EVAPORATION MEASURING DEVICES

1. EVAPORATION
Submitted to :
Dr. Aniruddha D Ghare
Professor, Civil Engineering
VNIT Nagpur
Submitted by:
Gaurav Gautam
MT21WRE012
Visvesvaraya National Institute of Technology , Nagpur
APPLIED SURFACE HYDROLOGY

2. CONTENT
• Atmometer
i. Design
ii. Types
iii. Use and Maintenance
iv. Advantages & Disadvantages
• Pan Evaporation
i. Observation on pan
ii. Pan coefficient
iii. Factors affecting Pan coefficient
iv. Types of pan
v. Numerical Problem

3. ATMOMETER
• An atmometer is a scientific instrument used for measuring the rate of water evaporation from
a wet surface to the atmosphere .
• This is a device that can give direct measurement of evaporation .
• Atmometers are mainly used by farmers and growers to measure evapotranspiration rates of
crops at any field location .
• Atmometers are not common because of there small size and also they do not have sufficient
exposure .
• The rate of evaporation observed from an atmometers is on higher side than that from any other
method .

4. DESIGN
• Atmometer basically consists of a wet , porous ceramic cup
mounted on the top a cylindrical water reservoir .
• The ceramic cup is covered with a green fabric that stimulates the
canopy of a crop .
• The reservoir is filled with distilled water that evaporates out of the
ceramic cup and is pulled through a suction tube that extends to the
bottom of the reservoir .
• Underneath the fabric , the ceramic cup is covered by the special
membrane that keeps rain water from seeping into the ceramic cup.
• A rigid wire extending from the top keep birds from perching on top
of the gauge .
Source :- Paper Atmometer – based irrigation scheduling system for drip – irrigated onion (Allium
cepa L.)

5. The following normal types of atmometers :-
1) Livingstone atmometer
• It consists of 50 mm diameter spherical surface of 2.5 mm thick
porous material .
• The bottle is filled with distilled water that is supplied
continuously to the porous bulb .
• The loss of water from the bottle is due to the evaporation .

6. 2) Piche atmometer
• It consists of graduated glass tube of 15 mm diameter and 300 mm in length .
• It is filled with water and covered with filter paper .
• The tube is kept in an inverted position so that there is a continuous supply of
water to the filter paper .
• The loss of water from the glass tube is the loss due to evaporation .

7. USE and MAINTENANCE
• An atmometer is fairly easy to install and use .
• It is usually mounted on a wooden post about 1000 mm above the ground in an area
representative of the weather and field conditions .
• The plate of the atmometer should be placed in direct sunlight so the evaporation rates are not
affected .
• It should not be placed near tall trees or buildings, as they can affect the amount of exposure
that the atmometer has to environmental factors, which affect evapotranspiration rates .
• To measure the amount of water that has evaporated, calculate the change in water level on the
gauge by subtracting the final water level from the initial water level.

8. ADVANTAGES
• Low cost
• Easy operation
• Convenience
• No computer or power required
DISADVANTAGES
• Bad weather can occur damage to
the device
• Constant need to refill water supply
• Gauge must be read manually (only
on manual model)

9. PAN EVAPORATION
• The estimation of observation can be done by taking actual evaporation from pan and
correlating these result with reservoir .
• Pan :- A pan is a metal container , square or circular , and of uniform cross-section . Normally,
it is circular having diameter ranging from 300 – 1500 mm .
• The pan is filled with water and the loss of water from this pan in a specific period is measured.
• The rate of evaporation observed here is correlated to the evaporation from reservoir .

10. OBSERVATIONS ON PAN
• In the first method , the pan is filled up to a specific level and the loss of water is calculated by
observing the level of water over a specified time. Knowing the cross-section of the pan and the
reduction in the water level , the loss due to evaporation over a specified time can be calculated .
The water levels in the pan are taken accurately by a hook gauge .
• In the second method , the pan is filled up to specific level . This level is maintained constant by
adding water to the pan periodically to meet the evaporation loss . The loss due to evaporation over
that period is the quantity of water added to the pan to maintain a constant level .

11. PAN COEFFICIENT
• The observation from a pan are extrapolated to a reservoir by using a pan coefficient .
• It may be defined as follows :
Pan coefficient = Rate of Evaporation from the reservoir
Rate of Evaporation from the pan
• The value of pan coefficient is always less than 1.0 ( because in case of a pan , a very small area
is exposed to atmosphere and secondly , the metal container absorb more energy and in turn is
utilized for evaporation .
• It is dimensionless .

12. Factors Affecting Pan coefficient
The following factor affect the pan coefficient :
• Type of Pan ( Diameter , Depth of water , Height of rim above
water level , colour , material )
• Pan Environment ( Location )
• Climate ( Humidity & windspeed ) .

13. TYPES OF PAN
• Class A Evaporation Pan
• ISI Standard Pan
• Colorado Sunken Pan
• US Geological Survey Floating Pan

14. Class A Evaporation Pan
• It is the standard pan of 1210 mm diameter and 255 mm depth used by the US weather Bureau and
is known as Class A Land Pan .
• The depth of water is maintained between 18 cm and 20
cm. The pan is normally made up of unpainted galvanised
iron sheet .
• Monel metal is used where corrosion is problem .
• The pan is placed on the wooden platform of 15 cm
height above the ground to allow free circulation of air
below the pan .
• Evaporation measurement are made by measuring the depth of water with a hook gauge in stilling
well .

15. ISI Standard Pan
• This pan evaporimeter specified by IS:5973-1970 , also
known as Modified Class A Pan .
• Pan of 1220 mm in diameter with 255 mm of depth .
• Pan is made up of copper sheet of 0.9 mm thickness , tinned
inside and painted white outside .
• The pan is placed is placed on the square wooden platform
of width 1225 mm height 100 mm above ground level to
allow free air circulation below the pan
• A fixed point gauge indicates the level of water .

16. Colorado Sunken Pan
• This pan , having 920 mm square side and 460 mm deep and it is made up of unpainted
galvanised iron sheet and buried into the ground with 50mm of the top .
• The chief advantage of the sunken pan is that radiation and
aerodynamic characteristics are similar to those of a lake .
• However, it has the following disadvantages also :
i. Difficult to detect leaks
ii. Extra care is needed to keep the surrounding area free
from tall grass, dust ,etc .
iii. Expensive to install .

17. US Geological Survey Floating Pan
• A square pan of 900 mm sides and 450 mm deep .
• It is supported by drum floats in the middle of the raft of size 4.25m 4.87 m ,
• It is set to float in a lake with a view to simulate the characteristics of large body of water .
• Water level in the pan is maintained at the same level as that in the lake , leaving a rim of 75 mm .

18. S.NO TYPES OF PAN VALUE RANGE
1. Class A Land Pan 0.70 0.60-0.80
2. ISI Pan ( Modified Class A ) 0.80 0.65-1.10
3. Colorado Sunken Pan 0.78 075-0.86
4. USGS Floating Pan 0.80 0.70-0.82
Value of Pan Coefficient

19. Ques : A Class A pan was setup adjacent to a lake . The depth of water in a pan at the
beginning of a certain week was 195 mm . In that week there was a rainfall of 45 mm and
15 mm of water was removed from the pan to keep the water level within the specified
range .
If the depth of water in the end of week was 190 mm .Calculate the pan Evaporation using a
suitable Pan coefficient .
Estimate Lake evaporation in that week .

20. Soln :
• Initial Reading = 195 mm
• Rainfall = 45 mm
• Removed Water = 15 mm
• Cp for Class A Pan = 0.7
• Total Water in the pan = 195+45 = 240 mm
• Water Remaining After removal = 240-15 =225 mm
• Water Evaporated = 225 -190 =35 mm
• Lake Evaporation = Cp × Pan Evaporation
= 0.7 × 35 = 24.5 mm

21. References :
• Elementary Engineering Hydrology: By Deodar .
• Elementary Hydrology: By K Subramanya .
• Google .

22. THANK YOU