Lecture 11 Evapotranspiration (4)

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Lecture 11 Evapotranspiration (4)
ET Measurements

• Lysimeters
• Evaporation Pans
• Pan size effects

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Lysimeter

• Allow determination of water balance of a block
  of soil
• Maintain wet soil to measure PET
• Allow soil moisture to fluctuate to measure AET
•  

http//lawr.ucdavis.edu/classes/
hyd143/hyd143week2.html
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Weighing Lysimeters

• Use a balance to assess changes in storage
• Small lysimeters can be removed and weighed
• Measure both PET and AET

http//www.kfa-juelich.de/icg/icg5/irapic/einlasse
.jpg
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Draining Lysimeters

• Soil block is isolated but must be undisturbed
  and in the same surroundings
•  
• Cant measure change in storage, only inputs and
  outputs

http//www.l-sadwrn.dircon.co.uk/ lysim.html
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Evaporation Pan (US Class A)standard of the
National Weather Service (NWS)
http//www.fao.org/docrep/S2022E/s2022e07.htm
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Sunken Colorado Pan
http//www.fao.org/docrep/S2022E/s2022e07.htm
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Evaporation Pans
Solve a simplified water-balance equation for a
given period (?t)
E W (V2 V1)
E Evaporation W Precipitation during ?t V1
Volume at beginning of ?t V2 Volume at end of
?t  
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Estimating the Effect of Pan Size
Empirical equations to account for energy
exchange through the side the pan, in order to
estimate free water evaporation
when Tspan gt Ta - when Tspan lt Ta  
Efw Evaporation from free-water surface Epan
Pan evaporation P Atmospheric pressure
(mb) vpan Avg. wind speed 15 cm above pan
(km/day) Tspan Water-surface temperature
(?C) Ta Air temperature (?C) ?pan Proportion
of energy lost to evaporation
Kohler et al. (1955)
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Evaporation Pans

• Free-water evaporation
• Expose a cylindrical (or rectangular) pan of
  water to the atmosphere
• Provide estimates of potential
  evapotranspiration (PET) can be more accurate
  than formulae
• Must be related to vegetation canopies using
  other methods (coefficients etc.)
• Precipitation is measured in an adjacent rain
  gauge
• Pan volume is measured by recording water height
  (which is maintained by adding or removing
  measured amounts of water)
• Pans can be placed on land in clearings or on
  floating platforms on open water and usually
  surrounded by fences

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Plot Studies

• Water balance approach applied to small
  undisturbed plots of land
• Analogous to small lake for simple evaporation
• Must select suitable sites where inputs/outputs
  can be measured
• Spatial uncertainty increases with size of plot
• Artificial rain (sprinklers) can be used to
  assess PET
• Soil moisture assessed by spot sampling and
  measurement by
• Gravimetric methods
• Soil block moisture
• Neutron probe
• Time domain reflectometry

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Catchment Studies

• Water balance of entire drainage basin
• Even greater spatial uncertainties
•   Variable precipitation, soil water capacity
• Ground water seepage
•  
• Better for long term estimates
• On a yearly basis, change in storage is
  approx. zero