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.
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