The rainfall‐runoff relationships on several impervious urban drainage areas varying in size from about 0.4 to 1.0 acre were investigated. The effective storm pattern, divided into a sequence of 1‐minute inputs, provided the basis for generation of a corresponding sequence of runoff outputs, which were combined to produce the total runoff hydrograph. The basic output is a unit hydrograph resulting from an effective rainfall input of 1 minute's duration. (Key words: Runoff; unit hydrographs; urban areas)