Abstractions of precipitation ppt

Abstractions of Precipitation
Prepared by
Pradeep Kumawat
Assistant Professor
Civil Engg. Department
Late G. N. Sapkal COE

Interception
Interception refers to precipitation that does not reach the soil,
but is instead intercepted by the leaves, branches of plants and
the forest floor. It occurs in the canopy and in the forest floor.
When precipitation reaches the surface in vegetated areas, a
certain percentage of it is retained on or intercepted by the
vegetation. Water that reaches the ground via the trunks and
stems of the vegetation is called stem flow. The interception
storage capacities of the vegetation vary with the type and
structure of the vegetation and with meteorological factors.
Interception losses are described by the following equation:
Li = S + K × E × t
Where, Li = total volume of water intercepted
S = interception storage
E = rate of evaporation
t = time
K = ratio of surface area of leaves to the area of entire canopy.

Depression Storage
Depression storage is the term applied to water that is
lost because it becomes trapped in the numerous small
depressions that are characteristic of any natural surface
or
Depression storage refers to small low points in undulating
terrain that can store precipitation. Depression storage
exists on pervious and impervious surface.
The volume water in depression storage at any time
during precipitation is given by:
V = Sd (1-e^-kPe)
Where, V = Volume of water in depression storage
Sd = Maximum storage capacity
Pe = Rainfall excess
K = constant equal to unity

EVAPORATION
Evaporation is the process by which liquid water is
converted to the water vapour by the transfer of water
molecules to the atmosphere.
OR
Evaporation is the process by which the precipitation that
falls on the earths surface is returned to the atmosphere as
vapour.
 Evaporation(and Transpiration) are small for a runoff
event and can be neglected.
 The bulk of these abstractions take place during the
time between runoff events, which is usually long.
 Hence, these are more important during this time
interval.

Factor Affecting Evaporation
 Difference in vapour pressure between the water
surface and air above.
 Temperature of air and water
 Atmospheric pressure
 Wind velocity
 Depth of water in the water body
 Water quality
 Size or surface area of the water body
 Radiation
 Humidity

Vapour-pressure difference
 The rate of evaporation is proportional to the difference
between the saturation vapour pressure at the water
temperature, ew and the actual vapour pressure in the air, ea
EL = C (ew-ea)
Where;
Dalton’s law of evaporation
EL= rate of evaporation (mm/day)
C= constant (or K)
ew and ea are in mm of mercury
 Evaporation continues till ew= ea
Temperature
 Other factors remaining the same, the rate of evaporation
increases with an increase in the water temperature.
 Increase in evaporation rate with increasing temperature
Atmospheric pressure
 A decrease in the barometric pressure, as in high altitudes,
increases evaporation.

Wind speed
 Wind aids in removing the evaporated water vapour from the zone
of evaporation and consequently creates greater scope for
evaporation.
Water depth/ Heat storage in water Bodies
 Deep water bodies have more heat storage than shallow ones.
 A deep lake may store radiation energy received in summer and
release in winter causing less evaporation in summer and more
evaporation in winter compared to a shallow lake exposed to a
similar situation.
Size of water body
 More exposed area leads to more evaporation and vice-versa.

Water quality
 When solute is dissolved in water, the vapour pressure of
solution is less than that of pure water.
 Hence causes reduction in the rate of evaporation.
 Thus, under identical condition evaporation from sea water is about
2-3 % less than that from fresh water.
 Turbidity also affects the rate of evaporation by affecting the heat
transfer within the depth of water body.

The amount of water evaporated from a water surface is
estimated by the following methods:
A) Mass Transfer Method
B) Actual Observations
C) Data collected from evaporimeters (Pan observations)
 U.S. Weather Bureau Class A Pan
 Standard IS Land Evaporation Pan
 Sunken Colorado Evaporation Pan
 Floating Evaporation Pan
Measurement/ Estimation of Evaporation

Measurement/ Estimation of Evaporation
D) Empirical evaporation equation
1) Meyer’s Formula 2) USBR Formula
3) Horton’s Formula 4) Thornthwaite Formula
E) Analytical Methods
1) Water Budget method
2) Energy Budget method

 When wind flows on the surface, a boundary is formed.
This method is based on turbulent mass transfer in the
boundary layer to calculate the mass of water vapor
transferred from surface to the surrounding atmosphere.
The evaporation is expressed as
E =
4 6 . 0 8 ( e 1  e 2 ) ( v 2  v 1 )
Where
E = Evaporation in mm/h
z1 & z2 = Arbitrary levels above water surface
e1 & e2 = Vapor pressure at z1 & z2 in km/h
v1 & v2 = windvelocity at in km/h
T = Average temperature in C between z1 & z2.
z 2
(T  2 7 3 ) ln (
z 1
)

Atmometers are provided with special surface which
are kept wet from where the evaporation takes place.
 There is continuous supply of water to the surface
for measuring the evaporation.
A variety of Atmometers are used in the world. The
most frequently used one are Piche and Bellani
Atmometer.
The different types of atmometers indicate different
amount of evaporation under different meteorological
conditions
However, they are not common because of their
small size.

The previous methods are not directly applicable in
design problems.
In most design problems, evaporation is measured by
evaporation pans which are called evaporimeters.
A evaporimeters or evaporometers are water circular pans
made of galvanized iron, zinc or copper.
The amount of water evaporated from a water surface is
estimated by the following Pan Observation methods:
 U.S. Weather Bureau Class A Pan
 Standard IS Land Evaporation Pan
 Sunken Colorado Evaporation Pan
 Floating Evaporation Pan

In most design problems, evaporation is measured by
evaporation pans which are called evaporimeters.
The most commonly used evaporimeters in India is US Weather
Bureau Class A Pan.
 A pan is a metal container (square or circular) with diameter
varying from 300 – 1500 mm.
It is filled water at depth in pan is maintained 180 mm to 200
mm.
The water loss is measured in a specified period (Normally twice
daily at 8:30 am and 5:30 pm).
The rate of evaporation is then correlated to the evaporation
from a reservoir.
U.S. Weather Bureau Class A Pan

120 cm
Wooden
support
15 cm
Galvanized
steel
25 cm
US Weather Bureau Class A Pan

Standard IS Land Evaporation Pan
The IS 5973-1970 standard evaporation pan (Class A pan) has
diameter of 1220 mm and depth of 255 mm.

Sunken Colorado Evaporation Pan
The sunken Colorado pan is square, 920 mm (3 ft.) on a side
and 460 mm (18 in.) deep and made of unpainted galvanized
iron sheet. As the name suggests, it is buried in the ground in
such a way that about 100 mm of the top projects above the
ground surface.
Fig. Sunken Colorado pan

Floating Evaporation Pan
The floating pan is 900 mm diameter size 450 mm deep and
supported on the raft floating in water.

Stilling well
This is an instrument to determine the water level in
the evaporation pan. The stilling well is installed in
the evaporation pan and leveled with the adjustable
leveling foot screw. By using the suspension measuring
rod (hook gauge) suspended in the smoothing pipe
(stilling well) the variation of water level can be measured
very accurately by using the micrometer scale on the
measuing rod. Daily the result of evaporation and
precipitation is measured within the still well, by means of a
high quality evaporation micrometer with a measuring
range of 01 mm and an accuracy of 0.02 mm.

Advantages:
 Cost of installation is reasonably low.
 It is easy for measurement.
Disadvantages:
 The pan gives higher rate of evaporation than that of large free water
surface.
Effects of wind and radiation are more which overestimate the
evaporation rate.
Pan observations

Empirical Formulae
Variousempirical formulae have been developed by different
investigators to estimate the evaporation.
Most of them are dependent on wind velocity, temperature and
atmospheric pressure.
Meyer’s equation (1915):
E = C ( 1 + V / 16 ) (es - ea)
Where, E = Evaporation from water body in mm/day
ea = Actual vapour pressure of overlying air mm Hg
es = Vapour pressure at water surface in mm
C = Coefficient varying from 0.36 to 0.50
Fitzgerald’s equation (1886):
E =( 0.4 + 0.124 V ) (es - ea)
V = Mean monthly Wind velocity at the water surface in km/hrs.
Hg

Empirical Formulae
 Horton’s equation (1917):
E = 0.635 [(θ ∙ vw) – v]
Where, E = Evaporation in mm/day
θ = Wind factor
vw = Maximum Vapour pressure at water surface in mm of mercury
v = Actual Vapour pressure in air in mm of mercury
 Thornthwaite Formula (1948):
E = 16 [(10T)/I]^a
Where, E = Evaporation in mm/month
T = Monthly Mean Temperature
a = Function of I
I = ∑ (T/S)^1.51

 The various variables used in the formulae are as
follows:
 USBR Equation :
E = 4.57 T + 43.3
E = Annual Evaporation of the water body in cm
T = Annual mean temperature in ̊C
Empirical Formulae
es = Vapour pressure at water surface in mm of mercury
ea = Actual Vapour pressure in air in mm of mercury

By Analytical Method
1. Water Budget Equation (i.e. Conservation of mass)
The Law of Conservation of Mass dates from Antoine Lavoisier's
1789 discovery that mass is neither created nor destroyed.
2. Energy Budget Equation (i.e. Conservation of energy)
The law of conservation of energy states that energy can neither be
created nor destroyed. Conservation of energy, principle of physics
according to which the energy in a closed system remains constant.

Subsurface
runoff - Qs
Outflow from surface- Q0
Subsurface seepage losses- Qd
Precipitation - P
Evaporation- E
Surface runoff - Qr
P Qs Qr  = Qo  Qd  E

2. Energy Budget Equation
 It is most accurate method
 A heat balance following the principle of the conservation
of energy is evaluate from incoming, outgoing and stored
energy as fallows:
HN = HS + HE + HF + HS + HO
Where,
HN = Net heat energy received in the water body
Hs = Sensible heat transfer from water body to air
HE = Heat energy consumed in evaporation process
HF = Heat flux to the ground
Hs = Heat stored in the water surface
Ho = Heat going out of system by flow of water

Methods Of Reducing Evaporation From Soil, Lake Or
Reservoirs
 By Keeping Free Water Surface Area Minimum
 By Spreading Certain Chemical Films On the Reservoirs and Lakes
Surface.
 By Suitable Wind Breakers
 Artificial Covers
 Other methods
 By Providing Mulch (cover) on the Land Surfaces
 By removing or cleaning weeds and water loving plants

Transpiration and its
Measurement

Transpiration
Transpiration is the process of water movement through a
plant and its evaporation from aerial parts, such as leaves,
stems and flowers. Water is necessary for plants but only a
small amount of water taken up by the roots is used for
growth and metabolism.

Environmental factors that affect the rate
of transpiration
1. Light
Plants transpire more rapidly in the light
than in the dark. This is largely because
light stimulates the opening of the stomata
(mechanism). Light also speeds up
transpiration by warming the leaf.

2. Temperature
Plants transpire more rapidly at
higher temperatures because water
evaporates more rapidly as the
temperature rises. At 30°C, a leaf
may transpire three times as fast
as it does at 20°C.

3. Humidity
At high humidity (moist air), the
stomata tends to close and thus limit the
exit of water vapour from the plant. In
addition, at high humidity the
atmosphere contains more water and
has low atmospheric demand, meaning
that it has limited capacity to absorb
more water.

4. Wind
When there is no breeze, the air
surrounding a leaf becomes increasingly
humid thus reducing the rate of
transpiration. When a breeze is present,
the humid air is carried away and replaced
by drier air.

5. Soil water
A plant cannot continue to transpire
rapidly if its water loss is not made up by
replacement
immediately
transpiration
from
reduces
(as
the soil. This
the rate of
well as of
photosynthesis).

6. Stage of plant development
Transpiration
growth as the
depends upon plant
water requirement is
different at different stage of its growth.

Measurement of Transpiration
Various methods of measurement of transpiration, the most common
method is by the ‘Phytometer’.
WT = WI + WA – WF
Where,
WT = Water loss due to transpiration
WI = Initial weight of apparatus in the beginning
WF = Final weight of apparatus at end of experiment
WA = Water added during plant growth
Fig. Phytometer

Evapotranspiration or consumptive use of water is the depth of
water consumed by evaporation and transpiration during crop
growth, including water consumed by accompanying weed growth.
Consumptive use of water includes the water deposited by rainfall
and subsequently evaporating without entering the plant system.
Its study is important in the design of reservoir, irrigation canals,
water balance on earth surface and projects relating to water.
The value of consumptive use of water varies from crop to crop
and also for the same crop it varies with time as well as place.

Factors Affecting Evapotranspiration
Meteorological factors:
It increases with the increase in temperature,
sunshine and wind velocity but decreases with
humidity.
Plant and soil factors:
• Greater the density of vegetation, greater is the
evapotranspiration.
•When the vegetative surface becomes dry and the
soil moisture decreases, the evaporation decreases.
• Evapotranspiration depends upon the stage of the
plant growth.

Measurement (or Estimation) of
Evapotranspiration (or Consumptive use)
The various methods adopted are broadly classified into:
a) Direct measurement of consumptive use of water.
o Lysimeter Method
o Field Experimental Method
o Integration Method
b) Empirical formula
o Blaney-Cridddle Equation
o Christiansen Equation

1.Lysimeter Method
Lysimeter is an evapotransporimeter, which is a circular tank
with pervious bottom whose diameter may be extended to 5m.
Tanks are watertight cylindrical containers open at one end and
are set into ground with their rim flush with the surface.
Consumptive use is determined by the difference of the total
water applied to the tank and that draining through the pervious
bottom and collected in a pan.
 This method is time consuming and expensive.

Lysimeter
ET = RW + IW – QD + δS
Where,
ET = Evapo-transpiration QD = Quantity of water drained
RW = Rainfall water IW = Irrigation water

2. Field Experimental Plots Method
In this method the irrigation water is applied to the selected field
experimental plots.
In the plot, all the elements of water budget are measured in a
known time interval and the evapotranspiration is determined as
Evapotranspiration = Precipitation + Irrigation input –
Runoff – Increase in soil storage – Groundwater loss
Since it is difficult to determine the ground water loss due to
deep percolation so it can be neglected by maintaining the moisture
condition in the plot at the field capacity.

In this method the consumptive use of water is determined by
the summation of the products of
i. Consumptive use of water for each crop times its area.
ii. Consumptive use of water for natural vegetation time its
area.
iii. Evaporation from water surface times water surface area.
iv. Evaporation from bare land times its area.
Note: It is necessary to know the division of total area under
irrigation crops, natural vegetation, water surface area and bare
land area.
3. Integration Method

1. Blaney-Cridddle Equation:
 It is based on the data collected from the arid Western Zone
of the United States.
 It is based on the assumption that
evapotranspiration depends only on the
the potential
mean monthly
temperature and the monthly daylight hours.
 ET = evapotranspiration (cm)
 t = mean monthly temperature (°C).
 p = monthly percentage of hours of bright day.
 k = monthly consumptive use coefficient for the crop.
b) Empirical Formula or Evapo-Transpiration Equation

2. Christiansen Equation:
The Christiansen equation for estimation of potential
evaporation
ET = 0.473 Qo C
Where
Qo = Solar radiation at the top of the atmosphere
converted to mm of equivalent evaporation.
C = Coefficient derived from series of climatic
measurements like temperature, humidity, wind,
sunshine, elevation etc.

Infiltration
Infiltration is the process by which surface water on the ground
surface enters in to the soil mass, through the pores of soil.
or Infiltration is defined as the flow of water from above ground
into the subsurface.
It is commonly used in both hydrology and soil Mechanics.
Percolation on the other hand is the passage of water within
the soil.

Infiltration Capacity (fp)
The infiltration capacity is the maximum rate at which water
can be absorbed by a given soil per unit area under given
conditions.
Or The infiltration capacity is defined as the maximum rate
of the absorb water that falls over the soil under given condition
and expressed in cm/hour.
Fp = fo + (fo - fc) ∙ e^-ki
Where,
fo = Initial rate of infiltration
fc = constant rate of infiltration after the saturation.
k = constant depend upon type of soil and vegetation.
i = rainfall intensity

Factors affecting infiltration
1. Vegetation Cover
2. Moisture Content
3. Temperature
4. Intensity of rainfall or Precipitation
5. Human Activity
6. Quality of water
7. Movement of man & animals
8. Presence of ground water table
9. Characteristics of soil (e.g. type, size, texture etc.)
10. Evapotranspiration
11. Slope of the land

1. Vegetation Cover – dense vegetation decrease the infiltration
rate as compare with Bare land.
2. Moisture content –
Infiltration rate depends on initial moisture condition of soil.
When soil moisture is high, infiltration rate is slow.
But Soil moisture is low, infiltration rate is high.
3. Temperature –
Viscosity of water changes with temperature. Increase in
temperature cause reduction in viscosity. So, Infiltration is
higher when temperature is high.
Factors Affecting Infiltration

4. Intensity of rainfall –
High intensity rainfall cause mechanical compaction of soil. So,
heavy intensity rainfall cause less infiltration,
Lesser intensity rainfall cause higher infiltration.
5. Human activity –
Cultivation on bare land will increase infiltration,
Construction of roads and buildings will decrease in infiltration
capacity.
6. Quality of water –
Turbidity, Silt and other impurities in water resulting in reduction
of infiltration.

7. Movement of man & animals –
Heavy movements cause compaction of soil, results in less
infiltration.
8. Presence of ground water table –
If ground water table is near to the earth surface, it reduce
infiltration.
For infiltration to continue, ground water table should not very
close.
9. Size and characteristics of soil particles –
Infiltration is directly proportional to the grain size/diameter, for
granular soils. However, if the soil has swelling minerals like illite
and montmorillonite, the infiltration rate will reduce drastically.

Measurement of Infiltration Rate
 By Infiltrometers
 By Rainfall simulators
 By hydrograph analysis

1. Single ring Infiltrometer
 Material of ring - Metal cylinder
 Ring diameter – 30 cm
 Ring length – 60 cm
 Ring driven into soil – 50 cm
 Water level maintain – 5 cm
 The volume of water added during different
time intervals, the plot of the infiltration
capacity vs time is obtained.
 Uniform Infiltration is obtained after 2-3 hrs.
Main drawback- infiltrated water spreads at the bottom of ring.

2. Double Ring Infiltrometer
 Material of ring - Metal cylinder
 Ring diameter – 30 cm & 60 cm
 Ring length – 25 cm
 Ring driven into soil – 15cm
 Water level maintain – 5 cm
 The water in both the rings should be
kept the same during the observation
period.
 Measurement is taken only from the
inner tube.

By Rainfall Simulators
The method is fist adopted by Robert E. Horton (American Geologist),
consists in applying water over an area by sprinkling at a rate which is
in excess of infiltration capacity.
The apparatus can
produce artificial rainfall
of various intensities and
desired duration.

By hydrograph Analysis
The method consists in determining the infiltration capacity from
the knowledge of intensities of rainfall occurring during a storm
and measuring the resulting run off form such storm.

INFILTRATION INDICES
The infiltration concept can be used for computation of surface
runoff by making use of simple relation.
Surface runoff = rainfall + (losses due to interception, depression
storage, evaporation, transpiration and infiltration)
In hydrological computations for computing surface runoff and
flood discharge, the use of infiltration capacity curve is not
convenient. Infiltration capacity of soil does not remain constant.

Fig. Infiltration capacity curve

Infiltration Indices
Constant Infiltration rate
Fig. Infiltration capacity curve with Φ index

Infiltration indices
It is defined as the average rate of infiltration such that the
volume of rainfall in excess of that rate will be equal to the
volume of observed runoff.
The two commonly used infiltration indices are the following:
1) The φ – index
2) The W – index
There are extremely used for the analysis of major floods when
the soil is wet and the infiltration rate becomes constant.

1. Ø - Index
For determination of Ø - Index , a
horizontal line is drawn on the
hyetograph such that the shaded
area above that line is equal to the
volume of surface runoff.
The unshaded area below the
horizontal line actually represents
all losses including interception,
depression storage and infiltration,
but it is assumed that all these
losses are due to infiltration only.
The amount of rainfall in excess of Ø – Index is
called rainfall excess.

2. W - Index
W – index is the average rate of infiltration during the
period when the rainfall intensity exceeds the infiltration
rate.
P = total rainfall (cm)
P – R – S
t
R = total runoff (cm)
S = total losses (cm)
t = total time period (hr)
W – index is average rate of infiltration (cm/hr)
W-index =

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