chapter 3 soil water and irrigation practice1

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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.

UseofSoilforplants

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/ cmginV

M

t

sdry

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 1050c 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

Water of Density

DBDsoil of Density Bulk DryAsg

,

Air

Water

Solids

Va

Vw

Vp

Vt

Vs

Mt

Ma

Mw

Ms

Volume RelationsMass 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

sp 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(1st

s

t

st

swa

wa

t

p

D

DBD

v

v

V

VV

VVV

VV

V

Vn

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

Ve

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

solid of Mass

water of Massm

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 1050c 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

www

wSWC

wwbasisweighton

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

Wv VV

Vx

V

Vv

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

ASGvvSMV 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 1050c 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 cmgminDwaterofDensity

cmgminDBD

3

3

)(

/

Volume wetness (%) or AsgwwSMCv

vSMCsvolumebasion

contentwaterSoil*

,%

Whxd

WW2

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.

LA

ALd 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) 33677.1

9

1000

1677cm

gcmv

wDBD

t

s

b) 33 87.11000

1870cm

gcm

gv

wWBD

t

t

c) Gdry = 677.11

677.1

3

3

cmg

cmg

DBD

w

d) Gwet = 87.11

87.1

3

3

cmg

cmg

WBD

w

e) Soil Wetness

(i) Mass wetness (SMC )ww

%5.116001677

193 x

w

w

s

wm

(ii) Volume wetness %vvSMC

%286.19677.1*5.11*% dryv GwwSMC

(f) Depth of water Area

waterof Volume

A

vd w

Air

Water

Solid

Air

Water

Solid



3

3

1931

193cm

cmgm

mg

wmV

w

w

w

ww

cmcm

cmDDepth

93.1100

193,

2

3

(g) Porosity of soil

wV

WG

V

Vn

s

sS

T

p

Volume of solids, 35.630

)1(66.2

1677cm

wG

WV

s

sS

Volume of pores or voids, VP

333 5.3695.6301000 cmcmcm

VVV

VVV

stp

spt

%95.361001000

5.3693

3

xcm

cm

V

Vn

T

p

h) Void ratio, e

586.0

5.630

5.3693

3

cm

cm

V

Ve

s

p

(i) Degree of Saturation, S

35.369

100

cmV

xv

vS

P

p

w

Volume of water

3

3

1931)1(

193cm

cmgm

wG

wV

v

wG

w

ww

ww

ww

When using grams & centimeters, ww WV

%2.52

1005.369

193

100

x

xV

VS

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

sdry V

M

Volume

wtDBD

soil of

soildry oven of .

Volume of soil = Volume of Core sampler = 322 66315*75.314.3 cmxhr

cmhheightandcmrradious 15,75.32

5.7

d=7.5cm

H=L=15cm



33 61.1663

1065

cmg

cmgDBD dry

Apparent specific gravity of dry soil, G dry 61.11

61.1

3

3

cmg

cmg

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

V soil, of Volume

M soil,dry oven of .

t

s

sM

wtDBD

Volume of soil = Volume of core sampler

32

2 57.6288*210*14.3 cmhr

3356.1

57.628

57.980cm

gcm

gDBD

(2) Apparent Specific gravity of dry soil, G dry

56.11

56.1

3

3

cmg

cmg

DBDG

wdry

(3) Soil water content on weight basis, Mass wetness

100.

.% x

core dry soil oven of wt

core dry soil oven of wt.-core soil wetof Wtw

wSMCm

%52.1310057.980

57.98014.1113

x

(4) Soil water content on volume basis, (Volume wetness)

%09.2156.1*%52.13*%% dryv GwwSMCv

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, 3395cmVt

Volume of water, 37575 cmmVw

Oven dry weight of soil core g625

Required %wwSMCm

Solution

3358.1

395

.625cm

gcm

g

cm core, soilof Volume

gcore, soilof dry weight OvenDBD

3

Apparent Specific gravity of dry soil, w

dryDBD

G

= 58.11

58.1

3

3

cmg

cmg

t

w

V core, soil of Volume

V water,of olume%

Vv

vSMC %0.19100395

753

3

xcm

cm

dryG

vvSMC

wwSMC

%% %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%

DASGDAsgPWPFCAW basis, depth in waterAvailable

PWPFCAWbasis, volume in waterAvailable

PWPFCAW basis, weightin waterAvailable

**)(* )()(%%

)()(%%

)()(%%

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.

%)( xdry soil oven ofwt

dry soil oven of wt.- soilmois of .wtw

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.

waterof Density

soilofDensity BulkxwwSMC

AsgwwSMCv

v SMCbasis, volume on

content moisture Soil

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

soil ofDensity ,G Soil,Dry ofGravity SpecificApparent dry

DryAsg

Dry Bulk Density Dry bulk density of soil is the ratio of oven dry weight of the soil to volume of soil.

soil of

soil of dry weight

Volume

OvenDBD

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

100soildry oven of.

soildry oven of wt.- soil mois of .%)( x

wt

wtw

wSMC %2510080

80100

x

33331

60

80cm

gmcm

gm

Volume

OvensityDryBulkDen .

soil of soil of dry weight

Density of water is = 31cm

gm



3311

3313

3

./

/.

cmg

cmgAsgdry

%3333.1*%25*%)/(%)/( AsgwwSMCvvSMC

soilofdepthmeterperwaterofcm33% x 100cm

basis) depth onSMC 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 1050c 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 soil of wt

dry soil of wt.- soilmoist of wtw

wSMCm

Volume of soil Volume of sampling tube/core/cylinder/ hr 2

Dry Bulk Density of Soil, DBD V

M

soilof Volume

dry soil oven of wt sdry

.

waterofDensity

soilofDensity Bulk DryAsgG Dry Soil, ofGravity SpecificApparent drydry ,



w

drymv waterof Density

soilofDensity BulkxwwSMC

AsgwwSMCv

v SMC

%

*%)(%

100cm

100cm*dbasis, Depth on SMC 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

waterofWeightm

s

w

xWd

wd

1*

*1*

mdSdormdw

wsdor 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

WS s

Thus, w

ss

w

WS

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, 1001*

1*x

d

dP w

V

or

dpd

ddwP

vw

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% 12-28 Content MoistureOptimum

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.c Optimum-m.cCapacity Fieldd

w

d

.

Readily Available Moisture

Available Moisture

Irrigation Interval/ frequency

PWP level / Pwp Mc

Optimum MC /critical level

Moisture content of soil

Field Capacity level

Moisture content of soil

Time



3.11

3.1

.

.

ccgm

ccgm

w

d

g

g

w

d

w

d

10.92cm 0.1092m 0.12m*0.7*1.3 0.16-0.280.7*1.3zone root the in stored waterof Depth

Hence the water available for evapotranspiration = 10.92 cm 12mm or 1.2cm of water is utilized by the plant in 1 day

days 9 y sa days days

1.2

1x10.92frequency Irrigation

in plant theby utilised be will waterof cm

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 = 272.14m

kNd

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 .dmoisture leor Availab capacity Storage Maximum

23 81.9,,72.14*m

KN0.8m zone root of depth d m

KN where wd

mccw

d WPFCdorCapacity Storage Maximumfore There *

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

mcmcw

d ExistingFCddeficiency moisture Soil *

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) fieldthetoedsupplibe to required waterof Quantity

13.5cm cm

NIR

FIRfieldthetosuppledbe to required waterof Quantitya

8.0

8.10



15.88cm cmFIR

outlet canal the at needed waterofQuantity 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)that retaining dry Soil of weight(i.e,

dry soil of volume samethe of Weight

soilof volume certain in contained waterof WeightFCContent, oistureCapacity M Field

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.the of wt.unitdry the is dd whereV .

Vd

vw V

soilvolume samethe of wt

soilof volume certain a in contained waterof wt.FC

.

.

.

11.136.0

4.0

.

F

n

nFC

V

Vn but

w

d

d

w

V

The max-quantity of water stored b/n field capacity (FC) & permanent wilting point (P.W)

mWWd PWPFCw

d 14.0)15.036.0(60.0*11.1

0.084m 0.14m*60%done is irrigation whencreated waterof Deficiency

(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*0.084mdeficencycreatedtheup fill to required waterirrigation of Vol.

Actual volume of irrigation water supplied = 800m3

%63800

504napplicatio field of Efficiency



SampleProblemonDeterminationofsoilMoisturecontentatFCandPWPandtheamountofAvailableMoistureinthesoilusingtheGravimetricMethod.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%or 181.0

30.385

30.385455@w

wSMC-

33.4%or 334.0385

385515@

PWP

FCwwSMC

V) Determination of Dry Bulk Density of soil

33

33

54.123.250

385.30gm PWP @ soil of

54.1250.23cm

385gmFC @ soil of

cmgm

cmDBD

cmgmDBD

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

54.11

cmgm1.54

PWP and @

3

3

cmgm

FCASgdry

vii) Computation of soil moisture content at FC and PWP in volume basis

dryAsgFCwwSMCFCv

vSMC *@@ 51.4% or 0.5141.54 x 334.0

dryAsgPWPwwSMCPwPv

vSMC *@@ 27.9% or 279.054.1*181.0

viii) Computation of Available soil moisture/ Available water



PWPvvSMCFCAM @@v

vSMC basis in volume 23.5% or 235.0279.0514.0

soil ofdepth meter Per water of mmin AM mmm

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.

FactorsAffectingInfiltrationRate 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 GL GL

10cm

30cm60cm

Section

A A

Plan

chapter 3 soil water and irrigation practice1

Documents

deficit of soil water

soil air

water nutrients

properties of soil

system soil

soil medium

soil components

soil solids