"This paper presents the new fundamental concept of the hyper-effective drainage area, the area of the high severity burn plus the area of the moderate severity burn, and provides an empirical formula to estimate the 5-year peak discharge from small post-burn watersheds to demonstrate the use of the concept. The equation uses the documented hydrologic response within the first two years after the occurrence of wildfire of ten watersheds in Southeast Arizona. These watersheds are within the forested steep terrain of the Santa Catalina, Santa Rita, and Pinaleno Mountains. After the burns, frequent flash floods and occasional debris flows have occurred. 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. To predict the 'likely' peak flow that can be expected before a given burned watershed is back to conditions that resemble pre-burn hydrology, an empirical equation was devised to estimate the post-burn 5- year peak flow. The developed equation works reasonably well (cross validation adjusted coefficient of determination of 0.90) for the documented watersheds. Its ability to deal with topographic and geomorphologic diversity lies in the use of a multivariate runoff index that utilizes the hyper-effective drainage area (determined from burn severity), average basin elevation, and an objective modified channel relief ratio"--Abstract.

National Weather Service (NWS)

Arizona