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Title: P1252109253Hxthq


1
Soil Water
2
  • 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

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4
  • 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)

5
Hydrogen Bonding H atom of one molecule
attracted to O molecule of another Causes high
boiling point, specific heat and viscosity
6
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
7
  • 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

8
  • 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

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10
  • 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
  • Matric force (attraction of water to solids)
  • (2) Osmotic force (attraction of water to ions
    and other solutes)
  • (3) Gravity (downward)

11
  • 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)

12
  • 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

13
  • 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

14
  • 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

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16
  • 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

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20
  • 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

21
Matric potential (cm water)
Volumetric Water Content, ?
22
  • 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

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25
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
26
Capacitance methods Electrical capacitance of
two electrodes thrust into soil varies with
water level Air gaps cause measurement errors
27
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
28
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/mod
ule1/sequence_20/1210_30_method_e.html
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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
31
Gypsum block data for a soil in the tropical
cloud forest of Tambito, Cauca, Colombia
(Letts, 2003)
Saturation ratio
Hourly Rainfall (mm)
Date (2000)
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33
  • Water Flow in Soils
  • Saturated flow
  • Unsaturated flow
  • 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

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
DARCYS LAW Q/t A ? Ksat ? ??/L
34
DARCYS 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)
35
See Letts (2000) for saturated hydraulic
conductivity of organic soils (fibric, hemic and
sapric peat) http//people.uleth.ca/matthew.lett
s/letts20et20al.pdf
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37
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/Rese
arch/pfweb/educators/pesticides/infmacro.htm
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39
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)
40
Micropores dominate in clays. Many are still
water-filled at relatively high suction, but
macropores (dominant in sands) are dry
41
Infiltration rate is highest early in a rainfall
event, before the macropores fill up
42
Infiltration (gravity dominates percolation near
surface after heavy rain)
Percolation (matric potential gradients
most important)
Wetting front
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45
Air fills macropores -10 to -30 kPa
Plant needs not met -1500 to -2000 kPa
Evaporation beyond W.P.
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48
Soils and the Global Hydrological Cycle
Distribution of Water on Earth
49
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
50
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
51
Fate of Precipitation and Irrigation
Water   Interception some precipitation is
captured by vegetation, temporarily stored, and
returned to the atmosphere via evaporation Includ
es 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
52
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/lettsmulliga
n_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.  
53
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
54
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.
55
Hourly Rainfall (mm)
Saturation ratio
Date (2000)
Field capacity
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57
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
58
Tight resists infiltration and percolation
59
The Soil-Plant-Atmosphere Continuum
? -20000 kPa
? -500 kPa
? -70 kPa
? -50 kPa
60
Points of Resistance to Water Loss
1. Soil-root 2. Leaf-atmosphere (boundary and
stomatal resistance)
61
  • 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, lt10 ?m wide
  • - 50-500 stomata mm-2

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63
  • 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

64
Determined as 0.65 pan evaporation PET varies
from lt40 mm to gt1500 mm per year  
65
  • 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

66
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)
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68
  • 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

69
  • 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)
  • Increased absorption of solar radiation by
    photosynthetic apparatae for transpiration
  • 2. Decreased absorption of solar radiation by
    surface for evaporation

70
Influence of Plant Community on
Evapotranspiration
  • Water loss from evaporation and transpiration
  • is determined by
  • Climate (temperature, humidity, seasonality)
  • Leaf area index
  • 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

71
  • 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

72
  • 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)

73
  • 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

74
Volumetric Water Content in the Lethbridge
Coulees (University of Lethbridge, Summer 2004)
NE-facing
S-facing
? (vol. water content)
Julian Day
75
Stomatal Conductance In Four Shrubs of
the Lethbridge Coulee (Summer 2004)
log (gs)
Julian Day
log (gs)
Julian Day
76
Transpiration Rate In Four Shrubs of
the Lethbridge Coulee (Summer 2004)
E (mmol?m-2?s-1)
Julian Day
E (mmol?m-2?s-1)
Julian Day
77
Stomatal Conductance vs. Volumetric Soil Moisture
log (gs)
Volumetric soil moisture (m3?m-3)
78
Net Photosynthesis vs. Volumetric Soil Moisture
PN (?mol?m-2?s-1)
Volumetric soil moisture (m3?m-3)
79
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82
Interception of nearly all PAR
Why does it decrease? Photosynthetic benefit of
higher LAI outweighed by respiration
83
Effect of LAI on Productivity in a Light Limited
Cloud Forest
84
Control of Evapotranspiration (ET) to Maintain
Adequate Soil Moisture
  • Sow fewer seeds per unit area
  • 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

85
Control of Surface Evaporation (E)
  • Mulch (small areas and high value crops)
  • Benefits
  • 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
86
Influence of Annual Temperature and Precipitation
on Partitioning of Water Loss
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92
Preferential By-pass Flow
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95
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 (?ETWT)
  • Reservoir construction required
  • Alters fish and wildlife habitat
  • Floods cultural/historical sites
  • Provides recreation opportunities

96
Centre Pivot Irrigation System (invented in
Burdett, Alberta)
Source Zimmatic
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Lateral Move Irrigation System
Source Zimmatic
99
Soil Aeration and Temperature
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101
  • Soil Respiration (at the Earth-atmosphere
    interface)
  • Requires O2 supply and CO2 removal
  • Factors affecting O2 availability
  • Soil macroporosity
  • Soil water content
  • O2 consumption by respiring organisms
  • Poor aeration
  • Impedes plant growth
  • Typically problematic when less than 20 of pore
  • space filled with air (gt80 water-filled)
  • Effect of high water content
  • Blocks pathways to gas exchange with atmosphere
  • Reduces air storage space

102
  • 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

103
Zone of highest CO2 production rate
High macropore density
CO2 concentration in a tropical rainforest soil,
Brazil
Low macropore density, greater distance to
atmosphere
104
  • How does soil-atmosphere gas exchange occur?
  • 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

105
  • Soil Aeration Status
  • Gaseous composition of the soil atmosphere
  • Air-filled soil porosity
  • Oxidation-reduction potential
  • Soil O2 Content
  • Atmosphere 21 O2
  • Soil Air Nearly 20 O2 near surface in
    well-structured
  • soils with high macropore volume
  • lt5 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

106
  • Soil CH4 Content
  • Concentrations of CH4 increase dramatically in
    persistently-
  • waterlogged 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)

107
  • Oxidation-reduction Potential
  • Well-aerated soils oxidated states are dominant
  • Poorly-aerated soils reduced forms are dominant
  • 2FeO 2H2O ? 2FeOOH 2H 2e-
  • 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

oxidized state
reduced state
3
2
108
  • 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

109
Solar Radiation and Soil Temperature
110
Key Temperature Ranges
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112
N
S-facing Site
NE-facing Site
The Study Site
113
Lethbridge Coulee Soil Temperature Patterns
January
December
Soil Temperature (?C)
10 cm Depth
114
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116
Soil Temperature Patterns
Temperature (Celsius)
January
October
117
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