An atmometer is used to directly measure evaporation rates. It consists of a porous ceramic cup attached to a water reservoir. Water evaporates from the cup through a tube into the reservoir. Pan evaporation involves measuring the water loss from a metal pan over time to estimate evaporation from larger bodies of water, like reservoirs. Factors like pan type, location, and climate affect the pan coefficient used to extrapolate pan evaporation rates to actual reservoir evaporation. In the example, a Class A pan recorded 35mm of evaporation over a week, corresponding to an estimated 24.5mm of reservoir evaporation based on the Class A pan coefficient of 0.7.

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