INFILTRATION UNIT 3

1. UNIT -3 (INFILTRATION )

2.  The process of entering rain water in to
soil strata of earth is called
INFILTRATION.
 The infiltrated water first meets the soil
moisture deficiency if any & excess
water moves vertically downwards to
reach the groundwater table. This
vertical movement is called
PERCOLATION.

3.  The infiltration capacity of soil is defined
as the maximum rate at which it is
capable of absorbing water and is
denoted by f.
 If i >= f then fa = f (depend upon soil capacity )
 If i < f then fa = i (depend upon rainfall intensity)
 where fa = actual infiltration capacity
i = rate of rainfall
f = infiltration capacity

4.  For
Dry Soil – (infiltration rate) f is more
Moist Soil – (infiltration rate) f is less
 Maximum rate of water absorption
by soil –Infiltration Capacity
 Maximum capacity of water absorption
by soil –Field Capacity

5.  The rate at which soil is able to absorb
rainfall or irrigation .
 It is measured in (mm/hr) or (inches/hr )
 Infiltrometer is used for measurement of
infiltration.
 If (i > f ) runoff occurs.
 Infiltration rate is connected to hydraulic
conductivity.

6.  Hydraulic conductivity is ability of a fluid to
flow through a porous medium.
It is determined by the size and shape of the
pore spaces in the medium & viscosity of
fluid.
OR
It is expressed as the volume of fluid that will
move in unit time under a unit hydraulic
gradient through a unit area measured
perpendicular to the direction of flow.

7.  SLOPE OF THE LAND:- The steeper the slope
(gradient), the less the infiltration or seepage.

8.  DEGREE OF SATURATION:- The more saturated
the loose Earth materials are, the less the infiltration.

9.  POROSITY:- Porosity is the percentage of
open space (pores and cracks) in a earth surface.
 The greater the porosity, the greater the amount
of infiltration.
SPONGE CLAY BRICK

11.  COMPACTION:- The clay surfaced soils are
compacted even by the impact of rain drops which
reduce infiltration. This effect is negligible in sandy
soils

12.  SURFACE COVER CONDITION:-
Vegetation:- Grasses, trees and other plant types
capture falling precipitation on leaves and branches,
keeping that water from being absorbed into the
Earth & take more time to reach in to the ground.
 MORE the vegetation
 Slower the Infiltration.

13.  Land Use:- Roads, parking lots, and buildings
create surfaces that are not longer permeable. Thus
infiltration is less.

14.  TEMPERATURE – At high temperature viscosity
decreases and infiltration increases
 Summer – Infiltration increases
 Winter – Infiltration decreases
FURROW IRRIGATION

15.  OTHER FACTORS –
a) Entrapped air in pores- Entrapped air can
greatly affect the hydraulic conductivity at or near
saturation
b) Quality of water-Turbidity by colloidal water
c) Freezing- Freezing in winter may lock pores.
d) Annual & seasonal changes –According to change
in land use pattern. Except for Massive
deforestation & agriculture.

16.  Infiltrometer is a device used to measure the rate
of water infiltration into soil.

17.  This consist of metal cylinder of
diameter 25 cm to 30 cm and
length of 50 cm to 60 cm, with both ends
open. length of cylinder= ( 2 x diameter )
 It is driven into a level ground such that about
10 cm of cylinder is above the ground.
 Water is poured into the top part to a depth of 5
cm & pointer is set inside the ring to indicate the
water level to be maintained.

18.  The single ring 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 means the rate of water supplied
corresponds to the infiltration capacity.
Falling head refers to condition where water is
supplied in the ring, and the water is allowed to
drop with time. The operator records how much
water goes into the soil for a given time period.

20.  The major drawback of the single ring
infiltrometer or tube infiltrometer is that the
infiltrated water percolates laterally at the
bottom of the ring.
 Thus the tube is not truly representing the
area through which infiltration is taking place.

21.  This is most commonly used flooding type
infiltrometer.
 it consists of two concentric rings driven into soil
uniformly without disturbing the soil to the least to a
depth of 15 cm. The diameter of rings may vary
between 25 cm to 60 cm.
 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.

24.  In this a small plot of land (2m X 4m) size, is provided
with a series of nozzles on the longer side with
arrangements to collect and measure the surface runoff
rate. The specially designed nozzles produce raindrops
falling from height of 2m and capable of producing
various intensities of rainfall. Experiments are conducted
under controlled conditions with various combinations
of intensities and durations and the surface runoff rates
and volumes are measured in each case. Using the water
budget equation infiltration rate and its variation with
time are estimate.
 P – R – G – E - T = ∆S
P = Precipitation, R = Surface runoff, G = net ground water
flow, E = Evaporation, T = Transpiration,
∆S = change in storage
P – R – G – E - T = ∆S

25.  plot of land (2m X 4m)
 The specially designed nozzles produce raindrops
falling from height of 2m
 under controlled conditions with various
combinations of intensities & durations and the
 surface runoff rates and volumes are measured in
each case.
P – R – G – E - T = ∆S

28.  The infiltration rate is the velocity or speed at
which water enters into the soil.
 It is usually measured by the depth (mm) of the
water layer that can enter the soil in one hour
Or
 rate at which water enters the soil at the surface.
It is denoted by f(t).
 CUMULATIVE INFILTRATION :- Accumulated
depth of water infiltrating during given time
period. It is denoted by F(t).

t
dttftF
0
)()(
dt
dF
tf )(

29.  INFILTRATION CAPACITY RATE CURVE
as obtained from infiltrometer is essentially
observed to be decaying curve (max to min)
 Some mathematical expressions to describe the
shape of curve, given by various investigators
are :-
a) Horton’s equation
b) Phillips equation
c) kostiakov equation
d) holtans equation

30. a) Horton’s equation :
ft= Infiltration capacity(inches/hour)
f0= Initial infiltration capacity.
fc= Minimum infiltration capacity.
t = Time since the start of rainfall.
k = Constant depending upon soil type & vegetable cover.
Note : fc is direct dependent upon hydraulic conductivity.

31. b) Phillips equation :
Here a = Minimum infiltration capacity.
s = Initial infiltration capacity.
c) kostiakov equation:
c) holtans equation :
 Here in above methods a & n are constants
depends on soil moisture & vegetable cover
F=[ A+(s/2) x t-0.5
]
F= (a x t n)
F = ( afn
p + fc )

32.  For consistency in hydrological calculations, a
constant value of infiltration rate for the entire
storm duration is adopted. The average
infiltration rate is called the INFILTRATION INDEX.
 The two commonly used infiltration indices are
the following:
o φ – index
o W – index
There are extremely used for the analysis of major
floods when the soil is wet and the infiltration
rate becomes constant.

33.  This is defined as the rate of infiltration above
which
rainfall volume = runoff volume(saturation).
 The shaded area below the horizontal line is
assumed that all losses are due to infiltration
only.
 For determination of Φ - index, a horizontal line
is drawn on the hyetograph such that the
unshaded area above that line is equal to the
volume of surface runoff.

34.  Φ – INDEX for a catchment, during a storm depends on
 Soil type
 vegetation cover
 Initial moisture condition
 Application – Estimation of flood magnitudes
due to critical storms.

35. For the soil conditions in India for flood
producing storms (C.W.C) has found
relationship
 R = Runoff in cm from a 24 hr rainfall of
intensity I (cm/hr).
 α = Coefficient depends upon soil type.
 In estimating maximum flood for design purpose
, in absence of any other data , a
Φ- index value of 0.10 cm/hr can be assumed
Φ = (I - R)/24 , R = (α X I 1.2 )

36.  Thisistheaverageinfiltrationrateduringthetimewhenthe
rainfallintensity>infiltrationrate.
where P=Total storm precipitation(cm)
R=Total surface runoff (cm)
Ia =Depression and interception losses (cm)
tf=Time periodofrunoff(inhours)
 The w- index is more accurate than Φ – index because it
excludestheDepression & interception.
W-index=(P–R–Ia)/tf=(F/tf)

37. INTERCEPTION : it is a part of water caught by
the vegetation and subsequently evaporated as
a) Surface flow
b) Stem flow
c) Evapotranspiration
For a given storm, the interception loss is
estimated as
Ii = Si + Ki Et

38. Where
 I i = Interception loss in mm.
 S i = Interception storage varies from 0.25 to 1.25
mm depending on the nature of vegetation
 K i =Ratio of vegetal surface area to its projected
area.
 E t = Evaporation rate in mm/h during the
precipitation.
 t = Duration of rainfall in hours.

39.  W-index is the refined version of Φ – INDEX.
 Initial losses I a are separated from total
abstractions.
 W-index = Φ–index I a
 The accurate estimation of W-index is rather
difficult to obtain hence Φ – index is most
commonly used.
 Since retention rate is very low both index W
& Φ are almost same.

40.  RUNOFF :- After infiltration remaining precipitation
on the surface is called runoff.
OR
Draining of precipitation from a catchment area
through a surface channel.
 According to source from which the flow is derived
the total runoff, consist of :-
 Surface runoff
 Subsurface runoff
 For the practical purpose of analysis of total runoff.
 Direct runoff
 Base flow
COMPONENTS OF RUNOFF

41. COMPONENTS OF
RUNOFF

42.  SURFACE RUNOFF :- Surface runoff is the water
flow that occurs when the soil is infiltrated to full
capacity and excess water from rain , melt water, or
other sources flows over the land.
 It is combination of overland flow and channel
precipitation.
 SUB SURFACE RUNOFF :- Lateral movement of
water occurring to the soil above the water table. It is
also known as INTERFLOW.
 Interflow is the portion of the stream flow
contributed by infiltrated water that moves laterally
in the subsurface until it reaches a channel.

43.  OVERLAND FLOW :- When excess precipitation
moves over the land surfaces to reach smaller stream
channel.
 CHANNEL PRECIPITATION :- The precipitation
falling on water surface is called channel
precipitation. It is also called as stream flow.

44.  DIRECT RUNOFF :- Direct Runoff, which is
composed of contributions from surface runoff and
quick interflow. Unit hydrograph analysis refers
only to direct runoff.
 BASE FLOW :- Base flow, which is composed of
contributions from delayed interflow and
groundwater runoff.

45.  Runoff area and Runoff volume from an area mainly
influenced by following two factors :-
 CLIMATIC FACTORS.
 PHYSIOGRAPHICAL FACTORS.
 Climate factors associate with characteristics
which includes the
 Type of precipitation.
 rainfall Intensity.
 rainfall Duration.
 Antecedent precipitation.
 Direction of storm movement.

46.  Physiographic Factors includes both
watershed and channel characteristics, such
as -
 Size of Watershed.
 Orientation of Watershed.
 slope of Watershed.
 Land Use.
 Soil type.
 Type of drainage network.
 Shape of catchment.

47.  TYPES OF PRECIPITATION:- state of precipitation
as liquid(rainfall), solid(hail) and gasseous(fog).
 RAINFALL INTENSITY:- Thus high intensities of
rainfall yield higher runoff,
where i>f (quick runoff) or i<f (slow runoff)
 DURATION OF RAINFALL:-
directly related to the volume of runoff be cause
infiltration rate of soil decreases with duration of
rainfall.
Therefore medium intensity rainfall even results in
considerable amount of runoff if duration is longer.

48.  DIRECTION OF PREVAILING WIND: - If the
direction of prevailing wind is same as drainage
system, it results in peak flow. A storm moving in
the direction of stream slope produce a higher peak
in shorter period of time than a storm moving in
opposite direction.
 ANTECEDENT MOISTURE OR SOIL
MOISTURE:- Magnitude of runoff yield depends
upon the initial moisture present in soil at the time of
rainfall. If the rain occurs after a long dry spell then
infiltration rate is more, hence it contributes less
runoff.

49.  SLOPE OF WATERSHED :- It has complex effect. It
controls the time of overland flow and time of
concentration of rainfall. E.g. sloppy watershed
results in greater runoff due to greater runoff
velocity.
 ORIENTATION OF WATERSHED :- This affects
the evaporation and transpiration losses from the
area. The north or south orientation, also affects the
time of melting of collected snow.
 LAND USE :- More vegetation ,Less runoff.
Less vegetation ,More runoff.

50.  SIZE OF WATERSHED:- A large watershed takes
longer time for draining the runoff to outlet than
smaller watershed.
 SOIL TYPE:- Infiltration rate vary with type of soil.
So runoff is great affected by soil type.
 Open textured soil or porous soil like sand have
high infiltration rate hence less runoff.
 fine grained soil and closely compacted soil such
as clay have high rate of runoff & less infiltration
rate.

51.  TYPE OF DRAINAGE NETWORK – More
tributaries and stream cause less overland flow
and surface runoff concentrates resulting in
high peaks quickly.
 SHAPE OF CATCHMENT :- Elongated
catchment are less subjected to high runoff
peaks.
the numerical indices like form factor,
circularity ratio, compactness coefficient will
express shape of catchment quantitatively.

52.  BASIN YIELD : Yield means to produce or
gain. Gaining any product from natural
resources is called Yielding.
 BASIN YIELD means quantity of water
available from a stream at a given point over a
specified duration of time.
 Time duration of yield would be a month
longer.

53.  Hydrological water balance equation for any
basin under consideration –
Q= instantaneous rate of flow from the basin.
P=Total average depth of precipitation over time t
E=Total evaporation and evapotranspiration from
basin.
∆ S=change in storage.
⌠Q dt = P- E - ∆ S