1.1) INTRODUCTION
Evaporation is the process by which water is transferred from
the land and water masses of the earth to the atmosphere. Transpiration is the evaporation counterpart for plants. It is the
process by which soil moisture taken up by vegetation is eventually evaporated
as it exits at plant pores. Evaporation and transpiration combined (evapotranspiration) generally,
constitute the largest component of
losses in rainfall-runoff sequences. Accordingly, good estimates of
evapotranspiration are a requisite for hydrologic modeling.
Infiltration
is that process by which precipitation moves downward through the surface
of the earth and replenishes soil moisture, recharges aquifier, and ultimately
supports streamflows during dry periods. Along with interception, depression
storage, and storm period evaporation, it determines the availability, if any
of the precipitation input for generating overland flows. Furthermore,
infiltration rates influences the timing of overland flow inputs to channelized
system. Accordingly, infiltration is an important component of any hydrologic
model.
1.2) EVAPORATION
Because there is a continuous exchange
of water molecules between an evaporating surface and its overlying atmosphere,
it is common in hydrologic practice to define an evaporation as the net rate of vapor transfer.
Evaporation from a particular surface
is directly related to the opportunity for evaporation (availability of water)
provided by that surface. For open bodies of water, evaporation opportunities
is 100% while for soils it varies from a high 100% to essentially 0% at stages
of very low moisture content.
Direct measurement of evaporation are
not easily obtained for large bodies of water because of the extensive surface
involved. In fact, of all variables included in the general hydrologic
equation, surface runoff is the only one that readily permits direct evaluation.
The choice of method used to determine evaporation depends on the required accuracy of results and the type
of instrumentation available.
1.3) ESTIMATING EVAPORATION
The most common method used in
estimating evaporation are water budget method and use of evaporation pans. The
pan method is less expensive method and will frequently provide good estimates
of annual evaporation.
1.1.1)
WATER BUDGET CALCULATIONS
The water budget method for
determining evaporation is very simple procedure, but it seldom produces
reliable result. In this method, reservoir (lake, pool, etc) evaporation, E can
be computed by the formula :
Es
= P + R1 – R2 + Rg –Ts – I - ∆Ss
All the terms are in volume
units for a time period of interest, and ∆t should be at least a week. In
general, however, the method would more likely to be used to estimate monthly
or annual evaporation from a particular reservoir. Precipitation, runoff and
changes in storage can often be determined within reasonable limits of
accucary, but evaluation of net seepage, Os is frequently subject to
appreciable errors. Seepage estimates usually come from measurement of
groundwater levels ands and/or soil permeability. The water budget is usable on
continuous basis if a stage-seepage relation for the lake can be established.
1.1.2)
MASS TRANSFER TECHNIQUE
Mass transfer equation are
based primarily on the concept of the turbulent transfer of water vapor (by
eddy motion) from an evaporating surface to the atmosphere. Many equations,
both theoretical and empirical, have been developed. Most are similar in form
to a relation between evaporation and vapor pressure first recognized by Dalton
:
E = k(eo
– ea)
where E = direct evaporation
k = coefficient dependent on the
wind velocity, atmospheric pressure, and other factor.
eo , ea = the
saturation vapor pressure at water surface temperature and air, respectively.
Theoretical mass transfer
equations are based on the concepts of discontinuous and continuous mixing air
at the air-liquid interface.
Empirical approaches often require
exacting and costly instrumentation and observations, so their general utility
is limited. The complexity of the equation varies from simple equation like E =
k(eo – ea) to complex relations like Sutton’s equation for
a circular lake of radius r :
Where
E = evaporation (cm/day)
Ρ
= atmospheris pressure
U
= average wind velocity (cm/s)
ρ
= mass density of air (g/cm3)
r
= radiusof lake (cm)
n
= empirical constant
G’
= a complex function
1.1.3)
USE OF EVAPORATION PANS
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 and the "Sunken Colorado Pan". In
Europe, India and South Africa, a Symon's Pan (or sometimes Symon's Tank) is
used. Often the evaporation pans are automated with water level sensors and a
small weather station is located nearby.
A) CLASS A EVAPORATION PAN
In the United
States, the National Weather Service has standardized its measurements on the
Class A evaporation pan, a cylinder with a diameter of 47.5 in (120.7 cm) that
has a depth of 10 in (25 cm). The pan rests on a carefully leveled, wooden base
and is often enclosed by a chain link fence to prevent animals drinking from
it. Evaporation is measured daily as the depth of water (in inches) evaporates
from the pan. The measurement day begins with the pan filled to exactly two
inches (5 cm) from the pan top. At the end of 24 hours, the amount of water to
refill the pan to exactly two inches from its top is measured.
If precipitation
occurs in the 24-hour period, it is taken into account in calculating the
evaporation. Sometimes precipitation is greater than evaporation, and measured
increments of water must be dipped from the pan. Evaporation cannot be measured
in a Class A pan when the pan's water surface is frozen.
The Class A Evaporation Pan is of limited use on days with
rainfall events of >30mm (203mm rain gauge) unless it is emptied more than
once per 24hours. Analysis of the daily rainfall and evaporation readings in
areas with regular heavy rainfall events shows that almost without fail, on
days with rainfall in excess of 30mm (203mm Rain Gauge) the daily evaporation
is spuriously higher than other days in the same month where conditions more
receptive to evaporation prevailed.
The most common and obvious error is in daily rainfall events of
>55mm (203mm rain gauge) where the Class A Evaporation pan will likely
overflow.
The less obvious, and therefore more concerning, is the influence
of heavy or intense rainfall causing spuriously high daily evaporation totals
without obvious overflow.
B) SUNKEN COLORADO PAN
The sunken
Colorado pan is square, 1 m (3 ft) on a side and 0.5 m (18 in.) deep and made
of unpainted galvanized iron. As the name suggests, it is buried in the ground
to within about 5 cm (2 in.) of its rim. Evaporation from a Sunken Colorado Pan
can be compared with a Class A pan using conversion constants. The pan
coefficient, on an annual basis, is about 0.8.
EXERCISE
1)
Consider a lake has a
surface area of 1.5mi2. If the average annual evaporation rate s
estimated to be 2/3 of the average daily rate calculated in example above, what
volume in million gallons per day and cubic meter per day would be lost to
evaporation?
SOLUTION
1)
The surface layer of
the lake in square feet,
52800 x 5280 x 1.5 = 41817760 ft2
41817760 x 0.21 x 2/3 = 731811
ft3/day
Converting to mgd,
731181 x 7.48 = 5473946 mgd
For 1 year, the total
would be,
5473946 x 365 = 1997990 mgy
Convert to cubic meter,
731811 x 0.0283 = 20710 m3
/ day
20710 x 365 = 7559242 m3
/ year
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