EVAPORATION Flashcards
the process in which a liquid
changes to the gaseous state at the free
surface, below the boiling point through the
transfer of heat energy. It is a cooling process in
that the latent heat of vaporization must be
provided by the water body
EVAPORATION
The rate of evaporation is dependent on
A. VAPOR PRESSURES AT THE WATER SURFACE AND AIR ABOVE
B. AIR AND WATER TEMPERATURES
C. WIND SPEED
D. ATMOSPHERIC PRESSURE
E. QUALITY OF WATER
F. SIZE OF THE WATER BODY
FACTOS AFFECTING EVAPORATION
VAPOR PRESSURE
TEMPERATURE
WIND
ATMOSPHERIC PRESSURE
SOLUBLE SALTS
HEAT STORAGE IN WATER BODIES
The rate of evaporation is
proportional to the
difference between the
saturation vapor pressure
at the water temperature,
and the actual vapor
pressure in the air.
VAPOR PRESSURE
The rate of evaporation
increases with an
increase in the water
temperature
TEMPERATURE
aids in removing the
evaporated water vapor
from the zone of
evaporation and
consequently creates
greater scope for
evaporation
WIND
A decrease in the
barometric pressure, as
in high altitudes,
increases evaporation
ATMOSPHERIC PRESSURE
When a solute is dissolved in
water, the vapor pressure of
the solution is less than that
of pure water and causes
reduction in the rate of
evaporation. For example,
evaporation from sea water is
about 2-3% less than that
from fresh water
SOLUBLE SALTS
Deep water bodies have
more heat storage than
shallow ones. The effect of
heat storage is essentially to
change the seasonal
evaporation rates and the
annual evaporation rate is
seldom affected.
HEAT STORAGE IN WATER BODIES
EVAPORATION RATE IS ESTIMATED USING THREE MAIN METHODS:
EVAPORIMETER DATA
EMPERICAL EQUATION
ANALYTICAL METHODS
- It is standard pan of 1210mm diameter and 225mm
depth used by the US Weather Bureau. The depth of water is maintained
between 18cm and 20cm.
CLASS A EVAPORATION PAN
This pan evaporimeter specified by IS: 5973-1970, also
known as modified Class A Pan, consist of a pan 1220mm in diameter with
255mm of depth. Made of copper sheet of 0.9mm thickness , tinned inside
and painted white outside.
ISI STANDARD PAN
This pan, 920mm square and 460mm deep is made
up of unpainted galvanished iron sheet and buried into the ground within
100mm of the top.
COLORADO SUNKEN PAN
With a view to simulate the
characteristics of a large body of water, this square pan (900mm side and
450mm depth) supported by drum floats in the middle of a raft (4.25m x
4.87m) is set afloat in a lake.
US GEOLOGICAL SURVEY FLOATING PAN
evaporation pans are not exact models of large
reservoirs
PAN COEFFICIENT CP
it is usual to instal evaporation pans in such locations
where other meteorological data are also simultaneously collected
EVAPORATION STATIONS
ANALYTICAL METHODS FOR THE DETERMINATION OF LAKE EVAPORATION CAN BE BROADLY CLASSIFIED INTO THREE CATEGORIES
- WATER BUDGET METHOD
- ENERGY-BALANCE METHOD
- MASS-TRANSFER METHOD
is the simplest of the three analytical methods and is also least reliable, it involves writing the hydrological continuity equation for the lake and determining the evaporation from a knowledge or estimation of other variables
WATER-BUDGET METHOD
an application of the law of conservation of energy. the energy available for evaporation is determined by considering the incoming energy, outgoing energy and energy stored in the water body over a known time interval
ENERGY BALANCE METHOD
Based on theories of turbulent mass transfer in boundary layer to calculate the mass water vapour transfer from the surface to the surrounding atmosphere
MASS TRANSFER METHOD
refers to the movement of mass from one location to another, often driven by differences in concentration, pressure, or temperature.
MASS-TRANSFER METHOD
PRIMARY METHODS OF MASS TRANSFER
- DIFFUSION
- CONVECTION
- INTERFACIAL MASS TRANSFER
the movement of molecules from high concentration to low concentration. It follows Fick’s laws
DIFFUSION
Mass transfer due to bulk fluid motion either natural or forced
CONVECTION
