Charter 5 Soil Water
Section 1
The types of soil water
and measuring soil moisture content
The importance of soil water：
? Effect on soil formation,erosion,and
structure stability
? It is the major constituent of plant
protoplasm.
? It is essential for photosynthesis and
conversion of starches to sugar,
?It is the solvent in which nutrients move
into and through plant parts.
一,The types and available
of soil moisture
1,Classification of soil water
? Adsorbed water
? Membranous water
? Capillary water
? Gravitational water
?Numerical
method
（ 1） Soil adsorbed water,
?held by strong electrical forces - low energy
?little movement- held tight by soil
?exists as a film
?unavailable to plants
?removed from soil by drying in an oven
（ 2） Soil Membranous water,
? held by hydrogen bonding
? liquid state in water film
? major source of water for plants
? greater energy than adhesion water
Sketch map of membranous water
（ 3） Soil capillary water,
Capillary water-The water held in the
“capillary” or small pores of a soil,
usually with a tension >60 cm of water.
Capillary water includes capillary
hanging water and capillary rise water.
Capillarity：
0.1-1mm
Capillarity obvious
0.05-0.1mm
Capillarity strong
0.05-0.005mm
Capillarity very strong
〈 0.001mm
Capillarity disappears
Water is drawn up into the capillary tube
Soil particle
Capillary
hanging water
sketch map
Field capacity-The amount of water
remaining in a soil after the free water
has been allowed to drain away (a day or
two) after the root zone had been
previously saturated; expressed as a
percentage.
Field capacity：
Soil particle
Capillary
water rise
sketch map
Groundwater table
The height of capillary water rise,
h（ cm),the hight of capillary water rise, d,the
diameter of the capillary tube（ mm）
（ 4） Gravitational water
Gravitational water -Water which
moves into,through,or out of the
soil under the influence of gravity.
二,The express methods of soil water content
（二） The volume water content （ ?v）
Volume water content = volume of water/bulk volume of
soil=(weight of water/ρW)/(weight of dry soil/ ρb)
?V=?m·?
（一） The mass water content（ ?m)
Percntage water = {[(wet soil weight)-(oven dry soil weight)]/
(oven dry soil weight)} × 100
(三） Relative water content（ %）
Relative water content= soil water content/ field capacity
（三） Soil water-storage capcity
2,Water fang(方 )（ m3）
whaF a n g DV 10/ ?
VFang/mu(亩 )＝ 2/3Dw
1,Water deepth（ DW）
DW=?V·h or
D hw
i
n
i
,100 1
1
? ?
?
? ?
mm
三,Estimating water contents
? Gravimetric method,The soil sample is dried in an
oven at 105° C and the mass of dry soil recorded.
? Neutron scattering method
? Time Domain Reflectometry (TDR)
Section 2 Soil Water Potential
一,Total soil water potential and
individual potentials
Soil
A
Sand
Soil
10%
Soil
B
Clay
Soil
15%
Where does water flow?
Water moves from areas of high potential
(wet soil, -2 or -4) to areas of low
potential (dry soil -8)
SoilSoilRoot
-7
-2
-3-.4
-8
Soil water potential- The amount
of work that must be done per unit of
a specified quantity of pure water in
order to transport reversibly and
isothermally an infinitesimal quantity
of water from a specified source to a
specified destination.
（一） The matric potential（ ?m）
This work is less than zero or negative
work,thus reported in negative values,
（二） The pressure potential（ ?p）
In saturated soil,the pressure potential is
always positive,In an unsatrated field
soil the pressure potential is always zero.
?p=?wgh
（三） The solute (osmotic) potential（ ?S）
The amount of work an infinitesmal quantity of water will do in
moving from a pool of free water the same composition as the soil
water to a pool of pure water at the same location,The solute
potential is usually very small and negative values.
（四） The gravitational potential（ ?g）
The amount of work an infinitesmal amount of pure free
water can do at the site of the soil solution as a result of the
force of gravity,
?g =± MgZ
Total soil water potential,
?t=?m+?p+?s+?g
Since soil matric and osmotic potentials are
always negative they are often onsidered as ‘suction’
or ‘tensions’,Suction and tensions are however
always expressed as positive values.
T＝ ?-?m ?
How do you use suction and
potentials to decide the direction
of soil water movement?
Absolute positive value二,Soil moisture suction
三,Soil water potential measurement
The popular unite of the soil water potential is,
Pa
1 Pa=0.0102-cm column of water
1 atmospheres=1033-cm column of
water=1.0133bar
1 bar=0.9896atm=1020-cm column of water
1 bar=105 Pa
Measuremets of
matric potential
above about -80
kPa
Tensionmeter method
四,Soil water characteristic curve
The relationship between the soil-water content
(by mass or volume) and the soil-water matric
potential.
S=a?b
S=a(?/?s)b
S=A(?s-?)n/?m
S,suction,Pa; θ:water content; a,b,A,n,m,experience constant.
0 10 20 30 40 50 60 70
Clay
SiltSand
Soil moisture content%
Affect the factors
?Texture
?Structure
?Temperature
?Phenomenon of
hysteresis
So
il m
ois
tur
e s
uc
tio
n
机理：墨水瓶效应
沙土比粘土明显
水分特征曲线的用途：
第四, 应用数学物理方法对土壤中的水运动进
行定量分析时, 水分特征曲线是必不可少的重
要参数 。
首先, 可利用它进行土壤水吸力 S和含水率 ?之
间的换算 (图 3.7)。
其次，土壤水分特征曲线可以间接地反映出土
壤孔隙大小的分布。
第三, 水分特征曲线可用来分析不同质地土壤
的持水性和土壤水分的有效性 。
Section3 Water Movement Soils
?The principle of water movement in soil
?Evaporation
?Infiltration
?Water redistrbution
一,Saturated Soil Water Flow
The rate of water flow through soil can be
described by Darcy’s Law which states that the flux
of water q is proportional to the hydraulic gradient
(the gravitational potential and the pressure
potential )multiplied by the conductivity or
permeability of the soil.
q K H
Ls
? ? ?
Saturated flow-The movement of water though a
soil that is temporarily saturated,Most of the water
moves downwards,and some move more slowly
laterally.
The rate of flow through a given amount of soil in a given time
equals the water quantity collected (Qw) divided by both the cross-
sectional area of soil used (A) and the time (t) of measurement.
Saturated hydraulic conductivity (Ks)
The characteristics of the saturated
hydraulic conductivity:
① The saturated hydraulic
conductivity is a constant
② It is maximum in
hydraulic conductivity
③ It is decided by the soil
texture and the soil
structure
The factors of affect the
saturated hydraulic
conductivity:
?The soil texture
?The soil structure
?The amount of organic
matter
?The clay mineral
At
QK w
s ??
二,Unsaturted soil water flow
The movement of water in soil in which the pores
are not filled to capacity with water,
The unsaturted soil water flow is decided by the
matric potential and the gravitational potential.
q K
d
dx
m? ? ( )?
?
Darcy’s Law can be extended to describe unsaturated
flow:
Unsaturated hydraulic conductivity
The relation between soil moisture suction and
hydraulic conductivity
K(?m),unsaturated hydraulic conductivity
d?/dx,water potential gradient
The unsaturated hydraulic conductivity
is a function of soil matric potential.
三,Vapor flow in soil
The form of movement of vaporous water in soil:
1.Water vapor diffusion
2,Water vapor coagulation
Vapor flow can be considered as a diffusion
mechanism in which the driving force is the vapor
pressure gradient.
1,Phenomenon of ?night wet?
四,Infiltration, water redistribution
and evaporation of soil surface
（一） Soil water infiltration
The entry of water into soil.
Affect the factors：
一,Velocity of
Supply water
二,Infiltration rate
The stable infiltration rate
in several different texture soils ( millimeter/ hour)
Soil Sand
Sandy
loam Silt Clay Alkalized
clay
Final
infiltration
rate >20 10-20 5-10 1-5 <1
（二） Redistribution of soil water
Redistribution of soil
water- The process of
soil-water movement
to achieve an
equilibrium energy
state of water
throughout the soil.
Soil water redistribution is
unsaturated flow of soil water.
(三 ) Evaporation in soil surface
?Evaporation- Water lose as vapor from a soil or open water
surface.
?1,The keeping the stable stage of evaporation in soil surface
?2, The stage of evaporate change with moisture content in
soil surface
?3, The stage of water vapor diffusion
五,Soil water balance in the field
Soil water balance in the field can be written as:
?W=P+I+U-E-T-R-In-D
P,precipitation; I,irrigation;
E,evaporation; T,transpiration;
R,runoff
Soil-plant-atmosphere continuum (SPAC)
Water moves from a relatively high potential energy level in the
soil (-100 kPa) and flows down a potential gradient into the plant
root (-500 kPa),plant stem (-800 kPa),and leaves (-1500 kPa),
where it is eventually evaporated into the atmosphere (-10000 kPa),
Desert plants
can live in
-2× 106 to
-8× 106 Pa.
Section 4,Control of soil water
Available soil water-The amount of water
released between in situ field capacity
and the permanent wilting point (usually
estimated by water content at soil matric
potential of -1.5 MPa).
一,Availability of soil water
二,Control of soil water
1,Cultivation measure, plow depth,
intertillage,roll etc.
2,Mulches,straw,plastic sheeting etc.
3,Irrigation,drip irrigation,sprinkler
irrigation etc.
4,Biological save water
以色列塑料坝
以色列花农
以色列沙地优质土豆
Section 1
The types of soil water
and measuring soil moisture content
The importance of soil water：
? Effect on soil formation,erosion,and
structure stability
? It is the major constituent of plant
protoplasm.
? It is essential for photosynthesis and
conversion of starches to sugar,
?It is the solvent in which nutrients move
into and through plant parts.
一,The types and available
of soil moisture
1,Classification of soil water
? Adsorbed water
? Membranous water
? Capillary water
? Gravitational water
?Numerical
method
（ 1） Soil adsorbed water,
?held by strong electrical forces - low energy
?little movement- held tight by soil
?exists as a film
?unavailable to plants
?removed from soil by drying in an oven
（ 2） Soil Membranous water,
? held by hydrogen bonding
? liquid state in water film
? major source of water for plants
? greater energy than adhesion water
Sketch map of membranous water
（ 3） Soil capillary water,
Capillary water-The water held in the
“capillary” or small pores of a soil,
usually with a tension >60 cm of water.
Capillary water includes capillary
hanging water and capillary rise water.
Capillarity：
0.1-1mm
Capillarity obvious
0.05-0.1mm
Capillarity strong
0.05-0.005mm
Capillarity very strong
〈 0.001mm
Capillarity disappears
Water is drawn up into the capillary tube
Soil particle
Capillary
hanging water
sketch map
Field capacity-The amount of water
remaining in a soil after the free water
has been allowed to drain away (a day or
two) after the root zone had been
previously saturated; expressed as a
percentage.
Field capacity：
Soil particle
Capillary
water rise
sketch map
Groundwater table
The height of capillary water rise,
h（ cm),the hight of capillary water rise, d,the
diameter of the capillary tube（ mm）
（ 4） Gravitational water
Gravitational water -Water which
moves into,through,or out of the
soil under the influence of gravity.
二,The express methods of soil water content
（二） The volume water content （ ?v）
Volume water content = volume of water/bulk volume of
soil=(weight of water/ρW)/(weight of dry soil/ ρb)
?V=?m·?
（一） The mass water content（ ?m)
Percntage water = {[(wet soil weight)-(oven dry soil weight)]/
(oven dry soil weight)} × 100
(三） Relative water content（ %）
Relative water content= soil water content/ field capacity
（三） Soil water-storage capcity
2,Water fang(方 )（ m3）
whaF a n g DV 10/ ?
VFang/mu(亩 )＝ 2/3Dw
1,Water deepth（ DW）
DW=?V·h or
D hw
i
n
i
,100 1
1
? ?
?
? ?
mm
三,Estimating water contents
? Gravimetric method,The soil sample is dried in an
oven at 105° C and the mass of dry soil recorded.
? Neutron scattering method
? Time Domain Reflectometry (TDR)
Section 2 Soil Water Potential
一,Total soil water potential and
individual potentials
Soil
A
Sand
Soil
10%
Soil
B
Clay
Soil
15%
Where does water flow?
Water moves from areas of high potential
(wet soil, -2 or -4) to areas of low
potential (dry soil -8)
SoilSoilRoot
-7
-2
-3-.4
-8
Soil water potential- The amount
of work that must be done per unit of
a specified quantity of pure water in
order to transport reversibly and
isothermally an infinitesimal quantity
of water from a specified source to a
specified destination.
（一） The matric potential（ ?m）
This work is less than zero or negative
work,thus reported in negative values,
（二） The pressure potential（ ?p）
In saturated soil,the pressure potential is
always positive,In an unsatrated field
soil the pressure potential is always zero.
?p=?wgh
（三） The solute (osmotic) potential（ ?S）
The amount of work an infinitesmal quantity of water will do in
moving from a pool of free water the same composition as the soil
water to a pool of pure water at the same location,The solute
potential is usually very small and negative values.
（四） The gravitational potential（ ?g）
The amount of work an infinitesmal amount of pure free
water can do at the site of the soil solution as a result of the
force of gravity,
?g =± MgZ
Total soil water potential,
?t=?m+?p+?s+?g
Since soil matric and osmotic potentials are
always negative they are often onsidered as ‘suction’
or ‘tensions’,Suction and tensions are however
always expressed as positive values.
T＝ ?-?m ?
How do you use suction and
potentials to decide the direction
of soil water movement?
Absolute positive value二,Soil moisture suction
三,Soil water potential measurement
The popular unite of the soil water potential is,
Pa
1 Pa=0.0102-cm column of water
1 atmospheres=1033-cm column of
water=1.0133bar
1 bar=0.9896atm=1020-cm column of water
1 bar=105 Pa
Measuremets of
matric potential
above about -80
kPa
Tensionmeter method
四,Soil water characteristic curve
The relationship between the soil-water content
(by mass or volume) and the soil-water matric
potential.
S=a?b
S=a(?/?s)b
S=A(?s-?)n/?m
S,suction,Pa; θ:water content; a,b,A,n,m,experience constant.
0 10 20 30 40 50 60 70
Clay
SiltSand
Soil moisture content%
Affect the factors
?Texture
?Structure
?Temperature
?Phenomenon of
hysteresis
So
il m
ois
tur
e s
uc
tio
n
机理：墨水瓶效应
沙土比粘土明显
水分特征曲线的用途：
第四, 应用数学物理方法对土壤中的水运动进
行定量分析时, 水分特征曲线是必不可少的重
要参数 。
首先, 可利用它进行土壤水吸力 S和含水率 ?之
间的换算 (图 3.7)。
其次，土壤水分特征曲线可以间接地反映出土
壤孔隙大小的分布。
第三, 水分特征曲线可用来分析不同质地土壤
的持水性和土壤水分的有效性 。
Section3 Water Movement Soils
?The principle of water movement in soil
?Evaporation
?Infiltration
?Water redistrbution
一,Saturated Soil Water Flow
The rate of water flow through soil can be
described by Darcy’s Law which states that the flux
of water q is proportional to the hydraulic gradient
(the gravitational potential and the pressure
potential )multiplied by the conductivity or
permeability of the soil.
q K H
Ls
? ? ?
Saturated flow-The movement of water though a
soil that is temporarily saturated,Most of the water
moves downwards,and some move more slowly
laterally.
The rate of flow through a given amount of soil in a given time
equals the water quantity collected (Qw) divided by both the cross-
sectional area of soil used (A) and the time (t) of measurement.
Saturated hydraulic conductivity (Ks)
The characteristics of the saturated
hydraulic conductivity:
① The saturated hydraulic
conductivity is a constant
② It is maximum in
hydraulic conductivity
③ It is decided by the soil
texture and the soil
structure
The factors of affect the
saturated hydraulic
conductivity:
?The soil texture
?The soil structure
?The amount of organic
matter
?The clay mineral
At
QK w
s ??
二,Unsaturted soil water flow
The movement of water in soil in which the pores
are not filled to capacity with water,
The unsaturted soil water flow is decided by the
matric potential and the gravitational potential.
q K
d
dx
m? ? ( )?
?
Darcy’s Law can be extended to describe unsaturated
flow:
Unsaturated hydraulic conductivity
The relation between soil moisture suction and
hydraulic conductivity
K(?m),unsaturated hydraulic conductivity
d?/dx,water potential gradient
The unsaturated hydraulic conductivity
is a function of soil matric potential.
三,Vapor flow in soil
The form of movement of vaporous water in soil:
1.Water vapor diffusion
2,Water vapor coagulation
Vapor flow can be considered as a diffusion
mechanism in which the driving force is the vapor
pressure gradient.
1,Phenomenon of ?night wet?
四,Infiltration, water redistribution
and evaporation of soil surface
（一） Soil water infiltration
The entry of water into soil.
Affect the factors：
一,Velocity of
Supply water
二,Infiltration rate
The stable infiltration rate
in several different texture soils ( millimeter/ hour)
Soil Sand
Sandy
loam Silt Clay Alkalized
clay
Final
infiltration
rate >20 10-20 5-10 1-5 <1
（二） Redistribution of soil water
Redistribution of soil
water- The process of
soil-water movement
to achieve an
equilibrium energy
state of water
throughout the soil.
Soil water redistribution is
unsaturated flow of soil water.
(三 ) Evaporation in soil surface
?Evaporation- Water lose as vapor from a soil or open water
surface.
?1,The keeping the stable stage of evaporation in soil surface
?2, The stage of evaporate change with moisture content in
soil surface
?3, The stage of water vapor diffusion
五,Soil water balance in the field
Soil water balance in the field can be written as:
?W=P+I+U-E-T-R-In-D
P,precipitation; I,irrigation;
E,evaporation; T,transpiration;
R,runoff
Soil-plant-atmosphere continuum (SPAC)
Water moves from a relatively high potential energy level in the
soil (-100 kPa) and flows down a potential gradient into the plant
root (-500 kPa),plant stem (-800 kPa),and leaves (-1500 kPa),
where it is eventually evaporated into the atmosphere (-10000 kPa),
Desert plants
can live in
-2× 106 to
-8× 106 Pa.
Section 4,Control of soil water
Available soil water-The amount of water
released between in situ field capacity
and the permanent wilting point (usually
estimated by water content at soil matric
potential of -1.5 MPa).
一,Availability of soil water
二,Control of soil water
1,Cultivation measure, plow depth,
intertillage,roll etc.
2,Mulches,straw,plastic sheeting etc.
3,Irrigation,drip irrigation,sprinkler
irrigation etc.
4,Biological save water
以色列塑料坝
以色列花农
以色列沙地优质土豆
