This paper discusses the validation of the physical-deterministic site model OPUS (SMITH and KNISEL 1985) which describes the water and matter fluxes in the soil and at the soil surface. In this study the submodels infiltration, water flux
in the soil and evapotranspiration have been tested using extensive data sets from a German research catchment. Furthermore, not only the applicability of this model but also the transferability have been examined by comparing different measurement sites.