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Hydrology and Irrigation Engineering
TOPIC –INFILTRATION AND INDICES
• Introduction to Infiltration
• Some related Definitions
• Principle behind infiltration
• Infiltration in waste water collection
• Factors affecting Infiltration
• Problems related to Infiltration
• Some research findings
• Hydrology means the science of water . It is
the science which deals with occurrence ,
circulation ,and distribution of the water on
the earth and it’s atmosphere.
When water falls on a given formation ,a small part of it is first of all absorbed
by the top thin layer of soil ,so as to replenish the soil moisture deficiency.
Thereafter ,excess water moves downward ,where it is trapped in the voids
and becomes ground water.
This process ,whereby the water enters the surface strata of the soil and
moves downward towards the watertable ,is known as infiltration.
Terms related to Infiltration-
• Infiltration capacity
• Infiltration rate
• Soil moisture
• Field capacity
• Equivalent moisture
The ground water stored in the under-ground soil depends mainly upon the
number of voids present in the soil ,which ,in turn ,does not depend upon the
size of the soil particles but rather upon the arrangement ,sorting ,shape and
degree of compaction .
Therefore different soils will have different no. of voids and hence ,different
capacity to absorb water.
The maximum rate at which a; soil in any given condition is capable of
absorbing water is called its infiltration capacity . denoted by f.
It is evident that the rain water will enter the ground soil at a full capacity
rate (f) only during the period when rainfall exceeds the infiltration capacity.
When the rain intensity is less than the infiltration capacity, the prevailing
infiltration rate is approximately equal to the rainfall rate. Hence, the actual
prevailing infiltration may be equal to or less than the infiltration capacity.
This actually prevailing rate at which the water will enter the soil at any given
time is known as the infiltration rate.
• If the rainfall intensity(p) exceeds the infiltration capacity(f)
,the difference is called the excess rainfall rate(pe).This excess
water is, first of all ,accumulated on the ground as surface
detention(D) and then flows overland into the streams.
The water below the watertable is known as the ground water and the water above
the watertable is known as the soil moisture.
• The region above the watertable is devided into 3 zones:
1. Capillary zone
2. Intermediate zone
3. Soil zone
• Extending above the water table , a distance usually
ranging from about 0.3 to 3m ,depending upon
texture, is a zone the moisture content is maintained
practically constant by capillary.
• Extending down from the ground surface is the soil
zone , which is defined as being the depth of
overburden that is penetrate by the roots of
• Throughout this root zone ,the m.c. varies
tremendously, ranging from a partly saturated state
during and immediately following the periods of
heavy rains to a minimum content after a spell of
long continued drought.
intermediate zone soil moisture
water table zone
ground water zone
• The zone between the capillary zone and the soil zone is called the intermediate zone.
Throughout this zone, except during the period of ground water accretion from rainfall , the
amount of water contained within any given space is nearly constant throughout the year.
• In many cases, the height of capillary fringe (or zone) may be more and may extent up to soil
zone. In this case, there will be no intermediate zone.
• Immediately after a rain, when all the gravity water has drained down to the water
table, a certain amount of water is retained on the surface of the soil grains by
molecular attraction. This is known as pellicular water. This water cannot be
easily removed. The degree of its resistance to the movement is expressed by the
surface tension. When this water, which any soil can retain indefinitely against the
force of gravity, is expressed in terms of depth of water (as of it were spread on
the basin), then it is known as the field capacity.
• A certain portion of this pellicular water which can be easily abstracted by the root
action of vegetation is called the available moisture.
• The remainder is called the hygroscopic water, which is unavailable moisture.
• The soil can extract water from the soil till the wilting point is reached. Therefore
the available moisture is the moisture between the field capacity(upper limit) and
the wilting point (lower limit).
• The wilting point is that moisture content at which permanent wilting takes place.
• The hygroscopic water is therefore, the m.c. of the soil, just after the soil has
The depth of the water required to bring the soil m.c. up to its field
capacity, is called the field moisture deficiency or soil moisture deficiency.
Just as the field capacity is the water retained by a saturated soil after acted upon by
gravity, similarly the equivalent moisture is the water retained by a saturated soil after
being centrifuged by a centrifugal force of 1000 times that of gravity. Therefore, it is
slightly less or at the most equal to the field capacity.
Infiltration in waste water collection-
• Waste water collection system consist of a set of lines, junctions
and lift stations to convey sewage to a waste water treatment plant.
When these lines are compromised by rupture, cracking or tree
root invasion, Infiltration/inflow of storm water often occurs. This
circumstance can lead to a sanitary sewer overflow ,or discharge of
untreated sewage to the environment.
Why is infiltration a concern??
• Proper management of the soil can help maximize.
infiltration and capture as much water as allowed by a
specific soil type
• If the water infiltration is restricted or blocked water does not
enter the soil and it either ponds on the surface or runs off
the land thus ,less water is stored in the soil profile for use by
• Run off can carry soil particles and surface applied
fertilizers and pesticides off the field.
O = origin – the time is counted from the start of the rain
t1 = Time during which there is no infiltration and the entire rain is used up in
interception and depression storage.
[ Interception refers to precipitation that does not reach the soil , but is instead
intercepted by the leaves and forest floor.
In occur in canopy (canopy interception) and in the forest floor or little layer (forest
floor interception) ]
[surface runoff ,also known as overland flow) is the flow of water that occurs when
excess storm water , meltwater or other sources flows over the earth’s surface.]
[ depression storage capacity is the ability of a particular area of land to retain water in
pits and depressions ,thus preventing it from flowing.]
• Standard infiltration capacity curve
The value of fo may be twice ,thrice or any other time of fc.