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Title: PARAMETERIZATION OF POTENTIAL EVAPORATION AND LONG-TERM WATER BALANCE TRENDS

Author: G. Papaioannou, A. Mitropoulou , G. Kitsara and P. Kerkides

Year: 2009

Publisher: European Water Resources Association (EWRA)

Description:

The water balance model underlying calculations of the Palmer Drought Severity index (PDSI) has been recently used on a worldwide long–term basis leading to the conclusion of continental drying during recent decades. In its traditional formulation potential evaporation (Ep) is estimated from Thornthwaite’s empirical formula, based solely on air temperature (Tair). Consequently, as Tair steadily increases with global warming, the calculated value of Ep in the model also steadily increases. In contrast, calculations of reference evapotranspiration from Penman-Monteith method have been reported as declining in some regions of the world, in general agreement with declining pan evaporation records. Thus, assessing the hydrologic impacts of climate change, the conflicting estimates of declining evaporative demand must be taken into account. In this paper, the effects of three different parameterisations of potential evaporation on long-term trends in actual evaporative flux (AET) and soil moisture (SM) are examined by running the PDSI water balance model across Greece, three times. In the first and second run, Ep is calculated from Thornthwaite’s or Penman-Monteith’s method, respectively, at 29 stations during the period 1974-2001. The third run uses measurements of evaporation from class-A pans available for 15 sites during the period 1979-1999, for more accurate outputs of the water balance model and hence modelling of drought. The sign of trends in the modelled soil moisture and actual evaporative flux is examined, according to the parameterisation used and the prevailing climatic regime. The analysis indicates that trends in soil moisture
and actual evaporative flux do not depend on Ep. Trends in AET and SM, are largely determined by trends in precipitation in Greece.