Hydrologic Response of Post-Burn Watersheds in Southeast Arizona
Southeast Arizona witnessed a large number of wildfires during 2002-2004. Several of these fires were on the forested steep terrain of the Santa Catalina and Pinaleno Mountains. Frequent flash floods and occasional debris flows followed. A few of the flash floods were particularly severe resulting in one fatality, several evacuations of flood prone areas, and the destruction of four stream gaging sites.

Post-burn flows were 1.3 to 6.5 times pre-burn flows in the Santa Catalina Mountains. Post-burn flows in the Pinaleno Mountains, where the average channel gradients are 2 to 3 times steeper, were 27 to 154 times pre-burn flows.

From these mountainous burn areas, rare hydrologic events occurred from rather common rainfall events. Half hour basin average precipitation frequencies of 2 years resulted in 30-year to 75-year return flows in the Santa Catalina Mountains. Half hour basin average precipitation frequencies of 5 to 7 years resulted in 250-year to 300-year return flows, containing significant debris entrainment in the Pinaleno Mountains.

Mike Schaffner, WFO Tucson, and Bill Reed, Colorado Basin River Forecast Center