Band 1 (C. Frühauf, 1998)
Cathleen Frühauf, 1998:
Methoden der Verdunstungsbestimmung am Beispiel eines hundertjährigen Fichtenbestandes im Tharandter Wald
(ISBN 3-86005-212-8)
Abstract
Evapotranspiration of a spruce stand was determined with several micrometeorological, hydrological and plant physiological methods at the Anchor Station Tharandter Wald between March and November 1996. These methods are analyzed and their results are compared.
Measurements of radiation components, eddy covariances (EC) and profiles of temperature, moisture and windspeed are performed continuously on a 41 m meteorological tower. On the same site soil temperature and moisture as well as xylem sap flow and circumference of several trees are recorded. Water gauge and stream flow measurements are available for the catchment Wernersbach approximately 5.6 km apart.
Results of the eddy covariance energy balance (ECEB) method based on carefully achieved available energy performed best and were used as reference for the other methods to estimate evapotranspiration. The Bowen ratio energy balance (BREB) method showed good agreement with ECEB for daytime hours (net radiation Rn>0). 91% of ECEB evapotranspiration was reached. The aerodynamic method gave acceptable results for sensible heat flux only, latent heat flux had to be determined as a residual of the energy balance. Also a linear correction for the aerodynamic method had been applied to utilize measurement heights within the roughness layer. Evapotranspiration estimated from soil moisture measurements reached 89% of ECEB. Another hydrological method has a wider data base: This method determines evapotranspiration in dry periods with the observed daily fluctuations of the water gauge of the small river Wernersbach. In comparison with the ECEB method evapotranspiration is underestimated (79%). Correlation between transpiration estimated from xylem sap flow data and ECEB evapotranspiration is high, but it amounts only to 40% of ECEB. The course of the data from the circumference measurements were only qualitatively evaluated. A determination of transpiration from reversible changes in circumference seems possible.
It is concluded that for reliable and continuous estimation of forest evapotranspiration EC methods should be used. If no direct EC measurement of water vapor flux is available, latent heat should be determined as a residual of the energy balance with help of sensible heat measured by EC. Acceptable agreement was found between ECEB and most other methods compared in this study with the exception of sap flow. The good agreement between ECEB evapotranspiration at the Anchor Station Tharandter Wald and evapotranspiration estimated from daily fluctuations in the water gauge allow a positive view of regionalization of meteorological evapotranspiration data for water balance studies.