25% of the French area (14 mio ha) and 30% of the surface of Germany (10,7 mio ha) are covered by forests. The evaporation and transpiration rates of these large ecosystems are scarcely exactly known. On the other hand we need quantitatively good information for water management purposes for instance. Besides that exact data are required to answer the questions in connection with global change. At the moment climatologists are working more and more with atmospherical and hydrological models as for one reason continuously performed monitoring is difficult and expensive, especially when carried out in forests and in particular when these measurements are aggrandized in space, time and different altitudes. But nevertheless, these measurements are necessary for the validation of simulations.
At both sides of the upper Rhine special studies were carried out to estimate forest evapotranspiration and some of them are still going on. As in Aubure in the Vosges mountains in France, in the valley plain near the village Hartheim and further east at the Schluchsee shore in the Black Forest, both in Germany.
In the mountains, the forests investigated are spruce forests, the wood in the plain is a pine forest. In the mountaineous sites hydrological methods were used. There sapflow was measured in addition and potential evapotranspiration was calculated. At the lower site hydrological methods were used as well, we used energy balance methods and aerodynamic approaches to estimate evapotranspiration in addition. We are able to compare the results of a synchroneous time series at all three sites from 1989 to 1991. The west to east profile Aubure - Hartheim - Schluchsee represents a distance of 50 miles. The paper includes results on the base of microclimatological measurements in Hartheim of the years 1987 to 1996 and hydrological simulations at Schluchsee from 1987 to 1994 where evaporation estimates by means of the eddy correlation method were taken as well.