Soil and the hydrologic cycle
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Globally 750mm of annual rainfall but not evenly distributed evenly
Floods and drought occur in abnormal years
Impacts food production, water and sanitation for human needs
Soil, plants and animals help moderate the adverse effects odf excess and deficiencies of
moisture
Role of the soil
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Serves as a moisture reservoir
Replenishes ground water and meets plant water needs
Soil is the conection channel between linking chemical pollutants with ground water
Sources of water
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Tot water effectively equivalent to 1400x 106 km3 spread on the earth’s surface -3km depth
spread on the surface (Brady, 2002)
97% is in oceans; 2% in glaciers, 0.7% is ground water
Only water in surface waters is actively cycled
The Hydrologic cycle
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Cycling of water from earth’s surface to the atm and back again
Driven by solar energy
500 000 km3 evaporated from earth surfaces and vegetation annually; 110 000 km3 fall as rain
and snow on continents; 70 000 km3 evaporated back; 40 000 km3 moves in as clouds from
ocean areas and R/o back to oceans is 40 000 km3
The water balance equation- the fate of precipitation and irrigation water
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5-10%- intercepted by plant foliage and evaporates to atm without reaching the soil. This can
be up to 10-30% in forest
Infiltration- water penetrates the soil especially on friable soils of negligible slope. Infiltration
can lead to saturation .
Runoff water may run off if the soil water intake rate is low especially in intense rainfall
Deep percolation infiltrated water may be lost in drainage. Deep percolation may move back
up in response to moisture gradients- esp when rainfall is low.
Fate of pptn is dependent on:
 Timing and form of precipitation –eg after soil is frozen or before-intemperate climates
 Rate or intensity of precipitation(mm/hr) rel to soil intake rate
 Vegetation and soil properties-
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Vegetation affects the amount of run off
Rate of penetration and amount
Vegtn provides a soil cover- slows down r/o encouraging infiltration
Soil properties affect infiltration rate- heavy clays with unstable strc resist
infiltration and increase r/o
o –factors that affect ground cover and soil intake rate affect the distribution of rainfall
between infiltration and r/o.
The Soil-Plant-Atmosphere-Continuum
The Soil-Plant-Atmosphere Continuum SPAC is the pathway for water moving from soil through the plant
to the atmosphere and also refers to the transport of water along this pathway ...
The transport of water along this pathway occurs in components, variously defined among scientific
disciplines:
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Soil Physics soil physics characterizes water in soil terms of tension,
Physiology of plants and animals characterizes water in organisms in terms of
diffusion pressure deficit, and
Meteorology uses vapour pressure or relative humidity to characterize atmospheric
water.
SPAC integrates these components and is defined as a:
...concept recognising that the field with all its components (soil, plant, animals and the ambient
atmosphere taken together) constitutes a physically integrated, dynamic system in which the
various flow processes involving energy and matter occur simultaneously and independently like
links in the chain.
 Principles governing water movt in SPAC are the same as those governing movt in soil
 Movement is in response to a potential difference
 Soil water potential must be such that potential in soil is > than potential in roots ie
-50kPa > -70kPa > -75kPa > -85kPa > -500kPa
 The main points of resistance are the soil/root and the leaf/atm interfaces
Atmosphere
-2000kPa
Precipitation and
irrigation water
100%
Transpiration
15-35%
Leaf surface
(-500kPa)
(-85kPa)
15-30%
Preciptn
intercepted by
plants
&evaporated
Loss thru
stoma
Evaporation
15-40%
(-75kPa)
Runoff
-30%
Absorption by
root hairs
15-30%
15-30%
40-75%
(-70kPa)
15-35%
0-10%
(-50kPa)
30-65%
10-30%
Water potential and water movement
Water potential is a term expressing the ability of the water to do work.
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According to thermodynamics - this is the Gibbs free energy (G) or the chemical potential (µw)
of the water
µw = µ0w + R T ln Nw
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Where µ0w is the chemical potential of pure water and has a value of zero
R is the gas constant
T is the temperature in K and
Nw is the mole fraction of water
Chemical potential can also be related to the vapour pressure of the water.
Raoult's law:
…. the vapour pressure of solvent vapour in equilibrium with a dilute solution of a nondissociating solute is proportional to the mole fraction of solvent in the solution.
e = e0. Nw
 where e0 is the vapour pressure of pure water
 The vapour pressure ratio is given by :
The chemical potential therefore becomes
factors affecting water potential ?
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Decreases in e due to:
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the presence of solutes
negative hydrostatic pressure
temperature but note it affects both e and e0 similarly
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increase in e due to:
o positive hydrostatic pressure
o temperature ( affecting both e and e0 similarly)
 Chemical potential is expressed per mole of water, an inconvenient unit.
 The chemical potential can be expressed on a per unit volume basis - the volumetric water
potential (w) by dividing by the partial molar volume of water: ie the volume occupied by
the water molecules in the solution.
The components of volumetric water potential are:
Where
s = potential due to solutes (-ve)
p = pressure potential (+ve or -ve)
m = matric potential (-ve)
g = potential due to gravity
The availability of soil water to plants is determined by:
 the soil water potential and
 the movement of water in the soil.
 Water moves in response to a potential gradient.
 The components of potential differ in effectiveness depending on the system - e.g presence or
absence of a selectively permeable membrane.
 soil water is held in pores and work has be done to remove water from the soil.
 The equation for capillary rise is used to identify the size of pores which hold water as the soil
dries.
 The change in potential across the meniscus p = 2/ r
where t = surface tension, and r is the meniscus radius of
curvature.
 This approximates to: p = 2 cos a /R
where R is the radius of the pore; p = hg , and
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 is the density of the liquid in the pore.
 R is given by 2/hg if the angle of contact is very small.
 Similarly, h is given by 2 /Rg
Eg. for a pore of diameter 300µm the capillary rise is 10 cm.
The soil moisture characteristic curve
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The relationship between the volume of
water in the soil and the work required to remove
the water is given by the moisture release curve.
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Pore necks mean that the curve for a
drying soil is not identical to that for a wetting
soil - a phenomenon known as hysteresis.
In the soil the water potential is given by :
 p is positive, so will only operate below a water table, ie when m increases to zero.
 Also, s only becomes really important for water movement to plant roots, or for vapour
transfer.
 s becomes important in soils for fertilizer bands. Water can move in vapour form across an
air-water interface towards the fertilizer band. The soil will eventually saturate in the region
of the fertilizer band and the water will flow away from the area due to the difference in
matric potential.
 The sign for the gravitational component depends on the reference point, but it is often
convenient to use the soil surface as the reference point.
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In this diagram there is a water table at 80 cm.
Using the soil surface as the reference level, g will
be zero at the soil surface and -80 at the water table.
If the soil water in the unsaturated zone is in
equilibrium with the water in the saturated zone, there
will be no net vertical movement. The hydraulic head
h is constant.
By definition,m will have a value of zero at the
water table.
The value of h must have a value of -80 cm at all
depths in the unsaturated layer.
Below the water table the value of h will be
influenced by the pressure potential, which will
increase as the gravitational potential decreases.
The available water content of a soil
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This is the difference in the volume of water that is held in the soil after it has stopped draining
under gravity (often referred to as field capacity) and the volume held when plants show
permanent wilting. The potential often chosen to determine the upper limit is -0.033 MPa. In
most soils a value closer to -0.02 MPa is more appropriate.
The lower limit is commonly taken to be -1.5 MPa.
The tendency is for there to be less water available in sands and clays, with most being available
in silt-loam soils.
The Field Water Balance
How water is redistributed in the soil-plant atmosphere continuum
Incoming precipitation (rain, snow, hail) - P may be augmented with irrigation – I and is given by:
P+I=R+D+S+E
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R. is water that will runoff the field into ditches or streams
S, is water that will add to the soil water content and be stored
D, is water that will drain through the soil and enter the water table,or be intercepted by a
subsurface drainage system, but in any case it and is lost to the crop
And some water - E, will be evaporated from the surface of leaves, or from the soil surface,
or will be taken up by plants and be used for cooling as it evaporates at the surface of the
mesophyll cells surrounding the sub-stomatal cavity
Water movement in the soil-plant-atmosphere continuum.
A temperature gradient has little effect on movement of water in the liquid phase. Water will move
in vapour phase from warmer to cooler regions in response to the lowering of vapour pressure
What force drives water moving through soil ?
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The total water potential gradient -d/dx
The resistance to water movt is expressed in reciprocal form as a conductance per unit
distance or conductivity k.
The flux of water (volume per unit area) is given by:
Hydraulic conductivity is however dependent on the water content of the soil.
Hydraulic conductivity can decrease by several orders of magnitude between field capacity and
wilting point.
The coarser the soil texture the greater the change in hydraulic conductivity with water content.
The potentials that contribute to the driving force for water moving through plants
Because we are often concerned with cell to cell movement we can ignore g. Similarly there is a
positive hydrostatic pressure inside the cell vacuole so m can also be ignored. Thus
w = p +s
The driving force for water flow in the soil-plant-atmosphere-continuum is evaporation of water
from leaf surfaces - transpiration.
Transpiration
The energy balance of a leaf can be represented as follows:
Rn = H + E + h
where:
Rn = net radiation; H= sensible heat exchange with atmosphere (+ve or -ve)
E= latent heat flux and is the product of the latent heat of vaporization and the mass of water
evaporated, h= energy used in photosynthesis (-ve) or released in other metabolic processes
(+ve), usually not more than 2-3% of Rn.
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Normally about 80% of the net radiation is dissipated by evaporation of water from the leaf
surface (transpiration).
If water not available for transpiration, leaf temperature rises and heat is transferred to air.
A well-watered crop may evaporate 5mm water per day. This is equivalent to 5 x 104 kg
water
The driving force for transpiration is the difference in vapour pressure between the
evaporating surface in the leaf and that in the air above the leaf :
eL - eair. Ie L - air
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The resistances to the transport of water is constituted as follows:
- Stomatal resistance (rst)
- Cuticular resistance (rc) - large compared with the other leaf resistances
Relative importance of components of resistance to water vapour in leaves
Components of resistance are:
- resistances in the mesophyll (rm)
- the boundary layer resistance of the air (ra)
- rst - varies according to the stomatal aperture.
- ra - the boundary layer resistance is determined by wind velocity, which makes the layer
thinner as velocity increases
- If instead of laminar flow, the velocity is such that tubulence is created, this breaks-up
the boundary layer. In reality, the increased flow will cool the leaf, and reduce
transpiration.
- The structure of the leaf surface also affects the thickness of the boundary layer.
Transpiration rate is controlled primarily by the stomatal resistance rst
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The degree of opening depends on the turgidity of the guard cells which in turn depends on:
 CO2 concentration in the sub-stomatal cavity
 light intensity (affects CO2 concentration)
 Water deficiency (through direct or triggered reaction)
At a potential of about -1.7 MPa in the guard cells, stomates close.
What is the vapour pressure in the sub-stomatal cavity relative to the saturated vapour pressure?
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The vapour pressure is always very close to saturation (98-99% relative humidity) because
of the wet surfaces of the mesophyll cells.
In free air the vapour pressure hardly change with temperature, but the relative humidity declines
because warm air can hold more water vapour than cooler air.
As the leaf warms, the vapour pressure in the leaf increases, because the vapour pressure of liquid
water increases markedly with temperature.
In a lighted growth chamber at 100% relative humidity, the leaf temperature will increase so vapour
pressure at the evaporating surface will increase, and water be transpired.
Movement in the plant
Driving force governing water movement between the soil and the leaf:
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IfL is the leaf water potential and S is the soil water potential
The driving force is
A typical value of water potential in a transpiring leaf is -1 MPa
The resistances to water movement in the plant are constituted as follows:
 In stem, petiole and leaf veins - xylem
 In root - root surface to xylem
 In soil - bulk soil to root surface
 The greatest resistance is in the root - water passes through membranes.
There may be a resistance to flow at the peridermis (if one is present). In most cases the greatest
resistance is at the endodermis
The flux of water movement in the plant may be summarized as:
 Water potential in the soil might be 0.05 MPa ie 50KPa (just below field capacity).
At the root-soil interface it might be 0.06 MPa
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Resistance in the soil is unimportant until the soil water content declines such that
hydraulic conductivity decreases.
In the xylem the water potential is -0.085 MPa.
eL, eair, rL, and rair are all very variable on a daily basis. S, rS, rrt, and rxyl are all relatively
constant.
L is the plant parameter that has to change in response to environmental factors.
NB leaves will curl to reduce the evaporative surface, as well as close the stomates
Q At night what happens to plant water potentials ?
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All tend to increase.
Q Why do leaves guttate ?
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Ions continue to be pumped into the xylem even though transpiration has ceased. The
increased concentration results in a lower osmotic potential in the xylem, and a gradient is
developed across the root. Water moves into the xylem in response.
This is the basis of root pressure
Absorption of Water from Soil
1) Transpiration of water from leaf surfaces lowers water potential in leaf, hence creating a
potential gradient between leaf and soil. Water flows in response to this gradient. Resistances to
water flow may occur in the soil, at the soil-root interface or in the root.
2) The ability of the plant to extract water from soil at a sufficient rate to prevent stress depends on:
Soil water availability and conductivity
Extent of root systems
Water Deficits and Plant Growth
Water stress affects the various plant processes differently. For example leaf expansion may be
reduced to very low levels at water potentials of -0.2 MPa whereas stomatal closure may not begin
until potential drops to -1.0 MPa.
The importance of root distribution
Adding branch roots to a single axis, or new axes all reduce root resistance because the new roots
are in parallel with the original root. The resistances of the two systems are given by:
In consequence, water needs to move less far through the soil to reach a root, and a larger volume of
soil is explored for water and other nutrients.
Q Why do soils dry from the top down ?
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As xylem resistance is small, though significant, root resistance increases with distance from
the stem.
Water Use Efficiency
Water use efficiency can be expressed in agronomic or physiological terms.
It makes more sense when considering the processes to determine the above ground dry matter
production.
For irrigation, the water used will include drainage and run-off as well as evaporation and
transpiration. Much of the effort in irrigation schemes is to minimize losses through drainage and
run-off.
We can also consider the physiological water use efficiency
If the CO2 concentration in the atmosphere increases, water-use efficiency increases.
If the atmosphere is is more humid, it also increases WUE.
Q Are there differences between plants in WUE ?
The resistances for CO2 and water are not the same in all species.
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Similarly, the effective concentration of CO2 in the sub-stomatal cavity is species
dependent.
Some plants have developed a novel method of increasing water use efficiency. They are
mainly succulents and use crassulacean acid metabolism (CAM).
CO2 enters the leaf and is fixed in the cytoplasm of mesophyll cells by PEP carboxylase,
which converts phosphoenolpyruvate to oxaloacetate. The latter is reduced to malic acid.
The malate is stored in the vacuoles. During the day, stomates are closed. Malate is released
from the vacuoles, decarboxylated, and the released CO2 is fixed by the Calvin cycle in the
chloroplasts.
This pathway can be switched-on as a result of water stress (or salt stress) in some
halophytes, the rapidity of the response being more efficient as plants age.
Because of the strong interdependence of C02 uptake and water transpired, for a given crop there is
a reasonably constant relation between yield (Y) and transpiration (T) at a given vapour pressure
deficit (e).
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Signalling of changes in the root zone water status
The traditional concept was that shoot growth was affected by soil water deficit because of the
changes in the water supply to the leaves and meristems.
Over the past 10-15 years it has been established that roots sense a drying soil and send a signal to
the shoot causing reduced leaf expansion or stomatal closure before there is any detectable change
in leaf water status. Davies and Zhang ( Annu. Rev. Plant Physiol. Plant Mol. Biol. 1991. 42, 5576) report one of the most elegant demonstrations of roots signalling changes in soil water status.
They measured daily increments in leaf area for apple trees which had their root systems divided
between two pots of soil. Plants were either watered through additions to both pots, or to one pot
only. After 24 days of these treatments, some plants, which had previously had been watered
through one pot had water restored to the roots in the dry soil. Another group of plants
that had also been watered through one pot only, had their roots in the dry soil excised. Growth was
followed over the next 14 days.
After two weeks the plants that had had
the roots excised grew as well as the
rewatered pots, and both grew better than
the half-watered plants. The evidence
seems unmistakable that messages from
the roots in the unwatered pot reduced the
growth of the shoot. Cutting-ff the supply
of the message