Pacific Southwest

Title: Frequency of floods from a burned chaparral watershed

Author: Nasseri, Iraj

Date: 1989

Source: In: Berg, Neil H. tech. coord. Proceedings of the Symposium on Fire and Watershed Management: October 26-28, 1988, Sacramento, California. Gen. Tech. Rep. PSW-109. Berkeley, Calif.: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station: 68-71

Station ID: GTR-PSW-109

Description: Effects of brush fire on hydrologic characteristics of chaparral watersheds were analyzed. An unburned chaparral produces moderate surface runoff. The vegetation promotes infiltration by retarding the runoff and providing temporary storage during intense rainfall. The hydrologic characteristics of chaparral watershed, however, are drastically changed by fires. The high rate of runoff following brush fires may result from the combined effects of denudation and formation of a water-repellent soil layer beneath the ground surface. This layer greatly decreases infiltration rates and reduces the hydrologically active portion of the watershed. Infiltrometer tests were performed on burned and unburned watersheds with similar soil types. The test results for the selected sites showed that for simulated rainfall intensities of one-inch per hour or more, the average ratio of runoff rate to rainfall intensity could be two times as great for the burned as for the unburned condition. To simulate floods following a brush fire, the Stanford Watershed Model was calibrated to a burned watershed using the hydrologic data of the postfire period. The floods were simulated by postulating scenarios that historical storms may occur following a brush fire. The study showed that the moderate storms may produce floods of considerable magnitude under a burned condition.

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Citation

Nasseri, Iraj  1989.  Frequency of floods from a burned chaparral watershed.   In: Berg, Neil H. tech. coord. Proceedings of the Symposium on Fire and Watershed Management: October 26-28, 1988, Sacramento, California. Gen. Tech. Rep. PSW-109. Berkeley, Calif.: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station: 68-71.