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ATMOMETER AND PAN EVAPORATION
- 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