Probable peak discharges and erosion rates from southern California watersheds as influenced by fire

Abstract

Damages from floods and erosion have been a serious problem in southern California since early pioneer days. The problem is becoming even more serious as the rapidly increasing population and expanding industrial and agricultural development encroach upon the flood plains and extend up the steep slopes and into canyons of the nearby mountains. Protection of forest vegetation from fire is an extremely important factor in maintaining damages at a minimum. The removal of the vegetative cover of the mountain watersheds by fire greatly increases the flood peaks and erosion rates with a consequent increase in flood and erosion damage. Estimates of the amount of damage, both actual and potential, caused by increased flood and erosion as a result of fires, therefore, are essential in checking the adequacy of present fire protection and in planning protection levels in keeping with the values involved. This publication presents the results of one phase of a study seeking a uniform method of making such estimates for the national forests of southern California. The immediate objective of this phase of the study was to provide basic hydrologic information by (1) establishing reasonable estimates of the average frequency and size of flood events and erosion rates with watershed vegetation in normal condition, that is, fully recovered from past burns, and (2) determining the effect of burning the vegetation on those flood peaks and erosion rates. Hydrologic data of this kind are critically needed for management of mountain watersheds in southern California. For this reason the results of this phase of the study are presented here, in advance of detailed discussion of the analysis, to make the data immediately available for those having need of such information. The estimates of peak discharge and erosion are given for 256 watershed units with a combined area of approximately 6800 square miles. These watershed units embrace a major portion of the higher mountain drainages in a 20- to 80-mile wide strip extending along the coast from the Mexican border to watersheds a few miles north of San Luis Obispo.

Citation

Rowe, P.B.; Countryman, C.M.; Storey, H.C. 1949. Probable peak discharges and erosion rates from southern California watersheds as influenced by fire. U.S. Department of Agriculture, Forest Service, California Forest and Range Experiment Station. 107 p.