Abstract: LASCAM (Large Scale Catchment Model) is a kind of large-scale catchment hydrological model with intermediate resolution between the fully lumped conceptual hydrological model and the fully physically-based distributed hydrological model. LASCAM first develops the lumped conceptual model at all subcatchments which form the building blocks for the hydrological modeling of the whole catchment. Each subcatchment includes 3 conceptual soil water stores: A representing the near-stream perched aquifer, B representing the permanent deeper groundwater store, and F representing the intermediate unsaturated infiltration store. Water balance and water flux processes of and within each soil water store are the most important for model construction.Lumped conceptual model at subcatchment scale is the building block to the catchment model. Water levels of each soil water store will control the main water fluxes and other hydrological processes within the subcatchment. Based on empirical equations resulted from regional case studies in southwest area of the Western Australia, at the scale of each subcatchment, LASCAM: 1) first establishes the functional relationship between each soil water store and ecological (Leaf Area Index) and climatic characteristics (precipitation); 2) constructs a series functions to model precipitation interception through canopy, surface runoff generation processes, processes of surface infiltration and subsurface runoff generation, subsurface infiltration to F store, which unveil the reallocation processes of ground precipitation to runoff (including surface and subsurface) and infiltration within each subcatchment; 3) builds water flux processes between each soil water store based on their water quantity (water level); 4) builds the links between the evaporation of each soil water store and ecological characteristics (LAI) to model evaporation of each store. In the end, LASCAM integrates runoff generated in each subcatchment into the whole catchment through stream network routing subroutine and the distribution status of each subcatchment within the whole catchment, which makes it possible to model the hydrological processes and water balance of large-scale heterogeneous catchment.