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Evapotranspiration

Evapotranspiration. Eric Peterson GEO 361 - Hydrology. Evaporation – process by which water is transferred from the land and water masses of the earth to the atmosphere.

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Evapotranspiration

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  1. Evapotranspiration Eric Peterson GEO 361 - Hydrology

  2. Evaporation – process by which water is transferred from the land and water masses of the earth to the atmosphere. • Transpiration – transfer of water from plants to the atmosphere, soil moisture taken up by vegetation is eventually evaporated as it exits plant pores. • Evapotranspiration – combination of evaporation and transpiration.

  3. Within the United States ~ 4,200 bgd of precipitation falls • 2/3 is returned to the atmosphere • Evaporation rates vary spatially and temporally.

  4. Where would you expect the highest evaporation rates? • Why? • Where would you expect the lowest evaporation rates?

  5. Globally, where would you expect the highest evaporation rates? • Globally, where would you expect the lowest evaporation rates? • Why?

  6. June 1998 Mean Surface Temperature

  7. Where Will Precipitation and Evaporation Occur?

  8. Temporal variation • Will evaporation occur at the same rate all year long? • Why

  9. Evaporation • Occurs when water changes state from a liquid to a gas • Function of: • Solar radiation • Differences in vapor pressure between the surface and the overlying air • Temperature • Wind • Atmospheric pressure

  10. Net Evaporation • A continuous exchange of water occurs between the atmosphere and the Earth’s surface • When more water evaporates than returns there is net evaporation • Net evaporation ceases when the air is saturated (water vapor pressure = saturated vapor pressure)

  11. Hornsberger et al., 1998

  12. Factors needed for Evaporation • For water to change states – energy is needed to supply latent heat of vaporization (the quantity of energy that is added when a unit mass of substance vaporizes • Concentration gradient in the water vapor, this is enhanced by air circulation.

  13. Evaporation and Fick’s First Law • Evaporation is a diffusive process that follows Fick’s first law that states molecules of a substance will move from a region of high concentration to a region of low concentration: • Where Jj is the flux of species j crossing a certain area per unit time • Dj is the diffusion coefficient of species j • is the concentration gradient of species j, and is the driving force for the movement

  14. Fick’s law applied to Evaporation • Applying Fick’s law • Where E is the evaporation rate (L/t) • es and ea are the vapor pressures of the evaporating surface and the overlying air (M/(Lt2)) • va is the wind speed (L/t) • KE is a coefficient that represents the efficiency of vertical transport of water vapor by turbulent eddies of wind (Lt2/M)

  15. KE is a function of the wind and is found using: • Where rais the density of air (M/L3) • P is the atmospheric pressure [M/(L*T2)] • zm is the height at which the wind speed and air vapor pressure are measured (L) • zd & z0 are the zero-plane displacement and the roughness height of the surface (L)

  16. zd (zero-plane displacement) = 0.7 zvegrepresents a height where wind velocity is minimal. • z0 (roughness height) = 0.1 zvegrepresents a height where wind is being influenced by surface features

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