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SOIL WATER- SATURATED AND UNSATURATED FLOWNAMITHA M R2015664502M. TECH (LWME)TNAUSAC 604. SOIL PHYSICS (2+1)

SOIL WATERSoilscontain considerable amounts ofwater.Soil can take in water, and will keep doing so until the rate at which they can transmit water into and through the pores is exceeded.Much of the water will beretained, away from the influence of gravity, for use ofplantsand other organismsto contribute to land productivity andsoil health

Soil water retentionPores provide for the passage and/or retention ofgasesandmoisturewithin thesoil profile.Soil water retention depends on:Particle sizeClay typeOrganic contentSoil structure

Types of soil water3 basic types:Gravitational water --- -1/3 barCapillary water --- -1/3 to -31 barsHygroscopic water --- -10,000 bars

ContdGravitational water:Moves through the soil due to the force of gravity.Found in the macropores.Not considered to be available to plants.Drains out of the soil in 2-3 days

ContdCapillary water:Found in the micropores.Most, but not all, of this water is available for plant growthHeld in the soil against the pull of gravityCapillary water is held by cohesion and adhesionAmount of water held is a function of the pore size and pore space.Tension (in bars) increases as the soil dries out.

ContdHygroscopic water:Forms very thin films around soil particles.Not available to the plant. Not held in the pores, but on the particle surface. Clay will contain much more of this type of water than sands because of surface area differences.Held very tightly, by forces of adhesion this water is not available to the plant.

Soil moisture constantsSoil moisture is always being subjected to pressure gradients and vapour pressure differences that cause it to move.Certain moisture contents are of particular significance in agriculture --- Soil moisture constants.

ContdSaturation capacity:When all the pores of the soil are filled with water, the soil is said to be under saturation capacity or maximum water holding capacity. Tension of water at saturation capacity is almost zero and it is equal to free water surface.

ContdField capacity:The field capacity is the upper limit of available moisture range in soil moisture and plant relations.Large soil pores are filled with air, the micro pores are filled with water.Soil moisture tension at field capacity varies from soil to soil, but it generally ranges from 1/10 to 1/3 atmospheres.

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ContdMoisture equivalent:Moisture equivalent is defined as the amount of water retained by a sample of initially saturated soil material after being subjected to a centrifugal force of 1000 times that of gravity for a definite period of time, usually half an hour.

ContdPermanent wilting percentage: Also known as permanent wilting point or wilting co-efficientSoil moisture content at which plants can no longer obtain enough moisture to meet transpiration requirements; and remain wilted unless water is added to the soil.Moisture tension of a soil at the permanent wilting point ranges from 7 to 32 atmospheres.

ContdWilting range:Range in soil-moisture content through which plants undergo progressive degrees of permanent or irreversible wilting, from wilting of the oldest leaves to complete wilting of all leaves.Moisture content at which the wilting is complete and the plants die is called the ultimate wilting.At the ultimate wilting point soil-moisture tension may be as high as 60 atmospheres.

ContdAvailable water:Soil moisture between field capacity and permanent wilting point It is the moisture available for plant use.Fine-textured soils have a wide range of water between field capacity and permanent wilting point than coarse textured soils.

Fig: Soil moisture constants

Table: Range of available water holding capacity of soils

Fig: Response of water to different soils

Movement of water within soilsMoves along gradients of decreasing water potential.Water in the liquid phase: flows through the water filled pore space under the influence of gravity.moves under the influence of surface tension forces (under unsaturated conditions).Water in vapour phase:moves through the air-filled pore spaces along gradients of decreasing vapour pressure.

ContdFlow rate of water depends on:Gradients in soil water potential (m) caused by differences in height, pressure, dissolved solutes and soil wetness.Hindrances: friction between water and particle surfaces as well as pore constrictions and other interruptions in flow path

ContdIt is generally recognized that three types of water movement occur in soil:SaturatedUnsaturatedVapour

SATURATED FLOW OF SOIL WATERSaturated flow occurs when soil pores are completely filled with water (i.e.m> 33 J/Kg).Occur in aquifers (water - bearing sediments and rock layers), in flooded soil and in lower horizons of soil with limited drainage Pore size is of outstanding significance, as its fourth power is proportional to the rate of saturated flow

ContdSaturated flow decreases as the pore size decreases. Generally the rate of flow in soils of various textures is in the following sequence.Sand > loam > clayPoiseuilles law forms the basis.

UNSATURATED FLOW OF SOIL WATERWater flow when larger pores in soil are filled with air is said to be unsaturated.Smaller pores hold and transmit water. Contribution of hydraulic head or the gravitational component to total potential becomes progressively lessNegligible effect of solute potential is due to the fact that both solutes and water are moving.

ContdDriving force for water flow in these conditions is through the matric potential gradient (i.e.m)Movement will therefore be in the direction of moist soil to dry soil and from thick moisture- to thin moisture-filmsIn horizontal movement, only m applies.In downward movement, capillary and gravitational potentials act together.

Contd

ContdAs the soil moisture content and soil moisture potential decreases, the k decreases rapidly, so that soil is 15 bars, k is only 10-3 of the value at saturation. Rapid decrease in conductivity occurs because the larger pores are emptied first, which greatly decreases the cross-section available for liquid flow.

ContdUnsaturated conductivity is a function of soil moisture content , number, size and continuity of soil poresWhen the continuity of the films is broken, liquid flow no longer occurs.Movement of unsaturated flow ceases in sand at a lower tension than in fine textured soils, as the water films lose continuity sooner between the larger particles.Wetter the soil, the greater is the conductivity for water

ContdIn the moist range, the range of unsaturated flow in soils of various textures is in the following order:Sand < loam < clayIn the wet range the unsaturated conductivity occurs in the same or similar order as saturated conductivity

WATER VAPOUR MOVEMENTVapour transfer increases as void space increases.At a soil moisture potential of about-15 bars, the continuity of the liquid films is broken and water moves only in the form of vapour.Vapour pressure gradient acts as the driving force.Vapour pressure of soil moisture increases with the increase in soil moisture content and temperature, it decreases with the increase in soluble salt content.

ContdWater vapour movement is significant only in the moist range.Rate of diffusion of water vapour through the soil is proportional to the square of the effective porosity, regardless of pore sizes.Finer the soil pores, the higher is the moisture tension under which maximum water vapour movement occurs

ContdIn a coarse textured soil pores become free of liquid water at relatively low tensions and when the soil dries out there is little moisture left for vapour transferFine textured soil retains substantial amounts of moisture even at high tensions, thus permitting vapour transfer

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