Evaporation from Bare Soil

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Evaporation from Bare Soil

• Presentation
• by
• David Kwaw-Mensah
• April 11, 2003

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Evaporation From Bare Soil

• Introduction
• Definition
• In the field occurs at
• Plant Canopies (transpiration) Soil
  Surfaces(evaporation) Evapotranspiration

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Physical conditions for Evaporation

• 1. Continual supply of heat to meet the
  latent heat requirement of water about2.5 x 106
  J/kg of water evaporated at 15oC.
• 2. Vapor pressure over evaporating bodypressure at surface of body(vapor pressure
  gradient).
• 3. Continual supply of water from or
  through the interior of the body to the site of
  evaporation. This depends on evaporativity
  (atmospheric demand) and evaporability of soil
  moisture.
• A proper formulation of an evaporation process
  should account for spatial and temporal
  variability, as well as for interactions with the
  above ground and belowground environment

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Circumstances under which evaporation may occur

• Capillary rise from water a water table
• This is tied to the capillary model, which
  regards the soil as analogous to a bundle of
  capillary tubes.
• These tubes are wider in sandy soils and narrow
  in clayey soils.
• The equilibrium height, hc of a capillary rise
  can be therefore be calculated from
• hc (2?cos?)/r?wg, (18.1) where
• ? surface tension,
• r capillary radius
• ?w density of water
• g acceleration
  due to gravity (9.8ms-2)
• ? wetting
  angle (normally taken as zero)

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The equation predicts a higher capillary rise in
clay than in sand.Reason clay has narrower
pores.Soil pores are have constrictions, dead
ends etc. Therefore capillarity differ in
different pores.Matric suction generally
increases with height above the water table.






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The number of water filled pores or wetness,
thus diminish in the soil as a function of
height.The soil is drier further above the
water table.The rate of capillary rise
generally decreases with time as the soil is
wetted to greater height and equilibrium is
approached.
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Capillary action is similar to infiltration,
except that it occurs in the opposite direction
against gravity.At the later stages of the
process, the flux tends to zero.Reason matric
suction gradient gravitational gradient which
occur in opposite directions.
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Steady Evaporation from Shallow Water
TableEquation q K (?) (d ?/dz 1)
(18.2) , or q D(?) d?/dz- K
(?). (18.3) q flux evaporation rate
under steady-state conditions? suction headK
hydraulic conductivityD hydraulic
diffusivity? volumetric wetnessz height
above the water table.
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When the suction profile is at equilibrium (d
?/dz 1), flow stops, i.e q 0.Another form of
Eq.(18.2)q/K (?) 1 d ?/dz . (18.4)The
integration of the equation gives the
relationship between depth and suction or
wetness, represented by z asz ? d ? /1 q/K
(?) ? ( K ?/ K (?) q) d ? (18.5)

.. (18.6)
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Gardner (1958) according to Hillel, gives an
empirical equation for K(?) K(?) a (?n
b)-1 (18.7), where a , b and n are
constants that must be determined for each soil.
The n is related to the steepness of the water
retention function.The suction head ? in this
relationship is expressed in centimeters.By
substituting Equation (18.7) in equation (18.2),
we haveq a(d ?/dz 1) / (?n b) e, the
evaporation rate.
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With equation (18.7) , Equation (18.5) can be
used to obtain suction distributions with heights
for different fluxes, and fluxes for different
surface-suction values. Hillel gives an example
in Fig. 18.2 on page 513, where the curves show
that the steady rate of capillary rise and
evaporation depends on depth of the water table
and on the suction at the soil surface.
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The suction at the soil surface is dictated by
the atmospheric evaporativity.The greater the
evaporativity, the greater will be the suction at
the soil surface.Increasing the suction at the
at the soil surface even to an infinite value can
increase the flux to an asymptotic maximal rate,
which depends on the depth of the water
table.According to the following equation, qmax
Aa/dn,where d depth of water table below soil
surface, and a and n are constants from equation
(18.7),The maximal evaporation rate decreases
with water-table depth more steeply in coarse
textured soils than in clays.
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Anat et al (1965) according to Hillel, indicate
maximal evaporation rate e max, varies inversely
with the depth of water table.With the equation
emax, 1 1.886/(n2 1)d-n
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Hazard of Salination due to high water table

• Capillary rise can supply water to the root zone
  of plants ( a positive side).
• The process can entail the hazard of salination
  where the ground water is brackish and potential
  evaporativity is high.
• Lowering the water table can decisively reduce
  the rate of capillary rise and evaporation

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Evaporation in the absence of a Water Table

• Initial constant-rate stage
• Occurs while soil is wet and
• conductive to supply water to the site of
  evaporation
• Evaporation rate is limited by and controlled by
  the following external meteorological conditions
• Radiation
• Wind
• Air temperature
• Humidity

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An intermediate falling stage also called
profile-controlled stage.A residual slow-rate
stage also called vapor-diffusion stage.