Evapotranspiration

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Evapotranspiration - Rate and amount of ET is
the core information needed to design irrigation
projects, managing water quality, predicting flow
yields, ground water recharge, etc. - Only
occurs when there is a vapor pressure gradient
between the evaporating surface and the air -
esat high during warm periods and actual vapor
pressure (e) in air is low when air is dry.
Evaporation increases with saturation deficit
(esat-e) and stops when it is zero - Daltons
Law never expressed mathematically by Dalton, but
simply states that if other factors remain
constant, evaporation is proportional to the wind
speed and vapor pressure deficit
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- evaporation from a particular surface is
directly related to availability of water at that
surface, or the opportunity for evaporation
(ranges from 100 at open water surfaces to
essentially 0 for very dry soils) - Within about
1 mm of the surface, water molecule transfer is
by molecular diffusion, after that it is by
turbulent eddy transfer (eddy diffusion) - as
heat is added to a water body the molecules
become increasingly energized and move more
rapidly, increasing the distance between the
molecules, and weakening the hydrogen bond. At
high temperatures more of the water molecules
will fly off into the adjacent layer of air -
condensation occurs at same time
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Factors affecting Evaporation from an open water
surface 1. Meteorology a) energy (mostly
solar) b) wind c) temperature at the air-water
interface d) humidity in the overlying air 2.
Geographical a) Water quality - saline water has
a reduced vapor pressure, and therefore
evaporation decreases about 1 for every 1
increase in salinity. Normally a small
consideration - turbidity of the water can affect
the heat budget, but generally not considered b)
Depth of water body - residence time and annual
turnover c) Size of the water surface (fetch)
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Additional factors specific to soils a) Soil
moisture content - Soil is different than open
water surface because opportunity for evaporation
is not always 100, can be more or less -
evaporation decreases as the soil surface dries,
therefore more rain is evaporated in small
frequent storms than in large infrequent
storms b) Soil capillary characteristics -
capillary rise is affected by soil texture. lt in
sandy soils gt in clay soils c) water table
depth d) soil colour - darker soils absorb more
heat e) presence of vegetation - shading
effects f) snow cover
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Physics of Evapotranspiration Ficks 1st law
of diffusion A diffusing substance moves from
where its concentration is larger to where its
concentration is smaller at a rate that is
proportional to the spatial gradient of
concentration where Fz(X) is the rate of
transfer of a constituent X in direction z per
unit area per unit time (the flux of X) C(X) is
the concentration of X D(X) is the diffusivity
of X in the fluid (also called diffusion
coefficient) Evaporation is a diffusive process
which follows Ficks first law of Diffusion
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Vapor pressure relations review 1. 2.
Latent-Heat Exchange Evaporation causes a
reduction in surface temperature due to latent
heat exchange, which may or may not be balanced
by transfer of radiative or sensible heat from
the air to the surface rate of latent transfer is
simply the rate of evaporation times the latent
heat of vaporization and the mass density of
water LE ?w?vE ?v (latent heat of vaporization)
decreases as the temperature of the evaporating
surface increases..
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Sensible-Heat Exchange Upward rate of sensible
heat exchange is given by the finite-difference
equation H is the upward rate of sensible
heat exchange by turbulent transfer KH is a
measure of the efficiency of vertical transport
of sensible heat by turbulent eddies..
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Bowen Ratio The ratio of sensible heat exchange
to latent heat exchange is called the Bowen
Ratio or combining the equations ca is
the heat capacity of air (1.005 J/g/C) P is
atmospheric pressure..
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Therefore the Bowen ratio depends on the ratio of
surface-air temperature difference to surface-air
vapor pressure difference times the psychrometric
constant (?) - ? is sometimes entered into ET
estimates as a separate term - not strictly a
constant because P and ?v vary with altitude and
temperature..
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Energy Balance for an evaporating surface LE K
L - G - H Aw - ?Q/?t LE through Aw are
average energy fluxes (energy gained or released
per unit area per unit time) through LE K net
shortwave radiation input L net longwave
input G net output via conduction to ground H net
output of sensible heat exchange with the
atmosphere Aw water advected energy (from inflows
and outflows) The term ?Q/?t is the heat change
in the evaporating body per unit area over the
given time period..