R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration
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Transcript of R 12013(ssc-411)-soil moisture constants,soil-water movement & infiltration
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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
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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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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
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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..
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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
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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
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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
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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”
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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
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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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