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  1. 1. EVAPOTRANSPIRATION Presented by: Jannat Iftikhar MS16-01 Presented to: Dr. Shakil Ahmad 1 Course title: Water Resources & their Management in Pakistan
  2. 2. Contents  Introduction  Difference between evaporation & transpiration  Types of evapotranspiration  Role of evapotranspiration in water cycle  Factors affecting evapotranspiration  How to determine evapotranspiration?  Conclusion 2
  3. 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. 4. 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. 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. 6. EvaporationTranspiration  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. 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. 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. 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. 10. Role of evapotranspir 10 Evapotranspiration is responsible for 15% of the atmosphere's water vapor.
  11. 11. Evaporation The transformation of water from liquid to gas phases as it moves from the ground or water bodies into the atmosphere Transpiration The release of water vapor from plants into the atmosphere Condensation The transformation of water vapor to liquid water droplets in the air, creating clouds and fog Precipitation Condensed water vapor that falls to the Earth's surface 11
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  13. 13. 13 SOIL FACTORS Factors effecting evapotranspiration
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  15. 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. 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. 18. Soil factor Soil characteristics  Soil capillary character  Water table depth  Soil moisture content ET 18 (when soil is lacking moisture, plants begin to transpire less water in an effort to survive, this in turn decreases evapotranspiration)
  20. 20. 20 Estimation of evapotranspiration Indirect method Catchment Water Balance Energy Balance Direct method Lysimetry method
  21. 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. 22. • The actual evapotranspiration is estimated by the energy balance. λE = Rn - G - H λE = the energy needed to change the phase of water from liquid to gas Rn = Net radiation G = Soil heat flux H = Sensible heat flux 22 ENERGY WATER BALANCE
  23. 23.  Using instruments like a scintillometer, soil heat flux plates or radiation meters, the components of the energy balance can be calculated. 23
  24. 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. 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. 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. 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. 29. References • Robert M. Devlin, Francis H. Witham, Plant Physiology, 4th edt. CBS publishers & distributers, jain bhawan 485. • Subhash Chandra Datta, Plant Physiology, 2nd 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. • • • • 29
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