Transcript
Page 1: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

SOIL MOISTURE CONSTANTS, MOVEMENT OF SOIL-WATER

& INFILTRATION OF WATER IN SOIL

SSC-411,4(0+4) : Village Attachment

DEPARTMENT OF HORTICULTURE,INSTITUTE OF AGRICULTURAL SCIENCES,

RAJIV GANDHI SOUTH CAMPUS,BANARAS HINDU UNIVERSITY,

BARKACHHA,MIRZAPUR

Page 2: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

Q. What is soil-water ? Water is retained by the soil-particles(on their surfaces) especially

colloidal particles and the pore spaces by the force of adhesion and

cohesion.

Such water present in the soil and is called as soil-water.

Classification of Soil Water

Gravitational water

Capillary water

Hygroscopic water

Physical classification Biological classification

Available water

Unavailable water

Super available water(Drainage water)

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Page 4: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

SOIL MOISTURE CONSTANTS

Varying degrees of SOIL-WETNESSWater contents present in soil under certain standard

conditions

represents definite soil moisture relationship and

retention of soil moisture in the field.  

The soil moisture tension is measured with

“TENSIOMETER”

Soil Water Content Soil Moisture Content

Page 5: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

While studying soil water and discussing its availability or other

wise to plant, some specific terms called as soil moisture

constants are used and they are as follows :

Maximum water holding

capacity

Field capacity

Maximum Capillary capacity

Moisture equivalents

Permanent wilting point

Hygroscopic coefficient

Available soil-moisture

Air capacity

Total pore volume

Continued..

Page 6: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

I. Field Capacity (FC or 0fc):It is the non-saturated but still very wet soil condition

where gravity drainage becomes negligible and only micropores retain water.

Factors affecting FC :

a.)Soil-texture b.)soil structure c.)type of clay d.)organic matter content

e.)soil-compaction f.) Impedition layer

Table : Influence of three soil textures on soil moisture constants

Page 7: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

I. Permanent Wilting point (WP or 0wp ): Also known as “wilting coefficient”. It

is the soil moisture content at which the plants can no longer be able to meet their

transpiration requirement, become water-stressed .There is still some water in the

soil but not enough to be used by the plants. WP is of two types :

• Temporary wilting point.

• Ultimate wilting point

FACTORS AFFECTING PWP

Soil properties Plant properties

Soil texture

&

structure

Types of clay Organic matter content

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SATURATION FIELD CAPACITY WILTING POINT

100% Moisture in soil pores (both macro and micro)

When water is no longer drained by gravity

When plants have extracted as much water as they can

Capillarity and surface attraction combine to pull more strongly than gravity on: 1) water in “micropores” and 2) water close to the “soil skin”

Some water is held too tightly to be pulled away by roots

Fig. : Diagrammatic representation of saturation, field capacity and wilting point

Page 10: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

Plant available water

Gravitational water

(Drainable)10-30micron

Oven-dryAir-dry

Wilting point Field capacity Saturation

Unavailable water~0.2micron

Hygroscopic coefficient

-10000 bar

-1000 bar

-31 bar -15 bar -0.33bar -0.00 bar

Capillary waterHygroscopic water

Increasing soil water constants

Increasing water potential

*-1000 bar to -300 bar depending on humidity

Fig. : Diagrammatic representation of soil moisture constants, soil water classifications & their potentials

Page 11: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

This characteristic

curve describes the

relationship between

water tension and

water content for a

specific soil.

Relationship between water tension (water potential) and water content using a “characteristic curve”

Page 12: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

Appearance of soil Type of Soil Soil Moisture ConstantMoisture Tension

in Atmosphere(in bar)

Wet soil Gravitational water Maximum water 0.00 (~ 0.001)

Moist soil Available water Field capacity 0.33 (1/3)

Water held in micro pores Wilting point 15

Dry soil Unavailable water tightly

held to the soil particlesHygroscopic coefficient 31

Air dry 1000

Oven dry 10,000

Table II : Moisture tension of soil moisture constants

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MOVEMENT OF SOIL-WATER

Saturated flow Unsaturated flow Water-vapour movement

The major principle of movement of soil-water is that it

is “along the gradient”.

•Wet soil to Dry Soil

•low soil moisture tension  to high SMT

•high soil water potential to low soil water potential

Page 14: R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration

Saturated flow• water move in the macropores since all of the pores are filled.

• Saturated flow is water flow caused by gravity’s pull.

• This water moves at water potentials larger than – 33 kPa.

• Factors affecting saturated flow :

1. Texture

2. Structure

3. Amount of organic matter

4. Temperature

5. Depth of soil to hard pan

6. Pressure

7. Amount of water in the soil

Saturated flow due to gravity

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Unsaturated flow

•Macropores full of air

•Micropores = water + air

•Moisture tension gradient creates unsaturated flow

•It is flow of water held with water potentials < -1/3 bar.

•Factors Affecting the Unsaturated Flow

i. Nature of soil

ii. Soil moisture content : The higher the percentage of water in the moist soil, the greater is the suction gradient and the more rapid is the delivery.

 

Distribution of pores

Size of pores

Unsaturated flow

Micropores

Macropores

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Water-vapour movement

Movement from high vapour pressure to a dry soil (low vapour pressure).

The movement of water vapour from soils takes place in two ways:

(a)Internal movement

(b)External movement

•Soil conditions affecting water vapour movement:

There are mainly two soil conditions that affect the water vapour movement.

i. Moisture regimes

ii. Thermal regime

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INFILTRATION (Hydrology)

Method of downward entry or

movement of water into the soil surface

Determines the part of the precipitation

that would become the surface runoff.

Infiltration is governed by two forces:

a.) Gravity

b.) capillary action

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Rate at which water enters the soil at the surface.

The rate of infiltration can be measured with Infiltrometer.

It is measured in inches per hour or millimeters per hour.

Potential infiltration rate - occurs when supply of water at surface is

not limited.

Cumulative infiltration rate - accumulated depth of water infiltrated

during a given period of time.

INFILTRATION RATE , f

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Fig. : Relation Between Rainfall Rate And Infiltration Capaciy

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Type of soil and its properties

- Porosity and hydraulic conductivity, soil-texture and

structure, soil-temperature

Moisture content of soil Condition of soil surface and its vegetative cover

rainfall intensity

FACTORS THAT INFLUENCE f

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TERMINOLOGY RELATED TO INFILTRATION PROCESS :

Percolation

Water intake

Overland flow

Interflow (or

Subsurface flow)

Seepage

Leaching

Permeability

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CONCLUSION Soil water is very essential for the proper plant growth and development.

Soil moisture constants are necessary to determine the moisture present in soil under any certain condition and at any instant of time.

Soil water movement is mainly of three types viz. saturated, unsaturated and water vapour movement.

Infiltration and other modes of water entry into the soil contributes to the formation of water reservoir in soil.

Higher the rainfall rate, lesser the infiltration and higher the runoff.

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