Characterizing the response of a catchment to an extreme rainfall event using hydrometric and isotopic data

[1] In this study we investigate an event-scale transit time distribution (TTD) for a catchment located in the Santa Catalina Mountains northeast of downtown Tucson, Arizona, USA, during a series of extreme rainfall events. Traditionally, TTD studies look at the long time response of a catchment to an input variation in tracer concentration. Few studies have determined the TTD at the scale of an event. Isotopic and hydrometric data collected during the event considered in this study provide the unique opportunity to characterize the response of the catchment under extreme conditions using both TTD modeling and hydrograph separation. This revealed similarity in the shapes of the event TTD and event water recession hydrograph. Also, the first moment of the event water recession hydrograph and the mean transit time of the TTD were similar. Because of the extreme nature of this event, it is likely that the catchment reaches and relaxes from the maximum possible storage, making its response a time-invariant characteristic of the catchment on the basis of hydraulic theory. The similarity between the event water recession and event TTD during this characteristic response of the catchment may, thus, allow for the derivation of a time-invariant event TTD. This result is especially valuable as it lays a basis for catchment similarity analysis linking a catchment's hydrological response and geomorphic properties.