1. A computer-based graphical user interface for graphically controlling the processing of hydrological data, comprising: a display menu interface listing executable tools from the group comprising one or more of: hydrological data input, hydrological data validity checking, hydrological data correction and hydrological modeling; a GUI for an operator to select, and arrange tools from said display menu on a graphical display to construct said hydrological process, the selected tools being represented by separate icons on said graphical display, and for the operator to establish a connection between at least one output of a first tool and an input of a second tool said tools by graphically displaying the connection between said tools representative icon; and input means for receiving user input to arrange and execute said constructed hydrological data processing.