Example Reference Biosphere 2B (ERB2B) is a hypothetical river catchment, described inthe IAEA-sponsored BIOMASS study on biosphere aspects of post-closure radiologicalsafety assessments for repositories for solid radioactive wastes. In ERB2B, a radioactivelycontaminated aquifer interacts with the soils and sediments of the river catchment. A‘semi-distributed’, lumped-parameter model (SDLP) was set up for the site as part of theBIOMASS study. In the model, empirically derived transfer functions are used to reducethe complexity of real hydrological transport systems to readily calculable mass-balanceaccounting routines. In this work, a physically based, spatially distributed modellingsystem SHETRAN was set up for the site and comparison made with the existing SDLPmodel.The work has shown that, using standard soil properties in SHETRAN, the soilrapidly saturates and much of the hydrologically effective rainfall (precipitation lessevapotranspiration) is lost as saturation-excess surface runoff. This is contrary to theassumptions in the SDLP model. The difficulty arose from the original formulation ofcatchment characteristics in BIOMASS. Specifically, there was a large water volumeentering the soils from precipitation together with an upward flux of groundwater acrossthe lower boundary of a substantial part of the catchment. This water had to be lost fromthe catchment in some way and the thinness of the soil zone precluded dominance ofsubsurface, lateral flow over surface runoff. Increasing the saturated conductivity from1 to 20 m d−1 reduced the surface flows to similar values to those assumed in the SDLP model (this couldalso have been achieved by increasing the soil depth). Even with the high saturatedconductivity there were still major differences between the two representations. In thewoodland on the upper slopes of the valley, the SHETRAN simulation was slightly wetterthan the SDLP model, whereas in the shrubland and marshland near the river it was drierthan the SDLP model. In the SDLP model, subsurface lateral flows are ignored ifthere is surface flow, and deep subsurface flows are ignored if there are shallowsubsurface flows. In the SDLP model, there is a major assumed change in flow regimebetween summer and winter. This is not the case in the SHETRAN simulation.Overall, this work illustrates the problems of using ‘semi-distributed’, lumped-parametermodels without prior calibration against a physically based model and the potential forimplying unexpected and possibly implausible hydrological characteristics through thespecification of flows without considering whether they could occur for realistic soil depthsand properties. As there is a need for application of such SDLP models, particularly whenundertaking probabilistic calculations, it is suggested that, in future, explicit hydrologicalmodelling should be undertaken first, so that a physically realistic representation can beproduced as a basis for assessment studies of the migration of radionuclides or othercontaminants.