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Surface Energy Balance and Evapotranspiration remote sensing

Surface Energy Balance and Evapotranspiration remote sensing. Lecture 2. Outline. 1. Energy balance 2. ET and in situ measurements 3. Remote sensing ET using energy balance method 4. Remote sensing ET using other methods. radiation. Longwave. Shortwave (1- α )S in. Longwave. (L out ).

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Surface Energy Balance and Evapotranspiration remote sensing

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  1. Surface Energy Balance and Evapotranspiration remote sensing Lecture 2

  2. Outline • 1. Energy balance • 2. ET and in situ measurements • 3. Remote sensing ET using energy balance method • 4. Remote sensing ET using other methods

  3. radiation Longwave Shortwave (1- α)Sin Longwave (Lout) (1 - ε0)Lin Longwave (Lin) Net radiation ( ) Latent Heat ( ) Sensible Heat (H) 1. Energy Balance Energy Balance Ground Heat (G) (Radiance Balance) . Net radiation absorbed by the land . Albedo Units : W/m2 . Emissivity (Heat Balance) . Latent Heat Flux G . Heat transfer to the ground (soil) H . Sensible heat flux loss to the air due to convection and conduction Rn-G called available energy (or Qa)

  4. Surface energy balance and micrometeorology Hornberger et al., 1998

  5. 2. Evapotranspiration • Evaporation: Liquid water is converted to water vapor and removed from evaporating surface (lake, river, pavement, soils, and wet vegetation) • Transpiration: Vaporization of liquid water contained in plant tissue and the vapor removal to the atmosphere through the stomata • Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. They are often measurement together. • Potential ET (PET): The evapotranspiration of a surface with optimal water condition (both on the surface and in the root zone) for E and T. It depends on micrometeorological condition, as well as the surface characteristics. It is the largest water availability (or loss) from a vegetated land surface under given climate and weather condition but not limited by soil moisture. • Reference ET is the PET of the reference (clipped) grass (h= 0.12m) or the (full-covered) alfafa (h>0.3 m).

  6. Quantifying Evapotranspiration • Coupling of energy balance and water balance equations • Water balance method • Energy balance method • (e.g., bowen ratio - energy balance) • Eddy covariance method • Scintillometry method • Aerodynamic method • Combination approach (energy balance + aerodynamic) • Penman-Monteith equation

  7. Coupling energy balance and water balance

  8. Water balance: Evaporation pan Evaporation Pan -- In hydrologic terms, a pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square. 121cm diameter x 24cm deep Write water balance equation for the pan. What variables do we measure to determine E What do you think of the pan evaporation measurement? Can it be a good estimate for lake E? Can you think of a better way to install evaporation pan? http://www.ametsoc.org/amsedu/WES/WESSlides/WESThumbnailHyperlinks/ch10/s041evaporation_pan.htm Data: http://www.wrcc.dri.edu/htmlfiles/westevap.final.html

  9. Water balance: Lysimeter Left and center, construction of the two large weighing lysimeters at KREC began in 1986. Each lysimeter consists of an underground chamber that houses a balancebeam weighing system with a rectangular “flower pot” measuring 6.5 feet wide by 13 feet long by 6.5 feet deep. Right, researchers enter the completed lysimeter. http://calag.ucop.edu/0502AMJ/pdfs/Lysimeters.pdf

  10. The Kearney lysimeters have been in constant use since 1987. One lysimeter measures water use in peaches (top), the other ‘Thompson Seedless’ grapes ( bottom). • The lysimeters have provided more accurate evapotranspiration values for peach trees and grape vines, information necessary in irrigation management decisions that ultimately affect plant health and fruit quality.

  11. Energy Balance: Bowen Ratio (EBBR) 5 over semiarid area, 0.5 over grassland, 0.2 over irrigated orchards, and 0.1 over sea EBBR

  12. Eddy covariance method http://horton.engr.uconn.edu/gamboa/images/IMG_0796-1.jpg ET from EBBR is usually 10% larger than that from Eddy covariance method

  13. Scintillometry method • Measurement of spatially average fluxes of sensible heat and water vapor (evaporation) on km scale • Measurements of spatially avergae rainfall rate Path length L, Aperture D, Wavelength l

  14. Aerodynamic approach ρa is air density, cp is the heat capacity of air, ra is the aerodynamic resistance, T0 is the aerodynamic surface temperature, Ta is the near-surface air temperature, e0 is the vapor pressure (Pa) at the aerodynamic surface, ea is the vapor pressure at near surface, γ is the psychrometric constant (≈ 66 Pa/K)

  15. Combination approach: PM equation Penman equation Penman-Monteith equation γ is the psychrometric constant (≈ 66 Pa/K)

  16. Field ET tower measurements Eddy Covariance ET Tower (2-3m) And Bowen Ratio ET tower both in the Sevilleta National WildLife Refuge

  17. Kurc and Small 2004, Small and Kurc 2001

  18. Components of the SEB at the grassland site using Bowen Ratio ET system. Left side shows a wet day (JD 233) and right side shows a dry day (JD 254). Hours from midnight are plotted on the x-axis, with time shifted one hour forward from the solar cycle due to daylight savings time. Small and Kurc 2003

  19. Small and Kurc 2001

  20. ET in this semi-arid region is only related to top 5 cm soil moisture and to root zone soil moisture as other climate regions Kurc and Small, 2004

  21. Large Aperture Scintillometry (LAS) network NMT Gomez, D. thesis, 2008

  22. Dr. Jan Hendrickx

  23. 3. Remote sensing ET using energy balance method

  24. recap emissivity where h=Planck constant, 6.6261E-34 J.sec k= Baltzmann constant, 1.38E-23 J/K, c= speed of light, 3E8 m/sec

  25. SEBAL-METRIC model Developed by Wim Bastiaanssen, The Netherlands Richard Allen, University of Idaho Based on Landsat imagery User Manual Can be downloaded from here: \\129.115.25.240\XIE_misc\EES5093 for learning purpose not for any other purpose.

  26. Estimating Net radiation

  27. Estimating Short-wave Albedo

  28. Estimating Short-wave Albedo Values for α of path_radiance range between 0.025 and 0.04 and for SEBAL. It is recommended a value of 0.03 based on Bastiaanssen (2000). Can we test it?

  29. Estimating Incoming short-wave radiation Can we test the transmissivity function for San Antonio area?

  30. Estimating surface emitted long-wave radiation Why are two emmissivities used? Explain Rc function

  31. Estimating Incoming long-wave radiation

  32. Estimating ground heat flux How can we test the empirical function for G? Why do deep water and snow have larger G/Rn?

  33. Estimating sensible heat

  34. ET (mm/d) estimation from Energy Balance Method June 16 2002 path/row: 33/36

  35. 4. Remote sensing ET using other methods • Priestley-Tayloy’s equation • Granger's complementary relationship and Priestley–Taylor's equation • … One paper for you to read if you are interested in

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