Soil Water Relationships: Soil Water Relationships: Permeability, Drainability Permeability, Drainability

and Hydraulic Conductivityand Hydraulic Conductivity

ObjectivesObjectives

• Define water holding capacity and Define water holding capacity and gravitational water.gravitational water.

• Explain how water holding capacity Explain how water holding capacity and permeability is affected by soil and permeability is affected by soil type.type.

• DefineDefine permeabilitypermeability..• DescribeDescribe thethe effectseffects ofof soilsoil typetype onon

permeability.permeability.• EvaluateEvaluate thethe drainabilitydrainability ofof soils.soils.

ObjectivesObjectives

• Define the forces that act on soil Define the forces that act on soil water.water.

• Discuss how water moves through Discuss how water moves through the soil.the soil.

• Measure the flow of water through Measure the flow of water through different soil textures.different soil textures.

TermsTerms

• AdhesionAdhesion

• Capillary RiseCapillary Rise

• CohesionCohesion

• DrainageDrainage

• Gravitational Gravitational FlowFlow

• Gravitational Gravitational WaterWater

• Hydraulic Hydraulic ConductivityConductivity

• PermeabilityPermeability

• SaturationSaturation

• Water Holding Water Holding CapacityCapacity

What is water holding What is water holding capacity and what is capacity and what is gravitational water?gravitational water?

- - Water holding capacity Water holding capacity is the is the amount of moisture that is held by amount of moisture that is held by the soil available to the plants.the soil available to the plants.

- - Gravitational water Gravitational water is free water is free water that moves downward through the that moves downward through the soil due to gravity.soil due to gravity.

• Soils have different levels of water Soils have different levels of water holding capacity largely determined holding capacity largely determined by the texture of the soil.by the texture of the soil.

• Soil Texture is the relative amount of Soil Texture is the relative amount of sand, silt and clay in the soil.sand, silt and clay in the soil.

• Hydraulic conductivity Hydraulic conductivity is a is a measure of the rate of water measure of the rate of water movement through the soil.movement through the soil.

• The soil texture also affects pore The soil texture also affects pore space, a portion of the soil not space, a portion of the soil not occupied by solid material but filled occupied by solid material but filled with air or water.with air or water.

• The larger the soil particle the larger The larger the soil particle the larger the pore spaces. the pore spaces.

• Small particles have more spaces but Small particles have more spaces but they are smaller in size.they are smaller in size.

How is water holding How is water holding capacity affected by soil capacity affected by soil type?type?

• A. Soils of different types affect the A. Soils of different types affect the water holding capacity.water holding capacity.- 1.Soils with fine texture are 1.Soils with fine texture are

composed mainly of clay and hold composed mainly of clay and hold water very firmly. There is less water very firmly. There is less gravitational water with fine gravitational water with fine textures.textures.

• B. Soils with medium texture have B. Soils with medium texture have intermediate amounts of sand and intermediate amounts of sand and clay, with a large representation of clay, with a large representation of silt.silt.– 1. Medium soils hold water well but 1. Medium soils hold water well but

still allow movement downward.still allow movement downward.

• C. Soils with coarse texture represent C. Soils with coarse texture represent those soils that are largely composed those soils that are largely composed of sand.of sand.– 1. Coarse soils will not hold water 1. Coarse soils will not hold water

very firmly and have fast very firmly and have fast permeability.permeability.

• WaterWater holding capacity is the amount holding capacity is the amount of moisture able to be held by the of moisture able to be held by the soil that is available to plants.soil that is available to plants.

• Gravitational water is the free water Gravitational water is the free water that moves downward through the that moves downward through the soil.soil.

• Both affect the workability and Both affect the workability and moisture availability to plantsmoisture availability to plants

What is permeability?What is permeability?

• Permeability Permeability is the movement of is the movement of water through the soil, synonymous water through the soil, synonymous with hydraulic conductivity and with hydraulic conductivity and drainability.drainability.

• PermeabilityPermeability allows movement of allows movement of water and in many cases nutrients water and in many cases nutrients through the soil.through the soil.

• The movement of the nutrients The movement of the nutrients through the soil can be significant if through the soil can be significant if the soil has much permeability.the soil has much permeability.

SOIL PERMEABILITY RATESSOIL PERMEABILITY RATES

How is permeability How is permeability affected by soil type?affected by soil type?

• Soil type can affect permeability.Soil type can affect permeability.• Medium texture allows for moderate Medium texture allows for moderate

permeability.permeability.• Coarse soils have rapid permeability.Coarse soils have rapid permeability.

• Fine soils have slow permeability and Fine soils have slow permeability and do not drain well.do not drain well.

• Drainage Drainage is the movement of is the movement of excess water through the soil, excess water through the soil, removal of water from the soil.removal of water from the soil.

How can you evaluate How can you evaluate the drainability of soils?the drainability of soils?

• Soil drainability is measured by the Soil drainability is measured by the speed of which a given amount of speed of which a given amount of water passes through the soil.water passes through the soil.

• In a laboratory situation one may In a laboratory situation one may take two different soils and set up an take two different soils and set up an experiment to measure the experiment to measure the drainability of each.drainability of each.

• On field conditions the drainability On field conditions the drainability may be done by measuring the time may be done by measuring the time it takes a soil to absorb 10 gallons of it takes a soil to absorb 10 gallons of water as compared to another soil. water as compared to another soil. – This procedure is done often in This procedure is done often in

landscaping situations to landscaping situations to determine if amendments need to determine if amendments need to be added. Soil in the field is be added. Soil in the field is difficult to amend.difficult to amend.

Texture classificationTexture classification

Textural TriangleTextural Triangle

What forces act on soil What forces act on soil water?water?• There are two main forces which act There are two main forces which act

on soil that relates to water.on soil that relates to water.•Adhesion Adhesion is an attractive force is an attractive force

that acts between two different that acts between two different substances. In soil water it is the substances. In soil water it is the attraction of soil to water.attraction of soil to water.

•Cohesion Cohesion is a force that acts is a force that acts between molecules of similar between molecules of similar substances. In the soil it is the substances. In the soil it is the attraction of water to itself.attraction of water to itself.

• The balance of these forces The balance of these forces determine the availability of water to determine the availability of water to plants and how water moves through plants and how water moves through the soil.the soil.

How does water move How does water move through the soil?through the soil?

• Water moves through the soil in Water moves through the soil in many ways.many ways.

• Soil pores fill with water as water Soil pores fill with water as water enters the earth’s crust. Water enters the earth’s crust. Water moves through the soil both by moves through the soil both by capillary action and by gravity.capillary action and by gravity.

• Capillary rise Capillary rise is the movement of is the movement of water through soil capillaries. Three water through soil capillaries. Three soil water types determine this soil water types determine this movement of water into the soil.movement of water into the soil.

• Capillary water is water that is held Capillary water is water that is held loosely by cohesive forces and is loosely by cohesive forces and is most readily available for plants.most readily available for plants.

• Hygroscopic water is held very firmly Hygroscopic water is held very firmly to the soil by adhesion.to the soil by adhesion.

• Gravitational water is water that is Gravitational water is water that is pulled downward through a process pulled downward through a process called called gravitational flowgravitational flow..

• Hydraulic conductivity Hydraulic conductivity is a trait of is a trait of soil that relates to the ease of water soil that relates to the ease of water movement through the soil. The movement through the soil. The higher the hydraulic conductivity the higher the hydraulic conductivity the more water that is moved through more water that is moved through the soil.the soil.

• Eventually all of the pores in the soil Eventually all of the pores in the soil are filled with water, which is are filled with water, which is saturationsaturation. The soil does not . The soil does not contain any air so plant growth is contain any air so plant growth is compromised.compromised.

• CapillaryCapillary water is water that moves water is water that moves freely through the soil.freely through the soil.

• Gravitational water is water that Gravitational water is water that leaves the soil caused by the forces leaves the soil caused by the forces of gravity.of gravity.

• The movement of water through the The movement of water through the soil can be measured in several soil can be measured in several ways.ways.

How can you measure How can you measure water through the soil?water through the soil?

• The movement of water through the The movement of water through the soil can be measured in several ways.soil can be measured in several ways.

• One method of measuring water One method of measuring water movement is to measure the capillary movement is to measure the capillary action through a test tube full of soil.action through a test tube full of soil.

• Other methods include measuring the Other methods include measuring the time it takes for soil to move through time it takes for soil to move through or by measuring permeability.or by measuring permeability.

Forces acting on soil and Forces acting on soil and waterwater• Two main forces act on soil and water.Two main forces act on soil and water.

– Adhesion is the force that acts upon two Adhesion is the force that acts upon two different substances, soil to water.different substances, soil to water.

– Cohesion is the force that that acts upon Cohesion is the force that that acts upon two like substances, water to water.two like substances, water to water.

• Both forces affect how water moves Both forces affect how water moves through the soil.through the soil.

Water movement through Water movement through soilsoil• There are three main types of soil water:There are three main types of soil water:

– Hygroscopic water is water held very Hygroscopic water is water held very tightly to the soil particle.tightly to the soil particle.

– Capillary water is water that moves Capillary water is water that moves freely through the soil.freely through the soil.

– Gravitational water is water that Gravitational water is water that leaves the soil caused by the forces of leaves the soil caused by the forces of gravity.gravity.

Soil Permeability RatesSoil Permeability Rates