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Arid zone, ephemeral rivers are characterised by discharge decreasing downstream in the lower reaches due to transmission losses. Modelling the flow regime of these rivers requires data on the spatial and temporal distribution of transmission losses in these reaches. In this study, a hydrological model is developed for a 330 km reach of the Diamantina River in southwestern Queensland. where the floodplain width varies from 5 to 60 km. Analysis of gauging station data at each end of the reach indicates that transmission losses are between 70 and 98% for floods with total discharge < 2300 GL. The flow routing and transmission losses are modelled using a grid-based, daily time-step conceptual model incorporating flow routing algorithms. Satellite images are used to identify the flow-paths used by the range of flood sizes and to identify threshold flow volumes for initiation of flow into new flow-paths. The grid-based approach allows for representative routing of flow through the reach and representation of spatial variability in transmission loss processes, including losses resulting from evaporation, channel/floodplain infiltration and terminal flow storage. A combination of gauging station data and satellite images is used in the calibration of the model parameters. (C) 2003 Elsevier Science Ltd. All rights reserved.