Chapter 3 soil water and irrigation practice1

Irrigation Engineering: Lecture Note
Civil Engineering Department, CET,JU Page 1
Chapter 3 Soil Water and Irrigation Practice
Introduction
 Soil plant water relationships relate to the properties of soil and plant that affect the movement retention
and use of water.
 Soil Serves as a storehouse of water.
 Irrigation water and rain water become available to plants through the soil.
 Irrigation water and rain water after due infiltration in to the soil get stored in micro & macro pores of
the soil.
 The water stored in the soil pores within the root zone constitutes the soil water.
 Water in soil medium is involved in many processes and soil characteristics influence those greatly.
 An understanding of the relation ship between soils and water is essential to make the most efficient use
of water in crop production.
Soil – A system
►Soil is a three-phase system consisting of solid, liquid and gases.
►The minerals and organic matters in soil constitute the solid phase.
►Water forms the liquid phase
►The soil air forms the gaseous phase.
►The mineral matters comprise the largest fraction of soil and exist in the form of particles of different
sizes and shapes encompassing the void space called soil pore space.
►Amount and geometry of soil pores, depend on the relative proportion of different sizes and shapes of
soil particles, their distribution and a management.
►The pore space remains filled with air and water in varying proportions, which are mainly manipulated by
the amount of water present in the soil.
►The soil air is totally expelled from soil when water is present in excess amount as in water logged soil,
while water in liquid form may be absent in dry sands of deserts.
►Volumes of the soil components vary widely. A typical silt loam soil contains about 50% soil solids 30%
water and 20% soil air.
►Soil serves as a medium of plant growth.
►Soil components when exists in proper amounts offer a favorable condition for plant growth.
Use of Soil for plants
 Reservoir of water
 Reservoir of Nutrients
 For anchorage
 Habitat for organisms
Water
Plants grow on soils that provide them with water & nutrients. They absorb the water from soils mainly through
roots and use only 1.0 to 1.5 percent of the volume of water absorbed for building their vegetative structures and
performing various physiological and biochemical activities. The rest of water absorbed is lost through
transpiration.
A close relationship exists between soil water and plant and that should be clearly understood to decide up
on the time and depth of irrigation and make the most efficient use of irrigation water.
An excess or deficit of soil water hinders the plant growth and reduces the yield.

Role of water in plants
 It is a structural constituent of plant cells.
 It is source of two essential elements oxygen, hydrogen required for synthesis of Carbohydrate during
photosynthesis.
 It serves as a solvent of substances and allow metabolic reactions to occur.
 It serves as a solvent of plant nutrients and helps in up take of nutrients from soils.
 It helps to transport manufactured to various parts of the plant in soluble form.
Soil Physical Properties Influencing Soil – Water Relationship
The important physical properties of soil affecting the soil-water relationship relate to soil characteristics
governing the entry of water in to the soil during irrigation or rain, water movement through the soil, retention
of water by the soil and availability of water to crop plants.
The two main physical properties of soil influencing soil-water relationship are soil texture and soil structure.
Soil Texture
Soil texture refers to the relative sizes of soil particles in a given soil. The sizes of particles making up a soil
determine its texture. In other words soil texture refers to the relative proportion of the various size groups (soil
separates) of mineral particles in a given soil.
According to their sizes soil particles are grouped in to gravel, sand, silt and clay sand, silt and clay are called soil
separates. The relative sizes of sand, silt and clay as proposed by the united state Department of Agriculture
(USDA) and international soil science society is given below.
Soil separates USDA
Particle diameter (mm)
Coarse Sand 1.0 – 0.5 2.0 – 0.2
Medium Sand 0.5 – 0.25 -
Fine sand 0.25 – 0.10 0.2 – 0.02
Very fine sand 0.10 – 0.05 -
Silt 0.05 – 0.002 0.02 – 0.002
Clay < 0.002 < 0.002
The percentage contents of soil separates in a soil are determined by Mechanical analysis. Based on the
percentage content of sand, silt and clay present, the textural class of soil is determined by using textural
triangle given below.
If a soil sample is analyzed for mechanical fractionalization and the result indicates that is made up of 25%
clay, 45% silt and 30% sand. Line may be traced on the textural triangle. Thus the above soil is indeed Loam
soil.
ISSS Very
Coarse Sand
ISSS Very Coarse Sand 2.0 – 1.0 -
Coarse Sand 1.0 – 0.5 2.0 –
0.2
Very Coarse Sand

Figure : Textural Triangle
Physical Characteristics of Textural Classes of soils
Sandy soil
 Loose and single grained
 Individual grains can be seen or felt.
 Give a rough feeling when rubbed between fingers
 Dry sands remain loose when pressed.
 Slightly moist soil tends to form a ball when pressed in palm but the same brakes when the pressure
is released.
 A moist soil forms a ball with impressions of fingers on it, but the same brakes at the release of the
pressure.
 Has a low water holding capacity and availability of water to plants is quite low.
 Has high infiltration rate
 Light soil and can be tilled very easily
A sand group includes all soils comprising sand fraction by 70 percent or more of the material weight. The
properties of such soils are characteristically sandy in nature.
Specific classes:- Sandy soil and loam sand.

Loam soil
►Contains sand, silt and clay fractions almost in equal proportions.
►When felt between fingers, it gives the feeling of the presence of small grits.
►When a lump of slightly moist soil is pressed in palm, it forms a ball and does not break when pressure is
released, but falls a part when dropped on the ground from above.
►A wet soil forms a ball that does not disintegrate when the pressure is released; it breaks when dropped
from a height with particles not separated out fully.
►Has a good water holding capacity and
►Can be tilled comfortably
►Provides favorable physical condition for crop growth.
►Specific classes:- Sandy loam
Silty loam
Clay loam
Clay soil
 It may have clay fraction more than 50 percent.
 The particles are fine and give a talcum powder feeling when rubbed between fingers.
 It forms very hard clods on drying.
 A wet soil can be puddled easily and it impounds water for a long time.
 Difficult to get good tilth during land preparation.
 Very elastic becomes very sticky when wet
 Water holding capacity is high
 Low infiltration rate
Soil Structure
►Soil structure refers to the manner in which soil particles are arranged in groups or aggregates.
►The structure of soil is dynamic and it changes constantly with soil management practices.
►Cementing/bounding agents – clay, organic matter, microbial glue mineral cementing agents.
►Soil aggregates may be temporary or stable depending on the amount and nature of cementing agents.
Three main types of soil structures
1.Single grained – Consists of one grain which is structure less
2.Massive grained – Consists of very large lumps of soil
3.Compound aggregates structure – Forms a small clods
► Depending on the shape, the structures are classified in to platy, columnar, prismatic, blocky, angular
blocky etc.
► A soil structure is important in plant growth as it influences
 The amount and nature of porosity
 Regulates water, air and heat regimes in the soil
 Mechanical properties of soil
► Soil management aims at obtaining soil structure favorable for plant growth yield besides ensuring
soil conservation and good infiltration and movement of water in soils.
► Common methods of soil structure management include addition of organic matter and adoption of
suitable tillage, soil conservation and cropping practices.

Volume and Mass Relationships of Soil Constituents
Soil has solids, liquid and air and their relative masses and volumes are often required for proper soil and crop
management.
A schematic diagram of soil shown below may be useful to define the volume and mass relationship of the three
soil phases. The diagram shows the presence of the three phases in relative proportions both in masses and
volumes.
Where
Ma = Mass of air (Negligible)
MW = Mass of water
Ms = Mass of solids
Mt = Total mass = Ma + Mw + Ms
Va = Volume of air
Vw = Volume of water
Vs = Volume of solids
Vp = Volume of pores = Va + Vw
Vt = Total volume = Vp + Vs = Va + Vw + Vs
Dry Bulk Density
Dry Bulk density is the weight of oven dry soil per unit volume of soil.
Dry bulk Density, DBD 3
/ cmgin
V
M
t
s
dry  
Where,
 DBD= dry = bulk density, g/cm3
 Ms = Mass of oven dry soil, g
 Vt = Volume of soil, cm3
Typical values: 1.1 - 1.6 g/cm3
For the determination of bulk an undisturbed soil core is taken from the field by a core sampler (sampling
cylinder) and dried in a hot air oven at 1050
c for 24 hrs to a constant weight. The weight of the soil per unit
volume is then calculated from the known volume of core sampler.
It is influenced by soil texture structure compactness, organic matter content and tillage practices.
It influences the water holding capacity of soils and hydraulic conductivity (permeability).
Its value ranges from 1.1 to 1.3 g/cm3
in fine textured surface soil and from 1.4 to 1.8 g/cm3
in coarse textured
soil. It decreases with an increase in looseness of soil and increase with compaction of soil.
Apparent Specific Gravity
Apparent specific gravity refers to the ratio of dry bulk density of soil to that of density of water. It is
dimensionless /unit less quantity/.
Apparent specific gravity,
w
dry
WaterofDensity
DBDsoilofDensityBulkDry
Asg



,
Air
Water
Solids
Va
Vw
Vp
Vt
Vs
Mt
Ma
Mw
Ms
Volume Relations
Mass Relations

Particle Density
Particle density denotes the mass of soil solid per unit volume of soil solids. It is also called true density or true
specific gravity of soil.
Particle density, Ds =
s
s
p
V
M

Particle density does not change with tillage practice or cropping practice.
Typical values: 2.6 - 2.7 g/cm3
Porosity
Porosity can be defined as the ratio of the volume of pores/voids to the total volume.
Porosity, 100*)1(1
st
s
t
st
swa
wa
t
p
D
DBD
v
v
V
VV
VVV
VV
V
V
n 





It is an index of the relative volume of pores. It is influenced by textural and structural characteristics of the soil.
The more finely divided are the individual soil particles, the greater is the porosity
Typical values: 30 - 60%
Void Ratio
Void ratio refers to the ratio of the volume of pores to the volume of soil solids.
It is also called relative porosity.
Void ratio, 1




s
t
S
st
s
wa
s
p
V
V
V
VV
V
VV
V
V
e
Soil Wetness
Soil wetness refers to the relative water content in the soil.
It is expressed on weigh basis (Mass Wetness), Volume basis (volume wetness) and depth basis.
1. Mass Wetness
It is the ratio of mass of water to mass of soil solids.
It is commonly called gravimetric soil moisture content on weight basis.
Mass wetness( soil moisture content on weight basis) =
Ms
Mw
solidofMass
waterofMass
m  
It is expressed in decimals or as a percentage.
The water content of the soil on weight basis (Mass wetness) can be found out by taking a soil sample from the
field with the help of core sampler or an auger, the sample is transferred to a previously weighed aluminum box
or container, weighed and then dried in a hot air over at 1050
c for 24 hours to a constant weight. Loss of weight
of soil sample is accounted on drying is accounted for the water present.
The weight of oven-dried soil is then determined and the percent soil water content on weight basis is calculated
as follows.
  100)(
),
(
(%)
13
32
ww
ww
w
wSWC
w
wbasisweighton
percentincontentwatersoil
wetnessMass
m

















Where
SMC (w/w) = m = Soil water content on weight basis, percent
W1 = Weight of empty container / box, g
W2 = Weight of box + moist soil sample, g
W3 = Weight of box + dried soil sample, g
2. Volume Wetness
It is the ratio of volume of water to total volume of soil. It is also termed as volumetric water content. Very
commonly the volume wetness is stated as soil water content on volume basis. It may be expressed in decimals
or as percentage.
Volume wetness   100)(100
,% PS
w
t
W
v
VV
V
x
V
V
v
vSMC
basis
volumeoncontent
watersoil













A relationship exists between mass wetness and volume wetness. This relationship is given by,
Volume wetness, = Mass wetness. x Apparent specific gravity
  ASG
v
vSMV mv * 
The soil water content on volume basis is determined by drawing a soil sample with core sampler. The sample
along with the core sampler is weighed and dried in a hot air oven at 1050
c to a constant weight. The loss of
weight of soil sample in the sampler on drying is accounted for the water present.
The weight of oven dried soil and the volume of soil core are then determined
The volume of the core, Apparent Specific gravity of the soil and the water content of the soil on volume basis
are calculated as follows.
Volume of Core = Volume of soil = h2

Dry Bulk Density of soil, DBD
t
S
V
M

Apparent Specific gravity, Asg =
w
dry
W
cm
gminDwaterofDensity
cmgminDBD



3
3
)(
/
Volume wetness (%) or     Asg
w
wSMC
v
vSMC
svolumebasion
contentwaterSoil
*
,%






Whxd
WW
2
21



Where,
SMC (v/v) = Soil water content on volume basis
W1 = Weight of core sampler/box + moist soil sample, g
W2 = Weight of core sampler/box + dried soil sample, g
Asg = Apparent Specific gravity
 = inside radius of core sampler / container, cm
h = height of core sampler/ container, cm
dw = density of soil water, = 1 gm/cm3

4.Equivalent depth of water
Equivalent depth of water is the volume of water per unit land area. It refers to the depth of water formed
if the water existing in the soil is squeezed and collected without affecting the soil structure. However, the soil
water exists distributed in the soil pores in a given volume of soil.
L
A
AL
d v
v



Where
– d = equivalent depth of water in a soil layer
– A= Area of the soil mass
– L = depth (thickness) of the soil layer
Figure: Soil wetness
Solved Problems on Mass-Volume Relationships of Soils
Problem 1
Given
 Weight of wet soil = W1 = 1870g
 Volume of soil = Vt = 1000cm3
= 10cm x 10cm x 10cm
 Weight of Oven dry soil = W2 = 1677g
 Apparent specific gravity of soil solids, Gs = 2.66

Required
1.Dry bulk density, DBD
2.Wet bulk density, WBD
3.Apparent specific gravity of dry soil, Asgdry
4.Apparent specific gravity of wet soil, Asgwet
5.Soil wetness
a.Mass wetness, SMC(w/w)
b.Volume wetness, SMC(v/v)
6.Depth of water
7.Porosity of soil, n
8.Void ratio, e
9.Degree of Saturation, s
Solution
Weight of water, Ww = Weight of wet soil - weight of oven dry soil
= W1 -W2 = 1870g -1677g= 193g
Weight of air, Wa = 0
Weight of solid, Ws = 1677g
Weight of water, Ww = 193g
Total weight of soil = WT = 1870g
Total volume of soil, VT = 1000cm3
Draw the block diagram of the soil.
a) 33
677.1
9
1000
1677
cm
g
cmv
w
DBD
t
s

b) 33 87.1
1000
1870
cm
g
cm
g
v
w
WBD
t
t

c) Gdry = 677.1
1
677.1
3
3

cm
g
cm
g
DBD
w
d) Gwet = 87.1
1
87.1
3
3

cm
g
cm
g
WBD
w
e) Soil Wetness
(i) Mass wetness (SMC )
w
w
%5.11600
1677
193
 x
w
w
s
w
m
(ii) Volume wetness  %
v
vSMC
  %286.19677.1*5.11*%  dryv G
w
wSMC
(f) Depth of water
Area
waterofVolume

A
v
d w

Air
Water
Solid
Air
Water
Solid

3
3
193
1
193
cm
cm
gm
m
g
wm
V
w
w
w
w
w 

cm
cm
cm
DDepth
93.1
100
193
, 2
3


(g) Porosity of soil
wV
W
G
V
V
n
s
s
S
T
p



Volume of solids,
3
5.630
)1(66.2
1677
cm
wG
W
V
s
s
S 

Volume of pores or voids, VP
333
5.3695.6301000 cmcmcm
VVV
VVV
stp
spt



%95.36100
1000
5.369
3
3
 x
cm
cm
V
V
n
T
p
h) Void ratio, e
586.0
5.630
5.369
3
3

cm
cm
V
V
e
s
p
(i) Degree of Saturation, S
3
5.369
100
cmV
x
v
v
S
P
p
w


Volume of water
3
3
193
1)1(
193
cm
cm
gm
wG
w
V
v
w
G
w
w
w
ww
w
w



When using grams centimeters,
ww WV 
%2.52
100
5.369
193
100



x
x
V
V
S
P
w
Problem 2
A soil sample was taken with core sampler from a field when soil reached field capacity. The oven dry sample
weighed 1.065 kg. The inside diameter of the core was 7.5cm and the length was 15cm. Determine the dry bulk
density and the apparent specific gravity of dry soil.
Given
 Weight of oven dry soil, Ws = 1.065kg = 1065g
 Diameter of core, d = 7.5cm
 Length of Core, h = 15cm
Required
 Dry bulk Density,  dry, DBD
 Apparent specific gravity of dry soil, G dry
Solution
t
s
dry
V
M
Volume
wt
DBD 
soilof
soildryovenof.

Volume of soil = Volume of Core sampler =
322
66315*75.314.3 cmxhr 
cmhheightandcmrradious 15,75.3
2
5.7

d=7.5cm
H=L=15cm

33 61.1
663
1065
cm
g
cm
gDBD dry  
Apparent specific gravity of dry soil, G dry 61.1
1
61.1
3
3

cm
g
cm
g
DBD
w
Problem 3
Given
 Diameter of Core Sampler , d= 10cm
 Length of Core sampler , h= 8 cm
 Wt. of wet soil /fresh core = 1113.14g
 Wt. of oven dry soil core = 980.57g
Required
1) Dry bulk Density of soil
2) Apparent Specific gravity of dry soil
3) Soil water content on weight basis (Mass Wetness)
4) Soil water content on volume basis (volume wetness)
Solution
1) Dry Bulk Density
579.980
Vsoil,ofVolume
Msoil,dryovenof.
t
s


sM
wt
DBD
Volume of soil = Volume of core sampler
  3
2
2
57.6288*
2
10*14.3 cmhr 
33
56.1
57.628
57.980
cm
g
cm
g
DBD 
(2) Apparent Specific gravity of dry soil, Gdry
56.1
1
56.1
3
3

cm
g
cm
g
DBD
G
w
dry

(3) Soil water content on weight basis, Mass wetness
  100
.
.
% x
coredry soilovenofwt
coredry soilovenofwt.-coresoilwetofWt
w
wSMCm 
%52.13100
57.980
57.98014.1113


 x
(4) Soil water content on volume basis, (Volume wetness)
    %09.2156.1*%52.13*%%  dryv G
w
wSMC
v
vSMC

Problem 4
The volume of water present in a 395cm3
soil core is m75 . The oven dry weight of the soil core is 625g.
Calculate the soil water content on weight basis.
Given
 Volume of soil, 3
395cmVt 
 Volume of water, 3
7575 cmmVw  
 Oven dry weight of soil core g625
Required  %
w
wSMCm 
Solution
33
58.1
395
.625
cm
g
cm
g
cmcore,soilofVolume
gcore,soilofdry weightOven
DBD 3

Apparent Specific gravity of dry soil,
w
dry
DBD
G

 = 58.1
1
58.1
3
3

cm
g
cm
g
 
t
w
Vcore,soilofVolume
Vwater,ofolume
%
V
v
vSMC  %0.19100
395
75
3
3
 x
cm
cm
   
dryG
v
vSMC
w
wSMC
%
%  %03.12
58.1
%0.19

Classification of Soil Water
The water below the water table is known as ground water and the water above the water table is known as soil
water. There are three kinds of soil water. These are:
1. Gravitational/Free water
2. Capillary water
3. Hygroscopic water
Gravitational/Free water
When sufficient water is added to soil, water gradually fills all the soil pore system expelling air completely from
soil, especially if drainage is impeded. At this stage the soil is said to be saturated with water.
The water tension at this stage is 1/3
atmosphere or less.
Gravitational water is that part of soil water moving through soil interstices under gravity. It is the water in the
soil macro pores that moves down ward freely under the influence of gravity. Gravitational water is not available
to plants because of the rapid disappearance of the water from the soil.
The upper limit or maximum level of gravitational water is when the soil is saturated.
For coarse sandy soil gravitational water will drain in one day but for fine clay soil it will drain with in 2 to 3
days.

Capillary Water
With increasing supply of water, the water film held around soil particles thickness. Water then enters the pore
system gradually filling the pores and wedges between adjacent soil particles until a stage is reached when the
water tension is in equilibrium with gravity. The soil water tension is now about 1/10
to 1/3
atm. Soil can not hold
any more water once this stage is reached and the excess water begins to move down wards under gravity as
gravitational water.
Capillarity water refers to water retained by soil after cessation of the down ward movement of water
(gravitational water). It is water held by forces of surface tension and continuous film around soil particles and in
capillary spaces. The water is held at a tension of 1/3
to 31 atm. and much of it is in fluid state.
The capillary water supplies the whole or largest part of water needed by plants. It also serves as soil solution and
as the medium of nutrient availability. It moves in any direction with in the soil but in the direction of greatest
tension or low potential.
Hygroscopic Water
Hygroscopic water refers to the soil water held tightly to the surface of soil particles by adsorption forces. It is
water than an oven dry soil absorbs when exposed to air saturated with water vapor. It occurs as a very thin film
over the surface of soil particles and held tenaciously at a tension of 31 atmospheres.
The water is held by adhesive force. Much of it is non-liquid and moves as vapor. It is unavailable water to
plants.
Figure: Diagrammatic Representation Of Kinds Of Soil Water
Soil Water Constants
Soil water content/ soil moisture varies constantly under natural conditions. Soil water is always subjected to
certain forces such as pressure gradients and vapor pressure differences that cause it to move. In order to
describe the soil water status under certain conditions of water equilibrium some terms referred to as soil water
constants are used.
Tension of thinnest film
about 10000 atm
Soil Solids
Solid-liquid interface
Hygroscopic Water
(Water of Adhesion)
Capillary
Water
(Water of
Cohesion)
Zone of progressive thickening
of water film
Tension of thickest
film around 1/3 atm
Gravitational water

The soil water constants include:
 Saturation Capacity
 Field Capacity
 Permanent wilting point
 Oven dry soil
These constants are important in soil-water relationships and have a direct bearing on plants.
Saturation Capacity
It is the percentage water content of a soil fully saturated with all its pores completely filled with water under
restricted drainage. It is also called maximum water holding capacity. Complete saturation occurs in surface soils
immediately after irrigation or rainfall.
The soil water is in free state and the tension at this stage is zero.
Field Capacity
Field capacity of a soil is the moisture content after gravitational water has drained off and/or has become very
slow and the moisture content of the soil become more stable.
It denotes the water content of a soil retained by an initially saturated soil against force of gravity. This stage is
reached when the excess water from a saturated soil after irrigation or rainfall has fully percolated down.
Field capacity refers to the moisture content of a soil 1 to 3 days after rainfall or irrigation depending up on the
soil texture.
It presupposes that the following conditions
 Evaporation transpiration are not active
 Down ward movement of water has practically ceased
 All the hydrostatic forces acting on soil water are in equilibrium.
Soil water tension at field capacity ranges from 0.1 to 0.33 atmospheres in different soils. It is 0.1 atmospheres
for sandy soil and 0.33 atmospheres for clay soils.
It is the highest point of available water range, as the soils cannot retain any more water above this point against
gravity.
Permanent Wilting Point (PWP)
It refers to the soil moisture content at which plants do not get enough water to meet the transpiration demand
and remain wilted unless water is added to the soil.
It is the moisture content of the soil when plants growing on that soil starts to show signs of wilting due to
moisture stress.
At the permanent wilting point the films of water around the soil particles are held so tightly that roots in
contact with the soil can not remove the water at a sufficiently rapid rate to prevent wilting of the plant leaves.
Permanent wilting point is considered as the lowest limit of available water range.
Soil water tension at PWP ranges from 7 to 32 atmosphere depending on soil texture, on the kind and condition
of the plants, on the amount of soluble salts in the soil solution and to some extent on climate and environment.
Oven Dry Soil
Oven dry soil is used to describe the soil water status when a soil sample is dried at 1050
c in a hot air over until
sample loses no more water.

The equilibrium tension of soil water at this stage is 10,000 atmosphere. All estimations of soil water content are
based on the oven dry weight of the soil and the soil at this stage is considered to contain zero amount of water.
See the following figure.
Figure: Schematic Representation of Soil Water Constants and Soil water Ranges
Determination of Field Capacity (FC) and Permanent Wilting Point (PWP)
Determination of Field Capacity (FC)
Method 1: Gravimetric Method – Field Method
In this method the FC is determined by pounding water on the soil surface in an area of two to five square
meters and allowing the water to drain for few days depending on the soil class. The drainage will take one day in
sandy soil and 2 days in Clay soil. Sufficient water is pounded over the area to ensure that the desired soil layers
get fully saturated. The soil surface is cleaned of weeds to prevent the possible transpiration loss Spreading a
black polythene sheet or sufficiently thick straw mulch over the area prevents surface evaporation.
Soil Samples are taken from the desired layers and the water content is determined gravimetrically. The value so
obtained thus represents the soil moisture content at field capacity.
Method 2: Pressure plate – Laboratory Method
The Pressure Plate is a laboratory procedure for estimating the field capacity. The pressure or suction, applied to
a saturated soil, is one-tenth (0.1) atmosphere for the sands, to one-third (0.3) atmosphere for the clays, and the
moisture remaining in the soil after equilibrium has been obtained is approximately the field capacity.
Oven dry/Absolute wilting
Permanent Wilting Point
Field Capacity
Saturation
Gravitational Water
Capillary Water
Hygroscopic Water
Unavailable
Water
Available Water
Unavailable
Water

Determination of Permanent Wilting Point (PWP)
Method 1: Gravimetric Method – Field Method
To determine the PWP under field conditions it is necessary to grow plants on soil that has been welted to field
capacity. When the plants have nearly reached their maximum vegetative growth, water is with held and they are
allowed to wilt.
At the time the plants show signs of wilting the soil is sampled and the soil moisture content is determined
gravimetrically.
The drawback of this method is the difficulty of taking undisturbed soil sample using soil-sampling cylinders.
Thus it may be derived by dividing the value of field capacity by a factor varying from 2.0 to 2.4 depending up
on the amount of silt in the soil. Dividing the FC with 2.4 derives the PWP for soil with high silt content.
Method 2: Pressure plate – Laboratory Method
With the pressure-membrane apparatus for the determination of the PWP, the principle is the same as for the
pressure plate for the determination of the FC, except much higher pressure are required 14 –15 atmosphere.
Typical Values of Soil Moisture at FC and PWP
The soil texture has a large influence on the amount of soil moisture present in the soil at FC and PWP.
Accordingly the ranges of soil moisture content on weight percentage at FC and PWP for various soil types have
been established. These values are shown in the following table.
Soil Moisture Ranges
The soil water ranges are the available water range and unavailable water range. See the above figure.
Available Water
The water held by soil between field capacity and permanent wilting point and at tension between 0.1 to 0.33 and
15 atm. Is available to plants and is termed as available water. It is the moisture available for plant use. It
comprises the greater part of capillary water. Availability of water to plants is more in the upper range of
available water that is, at field capacity or near to it. It decreases sharply as the water content approaches the
permanent wilting point.
In-order to calculate the amount of available water the following parameters must be known.
1. the soil moisture content in weight basis at FC and PWP
2. the dry bulk density of soil and apparent specific gravity
3. the soil moisture content in volume basis at FC and PWP
4. the effective root zone depth
Percent of Moisture Based on Weight of soil
Soil Type Field Capacity, FC Permanent Wilting
Point, PWP
Fine sand 3-5 1-3
Sandy Loam 5-15 3-8
Silt loam 12-18 6-10
Clay loam 15-30 7-16
Clay 25-40 12-20

The equations used for computing the available water are as under.
  RZvpwpVfcrzvvd
vpwpvfcvvv%
wpwpwfcwww%
DASGDAsgPWPFCAWbasis,depthinwaterAvailable
PWPFCAWbasis,volumeinwaterAvailable
PWPFCAWbasis,weightinwaterAvailable
**)(* )()(%%
)()(%%
)()(%%






Unavailable water
There are two situations at which soil water is not available to most plants
(i) When the soil water content falls below the permanent wilting point and is held at a tension of 15
atmospheres and above.
(ii) When the soil water above the field capacity and held at a tension between zero and 1/3
atmosphere.
Water in the former situation is held tightly or tenaciously by soil, while that in the latter situation moves down
ward under gravity. Water under both the situations is termed as unavailable water.
Root Zone Depth
Root zone depth is the maximum depth below the surface of soil from which a particular crop derives water for
use and develops its root system. Crops uses water for its growth in different proportions from the root zone
depth.
Root zone depth in irrigated fields are dependent on soil types, crop types, distance of water table from the
ground surface and the amount of water applied during irrigation. In general crop plants develop most of their
roots and derive most of their moisture supplies from the upper portion of the root zone depths.
Measurement of Soil Moisture Content
Definition
Soil moisture content refers to the amount of water stored and present in the soil at the time of measurement.
The significance of measuring soil moisture content are as under:
 For proper scheduling of irrigations
 For estimating the amount of water to apply in each irrigation
The principal methods of expressing soil moisture are
(i) by the amount of water in a given amount of soil
(ii) the stress or tension under which the water is held by the soil.
Expressing Amount of Soil Moisture
The amount of soil moisture that is held by a certain mass or volume of soil can be expressed as weight basis,
volume basis or depth basis.
On Weight Basis
Soil moisture on weight basis is based on dry weight of the soil sample.
100
.
%)( x
dry soilovenofwt
dry soilovenofwt.-soilmoisof.wt
w
wSMCw 
Expression of soil moisture content as a percentage of dry weight or on weight basis may not indicate the
amount of water available to plants unless the soil moisture characteristic curve or FC PWP are known. Also
the additions and losses of water from the soil are often measured in units of depth, cm, mm, etc. which on area

basis becomes the volume. Thus it is more useful to convert moisture content per unit weight or on weight basis
in to moisture content per unit volume or on volume basis.
On Volume Basis
Expression of soil moisture content on volume basis are necessary in order to calculate irrigation depth. This is
because irrigation depth is expressed as the amount of water needed to fill up a certain volume of soil over its
root zone (which equals area multiplied by rooting depth).
The soil moisture content percentage by weight and soil moisture content percentage by volume are related to
one another by apparent specific gravity of soil (i.e., bulk density of water and bulk density of water) as shown by
the following equation.
   
waterofDensity
soilofDensityBulkx
w
wSMC
Asg
w
wSMC
v
vSMCbasis,volumeon
contentmoistureSoil
v
%
*%)(%  
Apparent specific gravity of dry soil
Apparent Specific gravity of dry soil is the ratio of the dry bulk density of soil to density of water.
waterofDensity
soilofDensity
,GSoil,DryofGravitySpecificApparent dry
Dry
Asg 
Dry Bulk Density
Dry bulk density of soil is the ratio of oven dry weight of the soil to volume of soil.
soilof
soilofdry weight
Volume
Oven
DBD 
On the basis of depth
Like rainfall, irrigation depths are measured and expressed in units of depth in mm, cm, or m. Thus it is essential
to convert the soil moisture content on volume basis in to depth basis.
The square meter units used to express area can be cancelled from any equation dealing with percent water by
volume, because water is distributed across the same cross-sectional area as the soil. The amount of water
present can therefore be expressed simply in terms of depth
The simplest way to calculate this is to multiply the unit depth of soil by the volume percentage of water and
divide by 100 %
Example
Assume that 60cm3
of moist soil has been collected and found to weigh 100gm. It weighed 85 gm after air-
drying and 80 gm after oven drying. Given this information, the water content of the soil can be expressed as
below.
On Weight basis
100
soildryovenof.
soildryovenofwt.-soilmoisof.
%)( x
wt
wt
w
wSMC  %25100
80
80100


 x
33
331
60
80
cm
gm
cm
gm
Volume
Oven
sityDryBulkDen .
soilof
soilofdry weight

Density of water is = 31
cm
gm

331
1
331
3
3
.
/
/.

cmg
cmg
Asgdry
%3333.1*%25*%)/(%)/(  AsgwwSMCvvSMC
soilofdepthmeterperwaterofcm
33%x100cm
basis)depthonSMC 33
%100
( 
Methods of Determination
The most commonly used methods are:
 Gravimetric Method
 Measuring Instruments Method
 Touch and Feel Method
Here the gravimetric method is discussed.
Gravimetric Method
The gravimetric method provides the direct measurement of soil water content, which is expressed in percent
based on oven dry soil. The soil water contents can be expressed either on weight basis or volume basis. Bulk
density and apparent specific gravity of soil can also be determined.
It is the most accurate and reliable method of measuring water content of soil. It is relatively cheap and does not
require many equipment. It also used to calibrate other methods. It requires only an oven, a balance, a soil auger
or soil sampling cylinder/ core sampler and aluminum boxes.
The disadvantage with the method are that it is time consuming, laborious and requires several soil Samples to
avoid soil variability in obtaining accurate results.
The method generally involves drying a soil sample in hot air oven to drive out the water. The loss in weight of
the sample on drying is regarded as the measure of water present. Water content of the soil is found out by
taking a soil sample from the field with a soil auger or sampling tube (sampling cylinder or core). The Samples
are taken from the desired depth at several locations for each soil type. The sample is transferred to a previously
weighed aluminum box or container. The soil samples are weighed and they are dried in a hot air oven at 1050
c
for about 24 hours to a constant weight.
After removing from the oven they are cooled slowly to room temperature within a desiccator's and weighed
again. The difference in weight or loss of weight of the soil sample is the amount of moisture in the soil. The
weight of moist soil and oven dry soil is then determined. The water content of the soil on weight basis and
volume basis as well as the dry bulk density and apparent specific gravity of the soil are calculated by employing
the following formulas.
  100
.
.
% x
dry soilofwt
dry soilofwt.-soilmoistofwt
w
wSMCm 
Volume of soil  Volume of sampling tube/core/cylinder/ hr2

Dry Bulk Density of Soil, DBD
V
M
soilofVolume
dry soilovenofwt s
dry 
.


waterofDensity
soilofDensityBulkDry
AsgGDry Soil,ofGravitySpecificApparent drydry ,

   
w
drym
v
waterofDensity
soilofDensityBulkx
w
wSMC
Asg
w
wSMC
v
vSMC


 
%
*%)(%
100cm
100cm*
dbasis,DepthonSMC v

Depth of Available Water
The available water can be expressed in weight basis volume basis or as a depth of water.
The depth of available water can be determined as follows.
Let d be the depth of the root zone of the plant. Let wS be the unit weight of the dry soil and w be the unit
weight of water if we consider a unit area of soil their the volume of soil in the not zone will be ( d x 1)
Weight of soil = (d x 1) x Ws
If dW is the depth of water, in the soil depth d, the weight of water per unit area is given by
weight of water =(dw x 1) x w
Now from the definition of water content (moisture content) expressed as a ratio

soilofweight
waterofWeight
m
 
  s
w
xWd
wd
1*
*1*
mdSdormd
w
ws
dor ww **** 






Where S is the apparent specific gravity of the soil. It is equal to the ratio of the weight of the given volume of
soil to the weight of an equal volume of water, thus
w
W
S s

Thus,
w
ss
w
W
S



Depth of water at field capacity (F.C) )(**).(** mfcrzrzfc dSCFdSd 
Depth of water at the permanent wilting point (P.W.P)   )(**..** mpwprzrzpwp dSPWPdSd 
Therefore, depth of available water, )()((**)...*(* mpwpmfcrzrzw dSPWPCFdSd  
The depth of available water per meter depth of soil   )(*..* )()( vpwpvfcw SPWPFCSd  
Assuming the value of readily available depth of water to be 75% of the available water,
 PWPCFdSRAWWaterofDepthAvailableadily ...**75.0,Re 
If the water content of the soil at the lower limit of the readily available water is Mo ,
the readily available depth of water,   oMmfcrzdSMFCdS   )(0 (****
Now wateravailableadilyCFmo Re. 
Or  PWPCFCFmo ...*75.0. 
The moisture content mo is also called the optimum moisture content.
Percentage volume of available water.
The available water can be expressed as a percentage of total volume.
Volume of water per unit area = (dw x 1)
Volume of soil per unit area = d x 1

Percentage volume of available water, 100
1*
1*
x
d
d
P w
V 






or
 
d
p
d
d
dwP
v
w
v
*
100
100*







Substituting mdSdw ** in to the equation for PV
100**mSPV 
Soil moisture deficiency
The water required to bring the soil moisture content of a given soil to its field capacity is called the field
moisture deficiency or soil moisture deficiency. The soil moisture deficiency indicates the water required to bring
the soil moisture to the field capacity. Thus
Soil moisture deficiency = Field capacity - Existing water content or soil moisture deficiency = F.C.-m
Where m is the existing water content.
Estimating Depth and Frequency of irrigation on the basis of soil moisture regime
concept
Water or soil moisture is consumed by plants through their roots. It, therefore, becomes necessary that sufficient
moisture remains available in the soil from the surface to the root zone depth.
The soil moisture in the root zone can vary b/n Field capacity (upper limit) and wilting point moisture content
(lower unit).
It is necessary to note that the soil moisture is not allowed to be depleted up to the wilting point, as it would
result in considerable fall in crop yields.
The optimum level up to which the soil moisture may be allowed to be depleted in the root zone without fail in
crop yields has to be worked out with experimentation. Irrigation water should be supplied as soon as the
moisture falls up to the optimum level (fixing irrigation frequency) and its quantity should be just sufficient to
bring the moisture content up to its field capacity, making allowance for application losses (fixing depth). The
optimum soil water regime means the range of available soil water in which plants do not suffer from water
stress and all the plant activities occur at an optimal rate.
The optimum soil water range is also called Readily Available water, RAW. The readily available water is that
portion of the total available water, which can be easily extracted by plant roots. It differs from one crop to
another. It has been found in practice that about 20- 75% of the available water is readily available . But the
optimum level or critical soil water level or allowable depletion value (p) up to which the soil moisture may be
Field Capacity MC
Available
M.C(Capillary Water)
Non- Available
MC(Hygroscopic water)
Optimum MC
Permanent wilting point MC
Oven dry level
Readily
Available
Water
Moisture Content
Of soil
Time

allowed to be depleted in the root zone with out full in crop yield has to worked out for every crop and soil by
experimentation. The allowable depletion value (p) varies with the type of crop and evaporative demand.
Water will be utilized by the plants after irrigation and soil moisture will start falling. It will be recoupled or
refilled by a fresh dose of irrigation as soon as the soil moisture reaches the optimum level. This sequence of
operation can be shown in the following figure.
Examples
1.After How many days will you supply water to soil in order to ensure sufficient irrigation of the given crop, if
(i) Field Capacity of soil is =28%
(ii) Permanent wilting point = 13%
(iii) Density o f soil = 1.3gm/cm3
(iv) Effective depth of root zone = 70 cm
(v) Daily consumptive use of water by the given
Crop = 12mm = 12mm/day
Assume RAM = 80% of AM.
drz = 0.7m
Solution:-
16%12-28ContentMoistureOptimum
RAM
PWPFCAM



%12%15*8.0.2
%151328.1
It means that the moisture will be filled by irrigation b/n 16% 28%
Depth of water stored in the root zone b/n these two limits
 m.cOptimum-m.cCapacityField
d
w
d

 .

Readily
Available
Moisture
Available
Moisture
Irrigation Interval/ frequency
PWP level /
Pwp Mc
Optimum MC /critical
levelMoisture
content of
soil
Field Capacity level
Moisture
content of
soil
Time

3.1
1
3.1
.
.

cc
gm
cc
gm
w
d
g
g
w
d
w
d






  10.92cm0.1092m0.12m*0.7*1.30.16-0.280.7*1.3zoneroottheinstoredwaterofDepth 
 Hence the water available for evapotranspiration = 10.92 cm
 12mm or 1.2cm of water is utilized by the plant in 1 day
days9ysadaysdays
1.2
1x10.92
frequencyIrrigation
inplantthebyutilisedbewillwaterofcm


1.9
92.10
Hence, after 9 days, water should be supplied to the given crop.
2. Given
Given crop = wheat
FC = 27%
PWP = 13%
Depth, d = 80cm
Dry unit weight of soil = 272.14
m
kN
d 
Irrigation water is to be supplied when the average soil moisture falls to 18%
 Field application efficiency = 80%
 Water lost in the water- course the field channels is 15% of
Required
 Find the storage capacity
 Fine the water depth required to be supplied to the field
 What is the amount of water needed at the canal outlet
Solution:-





100100
WPmcFCmcd
w
 .d
moistureleor AvailabcapacityStorageMaximum
23 81.9,,72.14*
m
KN0.8mzonerootofdepthd
m
KNwhere wd  
 mcc
w
d
WPFCdorCapacityStorageMaximumforeThere  *


    cmmetersMoistureAvailableMaximum 8.16168.014.02.113.027.08.0*
81.9
72.14

Since the moisture is allowed to vary b/n 27% 18%, the deficiency created in this fall.





OMC
mcmc
w
d
ExistingFCddeficiencymoistureSoil *


  cmmetersNIR 8.10108.009.0*2.118.027.08.0*
81.9
72.14

Hence, 10.8cm depth of water is the net irrigation requirement
a
NIR
(FIR)fieldthetoedsupplibetorequiredwaterofQuantity


13.5cm
cmNIR
FIRfieldthetosuppledbetorequiredwaterofQuantity
a

8.0
8.10


15.88cm
cmFIR
outletcanaltheatneededwaterofQuantity
c

85.0
5.13

3. 800m3
of water is supplied to a farmer's rice field of 0.6 ha. When the moisture content of the soil falls to 40%
of the available water b/n FC 36% and PWP 15% of the soil crop combination. Determine the field appellation
efficiency. The root zone depth of rice is 60 cm. Assume density = 0.4
Given
 Volume of water supplied to the field = V = 800m3
 Area of field, A = 0.6ha = 6000m2
 FC = 36%
 PWP = 15%
 Irrigation water is supplied when moisture content of the soil falls to 40% of moisture content
available b/n FC PWP
 d = 0.6m (root zone depth)
 Porosity, n = 0.4
Required:- Field Application efficiency
Solution
water)thatretainingdry Soilofweight(i.e,
dry soilofvolumesametheofWeight
soilofvolumecertainincontainedwaterofWeight
FCContent,oistureCapacity MField 
If a saturated soil contains volume equal to V, the volume of its voids is VV then the weight of water
contained in this soil = ,VwV Where w is the unit wt. of water.
The wt. of this soil of Vm3
after it is oven dried to remove water and to fill the voids with air is given by
soil.theofwt.unitdrytheisdd whereV  .
Vd
vw V
soilvolumesametheofwt
soilofvolumecertainaincontainedwaterofwt.
FC
.
.
. 


11.1
36.0
4.0
.



F
n
nFC
V
V
nbut
w
d
d
w
V




The max-quantity of water stored b/n field capacity (FC) permanent wilting point (P.W)
  mWWd PWPFC
w
d
14.0)15.036.0(60.0*11.1 








0.084m0.14m*60%doneisirrigationwhencreatedwaterofDeficiency 
(Since irrigation water is applied when m.c falls to 40% of m.c available b/ FC PWP
Hence, irrigation water is supplied to fill up 0.084m depth of water
32
504m10,000)m*(0.6*0.084mdeficencycreatedtheupfilltorequiredwaterirrigationofVol. 
Actual volume of irrigation water supplied = 800m3
%63
800
504
napplicatiofieldofEfficiency

Sample Problem on Determination of soil Moisture content at FC and PWP and the amount of
Available Moisture in the soil using the Gravimetric Method.
An experiment is carried out in order to determine the soil moisture content at FC and PWP as well as the
available water of a soil located at the Eladale farm site. The data so obtained are as under.
 Weight of empty can for FC = 24.94 gm
 PWP = 24.84 gm
 Weight of moist soil and can at FC = 539.94 gm
 PWP = 479.84 gm
 Weight of oven dry soil and can at FC = 409.94 gm
 Weight of PWP = 410.14 gm
 Diameter of soil sampling cylinder, D = 6cm
 Height/Depth of soil sampling cylinder , h = 8.85 cm
Solution
The Process of computation of the required parameters are carried out as under.
i) Determination of weight of moist soil
 Wt. of moist soil at FC = 539.94 - 24.94 = 515 gm
 Wt. of dry soil at PWP = 479.89 - 24.84 = 455 gm
ii) Determination of weight of dry soil
 Wt. of dry soil @ FC = 409.94 - 24.94 = 385gm
 Wt. of dry soil @ PWP = 410.14 - 24.84 = 385.30 gm
iii) Volume of soil
Volume of soil = Volume of soil sampling cylinder
= h2
   32
23.25085.83 cmx 
iv) Determination of soil moisture content at FC and PWP in weight basis
 
  18.1%or181.0
30.385
30.385455
@
w
wSMC-
33.4%or334.0
385
385515
@






PWP
FC
w
wSMC
V) Determination of Dry Bulk Density of soil
33
33
54.1
23.250
385.30gm
PWP@soilof
54.1
250.23cm
385gm
FC@soilof
cm
gm
cm
DBD
cm
gmDBD


The values of DBD @ FC and PWP should be the same since the type of soil of the farm area does not vary
considerably. If the values obtained turns out to be different, an average value should be taken.
vi) Computation of Apparent Specific gravity of dry soil
54.1
1
cm
gm1.54
PWPand@
3
3

cm
gm
FCASgdry
vii) Computation of soil moisture content at FC and PWP in volume basis
    dryAsgFC
w
wSMCFC
v
vSMC *@@  51.4%or0.5141.54x  334.0
    dryAsgPWP
w
wSMCPwP
v
vSMC *@@  27.9%or279.054.1*181.0 
viii) Computation of Available soil moisture/ Available water

    PWP
v
vSMCFCAM @@
v
vSMCbasisin volume  23.5%or235.0279.0514.0 
soilofdepthmeterPerwaterofmminAM
m
mm
1000
1000
x 235235.0 
Infiltration of Water into Soils
Definition
Infiltration is the entrance or movement of water from the surface into the soil. It refers to the vertical entrance
of water from the surface in to the soil. The infiltration characteristics of the soil is one of the dominant
variables influencing irrigation. It essentially controls the amount of water entering the soil reservoir as well as
the advance and recession of the overland flow.
Infiltration rate is the soil characteristics determining the maximum rate at which water can enter the soil under
specific conditions. Accumulated infiltration or cumulative infiltration is the total quantity of water that enters
the soil in a given time. Infiltration rate accumulated infiltration are the two parameters commonly used in
evaluating the infiltration characteristics of soil.
Factors Affecting Infiltration Rate
 The initial moisture content of the soil
 Condition of soil surface
 Texture, porosity, degree of swelling organic matter
 Vegetative cover
 Duration of rainfall or irrigation
 Viscosity of water
Measurement of Infiltration
The measurement of infiltration is carried out in order to know the infiltration rate and accumulated infiltration
of soils. The measurement can be carried out in the laboratory or in the field. Field methods of measurements
are mainly preferred.
There are three field methods which are recognized for estimating infiltration characteristics of soils for the
design operation of irrigation methods.
These methods are
1) The use of cylinder infiltrometes
2) Measurement of subsidence of free water in a large basin (pounding)
3) Estimation of accumulated infiltration from the water front advance data
The use of Cylinder infiltrometer is the most commonly used. It will be described below.
Cylinder Infiltrometer
Cylinder infiltrometer are metal cylinders with a diameter of 30 cm or more and a height of 25 cm or more,
which are formed of 2mm rolled steel sheet metal. Two cylinders are mostly used, one outer and the other inner
cylinder.
The most commonly used cylinders are of the following dimensions.
Inner Cylinder
o Diameter = 30cm
o Height = 25 cm
Outer Cylinder
o Diameter = 60 cm
o Height = 25 cm

In this method the infiltration characteristics of soils can be determined by pounding water in a metal cylinder
installed on the field surface and observing the rate at which water level is lowered in the cylinder.
Since by definition infiltration is the vertical entrance of water from the surface in to the soils, the lateral
movement of water should be minimized. This can be achieved by using double ring cylinder infiltrometer. The
lateral movement of water from the inner cylinder is avoided or minimized by pounding water in an outer/
guard cylinder of buffer area around the inner cylinder.
Apparatus
Two cylinder infiltrometer, point gauge or meter rule, stop watch, plastic task for water storage, plastic buckets
of known volume, Large quantity of water, Driving plate of hammer.
Procedures
I. Selecting the sites
o Examine and select possible sites for the test. Avoid surfaces un usually disturbed, animal
burrows, stony soils paths, roads etc.
o The site should be level as much as possible
II. Installing the cylinder infiltrometer
o Set both cylinders in place and press them firmly in to the soil.
o Place the driving plate or wooden plank on top of the cylinder.
o Drive the cylinders into the ground by using striking on a driving plate using a hammer or mallet.
The cylinders are installed to about 5 to 10 cm deep in the soil. Make a mark on the out side of
the cylinders to the required depth.
o Tamp soil into the space between the soil and the cylinder.
III. Installation of point gauge or plastic rule
o Attach or fix a point gauge /hook gauge or any ordinary plastic or wooden rule/scale in the inner
cylinder.
IV. Conducting the test/measurement
o Measure the volume of cylinder above the soil (diameter and depth)
o Place a piece of folded jute matting or inner tube on the surface with in the inner and outer
surface. The matting is used to prevent puddling and sealing of the surface soil.
o Add water in to two containers of known volume (bucket or graduated jar) from the water
tanker.
o Add water to both cylinders on to the matting placed simultaneously and quickly to about three
fourth of the cylinder. remove the matting.
o Take the initial water depth reading and the initial time. Continue to note the instant and the time
the water level reaches the desired level.
V. The infiltration that occurs during the period between the start of the test and the first measurement is the
difference between the computed initial level and the first actual reading.
VI. Additional measurements should be recorded at periodic intervals, 5 to 10 minutes at start of the test
expanding to 15 to 30 minutes intervals after some readings.
When the water level has dropped about one half of the depth of the cylinder, water should be added to return
the surface to its approximate initial depth. The depth should be maintained in the cylinder between 6 and 12 cm
throughout the test. When water is added, it is necessary to record the level of water before and after filling.
The interval between these two readings should be as short as possible to avoid errors due to infiltration during
the refilling period.

A stopwatch is used to note the instant the addition of water begin and the time the water reaches the desired
level.
The total quantity of water added to the inner cylinder is determined by counting the number of full containers
of water and the fractional volume in the jar or tank, which is added last. Care is taken to fill the container
completely each time before adding water to the cylinder. The difference between the quantity of water added
and the volume of water in the cylinder at the instant it reaches the desired point is taken as the quantity of water
that infiltrates during the time interval between the start of filling to first measurement.
The buffer pond/outer cylinder is filled with water immediately after filling the inner cylinder. Water levels in the
inner cylinder and the buffer pond are kept approximately the same.
VII. Measurements and the experiments are continued until the intake rates are constant over 1 to 2 hour period.
VIII. The data are tabulated on standard form as shown in the following table.
IX. Analysis of data is carried out by plotting the data on normal or logarithmic paper cumulative depth Z on
the vertical axis, cumulative time, t on horizontal axis. Also Infiltration rate on the vertical axis, cumulative
time, t on horizontal axis.
It is necessary to conduct replicated tests at suitable locations in the filled.
25cm
GLGL
10cm
30cm
60cm
Section
A A
Plan

Chapter 3 soil water and irrigation practice1

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