration,
while the precipitation due to warm front is less intense but is spread over a comparatively
larger area which is more continuous.
 Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of
cold and warm air sliding against each other. This phenomenon is called ‘occlusion’ and the
resulting frontal surface is called an ‘occluded front’.
 The precipitation pattern is a combination of both cold and warm frontal distributions.
2. Convective Precipitation: -
 Caused by natural rising of warmer lighter air in colder, denser surroundings.
 The difference in temperature may result from unequal heating at the surface, unequal
cooling at the top of the air layer, or mechanical lifting when air is forced to pass over a
denser colder air masses.
 The vertical air currents develop tremendous velocities and are hazardous to aircrafts.
 Convective precipitation is spotty and its intensity may vary from light showers to cloud
bursts (high intensity and short duration).
 It is characterised by occurrence of hot weather conditions and it occurs in temperate zones
at low latitudes.
 This type is common in equatorial and tropical areas.
Fig.1.3 Convective Precipitation
3. Orographic Precipitation
 Due to the lifting of warm moisture laden air masses due to topographic barriers such as
mountains causing condensation and precipitation.
 The greatest amount of precipitation falls on the windward side, and the leeward side often
has very little precipitation (rain shadow region).
 The factors that are important in this process are land elevation, local slope, orientation of
landscape, and distance from the moisture source.
 The rainfall is composed of showers and steady rainfall.

Fig.1.4 Orographic Precipitation
Forms of Precipitation
 The common forms of precipitation are: -
a. Drizzle or mist: Water droplets of diameters less than 0.5 mm, intensity < 1
mm per hour
b. Rain: water drops of size between 0.5 mm and 6.0 mm
c. Glaze: when the drizzle or rain freezes as it comes in contact with cold
objects, it is known as glaze.
d. Sleet: rain water drops falling through air at or below freezing temperatures,
turned to frozen rain drops
e. Snow: ice crystals resulting from sublimation
f. Snowflakes: ice crystals combine to form flakes with average specific gravity
of about 0.1
g. Hail: precipitation in the form of ice balls of diameter more than about 8
mm formed by alternate freezing
 Most of the precipitation, generally, is in the form of rains. Therefore, the terms
precipitation and rainfall are considered synonymous.
 Rainfall, i.e., liquid precipitation, is considered light when the rate of rainfall is up to 2.5
mm/hr, moderate when the rate of rainfall is between 2.5 mm/hr and about 7.5 mm/hr, and
heavy when the rate of rainfall is higher than about 7.5 mm/hr.
MEASUREMENT OF RAINFALL
 Precipitation (of all kinds) is measured in terms of depth of water that would accumulate on
a level surface if the precipitation remained where it fell.
 A variety of instruments have been developed for measuring precipitation (or precipitation
rate) and are known as precipitation gauges or, simply, rain gauges which are classified as
either recording or non-recording rain gauges.

 Non-recording rain gauges: - also known as non-automatic rain gauges. It can only collect
rain water which, when measured suitably, gives the total amount of rainfall at the rain
gauge station during the measuring interval. The Indian Meteorological Department has
adopted Symon’s rain gauge.
 Non-recording rain gauges: - also known as automatic rain gauges. They are integrating type
recording rain gauges and are of three types – weighing bucket rain gauges, tipping bucket
rain gauge, float type rain gauge.
OPTIMUM NUMBER OF RAIN GAUGES
 The spatial variability of the precipitation, nature of the terrain and the intended uses of the
precipitation data govern the density (i.e., the catchment area per rain gauge) of the rain
gauge network.
 Obviously, the density should be as large as possible depending upon the economic and
other considerations such as topography, accessibility etc.
 The World Meteorological Organisation (WMO) recommends the following ideal densities of
the rain gauge network:
o For flat regions of temperate, Mediterranean, and tropical zones - 1 gauge
for 600 to 900 sq. km (900–3000 sq. km tolerable).
o For mountainous regions of temperate, Mediterranean, and tropical zones -
- 1 gauge for 100 to 250 sq. km (250 to 1000 sq. km tolerable).
o For small mountainous islands with irregular precipitation - - 1 gauge for 25
sq. km.
o For arid and polar zones - - 1 gauge for 1500 to 10,000 sq. km.
o At least ten per cent of rain gauge stations should be equipped with self-
recording gauges to know the intensities of rainfall.
 The Bureau of Indian Standards recommends the following densities for the precipitation
gauge network (as per IS 4987 – 1968):
o One gauge per 520 sq. km in plain areas, with denser network for the areas
lying in the path of low pressure systems.
o One gauge per 260 to 390 sq. km in regions of average elevation of 1000 m
above mean sea level.
o One gauge per 130 sq.km in predominantly hilly regions with heavy rainfall,
higher density being preferred wherever possible.
o 10% of the gauges are of recording type.

 For an existing network of rain gauge stations, one may need to know the adequacy of the
rain gauge stations and, therefore, the optimal number of rain gauge stations N required for
a desired accuracy (or maximum error in per cent) in the estimation of the mean rainfall.
 The optimal number of rain gauge stations N is given as
[ ]
Here,
= the coefficient of variation of the rainfall values at the existing stations
̅
X 100
= standard deviation = √
∑( ̅)
̅ = mean of rainfall values of existing stations
P = desired degree of error in estimating mean rainfall
o Both and p should be expressed as percentage.
o If N < n , where n is the number of existing stations, the existing network
estimates the average depth of rainfall with an error less than allowable
value p and no more gauges are required.
o If N > n, the number of additional rain gauges will be (N-n), and these should
be distributed in different zones (caused by isohyets) in proportion to their
areas, i.e. depending upon spatial distribution of the existing rain gauge
stations and the variability of the rainfall over the basin.
 For important projects, the network of rain gauges should be so set that any addition of rain
gauge stations will not appreciably alter the average depth of rainfall estimated. Such a
network is known as saturated network in which p will be small and thus N will be large so as
to estimate rainfall with greater accuracy.
ESTIMATION OF MISSING PRECIPITATION DATA
 The continuity of a record of precipitation data may have been broken with missing data due
to several reasons such as damage (or fault) in a rain gauge during a certain period or due to
the absence of the observer.
 The missing data is estimated using the rainfall data of the neighbouring rain gauge stations
using the following methods:
o Arithmetic mean method
o Normal ratio method

o Inverse distance method by US Weather Service
1. Arithmetic Mean Method
o Missing rainfall of the station X is computed by simple arithmetic average
of the rainfall at the nearby station known as index stations
∑ ⁄ ( )
where n = number of index stations
o This method is adapted under these conditions
a. The normal annual rainfall of the missing station is within
10% of the normal annual rainfall of the index stations.
b. Data of at least three index stations should be available.
c. The index stations should be evenly spaced around the
missing station and should be as close as possible.
2. Normal Ratio Method
o In this method, the rainfall ( ) of the surrounding index stations are
weighed by the ratio of normal annual rainfalls by using the following
equation:
[ ] [ ]
where = normal annual rainfall of index stations.
= normal annual rainfall of missing station
n = number of index stations
o This method is used when the normal annual precipitation of the index
stations differ more than 10% of the missing station.
o The data of at least three index stations should be available, and all these
index stations should be evenly spaced.
3. Inverse Distance Method ( US weather Service Method)
o In this method, a set of rectangular co-ordinate axes are passed through the
missing rain gauge station so that its co-ordinates are (0,0).
o The co-ordinates of each index stations ( ) are found.
o The weightage ( ) of each index stations surrounding the missing stations
is calculated as
o The missing rainfall data of the station X

∑
∑
o The most acceptable method for scientific analysis and its limitation is that it
estimates missing rainfall between the highest and lowest values of index
stations.
REPRESENTATION OF RAINFALL DATA
 Commonly used methods of presentation of rainfall data for interpretation and analysis are :
a. Mass curve
b. Hyetograph
1. Mass Curve of Rainfall
o It is a plot of cumulative depth of rainfall against time, plotted in
chronological order.
o Records of float-type and weighing bucket-type rain gauges are in this form.
o It is useful in extracting the information on the duration and magnitude of a
storm.
o The slope of mass curve gives the intensity of rainfall (i) at various time
intervals in a storm.
o Horizontal portion of the curve indicates that there was no rainfall during
that period.
o The mass curve of a design storm is obtained by maximising the mass curves
of the severe storms in the basin.
Fig.1.5 Mass curve of rainfall

2. Hyetograph
o Hyetograph is the plot of intensity of rainfall against time and is usually
represented as a bar chart.
o Hyetograph can be prepared either from the mass curve of rainfall, or
directly from the data obtained from automatic rain gauges.
o The area under a hyetograph represents the total precipitation received in
the period.
Fig.1.6 Hyetograph of a storm
COMPUTATION OF MEAN PRECIPITATION OVER A CATCHMENT AREA/BASIN
 For a small area the rainfall recorded at a single rain gauge station located in that area may
be taken as the average depth of rainfall over the area is determined.
 For large areas there will be a network of rain gauges suitably located in the area, the
computation of average precipitation or rainfall is done by the following methods.
a. Arithmetic mean method
b. Thiessen polygon method
c. Isohyetal method
a. Arithmetic Mean Method
o This is the simplest method in which average depth of rainfall is obtained by
obtaining the sum of the depths of rainfall (say P1, P2, P3, P4 .... Pn) measured
at stations 1, 2, 3, ....., n and dividing the sum by the total number of
stations, n.
∑
o This method is suitable if the rain gauge stations are uniformly distributed
over the entire area and the rainfall variation in the area is not large.
b. Theissen Polygon Method
o This method is based on the presumption that any point in the watershed
receives the same amount of rainfall as the nearest gauge and the rainfall

recorded at a gauge can be applied to any point at a distance half way to
the next station in any direction.
o Following steps are adopted in this method:
a. Draw lines joining adjacent gauges.
b. Draw perpendicular bisectors to the lines erected in step one.
c. Extend the lines erected in step b in both directions to form
representative area of gauges.
d. Compute area of each polygon constructed by steps a, b and c
e. Resolve the so-constructed polygons for each gauge station into
triangles
f. Calculate the area of each triangle in the polygon of the gauge by
erecting bisectors perpendicular to the base of each triangle of the
polygon.
g. Sum-up the area of all triangles to calculate the area of the polygon
representing the area of influence of each rain gauge station
h. Due weightage of each polygon area is given to the rainfall recorded
at each gauge
o Following the above steps, the rainfall at each rain gauges and the area of its
influence is determined.
o If P1, P2, P3... Pn are the rainfalls recorded at the stations 1, 2, 3, …., n, and
A1, A2, A3, .... An are the area of polygon enclosed by them respectively, then
the average depth of rainfall is given by
∑( )
∑
o The advantages of this method are as follows:
a. It makes use of data from nearby stations located outside the
catchment.
b. It allocates importance of measurement according to the station
spacing.
o The limitations of this method are s follows:
a. This method does not make any allowances for orographic
influences in basin. Thus, it is not suitable to compute the average
rainfall of mountainous catchments.
b. If a new rain gauge is added to the existing network or position of
rain gauge is changed in the catchment, then the network changes

and new polygons are required to be sketched. Thus, in this case, a
fresh computation of weights, and thus, average rainfall, is required.
c. Isohyetal Method
o An isohyet is a contour of equal rainfall.
o Knowing the depths of rainfall at each rain gauge station of an area and
assuming linear variation of rainfall between any two adjacent stations, one
can draw a smooth curve passing through all points indicating the same
value of rainfall.
o The area between two adjacent isohyets is measured with the help of a
planimeter.
o The average depth of rainfall P for the entire area A is given as:
∑[ ] [ ]
[ ] [ ] ( ) [ ]
∑
o Since this method considers actual spatial variation of rainfall, it is
considered as the best method for computing average depth of rainfall.
o This method can show orographic effects and thus it is adopted for pictorial
presentation.
o However, this method needs a fairly dense network of gauges to correctly
construct the isohyetal map and is time-consuming compared to the other
methods.
 Difference between Thiessen polygon method and isohyetal method
Sl. No. Thiessen Polygon Method Isohyetal Method
1.
This method does not make any
allowances for orographic influences
in basin.
This method is suitable to compute
the average rainfall of mountainous
catchments.
2.
This method is only a mechanical and
mathematical process and does not
need any expertise.
This method needs special expertise in
drawing of contours.
3.
This method is useful in case of less
number of stations.
In case of large number of stations,
isohyetal method is more feasible
than Thiessen polygon method.

DESIGN RAINFALL
 Design rainfall is a probabilistic representation of rainfall intensity (depth of rainfall over a
time period) at a given location for a given duration and average recurrence interval (ARI)
 It is an essential input to a hydrologic model, which is used to estimate design discharge that
is needed in the planning and design of many engineering infrastructure projects such as
street drainage systems, culverts, bridges and regulators.
 Design rainfall estimation is made using recorded rainfall data over many stations in a given
region.
 Recorded rainfall data at many stations are used to develop intensity-duration-frequency
(IDF) curves by adopting statistical techniques such as regional frequency analysis.
PROBABLE MAXIMUM RAINFALL /PRECIPITATION
 PMP can be defined as the maximum depth of precipitation for a given duration that may
possibly occur on a given catchment at any time of year - worst possible hydrologic
conditions in the area.
 Such storm is called Probable Maximum Storm and will be used for designing large hydraulic
structures.
 PMP is that magnitude of precipitation which is not likely to be exceeded for a particular
basin at any given time of a year in a given duration.
 Thus, PMP would yield a flood which would have virtually no risk of being exceeded in that
duration.
 Such a precipitation would occur under the most adverse combination of hydrological and
meteorological conditions in the basin/area.
 Estimation of PMP is useful for obtaining the design flood for the purpose of designing
hydraulic structures such as spillways failure of which would result in catastrophic damage
to life and property in the surrounding region.
 PMP can be estimated by using either meteorological methods or statistical studies of
rainfall data.
 One can derive a model (for predicting PMP) based on parameters (such as wind velocity
and humidity etc.) of the observed severe storms over the basin and then obtain the PMP
for maximum values of those parameters.
 Occasionally when enough storm data of given basin is not available, PMP can be estimated
by adopting a severe storm over a neighbouring catchment and transporting it to the
catchment under consideration.

INFILTRATION
 Infiltration is the downward movement of water from soil surface, into the soil mass through
the pores of the soil.
 When rain water falls on the ground, a small portion of it is initially absorbed by the top
layer of soil so as to replenish the soil moisture deficiency and thereafter, excess water
moves downwards to become a part of ground water.
 Once water enters into the soil, the process of transmission of water in the soil, the process
of transmission of water in the soil, known as percolation, takes place, thus removing the
water from near surface to down below.
 Infiltration and percolation are directly inter-related. When percolation stops, infiltration
also stops.
 At any instant, the maximum rate at which water will enter the soil in any given condition is
called infiltration capacity( ).
 The rate at which the water actually infiltrates through a soil at any instant during a storm is
known as infiltration rate and is equal to the infiltration capacity or the rainfall rate,
whichever is less.
 The rate of infiltration is affected by soil characteristics including ease of entry, storage
capacity, and transmission rate through the soil.
 The soil texture and structure, vegetation types and cover, water content of the soli, soil
temperature, and rainfall intensity all play a role in controlling infiltration rate and capacity.
Factors affecting infiltration
a. Condition of entry surface: Vegetative cover
b. Permeability/Percolation characteristics of soil formation
c. Soil moisture
d. Temperature
e. Intensity and duration of rainfall
f. Movement of man and animals
g. Change due to human activities
h. Quality of water
i. Presence of ground water
j. Size and characteristics of soil particles
k. Catchment parameters

HORTON’S EQUATION
 If infiltration rate is plotted with respect to time then a falling curve is obtained which is
known as infiltration capacity curve.
 Horton found that infiltration capacity curves approximate the form
( ) for 0 ≥ t ≥
where = infiltration capacity at any time t from the start of the rainfall
= final constant infiltration capacity at saturation occurring at t =
= initial infiltration capacity when t = 0
k = Horton’s decay coefficient which depends upon soil characteristics and vegetation cover
= duration of rainfall
Fig. 1.7 Infiltration capacity curve
 This equation assumes an infinite water supply at the surface i.e., it assumes saturation
conditions at the soil surface.
 The area under Horton’s curve for any time interval represents the total depth of water
infiltrated during that interval.
DOUBLE RING INFILTROMETER
 Most commonly used flooding type infiltrometer.
 Consists of two concentric rings driven into soil to a depth of about 15cm without tilt and
without disturbing the soil much.
 Diameters of the rings may vary from 25cm to 60cm
 Two sets of concentrating rings with diameters of 30cm and 60cm
 Water is applied in both the inner and outer rings to maintain a constant depth of 5cm.
 Water is replenished after the level falls by about 1cm.
 The water depth in the inner and outer rings should be kept same during the entire
observation period.
 Volume of water added to the inner ring at successive time intervals to maintain constant
depth is noted.

 Water added to the outer ring need not be measured.
 The experiment has to be carried out till a constant infiltration rate is observed.
Fig.1.8 Double Ring Infiltrometer
EVAPORATION
• Evaporation and evapo-transpiration processes transfer water to the atmosphere as water
vapour are the two most important phases of hydrologic cycle which redistribute the heat
energy between surfaces and atmosphere.
• It is a continuous natural process by which a substance changes from liquid to gaseous state.
• The main source of evaporation is the solar radiation.
• Evaporation is affected by air and water temperature, relative humidity, wind velocity,
surface area, barometric pressure and salinity of water.
• Measurement of evaporation-directly by IMD Land Pan
MEASUREMENT OF EVAPORATION-BY IMD LAND PAN
• Indian Meteorological Department (IMD) land pans are installed in vicinity of lake to
determine lake evaporation.
 Specified by IS:5973 and is a modified version of US Weather Bureau Class A Pan
 Pan is of diameter 1.22 m and depth 0.255 m.
 It is made of copper sheet 0.9mm thick, tinned inside and painted white outside.
 It is equipped with hexagonal wire netting of galvanised iron mesh covering it to protect
water from birds, and to make the water temperature more uniform during day and night.
 The pan is placed on a square wooden platform of width 1.225 m and height 10 cm above
ground level to allow free air circulation below the pan and also to thermally insulate the
pan completely from ground.
 A fixed point gauge placed inside a stilling basin 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.

Fig.1.9 Indian Standard Evaporation Pan
 Evaporation from this pan differs from that of a lake or reservoir.
Lake evaporation = Pan coefficient X Pan evaporation
 Pan coefficient ranges from 0.67 to 0.82 and pan coefficient for Indian Standard
Evaporimeter is around 0.8
Factors affecting evaporation losses
a. Nature of evaporation surface
b. Area of water surface
c. Depth of water in water body
d. Humidity
e. Wind velocity
f. Temperature of air
g. Atmospheric pressure
h. Quality of water
CONTROL OF EVAPORATION
The methods generally used for evaporation control are
a. Wind breakers
b. Covering the water surface
c. Reduction of exposed water surface
d. Integrated operation of reservoirs
e. Treatment with chemical Water Evapo Retardants (WER)

1. Wind breakers
o Planting of trees normal to windward direction-an effective measure for
checking of evaporation loss.
o Plants are grown in a row or rows to act as wind breaker.
o Plants to act as wind breakers are usually arranged in rows, with tallest
plants in the middle and the smallest along the end rows.
o Plants selected as wind breakers should be capable of resisting the stresses
due to wind, temperature, insects etc.
o The spacing between plants varies from place to place, depending upon the
climate and type of soil.
2. Covering the water surface
o Covering the surface of water bodies with covers considerably retards
evaporation loss.
o Covers reflect energy inputs from atmosphere and prevent transfer of water
vapour to outer atmosphere.
o For small storages – fixed covers are used
o For large storages - floating covers or spheres are used.
o Floating spheres of a polystyrol has reduced evaporation to 80% in small
experimental tanks.
o The effective evapo-retardants are mustard oil, thermocol, wax etc.
3. Reduction of exposed water surface
o In this method shallow portions of the reservoirs are isolated by
construction of bunds at suitable locations.
o Water accumulated during the monsoon season in such shallow portions is
diverted to appropriate deeper pocket in summer months, so that the
shallow water surface area exposed to evaporation is effectively reduced.
o In India, this method has been tried for Nayka reservoir, supplying water to
Surendranagar in Gujarat, which yielded good results.
4. Integrated operation of reservoirs
o This method is suitable for a system of reservoirs which can be operated in
an integrated way.
o It consists of operating the reservoirs in such a way that total exposed water
surface area is kept minimum for the system as a whole, thus reducing
evaporation.

o For achieving this objective water use should be planned in such a way that
shallow reservoirs with large water spread area are depleted first.
o This method has been successfully practiced by Mumbai Municipal
Corporation in their water supply scheme.
5. Treatment with chemical Water Evapo Retardants (WER)
o Chemicals capable of forming a thin mono-molecular film have been found
to be effective for reducing evaporation.
o The film so formed reflects energy inputs from atmosphere, as a result of
which evaporation loss is reduced.
o The film allows enough passage of air through it and hence, aquatic life is
not affected.
o The film developed by using fatty alcohols of different grades has been
found most useful for control of evaporation.
o They are generally termed as chemical Water Evapo-Retardants (WERs) and
these are available in the form of powder, solution or emulsion.
o These chemical water evapo-retardants have the disadvantage of high cost
of application.
o The economics of WERs application may however vary from site to site
depending on local factors.
o Following chemicals are generally used for water evaporation retardation:
• Cetyl Alcohol
• Stearyl Alcohol
• Linear Alcohols
• Cetyl Stearyl Alcohol
NB: For types of rain gauges, refer any standard textbooks and for factors affecting evaporation and
also infiltration in detail refer ‘Irrigation and water power engineering by Punmia et.al.
DRM Software Reviews

WATER RESOURCES ENGINEERING MODULE 1 NOTES

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to WATER RESOURCES ENGINEERING MODULE 1 NOTES

Similar to WATER RESOURCES ENGINEERING MODULE 1 NOTES (20)

Recently uploaded

Recently uploaded (20)

WATER RESOURCES ENGINEERING MODULE 1 NOTES