When rainwater falls on the ground, a small part of it is initially absorbed by the top thin layer of soil so as to replenish the soil moisture deficiency.

1. SUBJECT :- HYDROLOGY AND WATER RESOURCE AND
ENGINEERING
INFILTRATION

2. INFILTRATION
 Downward movement of water from soil surface, into soil mass through the
pores of the soil.
 When rain water falls on the ground, a small part of it is initially absorbed by the
top thin layer of soil so as to replenish the soil moisture deficiency.
 The excess water infiltration downward to join ground water table. In simple
terms, infiltration is the entry water into the soil through soil surface.

3.  The process of downward movement of water into soil, once water enters into
the soil, is known as percolation. Infiltration and percolation are directly into
related. When percolation stops, infiltration also stops.
 The evaluation of infiltration helps us in computing the runoff obtained from a
given rainfall. The infiltration, as a matter of fact, affects the short time runoff
caused by a single rainfall, as well as the long – time runoff evaluating over a
long period, such as the yearly runoff of a catchment.

4. FUNCTION OF INFILTRATION
 It reduce the magnitude of the flood.
 It delays the time of arrival of water to the channel.
 It recharge the ground water reservoir.
 It reduce the soil erosion.
 It fills the soil pores to its capacity, thus making water available to plants.
 It sustain green vegetation cover on the ground surface and thus help in
reducing dust storms

5. INFILTRATION INDICES
 The infiltration capacity of
soil can be determined
experimentally by subjected
an experimental plot to rain
fall rates in excess of
infiltration capacity and by
measuring the surface runoff.
 For small areas having
uniform infiltration
characteristic, the runoff
volume can be estimated by
subtracting the infiltration
from the design rainfall.

6. INFILTRATION INDEX
 Infiltration index is average rate of loss such that the volume of rainfall in excess
of that rate will be equal to direct runoff. Thus, the average constant value of
infiltration rate is called infiltration index.
 W – index :- it is average of infiltration during the period when the rainfall
intensity exceeds the infiltration rate.
W – index =
P−R−SR
tr
cm/hr

7.  ϕ – index :- It is average rate of rainfall such that the volume of rainfall in excess
of that rate is equal to the volume of surface runoff.
ϕ – index =
P−R
tr
cm/hr
W = rate of infiltration
P = Total precipitation
R = Total runoff
SR = Surface tension
tr = duration of rainfall

8. MEASUREMENT OF INFILTRATION
 Infiltration capacity of a soil can be measured in the field by conducting controlled
experiments on a small area, using infiltratometer, and rain simulators.
INFILTRATOMETER
FLOODING
SIMPLE RING
DOUBLE RING
RAIN

9. FLOODING TYPE INFILTRATOMETER
 Simple infiltratometer :-
 Diameter : 30cm
 Depth : 60cm
 Procedure :- This cylinder is driven into the ground to a depth of 50cm. Water is
poured into the cylinder to fill a depth of 55cm. A pointer is set to mark the water
level.
As the infiltration takes place, the water level goes down. The water level is
maintained constant by adding more water from burette. The volumes of water added
at different time intervals are noted. The volume of water added in a given time
interval is divided by the time interval and area of cylindrical ring to obtain infiltration
capacity.

10. The experiment is infact, continued till a uniform rate of infiltration is
obtained, which may take 2 to 3 hours.

11.  Double ring infiltratometer :-
 Inner ring dia : 22.5cm
 Outer ring dia : 35cm
 Procedure :- double ring infiltratometer consists of two cylinders, called ring. The
inner ring is used to determine the infiltration rate as in the case of a simple
infiltratometer.
The water level in the annular space between two rings is kept at the
same level as that in the inner ring to reduce the border effects due to water coming
out of the inner tube. Thus the outer ring provides a sort of water jacket to the inner
ring and the water in the inner ring infiltrates vertically downward without
spreading.

12.  INFILTRATION CAPACITY :-
VOLUME OF WATER ADDED
AREA OF INNER RING ∗ TIME INTERVAL

13.  Disadvantages of flooding type infiltratometer :-
I. It does not simulate the effect of the rain drop impact.
II. The soil gets distributed when cylinder is driven.
III. The results depends upon the size of the cylinder.
IV. Water spreads at the bottom in case of simple infiltratometer.

14. RAIN SIMULATORS
 A rain simulators is a type of infiltratometer in which, water is applied in the form
of spray or artificial rain. A plot of about 2m * 4m is selected, and water is
applied to it in the form of artificial rain at a uniform rate.
 Two rows of special types of nozzles are installed, one along each long side of the
plot for water spray. The nozzles are so adjusted that they direct spray upward
and slightly inward. The drops of water reach a height of about 2m above the
ground surface before falling on the plot.
 Various intensities of rainfall can be simulated by changing the nozzle openings.
 Infiltration = rainfall – surface runoff

15.  The rain simulators gives lower values of the infiltration index than a flooding
type infiltratometer. This is due to the effect of rainfall impact and the turbidity of
water which decrease the permeability.