Water Movement In Soils

Summary

This educational video script explores the principles of water movement in soil, emphasizing unsaturated flow and the forces of adhesion and cohesion. It uses time-lapse photography to illustrate how water moves upward, downward, and horizontally in soil profiles, influenced by soil texture and structure. Demonstrations show how water interacts with different soil layers, such as sands and clays, and how these interactions affect plant growth and irrigation practices. The script also discusses the practical implications for agriculture, turf management, and soil construction, highlighting the importance of understanding water movement for effective soil management.

Takeaways

• 💧 Water movement in soil is influenced more by the attraction of solid surfaces and water molecules to each other than by gravity, especially in unsaturated conditions.
• 🌱 Capillary action, driven by adhesion and cohesion, allows water to move against gravity, which is crucial for water uptake in dry soils.
• 📏 The size and type of soil pores play a significant role in water movement, with larger pores in sandy soils allowing faster movement compared to smaller pores in clay soils.
• 🏗️ Soil layers with different textures, such as a layer of coarse sand or gravel, can act as a check valve, holding water until the overlying soil is very wet before allowing water to pass through.
• 🌿 The presence of coarse materials like sand and gravel in soil can increase its water retention capacity, which is beneficial for plant growth and irrigation practices.
• 🚧 Compacted or layered soils can restrict water movement and root penetration, impacting agricultural practices and the construction of soil profiles in areas like golf courses.
• 🌈 Time-lapse photography is used to visualize and accelerate the process of water movement in soil, allowing for the observation of patterns that would otherwise be too slow to see.
• 🌊 Water tables can rise above the land surface during wet seasons if layers like clay pans restrict the downward movement of water, affecting land use.
• 🌱 The uniformity of soil mixtures is important for proper water infiltration and storage, as recognized in specifications for golf course putting greens.
• 🌿 The principles of water movement in soil have practical applications in agriculture and land management, such as the construction of soil profiles and the use of vertical aeration channels to improve water infiltration.

Q & A

What are the primary forces responsible for water movement in dry soil?

The primary forces responsible for water movement in dry soil are the adhesive forces between the solid mineral surfaces and water, and the cohesive forces between water molecules. These forces are responsible for capillarity, which allows water to move upward against the force of gravity.

How does the glass plate model in the demonstration represent a soil profile?

The glass plate model represents a vertical cross-section of a soil profile. It is designed to show the movement of water in the soil by allowing visual observation as the soil is wetted. The model is made with glass plates a foot high and 2 ft wide with about 1/2 in of space between for soil.

What is the significance of the time-lapse photography used in the study?

Time-lapse photography is used to speed up the observation of water movement in the soil. In nature, these movements would take many hours, but with time-lapse photography, these processes are sped up to a few minutes, allowing for easier study and understanding of water movement dynamics.

How does the presence of a sand layer in the soil affect water movement?

A sand layer in the soil acts like a check valve, holding water back until the soil above becomes very wet. The large pores in the sand cannot hold water against the forces in the smaller pores of the finer soil above, so water does not readily move into the sand. However, when the soil above the sand becomes excessively wet, water eventually moves into the sand.

What is the role of cohesive forces in water movement in soil?

Cohesive forces between water molecules are responsible for creating a membrane-like surface that pulls water upward beneath it. This force, along with adhesion, allows water to move against gravity and is crucial for capillary action in soil.

Why do water tables often rise above the land surface during wet seasons in soil with a clay pan?

Water tables often rise above the land surface during wet seasons in soil with a clay pan because the clay layer restricts the downward movement of water. The fine pores in the clay layer are not capable of transmitting water quickly enough, leading to a buildup of water above the clay pan.

How does the presence of a layer of fine clay in soil affect plant rooting depth?

A layer of fine clay in soil can restrict the rooting depth of plants. The extremely fine pores in the clay layer compared to the overlying soil make it difficult for water to penetrate, thus limiting the downward growth of plant roots.

What is the practical application of understanding water movement in soils for golf course construction?

Understanding water movement in soils is crucial for golf course construction, especially for putting greens. It helps in designing soil profiles with layers that can retain water effectively and ensure proper water infiltration and drainage, which are essential for maintaining the health and playability of the greens.

How does the presence of organic material affect water infiltration in soil?

The presence of organic material can improve water infiltration in soil by creating small aggregates that are stabilized and have large pores remaining open to the surface. This allows water to be taken up readily, thus maintaining a high infiltration rate.

What is the significance of the soluble dye used in the demonstration?

The soluble dye is used to visualize the pattern of water movement behind the wetting front. It helps to demonstrate how water moves in the soil and how soluble materials, like fertilizers, can be transported with the water flow, which is important for understanding nutrient placement in agricultural and turf management.

Outlines

00:00

💧 Water Movement in Soil

This paragraph explains the principles governing water movement in soil, particularly in dry conditions. The movement is not solely influenced by gravity but also by the attraction of solid surfaces and the cohesive forces between water molecules. A time-lapse study is described, where soil is held between glass plates to visualize water movement. The study uses different soil types and simulates non-uniformities in soil profiles. Water is added to the soil surface, and the movement is captured through time-lapse photography, with speed factors ranging from 24 to 500 times normal. The paragraph also discusses the role of capillarity in water movement and how it's influenced by adhesion and cohesion.

05:01

🌱 Soil Water Interactions and Layered Soils

This section delves into how water interacts with different soil layers, such as sand and clay, and how these interactions affect plant growth and water retention. It describes an experiment where water moves into a sand layer, acting like a check valve, and how water tables are influenced by the presence of coarse materials. The paragraph also discusses the impact of clay layers on water penetration and root development, using examples from agricultural land and golf course construction. It highlights the importance of understanding soil profiles for effective water management in various settings.

10:08

🌿 Water Flow Through Aggregates and Soil Textures

This paragraph focuses on how water flows through different soil textures and aggregate layers. It explains that water movement is restricted by the size of the pores in the soil and how the presence of large pores in aggregates can slow down water transmission. The text describes experiments showing water movement in coarse sand and aggregate layers, emphasizing the importance of contact with free water for effective water flow. It also discusses the impact of soil moisture on water infiltration and the differences in water movement patterns in dry versus moist sand.

15:08

🌱 Soil Water Retention and Movement Patterns

This section discusses the patterns of water movement in soil and how they are influenced by soil texture and structure. It highlights the differences in water retention and infiltration rates between sandy and clayey soils, and how these properties affect agricultural practices and soil management. The paragraph also addresses the challenges of wetting mounds or hills of soil, the impact of soil compaction on water movement, and the benefits of rolling to improve soil structure. It provides practical applications of these principles in managing soil conditions for optimal water infiltration and plant growth.

20:32

💧 Principles of Water Flow in Soil

The final paragraph summarizes the principles of unsaturated water flow in soil, emphasizing the importance of the attraction between solid surfaces and water molecules. It discusses how water movement is influenced by the nature of the soil pores and changes in the porous system. The text also touches on the practical applications of these principles in agricultural land management, particularly in the context of golf course putting greens and turf grass maintenance. It concludes by reiterating the importance of understanding water flow dynamics for effective soil and water management.

Keywords

💡Capillarity

Capillarity refers to the ability of a liquid to flow in narrow spaces without the assistance of, or even against, external forces like gravity. In the context of the video, capillarity is a key principle explaining how water moves into dry soil. The script illustrates this with a demonstration where water is pulled upward from a dish into a ceramic rod due to the attraction of solid mineral surfaces for water (adhesion) and the attraction of water molecules for each other (cohesion). This principle is crucial for understanding how water can move against gravity in soil profiles.

💡Adhesion

Adhesion is the attraction between different types of molecules, specifically in this case, the attraction of solid mineral surfaces for water. The video uses the term to explain how water is drawn into the soil due to the soil's adhesive forces. An example from the script is the upward movement of water in a ceramic rod, which is pulled by the adhesive forces between the glass and water.

💡Cohesion

Cohesion is the attraction between similar types of molecules, such as water molecules. In the video, cohesive forces are responsible for moving water upward against the force of gravity. This is demonstrated when water rises between two closely spaced glass plates, forming a membrane-like surface due to the cohesive forces between water molecules.

💡Unsaturated Flow

Unsaturated flow refers to the movement of water through soil when not all the pores are filled with water. The video explains that the forces governing this type of water movement are primarily due to the attraction of solid surfaces for water, rather than gravity. An example given is the rapid outward movement of water from an irrigation channel, indicating that adhesive forces are more influential than gravity in unsaturated conditions.

💡Wetting Front

A wetting front is the boundary between wet and dry soil as water infiltrates into the soil. The video uses time-lapse photography to show how the wetting front moves into the soil, illustrating the dynamics of water infiltration. The script mentions how the wetting front encounters different soil layers, such as coarse sand, and how this affects the movement of water.

💡Soil Profile

A soil profile refers to a vertical cross-section of soil layers, which can vary in composition and texture. The video discusses how different soil profiles, such as those containing sand layers or clay pans, affect water movement and retention. The script provides examples of how water behaves differently when it encounters these layers, such as the sand layer acting like a check valve.

💡Infiltration

Infiltration is the process by which water enters the soil surface and moves downward. The video script describes how infiltration rates can be affected by soil texture and structure. It mentions how sandy loam allows for greater infiltration due to larger pores, while clay loam restricts water flow due to finer pores.

💡Gravitational Forces

Gravitational forces play a role in water movement, especially in saturated conditions where water is free to move downward due to the pull of gravity. The script contrasts the influence of gravitational forces with the adhesive and cohesive forces, noting that as soil becomes wetter, gravity's role in water movement increases.

💡Sand Layer

A sand layer in the soil profile acts as a check valve for water, holding it back until the overlying soil becomes very wet. The video explains that the large pores in sand cannot hold water against the forces in the smaller pores above, thus restricting water movement until the soil is excessively wet. This concept is illustrated with a model showing how water eventually moves into the sand layer when the soil above it becomes saturated.

💡Clay Pan

A clay pan is a layer of fine clay within the soil that restricts rooting depth and water penetration. The video script describes how a clay pan can cause water tables to rise above the land surface, imposing limitations on land use. It contrasts the behavior of water in clay pans with that in sandy loam, noting that while a clay pan hinders water movement, it can absorb water as the soil above it wets.

💡Irrigation

Irrigation is the artificial application of water to land or soil. The video discusses how irrigation practices can be influenced by the principles of water movement in soil. It mentions how understanding these principles can improve irrigation efficiency, such as in the construction of soil profiles in turfed areas like golf courses.

Highlights

Water movement in soil is influenced by factors other than gravity, including capillary action and adhesion.

Time-lapse photography is used to visualize water movement in soil models, accelerating processes that would take hours in nature.

Adhesive and cohesive forces are key in moving water upward against gravity.

Water moves laterally as well as downward in soil, demonstrating the complexity of water flow.

The soil model represents a vertical cross-section of a soil profile, simulating real-world conditions.

Gravitational forces become more significant as soil saturation increases.

The soil's texture and structure, such as coarse sand layers, affect how water moves and is retained.

Clay layers can restrict water movement and root depth, impacting plant growth and water tables.

Soil with uniform textures transmits and retains water differently than stratified soils.

The presence of coarse materials like sand and gravel can increase a soil's water retention capacity.

Water movement in soil is crucial for plant growth and is influenced by soil preparation and management.

The principles of water movement in soil are applied in the construction of soil profiles in turfed areas.

The USGA's specifications for putting green construction recognize the importance of soil uniformity.

Water movement in soil is affected by the presence of organic materials and their impact on soil structure.

Vertical aeration channels can improve water infiltration in compacted or thatched areas of soil.

The position of water channels relative to the water table is critical for effective drainage in sports fields and golf courses.

The principles of water flow in soil have practical applications in agriculture and turf management, including irrigation strategies.

Observing water movement in soil can provide insights into the effectiveness of soil management practices.

Transcripts

when water moves into air dry soil it is

only slightly affected by gravitation

water moves upward and horizontally as

well as

downward the principles governing water

movement are graphically shown in this

time-lapse study in these demonstrations

soil is held between glass plates so

that you can see what happens as the

soil is wetted the glass plates are a

foot high high and 2 ft wide with about

1/2 in of space between for

soil think of this model as representing

a vertical cross-section of a soil

profile time-lapse photographic

processes used here permit speeding up

the action in nature it would require

many hours for the water movement which

you will observe in the film in only a

few

minutes using a motion picture camera

single pictures are Tak of the models at

Short Time intervals as the water moves

into the soil the time intervals are

from 1 second up to 22 seconds the

completed Motion Picture film is then

projected so that speed up times in

these sequences range from 24 times

normal to as much as 500 times normal

speed the speed Factor will be observed

on a card above the

model because water movement usually is

rapid when water is first applied

and is very slow at later times the

speed up Factor often is changed during

the sequence this is indicated by

changing the arrow on the time card

which points to the appropriate speed

Factor the soil used is an air dry silt

Loom which has been passed through a

fine screen Sands Clays and Aggregates

made from the soil are used to simulate

non uniformities in the soil profile

water will be added in a Furrow or on

the soil surface with a device which

keeps the water level at any desired

depth before you see the sequence using

time-lapse photography here are two

demonstrations that illustrate the

principle of capillarity this principle

is involved when water moves into dry

materials liquid is pulled upward from

free water in the dish into this por

ceramic Rod because of the attraction of

solid mineral surfaces for water called

adhesion and attraction of water

molecules for each other called cohesion

adhesive and cohesive forces then are

responsible for moving water upward

against the downward force of gravity

in the second demonstration water rises

between two closely spaced glass plates

because of the adhesive forces between

glass and water and the cohesive forces

between water molecules cohesive forces

near the airwater interface create a

membranelike surface and water is pulled

upward beneath this surface the pressure

beneath this negatively curved surface

is negative the opposite of the internal

pressure of a raindrop which has a

positive curvature and is positive water

in pores with a negative airwater

interface may be said to be under

tension now to the models and time-lapse

pictures it may be observed here that

water moves outward from an irrigation

Channel almost as rapidly as downward

this this is evidence that the forces

responsible for this type of water

movement are mainly due not to

gravitation but to the attraction of

solid surfaces for

water as the soil becomes wetter and

wetter however gravitation plays a

stronger role and if the soil becomes

completely saturated then gravitational

forces

predominate the horizontal layer you see

is course sand one of the important

principles of UN saturated flow of water

is Illustrated here as the wedding front

encounters the coarse sand the pores in

the soil are many times smaller than

those between sand grains water is held

in these small pores by large adhesive

and cohesive forces the pores in the

soil are like the pores in a piece of

blotting paper used to soak up ink the

huge pores in the sand cannot hold water

against the forces in the smaller pores

above hence the water does not move

readily into the sand

however as the soil above the sand

becomes very wet the water eventually

moves into the sand in the same way as

ink would drip from blotting paper which

was excessively wet the sand layer thus

acts something like a check valve

holding the water back until the soil

becomes very wet and then letting the

excess water pass

through what happens to water in soil

containing a sand layer is typical in

principle to what happens to water in

soil situations where Sands and gravels

occur as layers in fine soil material

much agricultural land as well as land

in Turf and other vegetation is layered

in this fashion in Washington State's

Columbia Basin there exists a quarter

million Acres of soil composed of 1 to 2

feet of fine Sandy LOM overlying coar

Sands and

gravels the ability of this soil to

support plant growth is greatly affected

by the presence of coar Sands and

gravels

because of these coarse materials the

overlying soil can retain more than

double the amount of water usually held

in a fine Sandy LOM this soil is one of

the best in the

Basin these principles are usefully

employed in the construction of soil

profiles in turfed areas such as plane

fields and particularly on golf courses

recognition of this principle is evident

in the specifications for putting green

construction adopted by the green

section of the United States Golf

Association now in this sequence you see

a layer of fine clay in an otherwise

uniform soil this clay layer is similar

to a clay pan or any type of layer in

which the pores are extremely fine

compared to the pores in the overlying

soil these layers often restrict rooting

depth of plants and are particularly

known for the trouble they cause in

preventing downward penetration of water

when excess water is added to the soil

water tables often are buil up over such

layers if they occur at shallow depths

water tables often rise above the land

surface during wet Seasons imposing

serious limitations on the use of the

land despite the fact that a clay pan

hinders downward movement of water it

does absorb water readily as the soil

above is wetted observe the wetting

front as it moves into the clay Pan the

pores in the clay are much finer than

than those in the overlining soil so

that they can pull water from the soil

water tables are not built up over clay

pans because of inability of water to

enter them instead water tables result

from slow transmission of water through

them the resistance to water movement in

the extremely fine pores of layers like

these is sufficiently great that even

over periods of weeks and months little

water is transmitted through them into

the soil below where a soil profile may

be artificially created as in Golf

Course putting greens uniformity of soil

mixtures is an important consideration

as is recognized in USGA putting green

specifications the extent to which

downward movement is restricted and

water storage is altered depends on the

finess of the pores and the thickness of

the restricting layer this is in

contrast to what was shown earlier in

soil overlying coar sand layers there

downward movement of water was

temporarily checked but water tables

could not be filled up as long as the

opportunity existed for free drainage

into the course material

this model has the sand layer on the

left and the layer of course Aggregates

on the right these Aggregates are about

the same size as the sand grains but are

made up of soil particles like those of

surrounding soil the large pores between

Aggregates are about the size of the

large pores between sand grains water

movement in so materials which wet

readily depends upon procity each

individual aggregate contains numerous

fine pores of a size similar to the

pores in the surrounding soil

note that the small Aggregates in the

aggregate layer will wet up as soon as

the wetting front reaches them however

pores between Aggregates are too large

to pull water from the soil pores in the

finer soil hence the large pores remain

empty all of the water passing through

the layer must first move through the

fine pores of the Aggregates and then

across the contact points between

Aggregates the small number of contacts

between Aggregates therefore restricts

the rate at which water can move through

the aggregate

layer if free water is supplied directly

to a layer of coarse sand water rushes

in rapidly filling all of the pores

these are conditions of saturated flow

the moving force is due to positive

pressure from the water in the channel

under saturated conditions large pores

can transmit water readily but the rate

of transmission depends upon the

hydrostatic pressure of the water supply

the positive pressure is dissipated

rapidly over a very short distance in

the fine pores giving way to absorptive

forces in the drier soil thus water

moves out into the soil from a sand

layer under unsaturated conditions it is

pulled into and through the soil because

of the attraction for water of the

mineral surfaces making up the fine

pores of the soil the sand in the layer

at the left is the same kind of sand

through which water is Flowing at the

right here however the layer is not in

contact with free water or water under

positive pressure hence the surrounding

soil is wetted under unsaturated

conditions where the water is present

only under negative pressure

the sand layer cannot wet until the

water pressure in the films of the

adjoining soil becomes nearly positive

which means that the soil becomes very

wet as this happens the layer takes

water porous materials with very large

pores Aid in movement only under

conditions where they contact free water

that is water under zero or positive

pressure where water exists only under

negative pressure such material stops or

materially retards water flow for

a question frequently is raised what

differences in flow might be expected if

underlying Sands were moist rather than

air

dry here you're looking at dry soil

overlying dry sand on the left and moist

sand on the right since water movement

has so far been detected by observing

color change a different technique is

needed to help you see water entering

sand which already is moist a chemical

substance has been added to a white sand

which when contacted by added water

containing another chemical will turn

pink the water content of the sand on

the right is about the quantity which

would be present in a sand just barely

wet enough to support plant growth the

presence of some water in the sand

should make a difference in the tension

when added water enters the

sand it may be seen here that water

enters the dry sand at nearly the same

time as it enters the

moist there is a difference in the rate

and pattern of water penetration into

the sand in the two cases but in both

the water retained above the sand is

about the

same a fingering pattern develops in the

moist sand because it already is wet and

a few of the smaller channels can

readily transmit a little water thus

reducing the buildup of water above and

flow in ad joining larger channels under

many conditions in nature including

situations where irrigation is practiced

Sands and gravels lying below finer soil

materials are naturally

moist it is important to illustrate

differences among uniform soils with

respect to their ability to transmit and

retain water apart from problems of

stratification note that the depth of

penetration at any given time is

greatest for the Sandy LOM which has the

largest pores and at least for the clay

LOM which has the finest pores the finer

the pores the more the rate of water

flow is

restricted but after the water source is

removed the forces causing continued

water movement are greatest in the clay

LOM which has the finest pores and least

in the Sandy LOM with the larger pores

despite this however the net useful

storage is greatest in the clay LOM and

least in the Sandy

Loom although a Sandy LOM retains less

useful water than does the clay LOM it

is nevertheless a good soil in an

irrigated area where lack of water

holding capacity can be compensated by

irrigation the infiltration properties

are generally good Clay loms on the

other hand often are difficult to

irrigate because of low infiltration

rates in dry climates with no irrigation

a Sandy LOM would not hold enough water

to carry most plants through the growing

season a clay loone by contrast would

retain more water over a long period of

time the principles Illustrated here

have important application to situations

where control of soil materials and

profiles is possible as in a golf course

putting green such as that specified by

the

USGA Sandy soils are favored because

they resist compaction and their use is

permitted because frequent irrigation is

practical also the presence of coar sand

and gravel layers in deeper soil

increases is the water storage capacity

of such a

soil in the demonstration so far your

attention has been mostly focused on the

movement of the wedding front now with

the aid of a soluble dieye the pattern

of water movement back of the wedding

front appears any water soluble material

which is not strongly absorbed by the

clay particle will have the net movement

you see here

the die traces are not streamlines the

reason for this is that geometry of the

absorptive force field responsible for

water movement is changing the die

traces include the effect of any change

in direction of water flow due to the

development of a non-uniform

field such a non-uniform field is

produced as the two wedding fronts

join in the beginning the Dy moves

radially away from the Water Source this

continues as long as the movement of the

wedding front remains radial and uniform

soluble fertilizer material such as

nitrates moving with the water will

Trace out patterns like these the die

traces show the importance of proper

fertilizer placement with respect to the

position of the wedding stream in an

irrigation

fural now consider what will happen to

the the dice spots midway between the

two furrows when the two wedding fronts

come together note particularly the

middle dice spot at the same level as

the water in the furrows when the

wedding fronts join there is a radical

change in the pattern of water movement

water continues to move upward into the

dry Hill above and downward into dry

soil below that D Spot in the middle

shows little movement because water flow

Above This level is upward and below it

is downward and after the soil in the

hill is entirely wetted only evaporation

will cause further upward flow such

upward flow is of much practical

importance for example soluble

fertilizers and salts in the upward

moving stream will tend to accumulate in

the surface out of wrench of plant roots

such phenomena have been observed in

numerous placement

studies a further illustration of how

water moves in soil may be seen in this

cross-section through an 18in Mound or

Hill simulated an irrigation fur in

agriculture are numerous planting

situations as with soil preparation for

Turf loose soil thrown up into a mound

like this can be difficult to wet

because of the presence of excessively

large por spaces excessive elevation

differences Complicated by increasing

procity from the bottom of a mound to

the top can lead to poor wedding at the

upper level in potato culture loose soil

in Hills has been observed to to remain

dry during an entire growing season with

important reductions in yield quantity

and

quality compacting the mound with a

roller at planting time as you see it on

the left can help in two ways first the

elevation is greatly reduced second the

reduced prosity will help to increase

the rate of upward water movement

applied to soil conditions on a much

smaller scale the advantages of rolling

are firming a newly planted seed bed or

for that matter ly settling of an entire

surface of a new putting green are

evident

a practical application of principles of

water flow is shown here water moves

rapidly into soil with good tilt

propertiy practices on the soil in the

center have produced numerous small

Aggregates which have been stabilized by

decomposing organic material the

resulting large pores which remain open

all the way to the surface take water

readily thus the infiltration rate

remains High the same amount of organic

material when turned under in a layer

does little to improve soil tilt and if

anything makes conditions worse the

straw layer like a sand layer checks

downward flow of water in this case not

only does less rainfall penetrate into

the root Zone but more water remains on

the surface making it vulnerable to

damage from foot and vehicle traffic and

the impact of falling

rain on the right an irregular Channel

filled with coarse sand simulates an

open Channel left by mechanical aeration

or perhaps a channel left by decayed

roots or burrowing insects or worms such

channels or cracks do not assist in

water movement when they are not open to

a source of free water they aid water

flow only when they connect with free

water at the surface the principle

involved also applies to tile drains

water can move into such drains only if

positive water pressure exists in the

surrounding soil hence tile placed in

wet soil for drainage must be located

below the water table if they are to be

effective on athletic fields or on Golf

Course putting greens where a soil

profile is artificially created with a

gravel layer and tile lines for drainage

tiles must be located below the gravel

layer

water moves rapidly into the soil

through a vertical ation Channel filled

with sand or coarse organic materials

but this is true only if the channel

extends all of the way to the surface

use of such vertical aration channels

has particular application on Golf

Course putting greens or tea surfaces it

is of great benefit on compacted or

thatched areas where infiltration rates

at the surface are extremely limited a

vertical channel is cut into the soil

with an aerifier a top dressing sand of

a desirable particle size is then worked

into the open ation holes until they are

filled to the surface this makes rapid

infiltration possible permitting water

to quickly reachen and move into the

underlying dry soil thus reducing

surface

runoff since the water in the

surrounding soil is under negative

pressure none has entered the channel on

the right this important point to

remember here is that the channel or air

fire hole must remain in contact with

free water or water under positive

pressure to do any good and if the top

of the channel is covered over or

becomes plugged for instance by the

application of a fine silt top dressing

material water flow into the channel is

restricted even though the soil might be

saturated

these demonstrations amplify the

principles of water flow under

unsaturated conditions conditions under

which crops are grown on agricultural

land and particularly where grass is

grown and managed especially on golf

courses each demonstration has its

counterpart in

nature except possibly this last

one in nature the demonstrations may be

less dramatic but the principles hold

and can be seen in operation if one

observes carefully in summary then

unsaturated flow of water in soil and

other porest materials takes place

because of the attraction of solid

surfaces for water and of water

molecules for each other how the water

moves depends upon the nature of the

pores and changes in the porous system