Comparison Of Different Methods For Estimation Of Potential Evapotranspiration

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Comparison Of Different Methods For Estimation Of
Potential Evapotranspiration

• Plamen Videnov, Anna Tzenkova, Julia Ivancheva

Investigation Of The Evapotranspiration Regime In
Some Regions Of Bulgaria
Anna Tzenkova, Julia Ivancheva, Plamen Videnov
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One of the tasks in the frame of the
international project Assessment of the climate
change impact on the elements of hydrological
cycle is to estimate the aerial
evapotranspiration and its space and time
variability. For this purpose we need to choose
one method for accurate estimation of aerial
evapotranspiration, which will make possible the
comparison between the results from all countries
participating in the project.
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Osam River Basin

• The river basin crosses four climatic regions in
  Bulgaria, all pertaining to the continental
  climatic subzone. A characteristic specificity of
  this climatic subzone is a well expressed summer
  maximum of precipitation and a minimum in the
  cold half-year.

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Struma River Basin

• The climate conditions in the valley of the river
  Struma are rapidly changing, beginning from
  moderate-continental (the field of Radomir and
  the surrounding mountains), going by
  transitional-continental (the fields of Dupnitza,
  Blagoevgrad and Kiustendil, and the surrounding
  mountains), and reaching to continental-Mediterran
  ean climate (the field of Petrich-Sandanski and
  the surrounding mountains)

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METHODS
The first step is to choose correct and
comparable method for calculation of the
potential evapotranspiration

• For selecting a method for the calculation of the
  potential evapotranspiration we take into account
  the following considerations
• a sufficient degree of reliability
• the selected method should be widely used in
  order to compare the results with those from
  other regions
• the formula should include accessible (available)
  meteorological information.

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Taking these principles into consideration, the
following methods were selected

• -                     Penman PETPM
• -                     PristlyTaylor PETPT
• -                     Thornthwaite PETTH
• -                Modified Eagleman PETEG.

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DATA
The mean monthly data for the Sandanski station
for the1998-2001 period were used. Because the
pan evaporation is measured only during the
April-October period, the comparison of the
results obtained by different methods was
conducted in two stages a) Comparison of the
calculated and measured values for the period
April-October 1998-2001 b) Comparison of the
results from the calculation with different
formulae by the data for January-December
1998-2001.
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The correlation coefficient between data obtained
by the Thornthwaite and Eagelman formulae on one
hand, and the data from the evaporator on the
other, is respectively 84 and 90.
Mean annual and seasonal sums of PET according
Thornthwaite mm for Struma river
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Mean annual and seasonal sums of PET according
Thornthwaite mm for Osam river
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PET according Thornthwaite mm
OSAM river
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STRUMA river
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PET According Thornthwaite
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PET according Eagleman mm
OSAM river
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STRUMA river
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PET According Eagleman
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The actual annual evapotranspiration is
determined by the Turc formula
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Conclusions

• The potential evapotranspiration has a
  well-expressed annual course, whereas its
  seasonal distribution in the river basins depends
  on the geographic characteristics of the separate
  sections of the river.
• For the warm part of the year the Penman formula
  for PET describes 94 of the pan measurements,
  the Eagleman and PriestlyTaylor formulae -81,
  and the Thornthwaite formula - 71.

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• The slight increase of annual and summer values
  of PET is observed in both investigated river
  basins.
• The Fisher LSD test shows that there are two
  groups of stations in the upper and lower river
  course of the Osam River, while there is no such
  distribution alongside the Struma River.
• A tendency for an ET decrease during the
  1961-2002 period is only observed in the Struma
  River basin.

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Acknowledgments The research presented in this
paper is results obtained during implementation
of on going project Assessment of Climate Change
impact on the hydrological cycle elements of SE
Europe supported by UVO-ROSTE Venice Office