Soil Water (Vadose Zone)

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Soil Water (Vadose Zone) Infiltration - movement
of water from soil surface to soil Capillary
rise - movement from saturated zone upwards into
unsat zone by surface tension Recharge -
movement of water into the saturated zone
(groundwater recharge) Interflow - flow in the
unsat zone downslope Percolation - downward flow
in unsat zone Understanding infiltration and
redistribution important for e.g. 1. 2.
movement of nutrients, pollutants, etc. 3.
estimating timing and amounts of ground-water
recharge 4. Predicting stream response to
snowmelt and rainfall Infiltration and
redistribution involve unsaturated flow in a
porous media (soil). Therefore, first must
understand the nature of the porous media
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P lt 0
P gt 0
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• Soil Properties
• Soil matrix composed of rock, air, water, and
  organic matter. To simplify things organic matter
  is often not considered, but does contribute to
  the soil hydraulic properties.
• Size of pores in soil to grain size
• soil is a mixture of grain sizes
• soil texture is a term used to describe the grain
  size distribution (by weight), less than 2mm
  (upper boundary for sand)
• Measurement
• -
• silt/clay measured by settling techniques
  (pipette analysis, sedigraph, lpsa)
• Stokes Law
• V 2r2g(?particle ?medium)
• 9?

viscosity
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Particle density (?m) is a weighted average
density of mineral grains. Can be measured by
displacement techniques where Ws is the mass
and Vs is the volume of the mineral grains in
practice this is difficult so usually assumed to
be around 2.65 g cm-3
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Bulk Density (rb) Weight of unit volume of soil
after oven-drying at 105oC for 24 hrs In
almost all cases we discuss dry bulk density, but
in rare occasions we might be interested in the
wet bulk density (bulk density in its natural
state with a given amount of soil moisture)
rb Ws/V Ws/(Va Vw Vs)
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• Porosity (n)
• Proportion of pore spaces in a volume of soil
• Like bulk density is constant over time in most
  cases, and decreases with depth (compaction down
  deep biological cavities shallow)
• - In general finer-grained soils have higher
  porosities
• Organic material can increase the porosity of a
  soil dramatically
• porosity increases with sorting

n (Va Vw)/V Vv/V
Void Ratio e Vv/Vs
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Volumetric Water Content (?) (or simply soil
moisture) ratio of water volume to soil
volume ranges from 0 (dry) to n
(saturated) Can be measured at the same time as
bulk density when drying Non-destructive
techniques for monitoring field conditions
include 1. Electrical resistance blocks -
measures resistance in an artifical block of
porous material inserted in the soil profile 2.
Neutron moisture meters - detect scattering of
neutrons by hydrogen atoms 3. Gamma-ray scanners
- measure absorption of gamma rays by
water Passive microwave techniques being
developed so that soil moisture can be measured
remotely. Resolution would only be a few square
kms
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Gravimetric Water Content ?g 100(Ww/Ws) Ww is
the weight of water in the soil, Ws is the weight
of solid particles Degree of Saturation (S) (or
wetness)
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Hydraulic properties of soils Darcys Law
defines flow in porous media Darcys Law is
where Vx is the volumetric flow rate in
the x direction per unit x-sectional area of
soil z is height above some arbitrary datum p is
water pressure (measured in relation to
atmosphericgage pressure) ?w is weight density
of water Kh is the hydraulic conductivity (or
permeability) of the soil or ability of the
soil (or rock) to conduct water through it. A
rate that depends on textural properties in
saturated conditions, or textural 2 in
unsaturated conditions dz/dx is the gradient of
gravitational potential energy per unit weight of
flowing water d(p/?w)/dx is the gradient of
pressure-potential energy
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Since ?w is effectively constant for situations
that do not involve major gradients in
temperature and salinity, we can express the
pressure-potential energy as pressure head (hp)
In unstaturated conditions both pressure head
and hydraulic conductivity depend on soil
moisture so Darcys Law for unsaturated flow
becomes
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n
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• Soil-Water Pressure
• - Pressure is force per unit area PF/A
• - conventionally measured in relation to
  atmospheric (gage)
• pgt0 and hpgt0 in saturated flows
• plt0 and hp lt0 in unsaturated flows
• - Negative pressure is tension or suction
• When hp goes below zero it is called tension
  head or moisture potential
• Fetter denotes moisture potential as ?
• In unsaturated soils, water is held to grains by
  surface-tension forces and p and ? will always be
  negative
• tension inversely proportional to the radius of
  capillary tubes (pores) and proportional to the
  radius of curvature of menisci
• therefore tension increases as soils get drier,
  and tension is higher in silty soils than in
  sandy soils

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hc 2scos?/?gr
s is surface tension of fluid r is the radius of
capillary tube
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What is ?g? Now simplify for soils
hc 2scos?/?gr
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• Capillary fringe is not an even surface

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Measurement of tension can be done directly
using 1. Tensiometers - practical range covers
coarse soils only (lower tensions) - use of
several at different depths can get hydraulic
gradients
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• 2. Thermocouple psychrometers
• - measures soil-water tension using transfer
  equations based on the humidity of soil air
• - at equilibrium potential of soil moisture is
  proportional to the relative humidity
• wet bulb and dry bulb temperature measured and RH
  calculated
• Relationship between Soil RH and tension is
• Where R is the gas constant (8.31 J/mol K), Ta is
  the Kelvin temperature of the air, and M is the
  molecular weight of water.

Relation between pressure head and water content
for a given soil is called the Moisture-Characteri
stic Curve - curves for different soil types
constructed on soil samples in the lab
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Fetters Moisture Potential
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Hysterisis - Problem arises because tension of
the soil for a given water content is not unique,
but depends on the soils history of wetting and
drying - hysterisis still a largely mysterious
phenomenon with many competing theories
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- - during wetting the small pores fill first and
during draining the large pores empty
first -others include 1. Meniscus radii is
greater in an advancing fluid than in a
retreating one 2. Entrapped air in a newly
wetted soil decreases water content per unit
suction 3. Clay-rich soils change geometry
through swelling and shrinking during wetting and
drying - As a standard, the draining
(desorption) curve is the one used
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Water Conditions in Natural Soils Field Capacity
(?fc) is defined as the amount of water in a soil
after initial draining causes the soil to reach a
state of quasi-equilibrium, after which removal
of water only occurs very slowly - this is the
water that can be held against the force of
gravity - pressure head for all soils -340 cm
for all soils - at this point water can only be
removed by evapotranspiration - ?fc for sands is
as low as 0.1 and gt0.3 for clays - plants can
only exert a suction of about -15,000 cm and when
water content (tension) exceeds this value the
Permanent Wilting Point has been reached -
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Available water content (?a) is the difference
between field capacity and the permanent wilting
point
Hygroscopic water is water that is in equilibrium
with the surrounding atmosphere. Can only be
removed through application of high heat in the
lab for prolonged periods. - in clay-rich soils,
even drying for 24 hours at 105oC may leave
behind some water that is strongly attracted to
the clay particles
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Pedologic Horizons - - sequence of horizons
makes up the soil profile - horizons delimited by
color, texture, organic matter, the degree of
deposition (illuviation) or removal (eluviation)
of material by physical and chemical processes -
development of horizons dependant on climate,
topography, disturbances (erosion, deposition),
parent material, and duration of horizon
development
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Hydrologic Horizons
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Hydrologic Horizons - horizons defined by water
content and pressures Ground-Water Zone
(phreatic zone) - saturated with pressure - if
no flow, pressure is only hydrostatic p
is zo is the height of the water table ?w is
weight density of water
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Capillary Fringe (tension-saturated zone) -
lowest portion of the vadose zone saturated or
near-saturated due to capillary rise of
ground-water - pressures will be negative even if
saturated We can approximate r with the average
grain size of the soil so for most soils so
that all we need to know is something about the
temperature and texture at depth to calculate hcr
(assuming relatively fresh water) -
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Intermediate Zone - zone where water largely
enters by percolation from above and leaves by
gravity drainage - moisture content will increase
after heavy rains and return to field capacity
after a while - tensions gt?ae - - may not
exist in certain situations (e.g. wetlands in
shallow bedrock regions)
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Root Zone (Soil Moisture Zone) - top is soil
surface and bottom is depth to which plants can
extract water - water enters by infiltration and
leaves by evapotranspiration or gravity
drainage - water content above permanent wilting
point - below field capacity much of the time
between rainfall events Comparison of pedologic
and hydrologic zones - root zone usually extends
into zone of eluviation and may occupy entire
solum - solum usually developes above capillary
fringe, but water table may move into solum
seasonally - gleying is a blue/grey/green
mottling of soil that indicates soil below water
table for long periods of time -Impervious and
semi-impervious layers in the soil profile can
created perched water tables
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Infiltration Infiltration rate f(t) - rate at
which water passes from the surface to the
soil Water-input rate w(t) - rate of delivery of
water to the soil surface (i.e. rain or
snowmelt) Infiltration capacity fc(t) - maximum
f(t) possible for a given soil Depth of ponding
Y(t) - depth of water standing on the surface
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Measurement Ring Infiltrometers -metal rings
inserted in soil to which water is added at
recorded rate - infiltration rates usually start
high and settle to a constant lower rate which is
taken as the infiltration capacity - fc(t) in
this case is . the saturated hydraulic
conductivity of surface soil - reduction of
lateral movement by double ring, or adjust single
ring values -
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Sprinkler Plot Studies - apply artificial
rainfall to an experimental plot and record the
amount that doesnt infiltrate and runs off f(t)
w - q(t) Soil-Water changes - install e.g a
series of tensiometers to measure change in
hydraulic gradient and relate that to
infiltration rate if w is known or controlled
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Factors affecting infiltration rate 1. 2.
Saturated hydraulic conductivity - vegetation,
burrowing fauna, etc. at surface tends to
increase K - leaf litter can have opposite
effect - freezing of wet surface greatly reduces
K - clays shrink and swell - raindrop
compaction - human intervention tilling, paving,
etc. 3. Soil moisture at start of water input -
left over moisture from previous events - raised
water table
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4. Roughness and inclination of surface - only
important to infiltration rates during ponding -
steep slopes and smooth surfaces remove water
faster (and lower infiltration rates overall 5
chemical characteristics of surface - 6.
Physical and chemical characteristics of water -
Salinity and temperature of water affect tension,
density, and viscosity - K significantly
increases with increasing temperature and
decreasing salinity
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• Modeling Infiltration
• Richards Equation
• - basic theoretical equation for vertical
  unsaturated flow in a homogeneous porous medium
• - numerical solutions and require detailed data
  usually not available, so
• Green-and-Ampt Model
• - model based on Darcys Law and the principal of
  conservation of mass
• - basic model assumes we are dealing with a block
  of soil that is homogeneous to an indefinate
  depth (porosity, and sat. hydraulic conductivity
  are consistant throughout
• - no water table, capillary fringe, or
  impermeable layer
• - horizontal surface with no evapotranspiration
  and no ponding at t0
• - initial moisture in entire profile at t0 is
  considerably below field capacity
• water is delivered to the surface by rainfall or
  snowmelt at a rate of w and continues for a
  duration of tw