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▣ 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
vertical
groundwater
movement
table.
is
This
called
PERCOLATION.

▣ 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

▣ 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

▣ 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.

▣ 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.

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

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

▣ 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

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

▣ 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.

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

▣ TEMPERATURE – At high temperature viscosity
decreases and infiltration increases
▣ Summer – Infiltration
▣ Winter – Infiltration
increases
decreases
FURROW IRRIGATION

▣ 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.

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

▣ 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.

▣ 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.

▣ 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.

▣ 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.

▣ 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 = Precipitation, R = Surface runoff, G = net ground water
flow, E = Evaporation, T = Transpiration,
∆S= change in storage
P –P R– R––GG ––EE- T- T=
∆=S ∆S

▣ 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

▣ 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
F ( t )   f ( t ) d t
0 d t
d F
f ( t ) 

▣ 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)
b)
c)
d)
Horton’s equation
Phillips equation
kostiakov equation
holtans equation

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.

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 )

▣ 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.

▣ This is defined as the rate of infiltration above
which
rainfall volume = runoff volume(saturation).
▣ The
assum that all lo only.
Φ
▣ For determ
is
unshaded that to the
of surface runoff.

▣ 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.

s oil conditions in India
For the
producing
for flood
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 valueof 0.10 cm/hr can
be assumed
storms (C.W.C)
Φ = (I - R)/24 , R = (α X I 1.2 )

▣ This is the average infiltration rate during the time
when the rainfall intensity > infiltration rate.
W-index = (P – R – I a )/tf = ( F/t f )
where P = Total storm precipitation (cm) R = Total
surface runoff (cm)
I a = Depression and interception losses (cm) t f= Time
period of runoff ( in hours)
▣ The w- index is more accurate than Φ – index because
it excludes the Depression & interception.

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

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.

▣ 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.

▣ RUNOFF :- After infiltration remaining precipitation
on the surface is called runoff.
OR
Draining of precipitation from a
catchment area through a surface channel.
COMPONENTS OF RUNOFF
▣ 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

▣ 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.

▣ 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.

▣ 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.

 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.

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

▣ 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.

▣ 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.

▣ 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.

▣ 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.

▣ 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.

▣ 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.

▣ 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

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