Effects of Water on Soils
•Shrinkage and swelling
•Particles adhere to one another
•Encourages aggregate formation (or break down soil structure)
•Chemical reactions releasing or tying up nutrients
•Chemical reactions causing acidity
•Chemical reactions that break down minerals
•Affects rate of change of temperature
•Leads to freeze-thaw activity
•Affects metabolism of soil organisms
•Basic requirement for all plants
One of only three inorganic liquids normally found on Earth
Polarity of H2O
•Exhibits polarity - side with hydrogen atoms
electropositive, oxygen side electronegative
•High boiling point for its molecular weight - molecules
cluster together due to uneven polarity
•Polarity explains electrostatic attraction to charged
ions and colloidal surfaces
•Polarity encourages dissolution of salts (attracted
more to H2O than to each other)
•Heat of solution - molecules more tightly packed when
attracted to electrostatically-charged ions or clay (energy
status lower than in pure water)
Hydrogen Bonding
H atom of one molecule attracted to O molecule of
another
Causes high boiling point, specific heat and
viscosity
Cohesion vs. Adhesion
Attraction of water molecules to one another = cohesion
Attraction of water molecules to solid surfaces =
adhesion
Adhesion = adsorption
These two forces
help soil retain water
Yields clay plasticity
Surface tension
• Water molecules
adhere to glass
(hydrophilic),
but not to a waxy
surface
(hydrophobic)
• H2O molecules
are more strongly
attracted to
themselves in
hydrophobic
case
Capillary Mechanism
Forces causing capillarity:
(1) attraction of water for the solid (adhesion)
(2) surface tension of the water (cohesion)
•Capillary movement occurs in all directions
•Determined by pore size and pore size distribution
Sandy soils: rapid rise, but does not rise as far
(larger pores)
Clay soils: slow rise, but water rises farther
Soil Water Energy
• Potential energy important in determining water
movement within soils
• Water movement is controlled by differences in
energy levels
• Moves from high energy to low energy state
Forces:
(1) Matric force (attraction of water to solids)
(2) Osmotic force (attraction of water to ions and other
solutes)
(3) Gravity (downward)
Wet soil
•Water not held very tightly because many particles far
from surfaces in larger pores
•Higher energy level
Dry soil
•Remaining water has little freedom of movement,
present in small pores, adhering strongly to surfaces
•Lower energy level (little freedom of movement)
Soil Water Potential
•Difference in energy level between soil water
and free water
Total soil water potential (t)
1. Gravitational potential (g)
g = gh
•G is the acceleration due to gravity and h is the
height of the soil above a reference level
•Plays an important role in removing excess
water after heavy precipitation or irrigation
2. Matric potential (m)
•Attraction of water to solid surfaces (adhesion or
capillarity)
•Synonymous with suction or tension
•Negative because water attracted to soil has a lower
energy state
•Above the water table
•Influences soil water retention and movement
•Very important in supplying water to drier regions
around plant roots!
3. Submergence or hydrostatic potential (s)
•Positive hydrostatic pressure due to weight of water
above
•Used only for saturated zones below the water table
4. Osmotic potential (o)
•The presence of solutes reduces the potential energy of
water
•Reduced freedom of movement of H2O around each ion
or molecule
•Solutes tend to redistribute themselves to equalize
concentrations
•Has little effect on soil water movement (no membranes
– solute moves instead)
•Important effect on uptake of water by plant roots
•If soil water is salty, o is more negative and it is more
difficult for plant to uptake H2O
•If soil water is very salty, water leaves the root cells and
wilting or even plasmolysis occurs
Water Characteristic Curves
•At a given moisture content, water is held much more
tightly by clays than by loamy or sandy soils
•Clay soils hold much more water at a given
potential than loams or sandy soils
•The amount of clay determines the proportion of
micropores
•Tightly held water cannot be used by plants
•Water content does not deplete as quickly if it is tightly
held
•Well-structured soils have a greater water holding
capacity (higher porosity)
•A compacted soil has lower porosity (lower water
holding capacity) and water is held more tightly
because macropores are removed
Hysteresis
•As a soil is wetted, some of the smallest pores are
bypassed
Water penetration is prevented
•During drying, some macropores cannot lose their
water until the matric potential is low enough to remove
water from the smaller pores surrounding them
•The large pore then does not lose its water until matric
suction is strong enough to remove water from the
smaller pores
•Shrinkage and swelling also affect soil-water
relationships
Matric potential (cm water)
Volumetric Water Content, 
Volumetric Water Content, 
Volume of water associated with a volume of soil
Gravimetric Method
1. Weigh soil
2. Dry (105 C for 24 h)
3. Weigh again (1kg loss = 1L)
Neutron Scattering
•Useful for mineral soils only
•Fast neutrons emitted
•Collisions with H2O slow them down
•Slow neutrons detected
TDR Probes (Time-domain Reflectometry)
Reflection by soil water of electromagnetic signals
travelling in transmission cables
TDR measures (1) the time it takes for an
electromagnetic impulse to travel down parallel metal
transmission rods buried in the soil and (2) the degree of
dissipation of the impulse at the end of the line. Transit
time is related to the amount of water in the soil. The
dissipation is related to the level of salts.
Determines moisture content and salinity
Capacitance methods
Electrical capacitance of
two electrodes thrust into
soil varies with water level
Air gaps cause
measurement errors
Tensiometer
Measures the strength with
which water is held in soils
Water-filled tube: Vacuum
at top, porous ceramic at
bottom
Water leaves until potential in
the tensiometer is the same as
the soil matric potential
Vacuum gauge measures
negative tension at top
Thermocouple psychrometer
Best in relatively dry soils ( 50kPa error)
Relative humidity of soil air affected by o+ m
Soil moisture potential is inversely related to the rate
of evaporation
Pressure membrane apparatus
Used to make soil water characteristic curves
Pressure applied to force water out (see diagram)
Pressure when downward flow stops gives water potential
See: http://hydra.unine.ch/doityoursoil/demo/e/module1/sequence_20/1210_30_method_e.html
Gypsum blocks
Porous block of gypsum
embedded with electrodes
Water absorbed in proportion
to soil water content
Resistance to flow of electricity
decreases with water content
Hourly Rainfall (mm)
12
rl y
fall
)
rain
0 .7
s o il mo is ture
0 .6 5
10
0 .6
Saturation ratio
Gypsum block data for a soil in the tropical cloud
forest of Tambito, Cauca, Colombia (Letts, 2003)
8
Hourl y
Saturation
0 .55 Ratio
6
0 .5
0 .4 5
4
0 .4
2
0
20-Sep
0 .3 5
4-Oct
16-Oct
Date (2000)
28-Oct
0 .3
9-Nov
Water Flow in Soils
(i) Saturated flow
(ii) Unsaturated flow
(iii) Vapour movement
1. Saturated Flow
Soil pores are completely filled with water
•lower portion of poorly drained soils
•above clay layers in well-drained soils
•upper soil zone after heavy downpours
DARCY’S LAW:
Q/t = A  Ksat  /L
Q = quantity of H2O
t = time
A = cross-sectional area of column of water
through which water flows
Ksat = saturated hydraulic conductivity
 = change in water potential between ends
of the column
L = length of the column
DARCY’S LAW
Q/t = A  Ksat  /L
Rate of flow is determined by
ease of water transmission &
force driving the water
[hydraulic gradient = (1-2)/L]
Rearrange:
Ksat = (Q  L)/(A    t)
Saturated hydraulic conductivity
is measured in units of distance
divided by time (eg. cm/h)
*See Letts (2000) for saturated hydraulic conductivity of organic soils
(fibric, hemic and sapric peat)
http://people.uleth.ca/~matthew.letts/letts%20et%20al.pdf
Preferential Flow
Normally, flow is proportional
to the fourth power of the
pore radius (macropores are
important)
Biopores, including earthworm
channels, root channels etc.
lead to preferential flow
Ped edges and shrinkage cracks
serve a similar function
Also may allow pesticides to
reach groundwater before
decomposition!
http://www.bee.cornell.edu/swlab/SoilWaterWeb/Research/pfweb/educators/pesticides/infmacro.htm
2. Unsaturated Flow in Soils
Macropores filled with air, so water movement occurs
in micropores
Water content and potential can be highly variable, causing
complex patterns in the rate and direction of water movement
Differences in matric potential m rather than gravity dominate
Movement from moist areas to dry areas along matric
potential gradient (eg. from –1kPa to –100kPa)
Micropores dominate
in clays. Many are
still water-filled at
relatively high
suction, but
macropores (dominant
in sands) are dry
Infiltration rate is highest
early in a rainfall event,
before the macropores
fill up
Infiltration
(gravity dominates
percolation near
surface after heavy
rain)
Percolation
(matric potential
gradients most
important)
Wetting front
Air fills macropores: Plant needs not met:
-10 to -30 kPa
-1500 to -2000 kPa
Evaporation
beyond W.P.
Soils and the Global Hydrological Cycle
Distribution of Water on Earth
The Global Hydrological Cycle
Watershed: An area of land drained by a
single system of streams and bounded by
ridges that separate it from adjacent
watersheds.
P = ET + SS + D
P = precipitation
ET = evapotranspiration
SS = soil storage
D = discharge
Drainage Basins
Red: selected
drainage
basins for first
order streams
(collection of
red areas should
fill the yellow
area but some
streams not
represented)
Yellow:
larger drainage
basins for river
Fate of Precipitation and Irrigation Water
Interception: some precipitation is captured by
vegetation, temporarily stored, and returned to the
atmosphere via evaporation
Includes evaporation and sublimation of snow
Sublimation of snow especially important in
coniferous forests
This water does not assist plant growth processes,
aside from temporarily reducing transpiration rates
via cooling.
Interception is significant! 30-50% of
precipitation lost in dense forests
Exception: Throughfall exceeds rainfall in cloud
forests due to intercepted water. Up to 1860 mm/yr
may be intercepted in tropical montane cloud forests
(Gonzalez, 2000; Letts, 2005)
http://people.uleth.ca/~matthew.letts/lettsmulligan_published.pdf
Infiltration: Most water that reaches the soil
penetrates downward into it, replenishing soil water
storage.
Ponding, runoff and erosion results if rainfall or
snowmelt rate exceeds infiltration capacity. Due to
erosion, surface runoff carries sediment, affecting
the turbidity of water courses.
Percolation: downward movement of water through
the soil profile
Drainage: loss of water downward from the rooting
zone (especially important in humid or irrigated
zones)
Capillary flow: movement of water (especially
upward) toward drier areas of soil
Uptake from plant roots very important, driven
partly by evaporation at the leaf surface
Effects of Precipitation
Timing of near-surface soil freezing affects infiltration of
meltwater in temperate ecosystems
A large amount of precipitation during a short period will
lead to runoff and erosion
The same amount of precipitation falling gently over a
long period leads to greater soil water and (eventually)
groundwater storage.
rain
0 .7
s o il mo is ture
0 .6 5
10
0 .6
Saturation ratio
Hourly Rainfall (mm)
Hourl y
Rainfall
(mm)
12
8
Hourl y
Saturation
0 .55 Ratio
6
0 .5
0 .4 5
4
0 .4
2
0
20-Sep
0 .3 5
4-Oct
16-Oct
Date (2000)
Date (2000)
Field capacity
28-Oct
0 .3
9-Nov
Effect of Vegetation and Soil Properties on Water
Balance
1. Interception
2. Reduction of rainsplash erosion: protects porous
structure (enhances infiltration)
3. Tree roots and litter slow and block runoff
(enhances infiltration)
4. Stemflow can focus rainfall, concentrating
infilitration toward roots
Tight = resists
infiltration and
percolation
The Soil-Plant-Atmosphere Continuum
 = -20000 kPa
 = -500 kPa
 = -70 kPa
 = -50 kPa
Points of Resistance to Water Loss
1. Soil-root
2. Leaf-atmosphere (boundary and stomatal
resistance)
Transpiration through stomata
•increases the water vapour flux
•prevents overheating
•induces moisture and nutrient transport
Stomata
-
-
open during the day for gas exchange
closed at night
stomata open when there is enough
light, and appropriate levels of moisture,
temperature, humidity and internal CO2
concentration
10-30 m long, <10 m wide
50-500 stomata mm-2
Evapotranspiration
Difficult to assess evaporation vs. transpiration
Evapotranspiration (ET) is easier to measure (combined
effect – by eddy correlation techniques)
Potential evapotranspiration rate (PET)
•Indicates how fast water would be lost from a densely
vegetated system if soil water content were maintained
at an optimal level
Determined as
0.65 * pan evaporation
PET varies from
<40 mm to >1500 mm
per year
Effect of Soil Moisture Supply on
Evapotranspiration
•Evaporation supplied by top 15-25 cm of soil
•E briefly rapid after rainfall
•E drops dramatically when surface soil dries
N.B.: Much of transpiration from subsoil layers
Water Deficit and Plant Water Stress
Difference between PET and ET increases
under drought stress
Stomatal closure causes:
(1) reduced transpiration rate
(2) decrease in plant growth rate due to lack of CO2
(3) reduction in evaporative cooling causing higher
leaf temperature (some heat dissipation by xanthophyll
cycle results from inability to photosynthesize)
Influence of Solar Radiation
•Evaporation not solely determined by temperature and
vapour pressure deficit
•Solar radiation provides additional energy
Each 2260 J needed to evaporate 1g of water
Influence of Plant Canopy
Effect of increasing leaf area per unit area
(leaf area index or LAI)
Partitioning of evapotranspirative flux shifts from
evaporation to transpiration (except immediately
following rainfall)
1. Increased absorption of solar radiation by
photosynthetic apparatae for transpiration
2. Decreased absorption of solar radiation by
surface for evaporation
Influence of Plant Community
on Evapotranspiration
Water loss from evaporation and transpiration
is determined by:
1. Climate (temperature, humidity, seasonality)
2. Leaf area index
3. Plant water use efficiency by distinct functional
groups and species (eg., grasses vs.
shrubs and cacti in southern Alberta)
4. Length and season of the growing season
1. Grasses
•
•
•
•
High water use in spring
Life cycle completed
very quickly with seeds
produced by midsummer
Dormancy in summer if
soil moisture low
C4 (eg. Bouteloua gracilis)
more efficient than C3
(eg. Agropyron cristatum)
Effect on Soil Moisture
•
•
Drawdown early
Less drawdown in late
summer unless rainfall
is heavier than usual
2. Woody shrubs
•
•
Less pronounced
seasonality than grasses
Stomata close when
soil moisture unavailable
(eg. Artemisia cana)
3. Small trees
•
•
•
•
Least pronounced
seasonality of all
Drawdown begins later,
but lasts longer
Lower transpiration rates
per unit leaf area, but
higher LAI
Found only at wet
microsites
(eg. Prunus virginiana)
4. Cacti
•Employ Crassulacean
Acid Metabolism (CAM)
•Stomata closed during
the day
•CO2 is taken up at night!
•Water is stored during
times of available soil
moisture and used during
both moist and dry periods
Volumetric Water Content in the Lethbridge Coulees
(University of Lethbridge, Summer 2004)
0.3
0.25
NE-facing
S-facing
0.2
0.15
0.1
0.05
0
150
200
250
Julian Day
300
4
Stomatal Conductance
In Four Shrubs of the
Lethbridge Coulee
(Summer 2004)
3.5
3
2.5
2
1.5
60
1
150
50
200
250
300
Julian Day
40
30
LEGEND
20
4
A. cana
P. virginiana
R. trilobata
R. aureum
3
10
0
150
10 250
3.5
2.5
200 300
250
300
2
1.5
1
150
200
250
Julian Day
300
30
Transpiration Rate
In Four Shrubs of the
Lethbridge Coulee
(Summer 2004)
25
20
15
10
5
0
150
-5
60
50
200
250
300
Julian Day
40
30
LEGEND
20
30
A. cana
P. virginiana
R. trilobata
R. aureum
20
10
0
150
10 250
25
15
200 300
250
300
10
5
0
150
200
250
Julian Day
300
60
60
50
50
40
40
30
430
20
20
10
3.510
Stomatal Conductance vs. Volumetric Soil Moisture
0
150
-10
LEGEND
A. cana
P. virginiana
R. trilobata
R. aureum
R2 = 0.90
R2 = 0.82
30
200 -10150 250
200 300
250
R2 = 0.87
300
2.5
R2 = 0.80
2
1.5
1
0.5
0
0
0.05
0.1
0.15
0.2
0.25
Volumetric soil moisture (m3m-3)
0.3
50
Net Photosynthesis
vs. Volumetric Soil Moisture
40
50
30
LEGEND
20
A. cana
P. virginiana
R. trilobata
R. aureum
R2 = 0.78
R2 = 0.88
10
40
PN (molm-2s-1)
150
60
0
200 -10150 250
200 300
250
R2 = 0.88
300
30
20
R2 = 0.85
10
0
0
-10
0.05
0.1
0.15
0.2
0.25
Volumetric soil moisture (m3m-3)
0.3
Interception of nearly all PAR
Why does it decrease?
Photosynthetic benefit of higher
LAI outweighed by respiration
Effect of LAI on Productivity in a Light Limited Cloud Forest
Relative productivity (%)
20
10
0
-10
-20
-30
-40
August
-50
November
-60
-70
0
1
2
3
4
5
6
Leaf Area Index (LAI)
7
8
Control of Evapotranspiration (ET) to
Maintain Adequate Soil Moisture
1. Sow fewer seeds per unit area
2. Eliminate weeds
(i) herbicides (plant residues can be left on surface)
(ii) cultivation of the soil (avoids toxic effects)
3. Limit nutrient supply to prevent excessive
early season water use
4. Fallow cropping (in arid regions).
5. Conservation tillage
Control of Surface Evaporation (E)
1. Mulch (small areas and high value crops)
Benefits:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
Reduces spread of disease
Reduces weed growth
Moderates temperature amplitude
Increases water infiltration
Provides organic matter
Encourages earthworm populations
Reduces soil erosion
2. Conservation Tillage (“stubble mulch”)
Eg. Wheat or corn stalks spread over surface and
tilled to very shallow depth.
Planting is carried out through the stubble
Influence of Annual Temperature and
Precipitation on Partitioning of Water Loss
Preferential
By-pass Flow
Effects of Irrigation
•Increases variety of crops that can be grown
•Dramatically increases yield
•Highly consumptive use of water
•Evapotranspiration
•Causes salinization in prone areas (ET&WT)
•Reservoir construction required
•Alters fish and wildlife habitat
•Floods cultural/historical sites
•Provides recreation opportunities
Centre Pivot
Irrigation System
(invented in Burdett, Alberta)
Source: Zimmatic
Lateral Move Irrigation System
Source: Zimmatic
Soil Respiration (at the Earth-atmosphere interface):
Requires O2 supply and CO2 removal
Factors affecting O2 availability:
1. Soil macroporosity
2. Soil water content
3. O2 consumption by respiring organisms
Poor aeration:
Impedes plant growth
Typically problematic when less than 20% of pore
space filled with air (>80% water-filled)
Effect of high water content
1. Blocks pathways to gas exchange with atmosphere
2. Reduces air storage space
Waterlogged soil
•Nearly all pores filled with water
•Occurs in (i) wetlands, (ii) depressions and flat areas,
and (iii) anywhere during and immediately following heavy
rainfall
Adaptation to waterlogging
•Most plants dependent on soil supply of O2 to roots
•Hydrophytes acquire O2 by alternative means:
aerenchyma tissues – hollow structures in stems and roots
Eg. Rice, marsh grasses, mangroves
Zone of highest CO2 production rate
High macropore
density
Low macropore
density, greater
distance to
atmosphere
CO2 concentration in
a tropical rainforest
soil, Brazil
How does soil-atmosphere gas exchange occur?
(i) Diffusion
• Each gas moves in a direction and rate determined by its
partial pressure
• All that is required is a concentration gradient (no pressure
differences are required)
• O2 enters due to its higher concentration in the
atmosphere compared to the soil, while CO2 and H2O move
outward.
(ii) Mass Flow
• Movement of air with the flow of water within the soil
• Air is expelled as the water table rises, and is ‘inhaled’ as
the water table is lowered
• Atmospheric pressure changes also cause this effect
Soil Aeration Status
(i) Gaseous composition of the soil atmosphere
(ii) Air-filled soil porosity
(iii) Oxidation-reduction potential
Soil O2 Content
• Atmosphere: 21% O2
• Soil Air: Nearly 20% O2 near surface in well-structured
soils with high macropore volume
• <5% O2 in B and C Horizons of poorly-drained soils
with few macropores
• Soil Water: Small quantities of O2, only sufficient for
temporary use by soil microorganisms. O2 depletion
results from persistent waterlogging.
Soil CO2 Content:
May reach 10-30 times atmospheric concentrations in
poorly-aerated soils conditions
Soil CH4 Content
•Concentrations of CH4 increase dramatically in persistentlywaterlogged soils
•Decomposition by anaerobic, methanogenic bacteria is
much slower, but the release of methane to the atmosphere
is of significance [Eg. Letts (1998)*]
•The radiative forcing of CH4 is 21 times that of CO2 on a
per-molecule basis
*Letts,
M.G. 1998. Modelling Peatland Soil Climate and Methane Flux using the Canadian Land Surface Scheme.
M.Sc. thesis. McGill University. 86 p.
Air-filled porosity
•Diffusion of O2 in air-filled pore is 10,000 times faster than
in water-filled pore
•Water-filled pores block the diffusion of oxygen into the soil
•Plant growth severely inhibited when air-filled porosity is
lower than 20% (about 10% of total soil volume)
Oxidation-reduction Potential
Well-aerated soils: oxidated states are dominant
Poorly-aerated soils: reduced forms are dominant
reduced state
oxidized state
2FeO +2H2O  2FeOOH + 2H+ +2e2+
3+
•Loss of electrons occurs, which creates the potential for
transfer of electrons between substances: redox potential
•Redox potential can be measured with a platinum electrode
•A substance that accepts electrons easily and gives away oxygen
is an oxidizing agent. Oxygen gas is an oxidizing agent (hence
the term)
•A substance that supplies electrons and receives oxygen
is a reducing agent
Relevance of Oxidation-reduction State to Plants
•Oxidized forms of N are more readily utilizable by plants
•Humid regions: reduced forms of Fe and Mn are very
soluble, resulting in toxicity
•Arid and semi-arid regions: where soils are neutral
to alkaline, oxidized forms of Fe and Mn are often incorporated
into highly insoluble compounds, resulting in deficiency
Solar Radiation
and
Soil Temperature
Key Temperature
Ranges
N
S-facing
Site
NE-facing
Site
The Study Site
Lethbridge Coulee Soil Temperature Patterns
15
Soil Temperature (C)
10
December
January
5
0
-5
S-facing
-10
NE-facing
-15
10 cm Depth
Soil Temperature Patterns
12
10
8
20 cm Temperature
10 cm temperature
6
4
2
0
0
2000
4000
6000
8000
-2
October
January
10000