CRUM3 is a distribution catchment runoff model that evolved from the CRUM2D model developed in my PhD at University of Leeds. The Catchment Runoff Model (CRUM3) is a distributed, physically based, catchment hydrological model designed to balance the conflicting demands of computational costs, data needs, predictive uncertainty and detailed output. The model contains three major sections: 1, detailed weather generation; 2, terrestrial hydrological processes; and 3, in channel flow. These process representations are combined in a grid spatial structure to represent the full catchment system.