Catchment response to precipitation is often investigated using two-component isotope-based hydrograph separation, which quantifies the contribution of precipitation (i.e., event water Q.sub.e) or water from storage (i.e., pre-event water Q.sub.pe) to total discharge (Q) during storm events. In order to better understand streamflow-generating mechanisms, two-component hydrograph separation studies often seek to relate the event-water fraction Q.sub.e /Q to storm characteristics or antecedent wetness conditions. However, these relationships may be obscured because the same factors that influence Q.sub.e also necessarily influence total discharge Q as well. Here we propose that the fractions of event water and pre-event water relative to total precipitation (Q.sub.e /P and Q.sub.pe /P), instead of total discharge, provide useful alternative tools for studying catchment storm responses. These two quantities separate the well-known runoff coefficient (Q/P, i.e., the ratio between total discharge and precipitation volumes over the event timescale) into its contributions from event water and pre-event water. Whereas the runoff coefficient Q/P quantifies how strongly precipitation inputs affect streamflow, the fractions Q.sub.e /P and Q.sub.pe /P track the sources of this streamflow response.