Abstract: Over a period of ten years, simultaneous measurement of storm rainfall and resulting runoff during individual storms were made in small basins in the San Francisco Bay Area, California. By simple measurement, without any recording devices, data collected define a relation of basin lag time, expressed as time between center of mass of rainfall and center of mass of runoff, is a specific measure of some basin characteristics including the effect of urbanization. Originally published: Catena; v. 18, no. 2 (April 1991).