HydroGIS 96: Application of Geographic Information Systems in Hydrology and Water Resources Management (Proceedings of the Vienna Conference, April 1996). IAHS Publ. no. 235, 1996. 649 Spatial évapotranspiration calculation on a microscale test site using the GIS-based PROMET-model STEPHAN SCHÀDLICH, WOLFRAM MAUSER Institute for Geography, Department of Geography and Geographical Remote Sensing, University of Munich, Luisenstrasse 37, D-80333 Munich, Germany Abstract Hourly actual évapotranspiration (aET) rates and soil moisture (top 1 m) were calculated for a microscale test site (100 km 2 ) using the PROMET-model, which is integrated in a GIS structure. PROMET is a physically based soil-vegetation-atmosphere-model, which was verified for different land use types using measurements with energy balance stations. PROMET can also be driven with data from regular climatic stations instead of data from energy balance stations. Assuming constant meteorological conditions over the microscale test site and using soil and plant parameters, which were measured or taken from literature, the spatial distribution of the hourly aET rates and soil water contents were calculated. Maps of the aggregated monthly aET show that the spatial distribution of the aET in the study area is mainly influenced by the vegetation distribution. INTRODUCTION Actual évapotranspiration (aET) plays an important role in the hydrologie cycle and biospheric productivity. It determines the river runoff, the groundwater discharge and the plant primary productivity. Evapotranspiration is a spatially inhomogeneous process, which depends on a large amount of different influencing factors, which have to be taken into account. The most important among those are climate, vegetation and soil, which lead to Soil-Vegetation-Atmosphere-Transfer Models (SVATs) to describe aET. The determination of the spatial distribution and temporal variation of aET also has to consider the distribution and temporal evolution of the parameters involved in the évapotranspiration process. This can best be done using Geographical Information Systems (GIS) in combination with SVAT-models. Since the commercially available GIS- systems cannot cope with the complex computational task of SVAT-modelling the com- bined GIS-model system PROMET was created. The following article will demonstrate first model results on two different scales, the field-scale and the microscale (100 km 2 ). METHOD PROMET (Mauser, 1990) is a process oriented multiscale évapotranspiration model developed at the Institute for Geography, University of Munich. It is integrated in a raster-GIS structure and consists of a physically based soil-vegetation-atmosphere- model, which calculates hourly évapotranspiration and soil moisture (top 1 m) at