INFILTRATION

GROUP 9
 INFILTRATION
Infiltration is the flow of water into
the ground through the soil surface.
 DISTRIBUTION OF SOIL
MOISTURE IN THE
INFILTRATION PROCESS
Zone 1: At the top, a thin
layer of saturated zone is
created.
Zone 2: Beneath zone 1,
there is a transition zone.
Zone 3: Next lower zone is the
transmission zone where the
downward motion of the moisture
takes place. The moisture
content in this zone is above field
capacity but below saturation.
Further, it is characterized by
unsaturated flow and fairly
uniform moisture content.
Zone 4: The last zone is the wetting zone.
The soil moisture in this zone will be at or
near field capacity and the moisture
content decreases with the depth. The
boundary of the wetting zone is the
wetting front where a sharp discontinuity
exists between the newly wet soil and
original moisture content of the soil.
Depending upon the amount of infiltration
and physical properties of the soil, the
wetting front can extend from a few
centimetres to metres.
 AN
ANALOGY
FOR
INFILTRATION
IT UNDERSCORES TWO
IMPORTANT ASPECTS:
(i) the maximum rate at which
the ground can absorb water,
the infiltration capacity

(ii) the volume of water that

the ground can hold, the field
capacity
AN INFILTRATION
MODEL
Since the infiltered water may
contribute to the ground water
discharge in addition to increasing
the soil moisture, the process can
be schematically modelled as in Fig.
3.10 (a) and (b) wherein two
situations, viz. low intensity
rainfall and high intensity rainfall
are considered.
 INFILTRATION CAPACITY
The maximum rate at which a given soil at a given time can absorb
water is defined as the infiltration capacity. It is designated as fp, and
is expressed in units of cm/h. The actual rate of infiltration f can be
expressed as
where i = intensity of rainfall. The infiltration
capacity of a soil is high at the beginning of a
storm and has an exponential decay as the
time elapses.
The infiltration capacity of an area is dependent on a
large number of factors, chief of them are:
Characteristics of the soil (Texture, porosity and hydraulic
conductivity)
Condition of the soil surface
Current moisture content
Vegetative cover
Soil temperature
 CHARACTERISTICS OF SOIL
The type of soil, viz. sand, silt or clay, its texture, structure,
permeability and underdrainage are the important characteristics
under this category. A loose, permeable, sandy soil will have a
larger infiltration capacity than a tight, clayey soil. A soil with
good underdrainage, i.e. the facility to transmit the infiltered
water downward to a groundwater storage would obviously have
a higher infiltration capacity. When the soils occur in layers, the
transmission capacity of the layers determines the overall
infiltration rate. Also, a dry soil can absorb more water than one
whose pores are already full. The land use has a significant
influence on fp . For example, a forest soil rich in organic matter
will have a much higher value of fp under identical conditions than
the same soil in an urban area where it is subjected to compaction.
SURFACE OF
ENTRY At the soil surface, the impact of
raindrops causes the fines in the soil to be
displaced and these in turn can clog the
pore spaces in the upper layers of the soil.
This is an important factor affecting the
infiltration capacity. Thus a surface
covered with grass and other vegetation
which can reduce this process has a
pronounced influence on the value of fp.
 FLUID
CHARACTERISTICS Water infiltrating into the soil will have many
impurities, both in solution and in suspension. The
turbidity of the water, especially the clay and
colloid content is an important factor and such
suspended particles block the fine pores in the soil
and reduce its infiltration capacity. The
temperature of the water is a factor in the sense
that it affects the viscosity of the water by which
in turn affects the infiltration rate. Contamination
of the water by dissolved salts can affect the soil
structure and in turn affect the infiltration rate.
 MEASUREMENT OF INFILTRATION

Infiltration characteristics of a soil at a given locationcan be

estimated by
Using flooding type infiltrometers
Measurement of subsidence of free water in a large basin or pond
Rainfall simulator
Hydrograph analysis
 MEASUREMENT are experimental devices used to
OF INFILTRATION obtain data relating to variation
of infiltration capacity with time.
Two types of flooding type
infiltrometers are in common use.
They are (a) Tube-type (or
FLOODING-TYPE
Simple) infiltrometer and (b)
INFILTROMETER Double- ring infiltrometer.
 SIMPLE (TUBE TYPE) INFILTROMETER
This is a simple instrument consisting essentially
of a metal cylinder.
Consists of a vertical tube inserted into the soil.
Water is poured into the tube and allowed to
infiltrate into the soil.
Measures infiltration rate based on the water
level decrease in the tube over time.
 DOUBLE-RING INFLITROMETER
This most commonly used infiltrometer is designed
to overcome the basic objection of the tube
infiltrometer, viz. the tube area is not
representative of the infiltrating area.
Consists of two concentric rings inserted into the
soil surface. Water is applied inside the inner ring,
creating a ponded area. Measures infiltration
rate based on the water level change in the inner
ring over time
 RAINFALL SIMULATOR
Rainfall simulator is a type of
infiltrometer that gives lower values
MEASUREMENT OF than flooding type infiltrometers. This
INFILTRATION is due to effect of the rainfall impact
and turbidity of the surface water
present in the former.
 COMPONENTS:
1. NOZZLES
MEASUREMENT OF 2. SOIL SAMPLE CONTAINERS
INFILTRATION 3. COLLECTION SYSTEM
4. CONTROL SYSTEM
 HYDROGRAPH ANALYSIS
Reasonable estimation of the infiltration
MEASUREMENT capacity of a small watershed can be
OF obtained by analyzing measured runoff
INFILTRATION hydrographs and corresponding rainfall
records.
MODELING INFILTRATION CAPACITY
Cumulative infiltration capacity Fp(t) is defined
as the accumulation of infiltration volume over a
time period since the start of the process and is
given by:
(eq. 3.21-a)
Thus the curve Fp(t) vs time in Fig. (3.13) is the
mass curve of infiltration. It may be noted
that from Eq. (3.21-a) it follow that:

(eq. 3.21-b)
 HORTON’S
EQUATION where:
fp = infiltration capacity at any time t
(1933) from the start of the rainfall.
fo = initial infiltration capacity at t = 0
fc = final steady state infiltration
Horton expressed the decay of capacity occurring at t = tc. Also, fc is
infiltration capacity with time as sometimes known as constant rate or
an exponential decay given by ultimate infiltration capacity.
Kh = Horton's decay coefficient which
depends upon soil characteristics and
vegetation cover.
The difficulty of determining the
variation of the three
parameters fo, fc and kh with
soil characteristics and
antecedent moisture conditions
preclude the general use of Eq.
(3.22).
 where a and b are local
KOSTIAKOV EQUATION (1932) parameters with a > 0 and
Kostiakov model expresses 0 < b < 1.
cumulative infiltration capacity The infiltration capacity
as would now b e expressed by
(eq. 3.25) Eq. (3.21-b) as
(eq. 3.26)
The coefficients of the Green– Ampt equations are m = 3.0256 and n = 10.042
CLASSIFICATION OF INFILTRATION CAPACITIES
 INFILTRATION INDICES
In hydrological calculations involving floods
it is found convenient to use a constant
value of infiltration rate for the duration
of the storm. The defined average
infiltration rate is called infiltration index
and two types of indices are in common use.
Thank you