2. Measurement of evaporation
Evaporation is measured by means of
evaporimeters. Evaporimeters are
pans containing water which are
exposed to atmosphere. The loss of
water by evaporation is measured at
regular intervals
3.
4. EVAPORATION
Change in the physical state of water from liquid
to vapour is known as evaporation Evaporation
from surface is influenced not only by
environmental factors but also by the depth, sie
state of evaporating surface and surrounding etc.
There are the four main types evaporations or
pans used for measuring evaporation
The amount of water lost by evaporation from the
free water surface in the pan at any given Interval
of time is measured by adding known quantities
of water to the pan and bringing it to the original
level
5. Operation and measurement
This is an instrument used to measure the amount of
water lost by evaporation per unit area at a given
meerval of time. The various of evaporation give a
measure of evaporative power of the air layers near the
ground
This is made up of galvanized iron or coppersheet of 20
gauge thickness 10 mm thick copper is the latest
recommended standard
The pan of the evaporimeter is 122 cm in diameter and
25.5 cm in deep. It is painted white and is covered with a
lid of hexagonal mesh to protect the water from birds
and squirrels.
6. This is used to measure the rate ofevaporation
mm/day with a precision of (0.1) mm
Water level in this application should be maintained
upto 20 centimeters
In order to provide undisturbed watersurface, a still-
well is used. It is kept in the pan at the based and is
provided with 3 small openings (120 degrees apart) at
its bottom so that the water level of the pan
corresponds to that of still-well.
7. The reference point is provided by the brass rod, fixed
at the centre of the still-well and is Lampered to end
at a point exactly 190 mm above the base of the pan
Measured quantities of water is added or removed to
bring the back the level of water to its original
position.
The rate of evaporation is determined by using the
equation a Volume-Arcax Depth
8. Situation I : On a normal day
On a pomal day water evaporates from "AA" and the
level comes to "R" Usky this situation known quantity
of water has to be added into the pan with the body of a
measuring eslinder till the water level comes to "AA"
For example, if the water adds to the pan is cans, then
the evaporation is equal to Smm ( can of water-2 mm)
Can height is 20 cm.
9. Situation II: On a slight rainy day
The rain fallen into the pan has to be accounted for. On a
normal day water evaporates from "AA" and the level
comes to "BB". Whereas on a slight rainy day, the
rainwater is added and the level of water in the pan
increases to CC Under this situation, known quantity of
water has to be added into the pan with help of a
measuring cylinder till the water level comes to "AA"
and the evaporation is calculated as follows
The water added to the pan is 3 cans 6 mm
The amount of rainfall received 4 mm
Then the Evaporation-6-4-10 mm
10. Situation III: On a heavy rainy day
On a heavy rainy day the rainwater is added and the
level of water in the pan increases to "DD" i.e. above
the level of the pointer. Under this situation, known
quantity of water has to be removed from the pan with
the help of a measuring cylinder till the water level
comes to "AA" and the evaporation is calculated as
follows
The water removed from the pan is 2 cans = 4 mm
The amount of rainfall received = 10 mm
Then the Evaporation=10-4-6 mm
11. Situation IV: On a heavy rainy day
Under this situation the water will overflow from the
pan. There is no known method available to measure the
evaporation under this condition. In the record, to
indicate the situation, put it as OFF
Repairs for nay leaks must be attended as and when
noticed
Clean the pan and stilling well regularly
Paint the evaporimeter with white enamel paint every
year
Use lemon juice to remove white deposits on the bulb of
the thermometer.
12. Consumptive use
Consumptive use of water by a crop is the depth of water
consumed by the plant in the process of interception,
transpiration, and evaporation, during crop growth.
These two processes (i.e. transpiration and
evaporation) collectively termed as evapo-
transpiration.Interception refers to precipitation that
does not reach the soil, but is instead intercepted by the
leaves and branches of plants
Consumptive use of water by a crop is expressed as the
depth of water per unit area for a specified period, such
as days, months or seasons.
The value of the consumptive use of water is needed to
calculate the irrigation requirement of the crop.
