Measurement of evaporation

This presentation will provide the knowledge on measurement of evaporation by using class A evaporation pan. In addition it will give you the knowledge regarding pan coefficient and crop coefficient.

1. Measurement of Evaporation Vijitha Vikneshwaran Lecturer (Temporary) Faculty of Technology University of Jaffna

2. Introduction Evaporation is the process of conversion of water from liquid into vapor. It is an important process in the hydrologic cycle. Measurement of evaporation is crucial in irrigation scheduling and water management. Loss of water due to evaporation is entirely lost from the usable supply. Evaporation is usually measured in meteorological and agro meteorological stations using a pan evaporimeter . 2

3. Introduction, cont …. The values obtained with the pan do not give directly the evaporation from a surface of natural conditions. It provides the measurement of the integrated effect of, Radiation Wind Temperature Humidity The rate of evaporation is referred as the amount of water lost by evaporation from a unit area of surface in unit time. Evaporation is usually expressed in depth basis as mm/day 3

4. Factors affecting the evaporation Degree of wetness of the surface Temperature of air Temperature of the soil Atmospheric humidity Wind velocity Vegetative cover 4

5. Class A evaporation pan It is the simplest, most reliable and standard device to measure the evaporation. It consists of a cylindrical reservoir made from copper sheet of fixed diameter. The water will be filled few centimeter below the rim and exposed on a flat form. A fixed point gauge in a stilling well serves to indicate the water level in the pan. A calibrated measuring cylinder is used to add or remove water at each observation to bring the water level to the fixed point. 5

6. Class A evaporation pan, cont The reservoir is covered with the wire mesh netting to protect the pan from birds and animal. The evaporation rate is calculated by the change in level of the free water surface and the recorded rainfall. 6 Figure 01: Class A evaporation pan

7. Methodology Select suitable location, which is away from shade and free from surrounding obstructions Install the pan in the field 15 cm above the ground level on the wooden support Fill the pan to exactly 5 cm below from the pan top Allow the water to evaporate Measure the rainfall conjunction with evaporation 7

8. Methodology cont …. Take the measurement of remaining water depth in the stilling well  after 24 hours by hook gauge Refill the pan at the end of 24 hours to exactly initial point from the top Calculate the amount of evaporation per time This will give pan evaporation, E pan (in mm/24 hours) 8 Figure 02: US class A evaporation pan

9. Deficiencies Reflection of solar radiation from water surface is only 5-8% but from most vegetative surfaces is 20-25% Storage of heat within the pan may cause evaporation in night as well but it is not possible in plants. The water loss from pan and plant may differ according to the differences in turbulence, temperature and humidity of the air immediately above the surface. Evaporation from the natural water body occurs comparatively at lower rate as it does not have metal surface to absorb heat. Color of the pan and the usage of screen also will affect the water loss. Light penetration in the natural water bodies decrease as depth increases, where as it is uniform in the pan. 9

10. Different sittings of pan Even with these deficiencies, it is accepted to use the pan to predict crop water requirement for periods of 10 days or longer along with proper sitting. 10 Figure 03: Two cases of pan sitting and their environment

11. Estimation of Evaporation Due to some deficiencies, the measured evaporation is higher than the actual evaporation. To correct this value, pan evaporation (E pan ) is multiplied by a pan coefficient (K pan ). For the Class A evaporation pan, the K pan varies between 0.35 and 0.85 and average K pan = 0.70 ET o = K pan × E pan ETc = ETo x Kc Epan – pan evaporation Kpan – pan coefficient ETo – reference crop evapotranspiration ETc – crop evapotranspiration Kc – crop coefficient 11

12. Pan coefficient 12 Figure 04: Pan Coefficients for class A pan

13. Crop coefficient 13 Figure 05: Crop coefficient (Single crop approach)

14. THANK YOU 14