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

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Prepared by :KritikaID no. : R-12013Enroll. no. : 351189B.Sc. (Ag.) 4th year, Ist Sem.Guided by :Dr. Ajai Kumar SinghSSC-411,4(0+4) : Village AttachmentDEPARTMENT OF HORTICULTURE,INSTITUTE OF AGRICULTURAL SCIENCES,RAJIV GANDHI SOUTH CAMPUS,BANARAS HINDU UNIVERSITY,BARKACHHA,MIRZAPUR

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 waterCapillary waterHygroscopic waterPhysical classificationBiological classificationAvailable waterUnavailable waterSuper available water(Drainage water)

SOIL MOISTURE CONSTANTS Varying degrees of SOIL-WETNESSWater contents present in soil under certain standard conditionsrepresents definite soil moisture relationship and retention of soil moisture in the field. The soil moisture tension is measured with TENSIOMETERSoil Water ContentSoil Moisture Content

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 capacityField capacity Maximum Capillary capacityMoisture equivalentsPermanent wilting pointHygroscopic coefficientAvailable soil-moistureAir capacityTotal pore volumeContinued..

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

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 pointFACTORS AFFECTING PWP Soil propertiesPlant propertiesSoil texture &structureTypes of clayOrganic matter content

SATURATIONFIELD CAPACITYWILTING POINT100% 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

Plant available waterGravitational water

(Drainable)10-30micronOven-dryAir-dryWilting pointField capacitySaturation

Unavailable water~0.2micronHygroscopic coefficient-10000 bar-1000 bar-31 bar-15 bar-0.33bar-0.00 barCapillary waterHygroscopic waterIncreasing soil water constantsIncreasing water potential*-1000 bar to -300 bar depending on humidityFig. : Diagrammatic representation of soil moisture constants, soil water classifications & their potentials

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

Appearance of soilType of SoilSoil Moisture ConstantMoisture Tensionin Atmosphere(in bar) Wet soil Gravitational waterMaximum water0.00 (~ 0.001) Moist soil Available waterField capacity0.33 (1/3) Water held in micro poresWilting point15 Dry soil Unavailable water tightly held to the soil particlesHygroscopic coefficient31Air dry1000Oven dry10,000

Table II : Moisture tension of soil moisture constants

MOVEMENT OF SOIL-WATERSaturated flowUnsaturated flowWater-vapour movementThe major principle of movement of soil-water is that it is along the gradient. Wet soilto Dry Soillow soil moisture tension to high SMThigh soil water potential tolow soil water potential

Saturated flowwater move in the macropores since all of the pores are filled.Saturated flow is water flow caused by gravitys pull.This water moves at water potentials larger than 33 kPa.Factors affecting saturated flow :Texture StructureAmount of organic matter Temperature Depth of soil to hard pan PressureAmount of water in the soil

Saturated flow due to gravity

Unsaturated flowMacropores full of airMicropores = water + airMoisture tension gradient creates unsaturated flowIt is flow of water held with water potentials < -1/3 bar.

Factors Affecting the Unsaturated Flow

Nature of soil

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 poresSize of pores

Unsaturated flow Micropores Macropores

Water-vapour movementMovement 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 movementSoil conditions affecting water vapour movement:There are mainly two soil conditions that affect the water vapour movement. Moisture regimes Thermal regime

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

Rate at which water enters the soil at the surface.The rate of infiltration can be measured withInfiltrometer.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

Fig. : Relation Between Rainfall Rate And Infiltration Capaciy

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 intensityFACTORS THAT INFLUENCE f

TERMINOLOGY RELATED TO INFILTRATION PROCESS : Percolation Water intake Overland flow Interflow (or Subsurface flow) Seepage LeachingPermeability

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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