3. Hyetograph
method
A hyetograph is a
bar graph showing
the intensity of
rainfall with time
It can be prepared
from the mass
curve of rainfall or
data from
automaticrain
gauges
4. Mass curve
method
• Mass curves are a
graphical representation
of cumulative depth of
rainfall over a period of
time.
• In the case of rainfall,
the vertical axis
represents the
cumulative rainfall for
the period of interest,
while the horizontal axis
represents the time.
5. Point rainfall
method
The rainfalldataof a station
is known as point rainfall
Point rainfalldatais
represented as daily, weekly,
monthly, and annualvalues.
Point rainfalldatais
graphicallyrepresentedas a
bar diagram of magnitude vs
chronologicaltime.
it is also used in intensity-
duration-frequencycurves
6. Interpretation of
rainfall data
Intensity of rainfall:
• The rate at which rainfall occurs and is
expressed as cm/h or mm/h.
• Non-recording type rain gauges
measure/record only rainfall depth in a
day, or over a duration.
Frequency of rainfall:
• Frequency of rainfall of a specified
period is determined using probability
theory
• Design of hydraulic structures such as
flood control structures, soil
conservation structures, waste-water
systems, drains, and culverts, etc is
based on the probability of extreme
rainfall and runoff
7. Intensity
duration
analysis
• Intensity of rainfall
decreases with increasing
duration
• Greater intensity of rainfall
has shorter duration
• Sherman proposed a
relation between intensity
and duration of rainfall:
8. Intensity
duration frequency
relationship
Intensity-Duration-Frequency (IDF)
relationship describes the
relationship between the intensity of
rainfall, duration, and frequency.
IDF relationship is typically in the
form of an equation or set of curves
derived from historical rainfall data.
Generally, as the duration of a
rainfall event increases, the intensity
required to achieve a given
frequency decreases.
Example: 1-hour rainfall event with a
frequency of once in 100 years may
require 100mm/hr, whereas a 24-
hour event with the same frequency
may only require 50mm/hr.
9. Depth area
relationship
• The depth-area relationship
describes the expected
relationship between
rainfall depth and area
• The equations or curves
used in the relationship are
specific to a particular
geographic region
• As rainfall depth increases,
the area over which it is
distributed decreases
• This is caused by increased
runoff resulting in less
infiltration
10. Depth
area duration
relationship
• Depth-area-duration curves show the
relationship between rainfalldepth, area, and
duration
• Curves show that the volume of runoff
increases as rainfall depth and duration
increase
• Relationship is due to the fact that as rainfall
intensity and duration increase, the soil's
ability to absorb water is exceeded
• DAD curves areimportant tools for
hydrologists and engineers
• By understanding therelationship, engineers
can estimate the volumeof runoff
• DAD curves areused to estimate the
maximum precipitation and volume of runoff
froma given catchmentarea during a rainfall
event
11. Evaporation
• Evaporation and evapo-transpiration are important
phases of the hydrologic cycle which redistribute heat
energy.
• Evaporation is a process in which liquid changes to
gaseous state below the boiling point with the
transfer of heat energy.
• Solar radiation is the main source of evaporation, and
in arid regions the loss due to evaporation can be up
to 90% of the annual precipitation.
12. Factors affecting evaporation loss
Nature of
evaporatingsurface
Area of water
surface
Depth of water in
water body
Wind speed Temperature ofair
Atmospheric
pressure
Qualityof water
13. Estimation or measurement of evaporation
Measurement using
evaporation pans
Use of empirical
equations
Water budget
method
Energy budget
method
14. Measurement using
evaporation pans
• Most reliable method for estimation of evaporation from large bodies of water is
through measurements from evaporation pans
• Pan is 1.22 m in diameter and 0.255 m deep. Made of 0.9 mm thick copper sheet
with hexagonal wire netting of galvanized iron mesh covering it
• Placed over a square wooden platform of 1.29 m in width and 10 cm in height
• The water level in the pan is recorded by a point gauge arrangement placed
inside a stilling basin
• Measurement is taken at least once a day by adding water to the pan in a
calibrated cylindrical glass jar
• If there is rainfall exceeding the depth of evaporation, water is taken out of the
pan in the same way by measuring jar
• Knowing the depth of rainfall from the rain gauge, the evaporation depth is found
by subtraction
15. Use of empirical equations
• Meyer's formula (1915)
E = evaporation from the water body, mm/day
es= saturation vapor pressure at the water surface
ea= actual vapor pressure of overlaying air at the sp. Height
V9= monthly mean wind velocity in km/h @ 9 m above the GL
Km= coefficient accounting for various other factors
= 0.36 for large deep waters
= 0.50 for small shallow waters
16. Use of empirical equations
• Rohwer's formula (1931)
E = evaporationfrom the water body, mm/day
es= saturationvaporpressure at the water surface
ea= actual vaporpressure of overlayingair at the sp. Height
V0.6= mean wind velocity in km/h @ 0.6 m above the GL
To find V0.6 in the numericalproblems using the following equation
Vh is the wind velocity at height h abovethe GL
17. Water budget method
P = Precipitation
Isf = surface water inflow Osf = surface water outflow
Igf = ground water inflow Ogf = ground water outflow
T = Transpiration loss
ΔS = change in storage
18. Energy budget method
• This method uses the conservation of energy by incorporating all the incoming, outgoing and
stored energy of aa lake/reservoir in the following form:
Hn = Ha + He + Hg + Hs + Hi ------ (1)
Hn = net heat energy received by water surface = Hc(1-r)-Hb
Hc(1-r) = incoming solar radiation into a surface of reflection coefficient r (approx. 0.05)
Hb = back radiation from the water body
Ha = sensible heat transfer from water surface to air
He = heat energy used up in evaporation =
19. Energy budget method
• La = latent heat of evaporation
• Hg = heat flux into the ground
• Hs = heat stored in waterbody
• Hi = net heat conducted out of the systemby waterflow
• All the energy terms are in cal/mm2/day
• Hs and Hi can be neglected forsmall time periods
• Tw = temperature of water surface
• Ta = temperature of air
• Pa = atmospheric pressure
Ha can be estimated by using Bowen's ratio(β)
------(2)
Combining eq 1 & 2, we obtain the following equation for evaporation E:
20. Infiltration
Infiltrationis the entry or passage of water into the soil through soil surface.
Process of transmission of water in the soil is known as percolation.
Terms infiltration wasfirst used by Horton (1935).
He defined it as the entry of water intosoil through the surface layer of the soil, vertically.
Infiltrationis a majorprocess continuouslyaffecting the magnitude, timing and distributionof surface run off at
any measured outlet of a basin.
Infiltrationandpercolationare directly interrelated.When percolationstops, infiltration alsostops.
21. Factors affecting infiltration
Condition of entry
surface: vegetation cover
versus bare land
Permeability/percolation
characteristicsof soil
formation
Antecedent moisture
conditions in soil
Temperature
Intensity and durationof
rainfall
Moment of man and
animals
Change due to human
activities
Quality of water
Presence of ground
water table
Size and characteristics
of soil particles
22. Measurement of infiltration
Single tube infiltrometer
• Consists of a hollow metal cylinder of 30 cm diameter and 60
cm length, both ends open
• 10 cm of cylinder driven into ground, water-filled to 7 cm head
above ground level
• Water level decreases due to infiltration, water added to
maintain a constantlevel
• Volume of water added over predetermined time interval
gives infiltration rate -
• Observations continued till a uniform rate is obtained (3-6
hours, depending on the type of soil) -
• Plot of time in abscissa againstthe rate of water added in
mm/h gives infiltration capacity curve -
• Drawback is infiltrated water percolates laterally at bottom of
the ring, does not represent area through which infiltration
takes place
23. Measurement of infiltration
Double tube infiltrometer
• Double tube infiltrometer consists of two concentric hollow
rings (or cylinders)
• Rings driven into soil without any tilt, to a depth of 15 cm.
Diameter of rings may vary from 25 to 60 cm
• Water applied in both inner and outer rings to maintaina
constant depth of about 5 cm
• Water level in inner and outer rings kept the same during
observation period
• Measurement includes recording of volume of water added
into inner compartment and corresponding elapsed time
26. Computation of runoff
Runoff by linear or exponential regression
• Linear regression is a statistical method used to determine the
relationship between two variables
• In the context of runoff prediction, linear regression is used to
determine the relationship between rainfall and runoff
• Exponential regression is a statistical method used to determine the
relationship between two variables when the data shows an
exponential pattern
• In the context of runoff prediction, exponential regression is used
when the relationship between rainfall and runoff is not linear
• Exponential regression can help to model and predict runoff when
rainfall has a greater impact on runoff as it increases
• Both linear and exponential regression can be used to predict runoff,
but they are typically used in different contexts
R = aP+b
Linear regression
equation
Coefficient of correlation
Exponential relationship
R = βPm
27. Computation of runoff
By empirical equations and tables
• Runoff coefficient
R = kP
• Inglis's formula
Derived from data collected from 37 catchments in the Bombay presidency
R = 0.85 P – 30.5 (For ghat areas)
R= 0.00394P2 – 0.0701 P (For plain regions)
Urban residents -0.3 - 0.5
Commercial and industrial -0.9
Forest areas -0.05 - 0.2
Parks, farm land, pasture -0.05 - 0.3
Asphalt or concrete pavements -0.85
28. Computation of runoff
Strange's tables and curves
• W.L Strange (1928) gavetables and curves
for runoff resulting from rainfall in the
plains of south India
29. Computation of runoff
• Lacey's formula
• Khosla's formula
Rm = Pm – 0.48 Tmj
• ICAR formula
R = 1.511 (P1.44)(Tm) – 1.34 (A – 0.0613)
Rm = monthlyrunoff in cm
T = mean temperature on the catchment
Pm = monthly precipitationin cm
R = runoff in cm
Tm = mean temperature on the catchment
P = precipitation incm
S = catchment factor
F = monsoon durationfactor
