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    Evaporation

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    Amraha Noor
    Evaporation is the process by which water changes from a liquid to a gas. It occurs below the boiling point through vapor pressure. Condensation is the opposite change of state from gas to liquid. Sublimation bypasses the liquid phase by going directly from solid to gas. The rate of evaporation depends on factors like temperature, humidity, wind, and pressure. It is measured using evaporation pans, which require a pan coefficient to estimate actual lake evaporation. Other estimation methods include empirical equations, water balance, and energy balance.

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    Evaporation

    Uploaded by

    Amraha Noor
    46 views43 pages
    Evaporation is the process by which water changes from a liquid to a gas. It occurs below the boiling point through vapor pressure. Condensation is the opposite change of state from gas to liquid. Sublimation bypasses the liquid phase by going directly from solid to gas. The rate of evaporation depends on factors like temperature, humidity, wind, and pressure. It is measured using evaporation pans, which require a pan coefficient to estimate actual lake evaporation. Other estimation methods include empirical equations, water balance, and energy balance.

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    Hydrogeology

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    Evaporation is the process by which water changes from a liquid to a gas. It occurs below the boiling point through vapor pressure. Condensation is the opposite change of state from gas to liquid. Sublimation bypasses the liquid phase by going directly from solid to gas. The rate of evaporation depends on factors like temperature, humidity, wind, and pressure. It is measured using evaporation pans, which require a pan coefficient to estimate actual lake evaporation. Other estimation methods include empirical equations, water balance, and energy balance.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    46 views43 pages

    Evaporation

    Uploaded by

    Amraha Noor
    Evaporation is the process by which water changes from a liquid to a gas. It occurs below the boiling point through vapor pressure. Condensation is the opposite change of state from gas to liquid. Sublimation bypasses the liquid phase by going directly from solid to gas. The rate of evaporation depends on factors like temperature, humidity, wind, and pressure. It is measured using evaporation pans, which require a pan coefficient to estimate actual lake evaporation. Other estimation methods include empirical equations, water balance, and energy balance.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 43

    Evaporation

    Evaporation

    • The conversion of water from a liquid to a gas.


    • Water is transferred from the surface to the atmosphere
    through evaporation, the process by which water changes
    from a liquid to a gas. It contributes to the humidity of the
    air.
    Evaporation
    • Evaporation is the change
    of a liquid into a vapor at
    a temperature below the
    boiling point. This occurs
    at any temperature
    between the melting
    point and the boiling
    point. However, the
    warmer the temperature
    the more quickly the
    water will evaporate.
    Condensation
    • Condensation is the
    change of a gas or vapor
    to a liquid, either by
    cooling it or by subjecting
    the substance to an
    increase in pressure.
    When water vapor cools
    in the atmosphere, it
    condenses into tiny drops
    of water which form
    clouds.
    Sublimation
    • Sublimation is the
    process of changing
    from a solid to a gas
    without passing through
    a liquid phase.
    Absolute humidity
    • The absolute humidity of a given air mass is
    the no of grams of water per cubic meter of
    air.
    The relative humidity
    • The relative humidity (RH) is calculated using the actual water
    vapor content in the air (mixing ratio) and the amount of
    water vapor that could be present in the air if it were
    saturated (saturation mixing ratio)
    • RH = w/ws x 100%
    • The relative humidity is simply what percentage the
    atmosphere is towards being saturated.
    • Relative humidity is not a good measure of exactly how much
    water vapor is present (50% relative humidity at a
    temperature of 80 degrees Fahrenheit will involve more water
    vapor than 50% relative humidity at -40 degrees)
    • Relative humidity can change even when the amount of water
    vapor has not changed (when the temperature changes and
    the saturation mixing ratio changes as a result)
    Dew Point
    • It is the temperature at which condensation will begin.
    • The dew point temperature is the temperature at
    which the air will become saturated if the pressure
    and water vapor content remain the same
    • The higher the dew point, the more water vapor that
    is present in the atmosphere.
    • The temperature is always greater than the dew point
    unless the air is saturated (when the temperature and
    dew point are equal)
    Vapor pressure
    • Vapor pressure (e) is simply the amount of
    pressure exerted only by the water vapor in
    the air.
    • The pressures exerted by all the other gases
    are not considered.
    • The unit for vapor pressure will be in units of
    pressure. (millibars and hectopascals are the
    same value with a different name)
    Rate of evaporation
    Rate of evaporation depends on the factors such as:
    • Water temperature
    • Absolute Humidity
    • Wind
    • Atmospheric pressure
    • Soluble salts
    • Wet soils
    • Deep and shallow water bodies
    How to measure Evaporation
    • It is rather impossible to measure evaporation
    directly in field. Evaporation from water
    surface its estimated by different methods and
    its values are correlated to field data .
    How to measure Evaporation
    • Evaporation pan
    • An evaporation pan is used to hold water during
    observations for the determination of the quantity of
    evaporation at a given location. Such pans are of
    varying sizes and shapes, the most commonly used
    being circular or square. The best known of the pans
    are the "Class A" evaporation pan . Often the
    evaporation pans are automated with water level
    sensors and a small weather station is located
    nearby.
    • The United States Class A pan is of cylindrical design,
    25.4 cm (10 in ) deep and 120.7 cm (4 ft ) in diameter.
    • The bottom of the pan is supported 3 to 5 cm above the
    ground level on an open-frame wooden platform, which
    enables air to circulate under the pan, keeps the
    bottom of the pan above the level of water on the
    ground during rainy weather, and enables the base of
    the pan to be inspected without difficulty.
    • The pan itself is constructed of 0.8 mm thick galvanized
    iron, copper , and is normally left unpainted.
    • The water level is measured by means of either a hook gauge
    or a fixed-point gauge. The hook gauge consists of a movable
    scale and Vernier fitted with a hook, the point of which
    touches the water surface when the gauge is correctly set.
    • A stilling well, about 10 cm across and about 30 cm deep,
    with a small hole at the bottom, breaks any ripples that may
    be present in the tank, and serves as a support for the hook
    gauge during an observation.
    • The pan is refilled whenever the water level, as indicated by
    the gauge, drops by more than 2.5 cm from the reference
    level.
    Pan Coefficient : Cp

    • Evaporation pans are not exact models of large reservoirs:


    • Their major drawbacks are the following:
    • They differ from reservoirs in the heat storage capacity and heat transfer
    characteristics from the sides and the bottom (sunken and floating pans aim to
    minimize this problem). Hence evaporation from a pan depends to some
    extent on its size (Evaporation from a pan of about 3m dia is almost the same
    as that from a large lake whereas that from a pan of about 1m dia is about 20%
    in excess of this).
    • The height of the rim in an evaporation pan affects wind action over the water
    surface in the pan. Also it casts a shadow of varying size on the water surface.
    • The heat transfer characteristics of the pan material is different form that of a
    reservoir.
    • Hence evaporation measured from a pan has to be corrected to get the
    evaporation from a large lake under identical climatic and exposure conditions.
    Pan Coefficient : Cp

    The actual evaporation from a nearby leak is


    less than that of pan evaporation

    Why ?

    • The sides of the pan is exposed to the sun.


    • The temperature over the pan is higher that over the lake.

    Lake evaporation = Cp pan evaporation


    E = Cp Ep

    Cp = pan coefficient = 0.7 for Class A Pan.


    Other Methods
    • Different methods are used for the
    computation of lake evaporation which are:
    • Empirical Equations
    • Water balance method
    • Energy budget and Energy balance method
    • Mass transfer method
    Dalton’s law
    E = C (es – e)

    E : rate of evaporation (mm/day)


    es : the saturation vapor pressure at the water surface; (milibar or mm of
    mercury) (mb or mm Hg)
    e : the actual vapor pressure of air (mb or mm Hg)

    C : a coefficient depends on wind speed, atmospheric pressure and other factors,


    C= a+ bu
    a , b are constants ;
    u : wind speed (m/s) ; ( u2 : wind speed at height 2m.)
    • Evaporation occurs till es = ea
    • If condensation takes place es ˃ ea
    Empirical Equations

    Most of empirical formulae are based on the Dalton-type equation:

    E = Kf(u) (ew - ea)

    E = lake evaporation in mm / day,


    es = saturated vapor pressure at the water-surface temperature;
    e = actual vapor pressure of over­lying air at a specified height;
    f(u) = wind-speed correction function and

    K = a coefficient.

    The term e is measured at the same height at which wind speed


    in measured.
    Empirical Equations

    Meyer's Formula (1915):

    E = KM (ew – ea ) ( I + u9/16 )

    u9 = monthly mean wind velocity about 8 m above ground


    KM = coefficient of 0.36 for large deep waters and 0.50 for
    small, shallow waters

    The limitations of the formula that at best be


    expected to give an approximate magnitude
    of the evaporation.
    Rohwer’s Formula
    Empirical Equations

    • Problem statement:
    A reservoir with average surface spread
    of 3.3 km2 in December has the water surface
    temperature of 22.5 C⁰ and relative humidity of
    35% .wind velocity measured at 2.0mabove the
    ground at the nearby observatory is 15km/h
    .calculate average evaporation loss from the
    reservoir in mm/day and the total depth and
    volume of evaporation loss for December.
    Water balance method
    • It balances all the incoming and outgoing of
    stored water in a lake or reservoir over a
    period of time.
    • Eq is in simplest form:
    ∑ inflow - ∑ outflow = change in storage +
    Evaporation loss
    ∑ I - ∑ O = E + ∆S
    Then E = ∑ I - ∑ O ± ∆S
    • It can be more generalized by taking all the factors of
    inflow and outflow:
    ∑I = P + Isf + Igf
    ∑O= T + Osf + Ogf
    Where :
    P = Precipitation
    T = Transpiration loss
    Isf=Surface inflow
    Igf =Ground water inflow
    Osf = Surface water outflow
    Ogf = Ground water outflow
    ∆S = Change in storage
    Measurement of all quantities is possible except
    Ogf , Igf and T
    This equation is fails to give accurate results since ground
    water inflow and outflow are very difficult to measure for
    a lake or reservoir.
    It gives us good results for annually measurements not for
    daily estimations.
    Energy Balance Method
    • Like water balance ,Energy balance for lakes or
    reservoirs can be carried out to calculate lake
    evaporation.
    • Energy of the lake is:
    Radiations = Stored energy
    Hli + Hsi - Hs - Hlo = Hi + Hif + Hs + He + Hlr + Hgf
    Energy Balance Method
    • Where:
    • Hli = Long wave radiations incident on the
    surface
    • Hsi = Incident Solar radiations
    • Hso = Reflected Solar radiations
    • Hlo = Reflected long wave radiations
    • Hi = Increased in stored heat energy of water
    • Hlf = Energy conducted due to the flow of the water
    • Hs = Sensible heat transfer between water and atmosphere
    • He = Energy used for evaporation
    • Hlr = Long wave radiations emitted from the water
    • Hgf = Heat absorbing into ground water
    Energy Balance Method
    • no water inflow/outflow to lake
    • no change in water temperature of lake
    • neglects sensible heat transfer to ground and
    atmosphere
    • neglects heat energy lost with water which
    leaves system as vapor
    • calculates evaporation on a daily time interval
    Energy Balance Method
    • Daily calculations are unreliable due to difficulties
    in measuring parameters but good estimate can
    be obtained if applied to monthly and yearly
    values .
    • Most difficult value to be measured is Hs ,it can
    be measured by Bowen’s ratio:
    • β = Hs / He
    • Ratio between heat lost by conduction to heat
    lost by evaporation.
    Energy Balance Method
    • Problem statement:
    Calculate for the month of august
    the solar radiations incident on earth’s surface
    and the net outgoing thermal (long wave)
    radiations for a place 20⁰ north latitude. Take
    no of sunshine hours recorded at the nearby
    IMD observatory as 11.5 and mean air
    temperature as 25 ⁰C .Take relative humidity
    for august as 98%.
    Table 4.6
    Table 4.7
    Mass transfer Method
    • Accurate estimation of the amount of water vapour transferred to
    atmosphere from a lake surface is still investigated. The equation
    proposed by the Thornthwaite and Holzman (1939) takes the following
    form:
    E = 0.000119 (e1 - e2 )(µ2 - µ1) / Pₓ ln (h2 / h1)
    E is in m/sec ,
    µ2 and µ1 are the velocities of wind in m/sec at height h 2 and h1 resp.
    e1 and e2 are the vapour pressure of air in pascal (pa) at height h 1 and
    h2 resp
    P is the mean atmospheric pressure in pa between height h 2 and h1 .
    h2 is the height taken close to the water surface level(lower height)
    and h1 is the upper height.
    Mass transfer Method
    • Problem statement:
    For air temperature of 25⁰ C,
    relative humidity of 98%, air pressure of 101.3
    × 103 Pa and wind speed of 3m/sec measured
    at 2m above the water surface, calculate the
    evaporation loss from the water surface .Take
    saturation height h1 as 0.03m.
    COMPARISON OF METHODS
    • Analytical methods can provide good results.
    However, they involve parameters that are
    difficult to assess.
    • Empirical equations can at best give
    approximate values of the correct order of
    magnitude.
    • In view of the above, pan measurements find
    wide acceptance in practice.

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