Conceptual models simulate conversion of rainfall to runoff by simulating some or all of the processes illustrated in . Thus, to use a conceptual model, rainfall data are required. These data include both observations of rainfall at gauges in the watershed and statistics on rainfall from which are developed.

With observations of rainfall at gauges, models can be calibrated and tested to ensure that they truly represent the behavior of the watershed.

With statistics of rainfall depths, a design storm can be developed, and the required design flow can be computed following the design storm assumption. This assumption is that “if median or average values of all other parameters are used, the frequency of the derived flood should be approximately equal to the frequency of the design rainfall” (Pilgrim and Cordery 1975).