Evapotranspiration

evapotranspiration and factors effecting it. method of measurement.

1. EVAPOTRANSPIRATION Presented by: J annat I ftikhar MS16-01 Presented to: Dr. Shakil A hmad 1 Course title: Water Resources & their Management in Pakistan

2. C ontents Introduction Difference between evaporation & transpiration Types of evapotranspiration Role of evapotranspiration in water cycle Factors affecting evapotranspiration How to determine evapotranspiration? Conclusion 2

3. Evaporation Movement of water to the air from sources such as the soil, canopy interception, and water bodies. Transpiration Movement of water within a plant and the subsequent loss of water as vapor through stomata of the leaves. 3 Introduction

4. EVAPOTRANSPIRATION The total amount of water loss in the form of water vapors into the atmosphere from surface of the soil, canopy interception, water bodies as well as from the aerial parts of plants in a process known as evapotranspiration. 4

5. Units of measurement The evapotranspiration rate is normally expressed in millimetres (mm) per unit time. The time unit can be an hour, day, decade, month or even an entire growing period or year . The rate expresses the amount of water lost from a cropped surface in units of water depth . 5

6. Evaporation Transpiration Physiological process & occurs only in plants Water moves through epidermis Living cells involved Various processes like vapor pressure, osmotic pressure, diffusion involved Physical process & occurs on any surface Any liquid can evaporate Both living & non living surfaces Not much forces involved 6

7. Formation of vapors continues for some time even after the saturation of air It depends upon absorption of water from the soil Evaporation stops when the air is fully saturated It continues as long as water is available on the surface 7

8. Types of evapotranspiration Evapotranspiration may be classified as: 1. Potential evapotranspiration (PET). 2. Actual evapotranspiration (AET). 8 Interception: Interception is that portion of the precipitation that, while falling on the Earth’s surface, may be stored or collected by vegetal cover and subsequently evaporated. The volume of water thus lost is called interception loss.

9. 9 Potential Evapotranspiration (PE) Actual Evapotranspiration (AE) PE is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply. AE is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration

10. Role of evapotranspiration in water cycle 10 Evapotranspiration is responsible for 15% of the atmosphere's water vapor.

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13. 13 Environmental factors GEOGRAPHICAL FACTORS SOIL FACTORS PLANT & CROP FACTORS Factors effecting evapotranspiration

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15. Plant and Crop Factor Physical attributes of the plant Vegetative cover Leaf area index Leaf shape & size Type of plant Stomatal resistance Plants regulate transpiration through adjustment of small openings in the leaves called stomata. As stomata close, the resistance of the leaf to loss of water vapor increases, decreasing to the diffusion of water vapor from plant to the atmosphere. 15

16. Geographical factors Evapotranspiration rates are also dependent upon geography, an area's latitude and climate. Regions on the globe with the most solar radiation experience more evapotranspiration. Evapotranspiration rates are also highest in areas with a hot and dry climate. Evaporation is less at higher latitude. 16

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18. Soil factor Soil characteristics Soil capillary character Water table depth Soil moisture content ET 18

19. 19 METHODS OF DETERMINATION OF EVAPOTRANSPIRATION

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21. Indirect method CATCHMENT WATER BALANCE Evapotranspiration may be estimated by creating an equation of the water balance. ET = P - ∆S - Q – D ET = Evapotranspiration P = Precipitation ∆S = Change in Storage Q = Stream flow D = Groundwater recharge 21

22. The actual evapotranspiration is estimated by the energy balance. λE = R n - G - H λE = the energy needed to change the phase of water from liquid to gas R n = Net radiation G = Soil heat flux H = Sensible heat flux 22 ENERGY WATER BALANCE

23. Using instruments like a scintillometer , soil heat flux plates or radiation meters, the components of the energy balance can be calculated. 23

24. Direct method Lysimetery method A lysimeter is a measuring device which can be used to measure the amount of actual ET which is released by plants, usually crops or trees. By recording the amount of precipitation that an area receives and the amount lost through the soil , the amount of water lost by evapotranspiration can be calculated . 24

25. Construction and method Select a piece of ground. Fix the rainguage for measuring the precipitation value in ml. Remove the turf and grasses from the small piece of land near the rainguage & dug the soil. Tank is fixed there in such a way that 1cm of it remains outside the ground surface and then fill the tank with soil. Upper surface of tank is filled with turf/grasses that the soil can not be seen. Then cover it with gravels or small stones & then with polythene sheet. 25

26. The receiver is best placed with its top about 5 cm above ground so that surface water cannot enter. Pack soil around the apparatus, relay turf and apply water to bed in the turf and settle the soil when construction is complete. Measurements can begin in a few days. Experiment may continue for few months or even for a year. In dry season, add water to the experimental area but if the precipitation rate is sufficient, no need of water. ET = water added – percolated water 26

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28. Significance of evapotranspiration Plays a major role in precipitation It is the most significant component of the hydrologic budget. A thorough knowledge of evapotranspiration is very important for planning and adjudicating the distribution of water resources. Managers of the crops can determine that how much supplemental water is needed to achieve maximum productivity by estimating PE & AE. 28

29. References Robert M. Devlin, Francis H. Witham, Plant Physiology, 4 th edt . CBS publishers & distributers, jain bhawan 485. Subhash Chandra Datta , Plant Physiology , 2 nd edt ., wiley eastern limited, new delhi , banglor . Frederik Schradera , Wolfgang Durnera , Johann Fankb , Estimating precipitation and actual evapotranspiration from precision lysimeter measurements Four Decades of Progress in Monitoring and Modeling of Processes in the Soil-Plant- Atmosphere System: Applications and Challenges. Brutsaert , W. 1982. Evaporation into the Atmosphere. D. Reidel Publishing Company, Boston. 299 pp. Penman, H. L. 1948. Natural Evaporation from Open Water, Bare Soil, and Grass. Royal Soc. Of London Proc. 193:122-145. https://en.wikipedia.org/wiki/ Evapotranspiration www.fao.org/docrep/x0490e/x0490e04.htm www.physicalgeography.net/fundamentals/8j.html www.sciencedirect.com/science/article/pii/S0378377405002805 29

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