Lecture 3: Soil Water Relationships

Prepared by

Husam Al-Najar

The Islamic University of Gaza- Civil Engineering Department

Irrigation and Drainage- ECIV 5327

Soil Properties

Texture: The relative size distribution of the mineral soil particles

US Department of Agriculture (USDA) classifications:

Sand: 0.05 – 2.0 mm

Silt: 0.002 - 0.05 mm

Clay: <0.002 mm

Textural triangle: USDA Textural Classes

Coarse vs. Fine, Light vs. Heavy Affects water movement and storage

Structure: how soil particles are grouped or arranged

Affects root penetration and water intake and movement

SAND COMPONENT

• Visible to the Naked Eye and Vary in Size.

• They are Gritty when rubbed between Fingers.

• Sand Particles do not Adhere to one another and are therefore not Sticky.

SILT AND CLAY COMPONENTS

Silt Particles are smaller than sand. The silt particles are toosmall to be seen without a microscope. It feels smooth but notsticky, even when wet.

Clays are the smallest class of mineral particles. They adheretogether to form a sticky mass when wet and form hard clodswhen dry.

SOIL TEXTURE

• Relative proportions of the various soil separates (sand, silt and clay) in a

soil.

• Terms such as sandy loam, silty clay, and clay loam are used to identify

soil texture.

• Soil Components are separated using Mechanical Analysis, Sieving for

Sand and Rate of Settling in Pipette for Silt and Clay.

• From the mechanical analysis, the proportions of sand, silt and clay are

obtained.

• The actual soil texture is determined using the Soil Textural Triangle e.g. for

a Soil with 50% sand, 20% silt and 30% clay, the texture is Sandy Clay Loam.

• Arranged in the increasing order of heaviness, there are 12 soil textures

namely: sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay

loam, silty clay loam, clay loam, sandy clay, silty clay and clay.

USDA Textural Triangle

COLLOIDAL MATERIAL

• The smaller particles (< 0.001 mm) of clay and similarsized organic particles) have colloidal properties and canbe seen with an electronic microscope.

• The colloidal particles have a very large area per unitweight so there are enough surface charges to whichwater and ions can be attracted. These charges makethem adhere together. Humus improves the water holdingcapacity of the soil.

soil structure affects root growth

Improved

infiltrationLow

infiltration

Bulk Density

Particle volume Pore volume

Bulk Soil = Particle volume + pore volume

Bulk density = dry weight (kg) / soil volume (m3)

Between 800 and 1500 kg/m3

Bulk Soil

Dry Bulk Density (b): Typical values 1.1 - 1.6 g/cm3

The mass of oven- dry soil (105oC during 24 hours)

b = soil bulk density, g/cm3

Ms = mass of dry soil, g

Vb = volume of soil sample, cm3

Particle Density (p): Typical values: 2.6 - 2.7 g/cm3

The density of solid material

P = soil particle density, g/cm3

Ms = mass of dry soil, g

Vs = volume of solids, cm3

b

sb

V

M

p

s

s

M

V

Porosity (): The fraction of the volume of the soil occupied by the

pores: Typical values: 30 - 60%

volume of pores

volume of soil

1 100%

b

p

• The water table, is the level in the soil where the pressure equals the

atmospheric pressure.

• The region above the water table is called unsaturated zone, although

just above the water table the soil may still saturated (capillary

fringes)

• Water in the unsaturated zone is termed soil moisture, while

groundwater usually refers to water below the water table.

• Important features of the unsaturated zone are:

1. Most crops require an unsaturated medium for growth

2. Reservoir for water for the crop during dry periods

3. Improvement of water quality

Soil Moisture

Saturated- Unsaturated

Saturated- Unsaturated system

Water retention force:

The force which retains moisture in the soil

against gravity (Capillary force).

The lifting force of water in the capillary tube

is the vertical component of the surface

tension F2 = δ Cos α, acting on the internal

circumstances of the capillary (2 π r).

Equating the lifting force to the downward

force (the weight of the water column) gives:

2 π r δ Cos α = π r2 h p g

The contact angle α between water and wall

of capillary tends towards zero (Cos α =1), so

that under equilibrium (no flow) conditions.

h = 2 δ / rpg where,

h = height of capillary rise (m), r = radius of capillary (m), p = density of water (1000),

g = acceleration due to gravity (9.81 m.s-2), δ = 0.075

Capillary rise in relation to the radius of the capillary tube

rh

410

15.0

Schematization of a clayey and a

sandy soil into a bundle of capillary

tube

Θ is The Moisture content

Soil Water Potential

• Description

– Measure of the energy status of the soil water

– Important because it reflects how hard plants must work to

extract water

– Units of measure are normally bars or atmospheres

– Soil water potentials are negative pressures (tension or

suction)

– Water flows from a higher (less negative) potential to a lower

(more negative) potential

Components

– t = total soil water potential

– g = gravitational potential (force of gravity pulling on the water)

– m = matric potential (force placed on the water by the soil matrix – soil water “tension”)

– o = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root)

– Matric potential, m, normally has the greatest effect on release of water from soil to plants

t g m o

Soil Water Potential

Water in Soils

Soil water content

Mass water content (m)

m = mass water content (fraction)

Mw = mass of water evaporated, g (24 hours at 105oC)

Ms = mass of dry soil, g

s

wm

M

M

Volumetric water content (v)

V = volumetric water content (fraction)

Vw = volume of water

Vb = volume of soil sample

At saturation, V =

V = As m

As = apparent soil specific gravity = b/w (w = density of water = 1 g/cm3)

As = b numerically when units of g/cm3 are used

m = mass water content (fraction)

v

w

b

V

V

Equivalent depth of water (d)

– d = volume of water per unit land area = (v A L) / A = v L

– d = equivalent depth of water in a soil layer

– L = depth (thickness) of the soil layer

Volumetric Water Content & Equivalent Depth

Equivalent

Depth

Volumetric Water Content & Equivalent Depth

Typical Values for Agricultural Soils

12.5 mm.

3.75 mm

5.0 mm

3.75 mm

Soil Solids (Particles): 50%

To

tal P

ore

Sp

ace:

50%

Very Large Pores: 15% (Gravitational Water)

Medium-sized Pores: 20% (Plant Available Water)

Very Small Pores: 15% (Unavailable Water)

Water-Holding Capacity of Soil

Effect of Soil Texture

Coarse Sand Silty Clay Loam

Gravitational Water

Water Holding Capacity

Available Water

Unavailable Water

Dry Soil

Field Capacity (FC or fc)

Soil water content where gravity drainage becomes negligible

Soil is not saturated but still a very wet condition

Traditionally defined as the water content corresponding to a soil water potential (SWP) of -1/10 to -1/3 bar

Permanent Wilting Point (WP or wp)

Soil water content beyond which plants cannot recover from water stress (dead)

Still some water in the soil but not enough to be of use to plants

Traditionally defined as the water content corresponding to -15 bars of SWP

Soil water potential curves

Available Water: Water held in the soil between field capacity and

permanent wilting point

“Available” for plant use

Available Water Capacity (AWC)= Field capacity - Permanent Wilting Point

AWC = fc - wp

Units: depth of available water per unit depth of soil,

“unitless” (in/in, or mm/mm)

Measured using field or laboratory methods

Soil Hydraulic Properties and Soil Texture

ggfhththtyj

…….

• Fraction available water depleted (fd)

(fc - v) = soil water deficit (SWD)

v = current soil volumetric water content

• Fraction available water remaining (fr)

(v - wp) = soil water balance (SWB)

wpfc

vfcdf

wpfc

wpvrf

• Total Available Water (TAW)

TAW = (AWC) (Rd)

– TAW = total available water capacity within the plant

root zone, (inches or centimeters)

– AWC = available water capacity of the soil,

(inches of H2O/inch or centimeter of soil)

– Rd = depth of the plant root zone, (inches or centimeter)

– If different soil layers have different AWC’s, need to

sum up the layer-by-layer TAW’s

TAW = (AWC1) (L1) + (AWC2) (L2) + . . . (AWCN) (LN)

- L = thickness of soil layer, (inches or centimeter)

- 1, 2, N: subscripts represent each successive soil layer

Example: A farm has a total area 1000 m2. The 30 cm layer is a clay loam texture

and the actual water content is 5% by weight. Find the required amount of water to

increases the water to the level of available water.

Actual field status:

Total soil mass = 1000 m2 X 0.3m X 1200 kg/m3 = 360,000 kg

Actual water content = 360,000 X 0.05 = 18,000 kg

The soil layer of 30 cm contains 18,000 kg water = 18,000 Litter = 18 m3

Level of available water

From the table AWC for clay loam 0.15 m/m

For 0.3 cm = 0.3 X 0.15 = 0.045 m

Water volume for the field (1000 m2) = 0.045 m X 1000 m2 = 45 m3

The required amount of water = 45 -18 = 27 m3

Water Infiltration: The entry of water into the soil

Influencing Factors:

• Soil texture

• Initial soil water content

• Surface sealing (structure, etc.)

• Soil cracking

• Tillage practices

• Method of application (e.g., Basin vs. Furrow)

• Water temperature

Cumulative Infiltration Depth vs. Time

For Different Soil Textures

Classification & Characteristics of Different Soil Types in Gaza Strip.

(Goris and Samain, 2001, Khalaf, 2005).

Local Classification TextureInfiltration rate

( mm / hr)

Loess soil Sandy loam (sand 58%, silt 34%, clay 6%) 404.5

Dark brown / reddish

brown

Sandy clay loam

(25% clay, 13% silt, 62% sand)963.42

Sandy loess soilSandy clay loam

(23% clay, 21% silt, 56% sand)258.66

Loessial sandy soil

The top layer is sandy loam (14% clay, 20%

silt, 66% sand). The lower profile is loam

(21% clay, 30% silt, 49% sand)

471.48

Sandy loess soil over

loess

Sandy loam (17.5% clay, 16.5% silt, 66%

sand)337.6

Sandy regosol

Top layer is loamy sand (9% clay, 4% silt,

87% sand). Deeper profile is sand (7.5% clay,

0% silt, 92.5% sand)

1079

Water Infiltration Rates and Soil Texture

Soil Map of Gaza Strip

Source, Ministry of Planning, 1998

Soil Infiltration Rate vs. Constant Irrigation Application Rate

Methods of Measuring Soil Water Content

• i) By Feel: This is by far the easiest method. Assessmentby feel is good for experienced people who have sort ofcalibrated their hands. The type of soil is important.

• ii) Gravimetric Method: This is equal to:

wm

s

M Mass of WaterP

M Mass of Dry Solids

• Weigh wet soil in a container, put in oven at 105 oC for about 48

hours; weigh again and obtain the weight of water by subtraction.

A good soil should have moisture contents between 5 and 60%

and for peat or organic soils, it can be greater than 100%.

Methods of Measuring Soil Water Content Contd.

(iii) Volumetric water content, Pv. This is equal to:

• Recall that volume = mass/density i.e.

SampleSoildUndisturbeofVolumeTotalWaterofVolume

wVaVsVwV

vP

soiltheofdensitybulktheisb

Dwhereb

DxmPvP

wDceb

DxsMwM

vPand

bD

sMwD

wM

vP

1sin

Methods of Measuring Soil Water Content Contd.

• (iv) Neutron Probe: It consists of a probe lowered down a hole

in the soil.

• A box (rate meter or rate scalar) is at the top.

• Within the probe is a radioactive source e.g. beryllium (435 years

life span).

• Close to the source is a detector.

• The source emits fast neutrons, some of which are slowed down

when they collide with water molecules (due to hydrogen

molecules).

• A cloud of slow neutrons (thermal neutrons) build up near the

probe and are registered by the rate meter or rate scalar which

measures the number of slowed down neutrons.

The method is quick but very expensive.

It is also dangerous since it is radioactive and must be used with care.

Diagram of Neutron Probe

Methods of Measuring Soil Water Suction

• i) Electrical Resistance Unit: This consists of a porous body with

two electrodes embedded into it.

• The porous body when buried equilibrates with the soil water and the

readings are obtained through the resistance meters attached to the

electrodes.

• Resistance units are measured and the instrument needs to be

calibrated against matric suction or volumetric moisture content (Pv).

• Various porous bodies needed are gypsum, nylon or fibreglass.

• The instrument is relatively cheap but it takes a long time to

equilibrate or react e.g. 48 hours. The method is insensitive in wet

soils <0.5 bars. It measures from 0.5 to 15 bars and more.

Electrical Resistance Blocks & Meters

Methods of Measuring Soil Water Suction Contd.

• ii) Tensiometer: Tensiometer operates on the principle that a

partial vacuum is developed in a closed chamber when water

moves out through the porous ceramic tip to the surrounding.

• A vacuum gauge or a water or mercury manometer can measure

the tension. The gauge is usually calibrated in centibars or

millibars.

• After the porous cup is put in the soil, the tensiometer is filled

with water. Water moves out from the porous tip to the

surrounding soil (as suction is more in the soil). A point is

reached when the water in the tensiometer is at equilibrium with

the soil water. The reading of the gauge is then taken and

correlated to moisture content using a calibration curve.

Tensiometer for Measuring Soil Water Potential

Porous Ceramic Tip

Vacuum Gauge (0-100 centibar)

Water Reservoir

Variable Tube Length (12 in- 48 in)

Based on Root Zone Depth

Assignment No. 4

a) Determine the actual soil texture using the Soil Textural Triangle for:

Soil with 60% sand and 10% clay.

Soil with 20% sand, 30% silt.

b) Find the percentage of clay in soil (2).

c) Use the soil texture triangle to arrange the following soil textures regarding to the

increase of gravitational water: (sandy loam- clay- sand- loam- clay loam).

Assignment No. 5

The sandy land surface level at Al-Mawasi area in Khanyounis is at 4 MSL, while the

water table is at 0.0 MSL. Most of the palm trees there are not irrigated, although it has

high evapotranspiration rate, but showing considerable yield and growth.

Find the minimum root depth of the Palm trees in Al-Mawasi area, assuming the radius of

the pores for Sand: 0.02mm

What is the maximum drop in water table, that the palm trees in (a) can survive in silt soil

(the radius of the pores for Silt: 0.01mm).

On the light of solutions a and b, could you explain the problem of Palm trees in the

inland desert of Libya after the transfer of groundwater by the great river to the coastal

area.