Sharon L. E. Desilets

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

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, AZ 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 provides 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. Due to 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 based on 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.

Impact of recent extreme Arizona storms

Heavy rainfall on 27–31 July 2006 led to record flooding and triggered an historically unprecedented number of debris flows in the Santa Catalina Mountains north of Tucson, Ariz. The U.S. Geological Survey (USGS) documented record floods along four watercourses in the Tucson basin, and at least 250 hillslope failures spawned damaging debris flows in an area where less than 10 small debris flows had been documented in the past 25 years. At least 18 debris flows destroyed infrastructure in the heavily used Sabino Canyon Recreation Area (http://wwwpaztcn.wr.usgs.gov/rsch_highlight/articles/20061 l.html). In four adjacent canyons, debris flows reached the heads of alluvial fans at the boundary of the Tucson metropolitan area. While landuse planners in southeastern Arizona evaluate the potential threat of this previously little recognized hazard to residents along the mountain front, an interdisciplinary group of scientists has collaborated to better understand this extreme event.